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Experimental research on wear of conical pick interacting with coal-rock
Engineering Failure Analysis 74 (2017) 172–187
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Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Experimental research on wear of conical pick interacting with coal-rock☆ Songyong Liu a,b,⁎, Huifu Ji a,b,⁎, Xiaohui Liu c, Hongxiang Jiang a,b a b c
School of Mechatronic Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China Jiangsu Key Laboratory of Mine Mechanical and Electrical Equipment, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China School of Construction Machinery, Chang'an University, Xi'an 710064, China
a r t i c l e
i n f o
Article history: Received 18 November 2016 Received in revised form 15 January 2017 Accepted 15 January 2017 Available online 16 January 2017 Keywords: Conical pick Coal-rock Interaction Wear Cutting performance
a b s t r a c t For severe wear of a conical pick on tunneling and mining equipment, the influence of pick cutting types, structure and working angle parameters on pick wear is studied using an experimental apparatus for cutting coal-rock. The influence of pick wear on cutting performance is analyzed, which provides an experimental basis for reducing pick wear. The results indicate that the pick wears on only one side under asymmetrical cutting types, which increases the pick's working life. The larger the height of carbide tip and cone angle of pick tip are, the easier it is to protect the head face of the pick-body from interference with coal-rock and avoid carbide tip loss due to wear of the pick-body. In the range of research, the suitable height of the carbide tip is found to be 20 mm ~ 24 mm, and the cone angle of the pick tip should be approximately 80°. The wear area on the pick-body increases with the head face diameter of the pick-body, and a suitable range for the head face diameter is found to be 20 mm– 22 mm. Pick wear decreases with cutting angle, but the cutting angle should be maintained at 45°–50°, considering the pick cutting load and coal-rock caving effect. For picks with an inclined angle, pick wear increases with incline angle, however, wear differences on the two sides of the pick form and increase gradually, which improves the pick's self-rotatory ability. The mean peak of pick cutting torque increases with wear degree, and the largest increase percentage was found to be 30%. Furthermore, a worn pick is unfavorable for the caving of coalrock and the coal-rock fragment size of b 2 mm increases by 14% compared to a pick without wear. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction Coal is one of the most widely used fuels, and its use is expected to be prolonged over decades. The shearer drums and the cutting heads of road-headers are the main components used in the coal mining process, which consumes 80%–90% of the power of the entire shearer and road-header. Conical picks, repeatedly interacting with coal in the mining process, are regularly arranged on the helix drums and cutting heads [1–2]. Their performance has direct influence on the cutting performance and efficiency of the working mechanism. They generally consist of a carbide tip, pick-body and pick-handle, which is installed on the
☆ This paper was recommended for publication in revised form by Associate Editor 000 000. ⁎ Corresponding authors at: School of Mechatronic Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China. E-mail address: [email protected] (H. Ji).
http://dx.doi.org/10.1016/j.engfailanal.2017.01.013 1350-6307/© 2017 Elsevier Ltd. All rights reserved.
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Fig. 1. Conical pick and its installation.
pick-holder and fixed by a spring flange, as shown in Fig. 1. They are used to break coal-rock when the pick-holder is welded on the cutting head. The working condition of a pick is severe, complex and dynamic. Conical picks work under high impact and stress conditions, which often results in failure. Many researchers have paid increasing attention to pick failure, and have noted that the failure styles mainly include premature wear, carbide tip drop off, tipping, fracture and normal wear. Among them, failure due to wear such as normal wear, premature wear and carbide tip drop off, account for approximately 75% of the failures. To study how conical picks wear during use, on one hand, researchers are gradually studying the load on pick cutting coal-rock by simulation and experiment. For simulation, researchers mainly use the finite element method [3–5] and discrete element method [6–8]. For experimentation, researchers established different testing apparatuses, such as a Coal-Rock Cutting Test-Bed [9–11], Automated Rotary Coal/rock Cutting Simulator (ARCCS) [12–14], Single Pick Cutting Test-Bed [15–16], full-scale cutting rig [17–18] and reconstructive test apparatus [19–20]. However, researchers have found that not all the load on a pick resulted in wear, and there was little study on load resulting in wear. This is the main reason that the wear mechanism of picks remains underexposed. On the other hand, researchers are gradually conducting more studies on pick wear directly [20–23]. Pick wear was tested under multiple cutting tests to mainly study the wear types of the picks. Under these conditions, pick wear was light with only some surface scratches, and the tested wear mass loss was under one mg. Due to serious limitations, the results of this study are not yet enough to show the wear characteristics of a pick. Pick wear characteristics should include pick wear states in cutting processes and the influence of wear on pick cutting performance. Based on this, the influence of pick cutting types, structural and working angle parameters on pick wear is studied in this paper using an experimental apparatus for cutting coal-rock, and the influence of pick wear on cutting performance is analyzed to provide an experimental basis for reducing pick wear. 2. Experiment methodology Experiments were conducted on a Coal-Rock Cutting Test-Bed to simulate the process of a conical pick cutting coalrock. The test-bed includes a main and auxiliary transmission system, cutting apparatus, hydraulic control system and
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(a) Principle diagram for experiment
(b) Cutting apparatus
(c) Pick installation Fig. 2. Coal-rock cutting test-bed.
signal acquisition system. The main principle diagram is shown in Fig. 2(a). The main transmission system is used to cause the cutting apparatus to rotate. The motor power is 30 kW and the motor rotating speed can be controlled between 0 and 180 r/min to change the cutting speed. The cutting apparatus can be pushed to change the cutting width, which is controlled by a hydraulic system. The auxiliary transmission system is used to move the coal-rock. The movement is implemented by a hydraulic motor driving a gear-rack device, and its speed can be controlled within 0– 10 m/min to change the feeding speed. The cutting apparatus as design includes an end plate, wheel hub, chuck and pick installation, as shown in Fig. 2 (b) and (c). This study was conducted without considering the self-rotatory performance of a conical pick to determine the influence of pick cutting types, structure and working angle parameters on pick wear. Thus, the pick should not rotate in the pick holder during the experiment, and therefore the pick fixed block was designed to limit pick rotation. This article mainly focuses on the arrangement parameters as well as the structural parameters; different types of coal-rock are not analyzed further. According to previous successes in imitating coal-rock, cement and sand was used as the basic material to imitate coal-rock, with the proportion 1:2, respectively. The imitated coal-rock is characterized by strong homogeneity, which can eliminate the influence of the secondary factors and make the experimental results more reliable. Uniaxial compressive strength tests and Brazilian disc tests were conducted on imitation samples and natural samples, as shown in Fig. 3. The failure modes and crack growth of imitation samples are consistent with natural rock. The compressive strength of imitated coal-rock is approximately 20– 25 MPa, and the tensile strength is approximately 2 MPa. The imitation samples were split at a displacement of
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Fig. 3. Failure mode of imitation and rock samples.
0.75 mm, with a local peaking occurring, which is similar to the brittleness of natural rock. Compared to the stress curves and failure mode, the results indicated that the imitation coal-rock can be used to accurately simulate the characteristics of natural rock.
(a) Asymmetrical cutting
(b) Symmetrical cutting
Fig. 4. Kerf shapes under symmetrical and asymmetrical cutting types.
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(a) Asymmetrical cutting
(b) Symmetrical cutting
Fig. 5. Macro profile of pick with wear under symmetrical and asymmetrical cutting types.
3. Results and discussion 3.1. Wear of conical pick 3.1.1. Influence of cutting types Symmetrical and asymmetrical cutting types are the two main cutting types used by picks on the cutting head, and the kerf shapes of the cutting types are shown in Fig. 4. From Fig. 4(a), the coal under asymmetrical cutting types has one free side and one non-free side. The kerf I cut by the previous pick created a free surface for the next cutting by adjacent picks, and the cutting for creating kerf II can be regarded as an asymmetrical cutting process. From Fig. 4(b), the reaction of coal acts on both sides of the picks, leading to a force balance of picks. The cutting for creating kerf III can be regarded as a symmetrical cutting process. To study the influence of cutting types on pick wear, the common pick was selected, and the height of the carbide tip was 18.6 mm; the cone angle of carbide tip was 80°; the head face diameters of pick-body was 24 mm. The incline angle and cutting angle were 0° and 40°, respectively. The rotating speed of the cutting apparatus was set at 150 r/min by adjusting the motor frequency; the movement speed of the coal-rock was 1.5 m/min, and the maximum cutting depth of the pick was 1 cm under this condition. The macro profile of a pick with wear under symmetrical and asymmetrical cutting types are shown in Fig. 5. As seen in Fig. 5, the pick wears on only one side under an asymmetrical cutting type, while on both side under a symmetrical cutting type, which indicates that the wear mass loss under an asymmetrical cutting type is half of that under a symmetrical cutting type, and pick working life increases. The analysis above was conducted without considering the self-rotatory performance of a conical pick. However, torque around the pick axis, causing a conical pick to rotate due to imbalanced load on it during the cutting process under real working conditions. This can be seen from pick wear under two cutting types in Fig. 5 where load on the pick must be imbalanced under asymmetrical cutting types, and the pick has self-rotatory potential. When viewing wear on the side of the pick under an asymmetrical cutting type (A view), the direction of friction on the pick interacted by coal-rock can be determined. The wear is formed with a certain angle to the pick axis, which shows that friction force can be resolved along the pick axis and its vertical direction, and the force resolved along the vertical direction can cause the pick to rotate. Thus, the pick wear will obviously decrease due to the pick's self-rotatory ability under asymmetrical cutting types.
(a) Height 13.6 mm
(b) Height 18.6 mm
(c) Height 23.6 mm
Fig. 6. Macro profile of pick with wear under different height of carbide tip.
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3.1.2. Influence of pick structural parameters (1) Height of carbide tip The height of the carbide tip studied in this paper refers to the height of the carbide tip outside of the pick-body head face. To study the influence of carbide tip height on pick wear, the heights of the pick for testing were selected to be 13.6 mm, 16.1 mm, 18.6 mm, 21.1 mm and 23.6 mm. The incline angle, cutting angle, rotating speed of the cutting apparatus and movement speed of the coal-rock remained the same as the previous test. A symmetrical cutting type was adopted for pick cutting, and the macro profiles of picks with wear under different heights of the carbide tip are shown in Fig. 6, all under the same cutting time. From Fig. 6, wear area on pick-body decreases with increased height of the carbide tip, which indicates that the larger the height of the carbide tip, the easier it is for carbide tip to avoid the pick-body head face interfering with the coal-rock in the cutting process and to ensure that the carbide tip is not easily dropped off due to wear on the pick-body. Moreover, interference between the pick and coal-rock decreases due to the increase of carbide tip height, which indicates that the pick cutting load would also decrease with increased height of carbide tip. To verify this conjecture, pick cutting torque was tested in the cutting process, as shown in Fig. 7. Fig. 7(a) shows the tested torque curve under a continuous feeding cutting process. In continuous feeding cutting, the pick continues to rotate and feed; it completes the cutting process several times when rotating for several cycles and the maximum cutting load in every cycle is essentially identical. Fig. 7(a) shows the tested torque curve in one cutting cycle. Then, pick cutting torques of five different carbide tip heights were tested, and the statistics on mean peak of pick cutting torque was conducted, shown in Fig. 8.
(a) Continuous feeding cutting
(b) Single feeding cutting Fig. 7. Pick cutting torque.
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Fig. 8. Mean peak of pick cutting torque under different heights of carbide tip.
From Fig. 8, it can be seen that the mean peak of pick cutting torque decreases first rapidly and then slowly with increased height of the carbide tip. The trend eventually becomes stable. The difference of interference between the pick and coal-rock is large when the height of carbide tip changes from 13.6 mm to 18.6 mm, which results in the cutting load decreasing rapidly. On the contrary, the cutting load decreases slowly because of the small difference of interference between the pick and coalrock when the height of carbide tip changes from 18.6 mm to 23.6 mm. It can be concluded that a large height of the carbide tip should be selected in pick design to protect the pick body and decrease the cutting load, though it is not necessary to choose too large of a height. The effect of protecting the pick-body and decreasing cutting load weakens gradually when it reaches a certain value. (2) Cone angle of pick tip The coal-rock caving performance is different along the cutting depth. The caving performance of coal-rock is good in low depth, and the caving fragments are almost sheet-like in shape. With the increase of cutting depth, especially near the pick tip, the caving performance is poor without free surfaces. Under this condition, the formed groove shape is almost consistent with the pick carbide tip, which indicates that there is an influence by pick tip cone angle on the caving angle, and ultimately on the wear of the pick carbide tip. To study the influence of the pick tip cone angle on pick wear, the cone angles were selected to be 60°, 70°, 80°, 90° and 100°. The incline angle, cutting angle, rotating speed of cutting apparatus, and movement speed of the coal-rock remained the same. A symmetrical cutting type was adopted for the pick cutting, and the macro profile of the pick with wear under different cone angles of the pick tip are shown in Fig. 9 at the same cutting time. From Fig. 9, the wear area on the pick-body decreases with the cone angle of the pick tip, which indicates that the larger the cone angle of the pick tip, the easier it is for the carbide tip to avoid pick-body head face interference with the coal-rock in the cutting process, and ensure that the carbide tip is does not easily drop off due to wear on the pick-body. The main reason for this is that the caving performance is poor due to the cutting depth being large near the carbide tip where it is far from the free surface, and the formed groove is almost consistent with the pick carbide tip in shape. It can be concluded that a small caving angle is formed by a small cone angle of the pick tip, while a large caving angle is formed by a large cone angle. Thus, under the same
(a) Cone angle 60°
(b) Cone angle 80°
(c) Cone angle 100°
Fig. 9. Macro profile of the pick with wear under different cone angles of pick tip.
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pick-body structure conditions, the pick is much causes more interference in a groove with a small caving angle resulting in more pick wear. This conclusion can be obtained from Fig. 10, and also from the ‘V’ Shaped grooves formed by different cone angles of pick tip in experiments, shown in Fig. 11. Fig. 10 shows the pick carbide tip structure and groove structure in A. Q. Rizwan's PhD dissertation [15]. It can be seen that the groove formed near the carbide tip is almost consistent with the pick carbide tip in shape, which verifies the theory analysis. From Fig. 11, it can also be seen that the cone angle of the pick tip is almost consistent with the caving angle of the ‘V’ Shaped groove. It should be noted that the caving angle of the ‘V’ Shaped groove exceeds 100° when the cone angle is 100°, and the caving line is a semicircular curve and not a straight line. The semicircular curve is formed by the circular column of the carbide tip cutting the coal-rock after the pick tip. From this, caving angles larger than the cone angle can be formed by a large cone angle of the pick tip, which helps protect the pick-body from wear. Similarly, pick cutting torques of five different pick tip cone angles were tested, and the statistics on the mean peak pick cutting torque was determined, as shown in Fig. 12. From Fig. 12, it can be seen that the mean peak pick cutting torque decreases first and then increases with the increasing pick tip cone angle. From 60° to 80°, though the contact area between coal-rock and pick rake face increases with cone angle, the interference between pick-body and coal-rock decreases. Thus, the mean peak of the cutting torque shows a slight drop with increasing cone angle. However, from 80° to 100°, the contact area between the coal-rock and pick rake face increases continuously, and the interference between the pick-body and coal-rock changes little, thought the interference between the carbide tip and coal-rock increases significantly. It can be concluded that the proper cone angle of the pick tip (approximately 80°) should be selected in pick design. Too small of a cone angle does not help avoid interference between the pick-body and coalrock, and too large of a cone angle increases pick cutting load. (3) Head face diameter of pick-body Surfacing welding picks have been used to solve the problem of pick failure due to wear in the cutting process. A normal pick and a surfacing welding pick are shown in Fig. 11. From Fig. 13, the essential aim of surfacing welding is to increase the head face diameter of the pick-body and its anti-wear capacity. Though the pick-body interacts with the coal-rock, the pick can be used for a long time. However, whether the essential aim of surfacing welding would be achieved should be discussed. There are mainly two questions: whether the usage time of a surfacing welding pick is longer than that of a normal pick, and whether surfacing welding technology changes the cutting performance of the pick. To investigate the first question, changing the head face diameter of pick-body was adopted to imitate a pick structural change by surfacing welding, and the influence of this change on pick wear was studied. The head face diameter of the pick-body was selected to be 20 mm, 22 mm, 24 mm, 26 mm and 28 mm, according to the increasing thickness of the pick-body, which is 2– 4 mm after the surfacing welding currently used in the industry. The incline angle, cutting angle, rotating speed of the cutting apparatus, and
Fig. 10. Shape of carbide tip and cutting groove (from A. Q. Rizwan's PhD dissertation).
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(a) Cone angle 60°
(b) Cone angle 80° (c) Cone angle 100°
Fig. 11. ‘V’ Shaped grooves formed by different cone angles of pick tip.
movement speed of the coal-rock remained the same. A symmetrical cutting type was adopted for the pick cutting, and the macro profile of the pick with wear under different head face diameters of the pick-body is shown in Fig. 14 at the same cutting times. From Fig. 14, the wear area on the pick-body increases with head face diameter of the pick-body, and there is almost no wear on the pick-body when the head face diameter is 20 mm. This indicates that the smaller the head face diameter is, the easier it is for the pick-body to avoid interfering with the coal-rock; otherwise, a large pick-body head face will wear with the carbide tip, which shows that it cannot avoid carbide tip drop off. Moreover, the surface hardness of the pick-body achieved 62 HRC after surfacing welding widely currently used in the industry, which is much lower than the hardness of a carbide tip, which should not be b85 HRC according to the Coal Industry Standard. Thus, the pick-body after surfacing welding wears more quickly than a carbide tip, and it does not help the carbide tip cut the coal-rock. In short, surfacing welding used on a pick is a method used at the expense of material and technology to increase pick life, but the pick life will not increase proportionally with surfacing welding strength and thickness, which does not conform to the principles of economy. For the second question, pick cutting torques of five different head face diameters of pick-bodies were tested, and the statistics of mean peak pick cutting torque were determined, as shown in Fig. 15. From Fig. 15, it can be seen that mean peak pick cutting torque increases first slowly and then rapidly with increasing head face diameter of the pick-body. The difference of interference between the pick and coal-rock is small when head face diameter of the pick-body changes from 20 mm to 24 mm, which results in a minor increase in cutting load. When the head face diameter changes from 24 mm to 28 mm, cutting load increases greatly due to the large difference of interference between the pick and coal-rock. It can be concluded that a small head face diameter should be selected in pick design to protect the pick-body and decrease the cutting load, though it should not be too small. The effect of protecting the pick-body and decreasing cutting load weakens gradually when decreases to a certain value.
Fig. 12. Mean peak of pick cutting torque under different cone angles of pick tip.
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(a) Normal pick
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(b) Surfacing welding pick
Fig. 13. Normal pick and surfacing welding pick.
3.1.3. Influence of pick cutting angle parameters (1) Cutting angle To study the influence of cutting angle on pick wear, the common pick was selected, and the incline angle was chosen to be 0°. The cutting angles were selected to be 35°, 40°, 45°, 50° and 55°. The rotating speed of the cutting apparatus and movement speed of the coal-rock remained the same. A symmetrical cutting type was adopted for pick cutting, and the macro profile of the pick with wear under different cutting angles is shown in Fig. 16 at the same cutting times. From Fig. 16, it can be seen that the wear length of the carbide tip interacting with coal-rock decreases with an increase in the cutting angle. The wear length decreases rapidly from 35° to 45°, then slowly from 45° to 55°. The wear length of the carbide tip is consistent with the height of the carbide tip when the cutting angle is 35°, while it is the same with as the height of the carbide tip cone section when 40°. Moreover, wear of the pick-body decreases with cutting angle; there is obvious wear on the pick-body, when the cutting angle is 35°, while wear is present on both sides of the pick-body when the cutting angle is 40° and 45°, and almost no wear on pick-body when the cutting angle is 50° and 55°. Similarly, the contact between the carbide tip, pick-body and coal-rock decreases with cutting angle, especially from 35° to 45°, which indicates that the cutting load would drop obviously. However, the contact between the carbide tip, pick-body and coal-rock changes little from 45° to 55°. The pick cutting load is influenced by caving performance of coal-rock, and the load change trend should be further studied. Thus, pick cutting torques of five different cutting angles were tested, and the statistics for mean peak pick cutting torque were determined, as shown in Fig. 17. From Fig. 17, mean peak of pick cutting torque shows first a large drop and then a slight increase with increasing cutting angle. The large drop from 35° to 45° is due to the reduction of interference between the carbide tip, pick-body and coal-rock, while the slight increase from 45° to 55° is due to the small change in between the pick and coal-rock, and the caving performance becomes poor with an increase in the cutting angle. To study the caving performance of coal-rock under different cutting angles, coal-rock fragments from the experiments were collected, and different fragment sizes of coal-rock are shown in Fig. 18. The percentage of coal-rock fragment size under different cutting angles is shown in Fig. 19.
(a) 20 mm
(b) 24 mm
(c) 28 mm
Fig. 14. Macro profile of pick with wear under different head face diameters of pick-body.
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Fig. 15. Mean peak of pick cutting torque under different head face diameters of pick-body.
From Fig. 19, first, the percentage of coal-rock fragment b 2 mm decreases with increased cutting angle. The fragment size in this range is in a powdered form, which is formed by friction between the pick and coal-rock. The results correspond to the pick wear, and indicate again that the increase of cutting angle reduces the contact between pick and coal-rock. Secondly, the percentage of coal-rock fragments between 2 and 10 mm increases with cutting angle. The fragment sizes in this range are formed by interference between the pick and coal-rock. Unlike friction, the interference causes coal-rock fragments to be massive and are found not just in powder form. The caving performance becomes poor with the increase of cutting angle and the interference between pick and coal-rock increases, which increases the percentage of coal-rock fragment of sizes between 2 and 10 mm. Thirdly, the percentage of coal-rock fragments between 10 and 20 mm changes little, which indicates that this is the main range of pick
(a) Cutting angle 35°
(c) Cutting angle 45°
(b) Cutting angle 40°
(d) Cutting angle 50° (e) Cutting angle 55°
Fig. 16. Macro profile of pick with wear under different cutting angles.
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Fig. 17. Mean peak pick cutting torque under different cutting angles.
caving coal-rock. Lastly, the percentage of coal-rock fragments N20 mm decreases gradually from 9.2% under a cutting angle of 35° to 0% under a cutting angle of 50° and 55°, which further shows that a small cutting angle is favorable for caving of coal-rock. (2) Cutting angle The drilling process of the cutting head can be realized under the interaction of different cutting angles of picks. To study the influence of cutting angle on pick wear, the common pick was selected, and the cutting angle was chosen to be 50°. The incline angle was selected to be 10°, 20°, 30°, 40° and 50°. The rotating speed of the cutting apparatus and the movement speed of the coal-rock remained the same. Symmetrical cutting types were adopted for pick cutting, and the macro profile of the pick with wear under different cutting angles is shown in Fig. 20 for the same cutting times.
(a) <2 mm
(b) 2~10 mm
(c) 10~20 mm
(d) >20 mm
Fig. 18. Different fragment sizes coal-rock.
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Fig. 19. Percentage of coal-rock fragment size under different cutting angles.
In the cutting process, pick left-side wear decreases and right-side wear increases with incline angle when the pick inclines to the left. The wear height difference of both sides of pick carbide tip changes from none to large, until there is almost no wear on the left when incline angle is 50°. The wear on the pick-body changes from none under 10°–20° to small under 30°, to large under 50°. Moreover, it can be seen from the macro profile of the pick with wear that load on the pick becomes more imbalanced with the increase of incline angle, which improves pick self-rotatory ability if it could rotate freely. Pick cutting torques of five different incline angles were tested, and the statistics on mean peak of pick cutting torque were conducted, as shown in Fig. 21.
(a) Incline angle 10°
(b) Incline angle 20°
(c) Incline angle 30° (d) Incline angle 40° (e) Incline angle 50° Fig. 20. Macro profile of pick with wear under different incline angles.
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Fig. 21. Mean peak pick cutting torque under different incline angles.
Because the cutting angles were all 50° under different incline angles, the caving performance of coal-rock changes little and it has no influence on pick load. From Fig. 21, mean peak of pick cutting torque first increases slowly and then rapidly with increasing incline angle. This is related to the interference between the pick and coal-rock. The increase of incline angle mainly results in the increase of interference with coal-rock on one side of the pick, and ultimately leads to the rapid increase of cutting load. 3.2. Influence of wear on pick cutting performance Pick wear is gradually becoming a serious issue in use, and the cutting load and coal-rock fragments distribution vary with pick wear degree. For example, picks wear due to contact with coal-rock, but the increase of contact area due to wear accelerates its wear and increases its cutting load. Moreover, pick wear leads to inactivation of the pick tip, thus it becomes difficult for the pick to wedge into coal-rock and for the coal-rock to break, which increases the cutting specific energy consumption.
(a) Initial wear
(b) Interim wear (Front view)
(c) Interim wear (side view)
(d) Upper wear
Fig. 22. Macro profile of pick under different wear degree.
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Fig. 23. Mean peak of pick cutting torque with different wear degree.
To study the influence of pick wear on cutting performance, the common pick was selected, and the height of the carbide tip is set at 18.6 mm; the cone angle of the carbide tip is 80°; the head face diameters of pick-body is 24 mm. The incline angle and cutting angle is 0° and 40°, respectively. The rotating speed of the cutting apparatus and the movement speed of the coal-rock remained the same. Picks with different wear degrees, such as initial wear, interim wear and upper wear, were obtained by interacting with the coal-rock. Macro profiles of picks under different wear degrees are shown in Fig. 22. From Fig. 22, it can be seen that interference on both sides of the pick with coal-rock is larger than that on the pick tip during the continuous wearing process. It can be seen clearly from the comparison between the pick with interim wear in side view and pick with no wear that the decreased height of the pick tip due to wear is within 2 mm, while that of the pick on both sides including the carbide tip and pick-body is larger than that value. On the other hand, the pick tip gradually becomes circular-shaped during the continuous wearing process, shown in Fig. 22 (d), which is the most vital part influencing the cutting performance. The obtained picks under different wear degrees were used to study the influence of pick wear on cutting load. Pick cutting torques of picks with different wear degrees were tested, and the statistics on mean peak of pick cutting torque were conducted, as shown in Fig. 23. From Fig. 23, the mean peak of pick cutting torque increases with wear degree of the pick, which indicates that the pick wear becomes severe and increases the contact area between the pick and coal-rock and accelerates the pick wear. There is only a small difference between the pick with initial wear and the pick without wear, which is within 50 N·m. The cutting load of the pick with interim wear increases by approximately 20% compared with that of the pick without wear, while the cutting load increases by N 30% for the pick with upper wear. This shows that some measures should be taken for the worn picks to avoid worn surfaces
Fig. 24. Percentage of coal-rock fragment size under different wear degrees.
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contacting the coal-rock. Otherwise, the pick wear would be much more severe, and the pick would fail when losing its cutting performance. To study the caving performance of coal-rock under different wear degrees, coal-rock fragments from the experiments were collected. The percentage of coal-rock fragment sizes under different wear degrees is shown in Fig. 24.
4. Conclusions (1) Picks wear only on one side under asymmetrical cutting types, which increases working life. Loads on picks must be imbalanced under asymmetrical cutting types, causing the pick to have self-rotatory potential. (2) The larger the height of the carbide tip and cone angle of pick tip are, the easier it is to protect the head face of the pickbody from interference with coal-rock and avoid carbide tip loss due to wear of the pick-body. In the range of this research, the suitable height of the carbide tip was found to be 20 mm–24 mm, and the cone angle of the pick tip should be approximately 80°. Wear area on the pick-body increases with head face diameter of the pick-body, and it is suitable at 20 mm– 22 mm. (3) Pick wear decreases with cutting angle, but the cutting angle should be 45°–50° when considering pick cutting load and coal-rock caving effects. For picks with incline angles, pick wear increases with incline angle, however wear difference on two sides of the pick forms and increases gradually, which improves the pick self-rotatory ability. (4) Lastly, the influence of pick wear on cutting performance is analyzed. It indicates that the mean peak pick cutting torque increases with wear degree, and the largest increase percentage is 30%. Worn picks are unfavorable for caving of coal-rock. Compared with picks without wear, the coal-rock fragment size b2 mm increased by 14%.
Acknowledgment This work was supported by the Fundamental Research Funds for the Central Universities (2014ZDPY12), the Top-notch Academic Programs Project of Jiangsu Higher Education Institutions (TAPP) and the Priority Academic Program Development of Jiangsu High Education Institution of China (PAPD).
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Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Experimental research on wear of conical pick interacting with coal-rock☆ Songyong Liu a,b,⁎, Huifu Ji a,b,⁎, Xiaohui Liu c, Hongxiang Jiang a,b a b c
School of Mechatronic Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China Jiangsu Key Laboratory of Mine Mechanical and Electrical Equipment, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China School of Construction Machinery, Chang'an University, Xi'an 710064, China
a r t i c l e
i n f o
Article history: Received 18 November 2016 Received in revised form 15 January 2017 Accepted 15 January 2017 Available online 16 January 2017 Keywords: Conical pick Coal-rock Interaction Wear Cutting performance
a b s t r a c t For severe wear of a conical pick on tunneling and mining equipment, the influence of pick cutting types, structure and working angle parameters on pick wear is studied using an experimental apparatus for cutting coal-rock. The influence of pick wear on cutting performance is analyzed, which provides an experimental basis for reducing pick wear. The results indicate that the pick wears on only one side under asymmetrical cutting types, which increases the pick's working life. The larger the height of carbide tip and cone angle of pick tip are, the easier it is to protect the head face of the pick-body from interference with coal-rock and avoid carbide tip loss due to wear of the pick-body. In the range of research, the suitable height of the carbide tip is found to be 20 mm ~ 24 mm, and the cone angle of the pick tip should be approximately 80°. The wear area on the pick-body increases with the head face diameter of the pick-body, and a suitable range for the head face diameter is found to be 20 mm– 22 mm. Pick wear decreases with cutting angle, but the cutting angle should be maintained at 45°–50°, considering the pick cutting load and coal-rock caving effect. For picks with an inclined angle, pick wear increases with incline angle, however, wear differences on the two sides of the pick form and increase gradually, which improves the pick's self-rotatory ability. The mean peak of pick cutting torque increases with wear degree, and the largest increase percentage was found to be 30%. Furthermore, a worn pick is unfavorable for the caving of coalrock and the coal-rock fragment size of b 2 mm increases by 14% compared to a pick without wear. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction Coal is one of the most widely used fuels, and its use is expected to be prolonged over decades. The shearer drums and the cutting heads of road-headers are the main components used in the coal mining process, which consumes 80%–90% of the power of the entire shearer and road-header. Conical picks, repeatedly interacting with coal in the mining process, are regularly arranged on the helix drums and cutting heads [1–2]. Their performance has direct influence on the cutting performance and efficiency of the working mechanism. They generally consist of a carbide tip, pick-body and pick-handle, which is installed on the
☆ This paper was recommended for publication in revised form by Associate Editor 000 000. ⁎ Corresponding authors at: School of Mechatronic Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China. E-mail address: [email protected] (H. Ji).
http://dx.doi.org/10.1016/j.engfailanal.2017.01.013 1350-6307/© 2017 Elsevier Ltd. All rights reserved.
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Fig. 1. Conical pick and its installation.
pick-holder and fixed by a spring flange, as shown in Fig. 1. They are used to break coal-rock when the pick-holder is welded on the cutting head. The working condition of a pick is severe, complex and dynamic. Conical picks work under high impact and stress conditions, which often results in failure. Many researchers have paid increasing attention to pick failure, and have noted that the failure styles mainly include premature wear, carbide tip drop off, tipping, fracture and normal wear. Among them, failure due to wear such as normal wear, premature wear and carbide tip drop off, account for approximately 75% of the failures. To study how conical picks wear during use, on one hand, researchers are gradually studying the load on pick cutting coal-rock by simulation and experiment. For simulation, researchers mainly use the finite element method [3–5] and discrete element method [6–8]. For experimentation, researchers established different testing apparatuses, such as a Coal-Rock Cutting Test-Bed [9–11], Automated Rotary Coal/rock Cutting Simulator (ARCCS) [12–14], Single Pick Cutting Test-Bed [15–16], full-scale cutting rig [17–18] and reconstructive test apparatus [19–20]. However, researchers have found that not all the load on a pick resulted in wear, and there was little study on load resulting in wear. This is the main reason that the wear mechanism of picks remains underexposed. On the other hand, researchers are gradually conducting more studies on pick wear directly [20–23]. Pick wear was tested under multiple cutting tests to mainly study the wear types of the picks. Under these conditions, pick wear was light with only some surface scratches, and the tested wear mass loss was under one mg. Due to serious limitations, the results of this study are not yet enough to show the wear characteristics of a pick. Pick wear characteristics should include pick wear states in cutting processes and the influence of wear on pick cutting performance. Based on this, the influence of pick cutting types, structural and working angle parameters on pick wear is studied in this paper using an experimental apparatus for cutting coal-rock, and the influence of pick wear on cutting performance is analyzed to provide an experimental basis for reducing pick wear. 2. Experiment methodology Experiments were conducted on a Coal-Rock Cutting Test-Bed to simulate the process of a conical pick cutting coalrock. The test-bed includes a main and auxiliary transmission system, cutting apparatus, hydraulic control system and
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(a) Principle diagram for experiment
(b) Cutting apparatus
(c) Pick installation Fig. 2. Coal-rock cutting test-bed.
signal acquisition system. The main principle diagram is shown in Fig. 2(a). The main transmission system is used to cause the cutting apparatus to rotate. The motor power is 30 kW and the motor rotating speed can be controlled between 0 and 180 r/min to change the cutting speed. The cutting apparatus can be pushed to change the cutting width, which is controlled by a hydraulic system. The auxiliary transmission system is used to move the coal-rock. The movement is implemented by a hydraulic motor driving a gear-rack device, and its speed can be controlled within 0– 10 m/min to change the feeding speed. The cutting apparatus as design includes an end plate, wheel hub, chuck and pick installation, as shown in Fig. 2 (b) and (c). This study was conducted without considering the self-rotatory performance of a conical pick to determine the influence of pick cutting types, structure and working angle parameters on pick wear. Thus, the pick should not rotate in the pick holder during the experiment, and therefore the pick fixed block was designed to limit pick rotation. This article mainly focuses on the arrangement parameters as well as the structural parameters; different types of coal-rock are not analyzed further. According to previous successes in imitating coal-rock, cement and sand was used as the basic material to imitate coal-rock, with the proportion 1:2, respectively. The imitated coal-rock is characterized by strong homogeneity, which can eliminate the influence of the secondary factors and make the experimental results more reliable. Uniaxial compressive strength tests and Brazilian disc tests were conducted on imitation samples and natural samples, as shown in Fig. 3. The failure modes and crack growth of imitation samples are consistent with natural rock. The compressive strength of imitated coal-rock is approximately 20– 25 MPa, and the tensile strength is approximately 2 MPa. The imitation samples were split at a displacement of
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Fig. 3. Failure mode of imitation and rock samples.
0.75 mm, with a local peaking occurring, which is similar to the brittleness of natural rock. Compared to the stress curves and failure mode, the results indicated that the imitation coal-rock can be used to accurately simulate the characteristics of natural rock.
(a) Asymmetrical cutting
(b) Symmetrical cutting
Fig. 4. Kerf shapes under symmetrical and asymmetrical cutting types.
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(a) Asymmetrical cutting
(b) Symmetrical cutting
Fig. 5. Macro profile of pick with wear under symmetrical and asymmetrical cutting types.
3. Results and discussion 3.1. Wear of conical pick 3.1.1. Influence of cutting types Symmetrical and asymmetrical cutting types are the two main cutting types used by picks on the cutting head, and the kerf shapes of the cutting types are shown in Fig. 4. From Fig. 4(a), the coal under asymmetrical cutting types has one free side and one non-free side. The kerf I cut by the previous pick created a free surface for the next cutting by adjacent picks, and the cutting for creating kerf II can be regarded as an asymmetrical cutting process. From Fig. 4(b), the reaction of coal acts on both sides of the picks, leading to a force balance of picks. The cutting for creating kerf III can be regarded as a symmetrical cutting process. To study the influence of cutting types on pick wear, the common pick was selected, and the height of the carbide tip was 18.6 mm; the cone angle of carbide tip was 80°; the head face diameters of pick-body was 24 mm. The incline angle and cutting angle were 0° and 40°, respectively. The rotating speed of the cutting apparatus was set at 150 r/min by adjusting the motor frequency; the movement speed of the coal-rock was 1.5 m/min, and the maximum cutting depth of the pick was 1 cm under this condition. The macro profile of a pick with wear under symmetrical and asymmetrical cutting types are shown in Fig. 5. As seen in Fig. 5, the pick wears on only one side under an asymmetrical cutting type, while on both side under a symmetrical cutting type, which indicates that the wear mass loss under an asymmetrical cutting type is half of that under a symmetrical cutting type, and pick working life increases. The analysis above was conducted without considering the self-rotatory performance of a conical pick. However, torque around the pick axis, causing a conical pick to rotate due to imbalanced load on it during the cutting process under real working conditions. This can be seen from pick wear under two cutting types in Fig. 5 where load on the pick must be imbalanced under asymmetrical cutting types, and the pick has self-rotatory potential. When viewing wear on the side of the pick under an asymmetrical cutting type (A view), the direction of friction on the pick interacted by coal-rock can be determined. The wear is formed with a certain angle to the pick axis, which shows that friction force can be resolved along the pick axis and its vertical direction, and the force resolved along the vertical direction can cause the pick to rotate. Thus, the pick wear will obviously decrease due to the pick's self-rotatory ability under asymmetrical cutting types.
(a) Height 13.6 mm
(b) Height 18.6 mm
(c) Height 23.6 mm
Fig. 6. Macro profile of pick with wear under different height of carbide tip.
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3.1.2. Influence of pick structural parameters (1) Height of carbide tip The height of the carbide tip studied in this paper refers to the height of the carbide tip outside of the pick-body head face. To study the influence of carbide tip height on pick wear, the heights of the pick for testing were selected to be 13.6 mm, 16.1 mm, 18.6 mm, 21.1 mm and 23.6 mm. The incline angle, cutting angle, rotating speed of the cutting apparatus and movement speed of the coal-rock remained the same as the previous test. A symmetrical cutting type was adopted for pick cutting, and the macro profiles of picks with wear under different heights of the carbide tip are shown in Fig. 6, all under the same cutting time. From Fig. 6, wear area on pick-body decreases with increased height of the carbide tip, which indicates that the larger the height of the carbide tip, the easier it is for carbide tip to avoid the pick-body head face interfering with the coal-rock in the cutting process and to ensure that the carbide tip is not easily dropped off due to wear on the pick-body. Moreover, interference between the pick and coal-rock decreases due to the increase of carbide tip height, which indicates that the pick cutting load would also decrease with increased height of carbide tip. To verify this conjecture, pick cutting torque was tested in the cutting process, as shown in Fig. 7. Fig. 7(a) shows the tested torque curve under a continuous feeding cutting process. In continuous feeding cutting, the pick continues to rotate and feed; it completes the cutting process several times when rotating for several cycles and the maximum cutting load in every cycle is essentially identical. Fig. 7(a) shows the tested torque curve in one cutting cycle. Then, pick cutting torques of five different carbide tip heights were tested, and the statistics on mean peak of pick cutting torque was conducted, shown in Fig. 8.
(a) Continuous feeding cutting
(b) Single feeding cutting Fig. 7. Pick cutting torque.
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Fig. 8. Mean peak of pick cutting torque under different heights of carbide tip.
From Fig. 8, it can be seen that the mean peak of pick cutting torque decreases first rapidly and then slowly with increased height of the carbide tip. The trend eventually becomes stable. The difference of interference between the pick and coal-rock is large when the height of carbide tip changes from 13.6 mm to 18.6 mm, which results in the cutting load decreasing rapidly. On the contrary, the cutting load decreases slowly because of the small difference of interference between the pick and coalrock when the height of carbide tip changes from 18.6 mm to 23.6 mm. It can be concluded that a large height of the carbide tip should be selected in pick design to protect the pick body and decrease the cutting load, though it is not necessary to choose too large of a height. The effect of protecting the pick-body and decreasing cutting load weakens gradually when it reaches a certain value. (2) Cone angle of pick tip The coal-rock caving performance is different along the cutting depth. The caving performance of coal-rock is good in low depth, and the caving fragments are almost sheet-like in shape. With the increase of cutting depth, especially near the pick tip, the caving performance is poor without free surfaces. Under this condition, the formed groove shape is almost consistent with the pick carbide tip, which indicates that there is an influence by pick tip cone angle on the caving angle, and ultimately on the wear of the pick carbide tip. To study the influence of the pick tip cone angle on pick wear, the cone angles were selected to be 60°, 70°, 80°, 90° and 100°. The incline angle, cutting angle, rotating speed of cutting apparatus, and movement speed of the coal-rock remained the same. A symmetrical cutting type was adopted for the pick cutting, and the macro profile of the pick with wear under different cone angles of the pick tip are shown in Fig. 9 at the same cutting time. From Fig. 9, the wear area on the pick-body decreases with the cone angle of the pick tip, which indicates that the larger the cone angle of the pick tip, the easier it is for the carbide tip to avoid pick-body head face interference with the coal-rock in the cutting process, and ensure that the carbide tip is does not easily drop off due to wear on the pick-body. The main reason for this is that the caving performance is poor due to the cutting depth being large near the carbide tip where it is far from the free surface, and the formed groove is almost consistent with the pick carbide tip in shape. It can be concluded that a small caving angle is formed by a small cone angle of the pick tip, while a large caving angle is formed by a large cone angle. Thus, under the same
(a) Cone angle 60°
(b) Cone angle 80°
(c) Cone angle 100°
Fig. 9. Macro profile of the pick with wear under different cone angles of pick tip.
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pick-body structure conditions, the pick is much causes more interference in a groove with a small caving angle resulting in more pick wear. This conclusion can be obtained from Fig. 10, and also from the ‘V’ Shaped grooves formed by different cone angles of pick tip in experiments, shown in Fig. 11. Fig. 10 shows the pick carbide tip structure and groove structure in A. Q. Rizwan's PhD dissertation [15]. It can be seen that the groove formed near the carbide tip is almost consistent with the pick carbide tip in shape, which verifies the theory analysis. From Fig. 11, it can also be seen that the cone angle of the pick tip is almost consistent with the caving angle of the ‘V’ Shaped groove. It should be noted that the caving angle of the ‘V’ Shaped groove exceeds 100° when the cone angle is 100°, and the caving line is a semicircular curve and not a straight line. The semicircular curve is formed by the circular column of the carbide tip cutting the coal-rock after the pick tip. From this, caving angles larger than the cone angle can be formed by a large cone angle of the pick tip, which helps protect the pick-body from wear. Similarly, pick cutting torques of five different pick tip cone angles were tested, and the statistics on the mean peak pick cutting torque was determined, as shown in Fig. 12. From Fig. 12, it can be seen that the mean peak pick cutting torque decreases first and then increases with the increasing pick tip cone angle. From 60° to 80°, though the contact area between coal-rock and pick rake face increases with cone angle, the interference between pick-body and coal-rock decreases. Thus, the mean peak of the cutting torque shows a slight drop with increasing cone angle. However, from 80° to 100°, the contact area between the coal-rock and pick rake face increases continuously, and the interference between the pick-body and coal-rock changes little, thought the interference between the carbide tip and coal-rock increases significantly. It can be concluded that the proper cone angle of the pick tip (approximately 80°) should be selected in pick design. Too small of a cone angle does not help avoid interference between the pick-body and coalrock, and too large of a cone angle increases pick cutting load. (3) Head face diameter of pick-body Surfacing welding picks have been used to solve the problem of pick failure due to wear in the cutting process. A normal pick and a surfacing welding pick are shown in Fig. 11. From Fig. 13, the essential aim of surfacing welding is to increase the head face diameter of the pick-body and its anti-wear capacity. Though the pick-body interacts with the coal-rock, the pick can be used for a long time. However, whether the essential aim of surfacing welding would be achieved should be discussed. There are mainly two questions: whether the usage time of a surfacing welding pick is longer than that of a normal pick, and whether surfacing welding technology changes the cutting performance of the pick. To investigate the first question, changing the head face diameter of pick-body was adopted to imitate a pick structural change by surfacing welding, and the influence of this change on pick wear was studied. The head face diameter of the pick-body was selected to be 20 mm, 22 mm, 24 mm, 26 mm and 28 mm, according to the increasing thickness of the pick-body, which is 2– 4 mm after the surfacing welding currently used in the industry. The incline angle, cutting angle, rotating speed of the cutting apparatus, and
Fig. 10. Shape of carbide tip and cutting groove (from A. Q. Rizwan's PhD dissertation).
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(a) Cone angle 60°
(b) Cone angle 80° (c) Cone angle 100°
Fig. 11. ‘V’ Shaped grooves formed by different cone angles of pick tip.
movement speed of the coal-rock remained the same. A symmetrical cutting type was adopted for the pick cutting, and the macro profile of the pick with wear under different head face diameters of the pick-body is shown in Fig. 14 at the same cutting times. From Fig. 14, the wear area on the pick-body increases with head face diameter of the pick-body, and there is almost no wear on the pick-body when the head face diameter is 20 mm. This indicates that the smaller the head face diameter is, the easier it is for the pick-body to avoid interfering with the coal-rock; otherwise, a large pick-body head face will wear with the carbide tip, which shows that it cannot avoid carbide tip drop off. Moreover, the surface hardness of the pick-body achieved 62 HRC after surfacing welding widely currently used in the industry, which is much lower than the hardness of a carbide tip, which should not be b85 HRC according to the Coal Industry Standard. Thus, the pick-body after surfacing welding wears more quickly than a carbide tip, and it does not help the carbide tip cut the coal-rock. In short, surfacing welding used on a pick is a method used at the expense of material and technology to increase pick life, but the pick life will not increase proportionally with surfacing welding strength and thickness, which does not conform to the principles of economy. For the second question, pick cutting torques of five different head face diameters of pick-bodies were tested, and the statistics of mean peak pick cutting torque were determined, as shown in Fig. 15. From Fig. 15, it can be seen that mean peak pick cutting torque increases first slowly and then rapidly with increasing head face diameter of the pick-body. The difference of interference between the pick and coal-rock is small when head face diameter of the pick-body changes from 20 mm to 24 mm, which results in a minor increase in cutting load. When the head face diameter changes from 24 mm to 28 mm, cutting load increases greatly due to the large difference of interference between the pick and coal-rock. It can be concluded that a small head face diameter should be selected in pick design to protect the pick-body and decrease the cutting load, though it should not be too small. The effect of protecting the pick-body and decreasing cutting load weakens gradually when decreases to a certain value.
Fig. 12. Mean peak of pick cutting torque under different cone angles of pick tip.
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(a) Normal pick
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(b) Surfacing welding pick
Fig. 13. Normal pick and surfacing welding pick.
3.1.3. Influence of pick cutting angle parameters (1) Cutting angle To study the influence of cutting angle on pick wear, the common pick was selected, and the incline angle was chosen to be 0°. The cutting angles were selected to be 35°, 40°, 45°, 50° and 55°. The rotating speed of the cutting apparatus and movement speed of the coal-rock remained the same. A symmetrical cutting type was adopted for pick cutting, and the macro profile of the pick with wear under different cutting angles is shown in Fig. 16 at the same cutting times. From Fig. 16, it can be seen that the wear length of the carbide tip interacting with coal-rock decreases with an increase in the cutting angle. The wear length decreases rapidly from 35° to 45°, then slowly from 45° to 55°. The wear length of the carbide tip is consistent with the height of the carbide tip when the cutting angle is 35°, while it is the same with as the height of the carbide tip cone section when 40°. Moreover, wear of the pick-body decreases with cutting angle; there is obvious wear on the pick-body, when the cutting angle is 35°, while wear is present on both sides of the pick-body when the cutting angle is 40° and 45°, and almost no wear on pick-body when the cutting angle is 50° and 55°. Similarly, the contact between the carbide tip, pick-body and coal-rock decreases with cutting angle, especially from 35° to 45°, which indicates that the cutting load would drop obviously. However, the contact between the carbide tip, pick-body and coal-rock changes little from 45° to 55°. The pick cutting load is influenced by caving performance of coal-rock, and the load change trend should be further studied. Thus, pick cutting torques of five different cutting angles were tested, and the statistics for mean peak pick cutting torque were determined, as shown in Fig. 17. From Fig. 17, mean peak of pick cutting torque shows first a large drop and then a slight increase with increasing cutting angle. The large drop from 35° to 45° is due to the reduction of interference between the carbide tip, pick-body and coal-rock, while the slight increase from 45° to 55° is due to the small change in between the pick and coal-rock, and the caving performance becomes poor with an increase in the cutting angle. To study the caving performance of coal-rock under different cutting angles, coal-rock fragments from the experiments were collected, and different fragment sizes of coal-rock are shown in Fig. 18. The percentage of coal-rock fragment size under different cutting angles is shown in Fig. 19.
(a) 20 mm
(b) 24 mm
(c) 28 mm
Fig. 14. Macro profile of pick with wear under different head face diameters of pick-body.
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Fig. 15. Mean peak of pick cutting torque under different head face diameters of pick-body.
From Fig. 19, first, the percentage of coal-rock fragment b 2 mm decreases with increased cutting angle. The fragment size in this range is in a powdered form, which is formed by friction between the pick and coal-rock. The results correspond to the pick wear, and indicate again that the increase of cutting angle reduces the contact between pick and coal-rock. Secondly, the percentage of coal-rock fragments between 2 and 10 mm increases with cutting angle. The fragment sizes in this range are formed by interference between the pick and coal-rock. Unlike friction, the interference causes coal-rock fragments to be massive and are found not just in powder form. The caving performance becomes poor with the increase of cutting angle and the interference between pick and coal-rock increases, which increases the percentage of coal-rock fragment of sizes between 2 and 10 mm. Thirdly, the percentage of coal-rock fragments between 10 and 20 mm changes little, which indicates that this is the main range of pick
(a) Cutting angle 35°
(c) Cutting angle 45°
(b) Cutting angle 40°
(d) Cutting angle 50° (e) Cutting angle 55°
Fig. 16. Macro profile of pick with wear under different cutting angles.
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Fig. 17. Mean peak pick cutting torque under different cutting angles.
caving coal-rock. Lastly, the percentage of coal-rock fragments N20 mm decreases gradually from 9.2% under a cutting angle of 35° to 0% under a cutting angle of 50° and 55°, which further shows that a small cutting angle is favorable for caving of coal-rock. (2) Cutting angle The drilling process of the cutting head can be realized under the interaction of different cutting angles of picks. To study the influence of cutting angle on pick wear, the common pick was selected, and the cutting angle was chosen to be 50°. The incline angle was selected to be 10°, 20°, 30°, 40° and 50°. The rotating speed of the cutting apparatus and the movement speed of the coal-rock remained the same. Symmetrical cutting types were adopted for pick cutting, and the macro profile of the pick with wear under different cutting angles is shown in Fig. 20 for the same cutting times.
(a) <2 mm
(b) 2~10 mm
(c) 10~20 mm
(d) >20 mm
Fig. 18. Different fragment sizes coal-rock.
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Fig. 19. Percentage of coal-rock fragment size under different cutting angles.
In the cutting process, pick left-side wear decreases and right-side wear increases with incline angle when the pick inclines to the left. The wear height difference of both sides of pick carbide tip changes from none to large, until there is almost no wear on the left when incline angle is 50°. The wear on the pick-body changes from none under 10°–20° to small under 30°, to large under 50°. Moreover, it can be seen from the macro profile of the pick with wear that load on the pick becomes more imbalanced with the increase of incline angle, which improves pick self-rotatory ability if it could rotate freely. Pick cutting torques of five different incline angles were tested, and the statistics on mean peak of pick cutting torque were conducted, as shown in Fig. 21.
(a) Incline angle 10°
(b) Incline angle 20°
(c) Incline angle 30° (d) Incline angle 40° (e) Incline angle 50° Fig. 20. Macro profile of pick with wear under different incline angles.
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Fig. 21. Mean peak pick cutting torque under different incline angles.
Because the cutting angles were all 50° under different incline angles, the caving performance of coal-rock changes little and it has no influence on pick load. From Fig. 21, mean peak of pick cutting torque first increases slowly and then rapidly with increasing incline angle. This is related to the interference between the pick and coal-rock. The increase of incline angle mainly results in the increase of interference with coal-rock on one side of the pick, and ultimately leads to the rapid increase of cutting load. 3.2. Influence of wear on pick cutting performance Pick wear is gradually becoming a serious issue in use, and the cutting load and coal-rock fragments distribution vary with pick wear degree. For example, picks wear due to contact with coal-rock, but the increase of contact area due to wear accelerates its wear and increases its cutting load. Moreover, pick wear leads to inactivation of the pick tip, thus it becomes difficult for the pick to wedge into coal-rock and for the coal-rock to break, which increases the cutting specific energy consumption.
(a) Initial wear
(b) Interim wear (Front view)
(c) Interim wear (side view)
(d) Upper wear
Fig. 22. Macro profile of pick under different wear degree.
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Fig. 23. Mean peak of pick cutting torque with different wear degree.
To study the influence of pick wear on cutting performance, the common pick was selected, and the height of the carbide tip is set at 18.6 mm; the cone angle of the carbide tip is 80°; the head face diameters of pick-body is 24 mm. The incline angle and cutting angle is 0° and 40°, respectively. The rotating speed of the cutting apparatus and the movement speed of the coal-rock remained the same. Picks with different wear degrees, such as initial wear, interim wear and upper wear, were obtained by interacting with the coal-rock. Macro profiles of picks under different wear degrees are shown in Fig. 22. From Fig. 22, it can be seen that interference on both sides of the pick with coal-rock is larger than that on the pick tip during the continuous wearing process. It can be seen clearly from the comparison between the pick with interim wear in side view and pick with no wear that the decreased height of the pick tip due to wear is within 2 mm, while that of the pick on both sides including the carbide tip and pick-body is larger than that value. On the other hand, the pick tip gradually becomes circular-shaped during the continuous wearing process, shown in Fig. 22 (d), which is the most vital part influencing the cutting performance. The obtained picks under different wear degrees were used to study the influence of pick wear on cutting load. Pick cutting torques of picks with different wear degrees were tested, and the statistics on mean peak of pick cutting torque were conducted, as shown in Fig. 23. From Fig. 23, the mean peak of pick cutting torque increases with wear degree of the pick, which indicates that the pick wear becomes severe and increases the contact area between the pick and coal-rock and accelerates the pick wear. There is only a small difference between the pick with initial wear and the pick without wear, which is within 50 N·m. The cutting load of the pick with interim wear increases by approximately 20% compared with that of the pick without wear, while the cutting load increases by N 30% for the pick with upper wear. This shows that some measures should be taken for the worn picks to avoid worn surfaces
Fig. 24. Percentage of coal-rock fragment size under different wear degrees.
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contacting the coal-rock. Otherwise, the pick wear would be much more severe, and the pick would fail when losing its cutting performance. To study the caving performance of coal-rock under different wear degrees, coal-rock fragments from the experiments were collected. The percentage of coal-rock fragment sizes under different wear degrees is shown in Fig. 24.
4. Conclusions (1) Picks wear only on one side under asymmetrical cutting types, which increases working life. Loads on picks must be imbalanced under asymmetrical cutting types, causing the pick to have self-rotatory potential. (2) The larger the height of the carbide tip and cone angle of pick tip are, the easier it is to protect the head face of the pickbody from interference with coal-rock and avoid carbide tip loss due to wear of the pick-body. In the range of this research, the suitable height of the carbide tip was found to be 20 mm–24 mm, and the cone angle of the pick tip should be approximately 80°. Wear area on the pick-body increases with head face diameter of the pick-body, and it is suitable at 20 mm– 22 mm. (3) Pick wear decreases with cutting angle, but the cutting angle should be 45°–50° when considering pick cutting load and coal-rock caving effects. For picks with incline angles, pick wear increases with incline angle, however wear difference on two sides of the pick forms and increases gradually, which improves the pick self-rotatory ability. (4) Lastly, the influence of pick wear on cutting performance is analyzed. It indicates that the mean peak pick cutting torque increases with wear degree, and the largest increase percentage is 30%. Worn picks are unfavorable for caving of coal-rock. Compared with picks without wear, the coal-rock fragment size b2 mm increased by 14%.
Acknowledgment This work was supported by the Fundamental Research Funds for the Central Universities (2014ZDPY12), the Top-notch Academic Programs Project of Jiangsu Higher Education Institutions (TAPP) and the Priority Academic Program Development of Jiangsu High Education Institution of China (PAPD).
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