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Chapter 8 Lund high pressure chamber
Chapter 8 Lund high pressure chamber 8.1
Introduction
The analysis in chapter 7 showed that the lubricant parameters needed as input data for the determination of oil film thickness, pressure distribution, and power loss, are not only the viscosity and viscosity pressure coefficients [Jacobson 19701. Also needed are a number of other parameters, namely compressibility in the liquid and the solid states, the solidification pressure for different temperatures, and the increase of the solidification pressure with temperature. In chapter 9 the increase of the shear strength with pressure for the solidified oil will be treated, and in chapter 10 a very accurate model for the compressibility of both the liquid and the solid state will be treated.
8.2
Experimental investigation of the shear strength of solidified oil
To be able to solve the pressure build-up in the rolling contact, it is necessary to know the . increase of the shear strength solidification pressure of the oil and the shear strength T ~ The with pressure, which is treated in chapter 9, is very important for the calculation of shear stresses in sliding, and combined sliding and rolling situations, because there are high shear stresses present in the whole of the contact. In a pure rolling situation, like the one treated in chapter 7, high shear stresses are only present in a narrow band around the periphery of the contact, at a pressure reasonably constant around the contact. Therefore the analysis in chapter 7 could presume a constant shear strength of the oil. It was necessary to know this shear strength for the analysis. For this purpose, a test apparatus was constructed to measure the solidification pressure and the shear strength of the solidified oil. The oil sample was put under such a hydrostatic pressure that it solidified. The sample was in contact with a steel surface of 628 mm'. To this surface a shear force could be applied until the oil broke. The solidification pressure was said to be that pressure at which the shear strength of the oil parted from zero. The accuracy of this measurement was influenced by the friction forces acting on the compression plungers and on the seals on the central piston. 119
120
8.3
C H A P T E R 8. LUND HIGH PRESSURE C H A M B E R
Detailed description of the high pressure chamber
A drawing of the central part of the high pressure chamber is shown in figure 8.1. The central pressure containing part of the high pressure chamber was made of a 125 mm diameter steel slab. Two holes were drilled along the centre line of the cylinder, one from each end of the cylindrical steel block. These holes, each 25 mm deep, were precision honed to a very fine surface finish and to a dimensional accuracy of about f l pm. The plungers A and B were first made of solid pieces of steel, and turned and ground to the final size and form. The hardening of these steel plungers was very difficult, because the volume of the head of each plunger was so large compared to the volume of the active cylindrical plunger part. After having cracked and plastically deformed a number of these plungers during experiments, a new type of plunger was constructed. It had a loose head and the active plunger was a straight cylinder which was much easier to harden and hone to the required hardness and precision. The nominal diameter of the finished plungers was equal to the nominal diameter of the cylindrical holes in the high pressure chamber. This gave some of the plungers a radial clearance of up to one micron, and some a press fit of the same size. This could of course give a difference in friction force for the two plungers A and B and thereby a slight difference in the pressures inside the plungers. From the holes inside the plungers a system of channels was drilled through the high pressure chamber to connect the plungers A and B, see figure 8.1. The two large threaded holes shown in figure 8.1 were used one for the pressure gauge (right) and one for the active shear strength measurement cylinder, in combination with the transducer indicating when the investigated oil had reached its shear strength (left). The fact that the shear stress in the oil had reached its shear strength was indicated by the downward motion of the piston E, see figure 8.1, which broke an electric contact between the plunger E and the threaded plug above it. To make the apparatus less sensitive to frictional variations in the contacts between the high pressure chamber and the two plungers A and B, a spring was used to lightly press the plunger E, upwards to make sure that the electric contact was not broken during the pressurization of the high pressure chamber. A steel cylinder was placed inside this spring, and outside it a ring was placed to fill up as much of the volume as possible with steel. This was done because the very high compressibility of the pressurization medium and the oil being tested (about 20 per cent at the highest pressures), together with the slight leakage along the plungers A and B otherwise made it impossible to reach the maximum pressure before the plungers A and B had reached the bottom of the holes. In the same way, the vertical hole under the pressure gauge to the right, see figure 8.1, was also filled with a steel bar slightly smaller than the hole. The horizontal channel below plunger B was sealed off from the atmosphere by a high strength screw. The central active part of the high pressure chamber was the plunger E, the cylindrical wall around it and the insert plunger D. When the plungers A and B were pressed into the high pressure chamber with equally large forces, the volume around the plunger E was hydrostatically compressed with the same pressure acting on the top and the bottom surface of it. Plunger E had two O-ring seals which sealed off a thin cylindrical volume around it. This cylindrical volume was connected via four holes to the central volume in the plunger E, then the cylindrical volume, the four holes, and the oil sample chamber were filled with the oil to be tested. The sealing plunger D was inserted so that the oil would not be mixed with the pressurizing liquid outside the plunger
8.4. FORCE ANALYSIS OF THE HIGH PRESSURE CHAMBER
121
E. This liquid was chosen to have a higher solidification pressure than the lubricant under investigation, so the shear forces in the pressurizing liquid could be neglected at the very low velocity gradients present there. Two hydraulic actuators were used, one to build up the hydrostatic pressure, the other to give the difference in force between the plungers A and B and thereby giving the shear stress in the solidified oil. To make sure that the forces acting on the plungers A and B were exactly the same, both forces were generated by the same hydraulic actuator in such a way that one force was the reaction force of the other. This was done by mounting the large hydraulic actuator in a loose frame, which was placed on roller bearings. When the actuator pressed against plunger B the force was taken up by the heavy frame and transmitted to plunger A, giving the same force on this plunger, see figure 8.2. The whole frame and the hydraulic actuator pressing the plungers A and B into the high pressure chamber were then pushed to the left by the small hydraulic actuator outside the frame. This actuator is clearly seen in figure 8.2, while the large actuator is hidden by the frame. The small hydraulic actuator was mounted on the same basic frame as the high pressure chamber and the bearings lifting the heavy upper frame. With this arrangement the force difference between the plungers A and B could be accurately controlled by the small hydraulic actuator.
8.4
Force analysis of the high pressure chamber
The hydrostatic pressure was measured by a pressure gauge with an effective range from zero to 6 x 1O'Pa. The plungers A and B gave the hydrostatic pressure when they were forced into the high pressure chamber. The oil sample in chamber C was then compressed to the same pressure because plunger D could easily move into chamber C. When the pressure was high enough for the oil to solidify, plunger A was provided with a larger force than plunger B. Plunger E was then exposed to a force
where P A and p~ were the pressures on the upper face and lower face of the plunger E which . force was taken up by shear forces in the solidified oil and to a had the diameter 4 ~ This minor part of the spring in the bottom of the pressure chamber and the friction in the O-ring seals. When the pressure difference was large enough, the oil would break. The shear stress in the oil was then
F - F. dEh where F,, the force from the spring and the seals was almost equal to zero. r 4 E h is the stressed oil area. Neglecting F, compared to F gives T=-
- F
(PA
- PB)&
- ( P A - PBME
"4Eh 4a4~h 4h But the pressure difference between the top and the bottom surfaces of plunger E could be calculated using the force difference A F between the plungers A and B.
122
C H A P T E R 8. LUND HIGH PRESSURE C H A M B E R
D C E
Figure 8.1 Test apparatus for measurement of the shear strength of solidified oil.
PA -PB
4AF
=-
r4i
so the shear stress in the solidified oil was 7
A F QJE = --
7rh 45: where 4 A was the diameters of the plungers A and B and A F was the force difference between the plungers A and B minus friction forces. Three different oils were tested in this first high pressure chamber: Shell Valvata Oil 578, Shell Macoma Oil 69, and Mobil Oil Compound BB. All of them had a shear strength of approximately 15 MPa.
8.5
The weak points of the high pressure chamber
During the experimental work with the high pressure chamber in Lund Technical University, a number of modifications had to be incorporated to reach high enough pressures to convert the oils being tested into solids. The first parts to fail were the plungers. When the pressure in the high pressure chamber was increased above the plasticity limit for the plunger material, the plungers were plastically deformed. This made the plungers conical, with the top of the plungers undeformed and the parts of the plungers outside the high pressure chamber deformed to become shorter and having a larger diameter. When the load on the plungers A and B was increased above the plastic limit, and the diameter of the plungers increased, very high normal pressures between the plungers and the high pressure chamber walls caused scoring of the surfaces. Despite the leakage of pressurization fluid past the plungers, it was not possible to avoid scoring as soon as the plungers were plastically deformed. The deformation probably broke up the oxide layer on the plunger surface and thereby made it possible for the two surfaces to adhere to each other. This knowledge was later used when constructing the high pressure chamber at Luleg Technical University. There the contacting parts were made of cemented carbide to avoid both plastic deformation and scoring of the surfaces.
8.5. THE WEAK POINTS O F THE HIGH PRESSURE CHAMBER
123
Figure 8.2 Photograph of the test rig, When a number of steel plungers had been destroyed by plastic deformation, both plungers, shown in figure 8.1 and the ones described earlier, where t h e thick part was loose, the walls of the pressurization cylinders were scored so much that it was no longer possible to re-hone them to a straight cylindrical shape. The decision was then made to drill out larger cylindrical holes in the high pressure chamber ends and to make threaded inserts with the proper inner diameter to fit the plungers. To make sure that the pressurization oil did not leak out through the threads on the outside of the inserts, the bottom of each insert was formed as a sharp edge. This edge was plastically deformed together with the seating at the bottom of the drilled hole when the inserts were screwed into the threaded holes and tightened. The final honing operation was done before the inserts were screwed into the threaded holes. The forces from the threads then elastically deformed the inserts to give a slightly smaller inner diameter at the bottom end. This conicity helped to maintain a more constant play between the plunger and the hole when the high pressure chamber was pressurized. The three oils mentioned earlier, Shell Valvata Oil 578, Shell Macoma Oil 69, and Mobil Oil Compound BB, were used because of their high viscosity and low solidification pressure. This made it possible to reach solidification before the plungers were plastically deformed, even at room temperature. To be able to investigate other oils with lower viscosity and higher solidification pressure, tests were performed in a cool chamber. The cool chamber temperature could be decreased to -40°C. This made it much easier to reach the solidification pressure as it decreases strongly with decreasing temperature. In chapter 10 some experiments and a theoretical investigation are described which show that the solidification for mineral oils takes place when the density of the fluid has a certain value, specific to that lubricant. When the oil temperature is changed,
124
CHAPTER 8. LUND HIGH PRESSURE CHAMBER
the density also changes owing to the thermal expansion or contraction of the oil. This means that the colder the oil is, the less it has to be mechanically compressed to convert from a liquid to a solid. This phenomenon makes it possible to reach solidification of the lubricant at very low pressures provided the temperature is low enough. Some low viscosity oils could not be compressed to solidification, even at the low temperatures in the cold chamber. This was mainly caused by leakage of the pressurization fluid in the channels between plunger A and plunger B outside plunger E in figure 8.1, because this fluid had to be liquid through the experiments. As the solidification pressures for the different oils were not known when the experiments started, some surprising results were found for low viscosity oils when the pressurization liquid solidified before the test oil, and therefore made it impossible to move the plunger E downwards even with a very high force difference between the plungers A and
B. To check whether even those low viscosity fluids could be solidified, experiments were tried
at the temperature of liquid nitrogen. These experiments failed because all test oils at that temperature became hard and brittle solids. If the test oils were cooled down hy direct contact with the boiling liquid nitrogen, they first became solid and after a few seconds cracked into sharp splinters when the surface of the oil had shrunk enough to give tensile stresses larger than the tensile strength of the solidified oil. The O-ring seals around the plungers D and E gave rather high friction during the mounting of the plungers at zero pressure. This strong press fit at room pressure was necessary to make sure that the seals would be tight also under the highest pressure in the high pressure chamber. At those pressures the rubber material in the seals was compressed by about 10 per cent, and this could change the nominal interference to a leakage gap if the interference at ambient pressure was too low. This gave a slightly varying friction force in the plunger seals for different pressures, but this variation could not be measured experimentally. All these weak points were considered when the new high pressure chamber was built at LuleH Technical University. The new high pressure chamber is described in chapter 9.
Bibliography [Jacobson 19701
Jacobson B.O., “On the Lubrication of Heavily Loaded Spherical Surfaces Considering Surface Deformation and Solidification of the Lubricant”, Acta Polytechnica Scandinavica, Mech. Eng. Series No. 54.
125
Introduction
The analysis in chapter 7 showed that the lubricant parameters needed as input data for the determination of oil film thickness, pressure distribution, and power loss, are not only the viscosity and viscosity pressure coefficients [Jacobson 19701. Also needed are a number of other parameters, namely compressibility in the liquid and the solid states, the solidification pressure for different temperatures, and the increase of the solidification pressure with temperature. In chapter 9 the increase of the shear strength with pressure for the solidified oil will be treated, and in chapter 10 a very accurate model for the compressibility of both the liquid and the solid state will be treated.
8.2
Experimental investigation of the shear strength of solidified oil
To be able to solve the pressure build-up in the rolling contact, it is necessary to know the . increase of the shear strength solidification pressure of the oil and the shear strength T ~ The with pressure, which is treated in chapter 9, is very important for the calculation of shear stresses in sliding, and combined sliding and rolling situations, because there are high shear stresses present in the whole of the contact. In a pure rolling situation, like the one treated in chapter 7, high shear stresses are only present in a narrow band around the periphery of the contact, at a pressure reasonably constant around the contact. Therefore the analysis in chapter 7 could presume a constant shear strength of the oil. It was necessary to know this shear strength for the analysis. For this purpose, a test apparatus was constructed to measure the solidification pressure and the shear strength of the solidified oil. The oil sample was put under such a hydrostatic pressure that it solidified. The sample was in contact with a steel surface of 628 mm'. To this surface a shear force could be applied until the oil broke. The solidification pressure was said to be that pressure at which the shear strength of the oil parted from zero. The accuracy of this measurement was influenced by the friction forces acting on the compression plungers and on the seals on the central piston. 119
120
8.3
C H A P T E R 8. LUND HIGH PRESSURE C H A M B E R
Detailed description of the high pressure chamber
A drawing of the central part of the high pressure chamber is shown in figure 8.1. The central pressure containing part of the high pressure chamber was made of a 125 mm diameter steel slab. Two holes were drilled along the centre line of the cylinder, one from each end of the cylindrical steel block. These holes, each 25 mm deep, were precision honed to a very fine surface finish and to a dimensional accuracy of about f l pm. The plungers A and B were first made of solid pieces of steel, and turned and ground to the final size and form. The hardening of these steel plungers was very difficult, because the volume of the head of each plunger was so large compared to the volume of the active cylindrical plunger part. After having cracked and plastically deformed a number of these plungers during experiments, a new type of plunger was constructed. It had a loose head and the active plunger was a straight cylinder which was much easier to harden and hone to the required hardness and precision. The nominal diameter of the finished plungers was equal to the nominal diameter of the cylindrical holes in the high pressure chamber. This gave some of the plungers a radial clearance of up to one micron, and some a press fit of the same size. This could of course give a difference in friction force for the two plungers A and B and thereby a slight difference in the pressures inside the plungers. From the holes inside the plungers a system of channels was drilled through the high pressure chamber to connect the plungers A and B, see figure 8.1. The two large threaded holes shown in figure 8.1 were used one for the pressure gauge (right) and one for the active shear strength measurement cylinder, in combination with the transducer indicating when the investigated oil had reached its shear strength (left). The fact that the shear stress in the oil had reached its shear strength was indicated by the downward motion of the piston E, see figure 8.1, which broke an electric contact between the plunger E and the threaded plug above it. To make the apparatus less sensitive to frictional variations in the contacts between the high pressure chamber and the two plungers A and B, a spring was used to lightly press the plunger E, upwards to make sure that the electric contact was not broken during the pressurization of the high pressure chamber. A steel cylinder was placed inside this spring, and outside it a ring was placed to fill up as much of the volume as possible with steel. This was done because the very high compressibility of the pressurization medium and the oil being tested (about 20 per cent at the highest pressures), together with the slight leakage along the plungers A and B otherwise made it impossible to reach the maximum pressure before the plungers A and B had reached the bottom of the holes. In the same way, the vertical hole under the pressure gauge to the right, see figure 8.1, was also filled with a steel bar slightly smaller than the hole. The horizontal channel below plunger B was sealed off from the atmosphere by a high strength screw. The central active part of the high pressure chamber was the plunger E, the cylindrical wall around it and the insert plunger D. When the plungers A and B were pressed into the high pressure chamber with equally large forces, the volume around the plunger E was hydrostatically compressed with the same pressure acting on the top and the bottom surface of it. Plunger E had two O-ring seals which sealed off a thin cylindrical volume around it. This cylindrical volume was connected via four holes to the central volume in the plunger E, then the cylindrical volume, the four holes, and the oil sample chamber were filled with the oil to be tested. The sealing plunger D was inserted so that the oil would not be mixed with the pressurizing liquid outside the plunger
8.4. FORCE ANALYSIS OF THE HIGH PRESSURE CHAMBER
121
E. This liquid was chosen to have a higher solidification pressure than the lubricant under investigation, so the shear forces in the pressurizing liquid could be neglected at the very low velocity gradients present there. Two hydraulic actuators were used, one to build up the hydrostatic pressure, the other to give the difference in force between the plungers A and B and thereby giving the shear stress in the solidified oil. To make sure that the forces acting on the plungers A and B were exactly the same, both forces were generated by the same hydraulic actuator in such a way that one force was the reaction force of the other. This was done by mounting the large hydraulic actuator in a loose frame, which was placed on roller bearings. When the actuator pressed against plunger B the force was taken up by the heavy frame and transmitted to plunger A, giving the same force on this plunger, see figure 8.2. The whole frame and the hydraulic actuator pressing the plungers A and B into the high pressure chamber were then pushed to the left by the small hydraulic actuator outside the frame. This actuator is clearly seen in figure 8.2, while the large actuator is hidden by the frame. The small hydraulic actuator was mounted on the same basic frame as the high pressure chamber and the bearings lifting the heavy upper frame. With this arrangement the force difference between the plungers A and B could be accurately controlled by the small hydraulic actuator.
8.4
Force analysis of the high pressure chamber
The hydrostatic pressure was measured by a pressure gauge with an effective range from zero to 6 x 1O'Pa. The plungers A and B gave the hydrostatic pressure when they were forced into the high pressure chamber. The oil sample in chamber C was then compressed to the same pressure because plunger D could easily move into chamber C. When the pressure was high enough for the oil to solidify, plunger A was provided with a larger force than plunger B. Plunger E was then exposed to a force
where P A and p~ were the pressures on the upper face and lower face of the plunger E which . force was taken up by shear forces in the solidified oil and to a had the diameter 4 ~ This minor part of the spring in the bottom of the pressure chamber and the friction in the O-ring seals. When the pressure difference was large enough, the oil would break. The shear stress in the oil was then
F - F. dEh where F,, the force from the spring and the seals was almost equal to zero. r 4 E h is the stressed oil area. Neglecting F, compared to F gives T=-
- F
(PA
- PB)&
- ( P A - PBME
"4Eh 4a4~h 4h But the pressure difference between the top and the bottom surfaces of plunger E could be calculated using the force difference A F between the plungers A and B.
122
C H A P T E R 8. LUND HIGH PRESSURE C H A M B E R
D C E
Figure 8.1 Test apparatus for measurement of the shear strength of solidified oil.
PA -PB
4AF
=-
r4i
so the shear stress in the solidified oil was 7
A F QJE = --
7rh 45: where 4 A was the diameters of the plungers A and B and A F was the force difference between the plungers A and B minus friction forces. Three different oils were tested in this first high pressure chamber: Shell Valvata Oil 578, Shell Macoma Oil 69, and Mobil Oil Compound BB. All of them had a shear strength of approximately 15 MPa.
8.5
The weak points of the high pressure chamber
During the experimental work with the high pressure chamber in Lund Technical University, a number of modifications had to be incorporated to reach high enough pressures to convert the oils being tested into solids. The first parts to fail were the plungers. When the pressure in the high pressure chamber was increased above the plasticity limit for the plunger material, the plungers were plastically deformed. This made the plungers conical, with the top of the plungers undeformed and the parts of the plungers outside the high pressure chamber deformed to become shorter and having a larger diameter. When the load on the plungers A and B was increased above the plastic limit, and the diameter of the plungers increased, very high normal pressures between the plungers and the high pressure chamber walls caused scoring of the surfaces. Despite the leakage of pressurization fluid past the plungers, it was not possible to avoid scoring as soon as the plungers were plastically deformed. The deformation probably broke up the oxide layer on the plunger surface and thereby made it possible for the two surfaces to adhere to each other. This knowledge was later used when constructing the high pressure chamber at Luleg Technical University. There the contacting parts were made of cemented carbide to avoid both plastic deformation and scoring of the surfaces.
8.5. THE WEAK POINTS O F THE HIGH PRESSURE CHAMBER
123
Figure 8.2 Photograph of the test rig, When a number of steel plungers had been destroyed by plastic deformation, both plungers, shown in figure 8.1 and the ones described earlier, where t h e thick part was loose, the walls of the pressurization cylinders were scored so much that it was no longer possible to re-hone them to a straight cylindrical shape. The decision was then made to drill out larger cylindrical holes in the high pressure chamber ends and to make threaded inserts with the proper inner diameter to fit the plungers. To make sure that the pressurization oil did not leak out through the threads on the outside of the inserts, the bottom of each insert was formed as a sharp edge. This edge was plastically deformed together with the seating at the bottom of the drilled hole when the inserts were screwed into the threaded holes and tightened. The final honing operation was done before the inserts were screwed into the threaded holes. The forces from the threads then elastically deformed the inserts to give a slightly smaller inner diameter at the bottom end. This conicity helped to maintain a more constant play between the plunger and the hole when the high pressure chamber was pressurized. The three oils mentioned earlier, Shell Valvata Oil 578, Shell Macoma Oil 69, and Mobil Oil Compound BB, were used because of their high viscosity and low solidification pressure. This made it possible to reach solidification before the plungers were plastically deformed, even at room temperature. To be able to investigate other oils with lower viscosity and higher solidification pressure, tests were performed in a cool chamber. The cool chamber temperature could be decreased to -40°C. This made it much easier to reach the solidification pressure as it decreases strongly with decreasing temperature. In chapter 10 some experiments and a theoretical investigation are described which show that the solidification for mineral oils takes place when the density of the fluid has a certain value, specific to that lubricant. When the oil temperature is changed,
124
CHAPTER 8. LUND HIGH PRESSURE CHAMBER
the density also changes owing to the thermal expansion or contraction of the oil. This means that the colder the oil is, the less it has to be mechanically compressed to convert from a liquid to a solid. This phenomenon makes it possible to reach solidification of the lubricant at very low pressures provided the temperature is low enough. Some low viscosity oils could not be compressed to solidification, even at the low temperatures in the cold chamber. This was mainly caused by leakage of the pressurization fluid in the channels between plunger A and plunger B outside plunger E in figure 8.1, because this fluid had to be liquid through the experiments. As the solidification pressures for the different oils were not known when the experiments started, some surprising results were found for low viscosity oils when the pressurization liquid solidified before the test oil, and therefore made it impossible to move the plunger E downwards even with a very high force difference between the plungers A and
B. To check whether even those low viscosity fluids could be solidified, experiments were tried
at the temperature of liquid nitrogen. These experiments failed because all test oils at that temperature became hard and brittle solids. If the test oils were cooled down hy direct contact with the boiling liquid nitrogen, they first became solid and after a few seconds cracked into sharp splinters when the surface of the oil had shrunk enough to give tensile stresses larger than the tensile strength of the solidified oil. The O-ring seals around the plungers D and E gave rather high friction during the mounting of the plungers at zero pressure. This strong press fit at room pressure was necessary to make sure that the seals would be tight also under the highest pressure in the high pressure chamber. At those pressures the rubber material in the seals was compressed by about 10 per cent, and this could change the nominal interference to a leakage gap if the interference at ambient pressure was too low. This gave a slightly varying friction force in the plunger seals for different pressures, but this variation could not be measured experimentally. All these weak points were considered when the new high pressure chamber was built at LuleH Technical University. The new high pressure chamber is described in chapter 9.
Bibliography [Jacobson 19701
Jacobson B.O., “On the Lubrication of Heavily Loaded Spherical Surfaces Considering Surface Deformation and Solidification of the Lubricant”, Acta Polytechnica Scandinavica, Mech. Eng. Series No. 54.
125