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Delivery, Unloading and Handling of Cane
1. Delivery, Unloading and Handling of Cane The factory takes delivery of the cane, either directly at the factory weighbridge, or at auxiliary weighbridges serving certain important or remote points in the area from which the mill draws its supplies. Transport is arranged by the factory, either by railway, or more often by lorries, or by tractors and trailers. The bulk density or weight per unit volume, of cane in the truck depends on the manner in which it is loaded. In Hawaii, for instance, it is often picked up in bulk in the field by a mobile crane mounted on caterpillar tracks and fitted with a grab. This simply diops the cane in a tangled mass into the trailer; in this case the bulk density may be taken as about 12.5 lb./cu.ft. If the loading is done rather more carefully: 20 lb./cu.ft. If the cane is loaded by hand, the stalks being placed parallel to each other in bundles or packages, as is the custom in Reunion, a value of 25 lb./cu.ft. may be attained. This bulk density depends on the stand of the cane. Straight and erect stalks will give a more compact loading than curved or lodged canes. ORGANISATION OF CANE SUPPLY THROUGHOUT THE DAY
A cane sugar factory generally operates continuously from Monday morning till Saturday evening. It shuts down for about 36 hours, including Sunday, for cleaning the multiple effects and for minor repairs. It operates then for about 132 hours per week. During the day, cane transport is generally carried on for only 12 hours, from 6 a.m. to 6 p.m. So that the mill will not run out of cane, it is necessary that the factory should receive in 12 hours, during the day, the tonnage which it crushes in 24 hours. Towards 6 p.m. then there will have accumulated a stock at least equal to half the daily tonnage: Overnight provision = 12C -f a (1) C = quantity of cane crushed by the factory per hour a = margin of safety, which should always be maintained to avoid being obliged to stop the mill for lack of cane. This margin a is necessary to make provision for accidental fluctuations in supply: rain retarding the cutting and restricting transport, irregularities in supply by the farmers, etc. It should not be too small, nor too large. In the latter case, the cane yard will be uselessly encumbered, and the delay occurring between the cutting of the cane and its passage through the mills will be unnecessarily increased. A suitable quantity is that corresponding to three hours' crushing: a = 3C (2) If this value be adopted, the yard should be able to accommodate an overnight stock of: P = 12C + 3 C = 15C
(3)
2
DELIVERY, UNLOADING AND HANDLING OF CANE
1
UNLOADING OF CANE
From the handling point of view, cane arriving at the factory may be divided into two cate gories : 1. Cane transported by mechanical means: trailers, lorries and railway wagons. 2. Cane transported by carts or drays. 1. Cane arriving by lorries
This cane is generally loaded in "packets" or bundles, bound by three chains or "slings" (Fig. 1). At one end these slings slide in a special hook fitted with a pawl, while a ring is attached to the other end.
Π
Fig. 1. Sling with automatic hook.
The unloading device at the factory, generally a crane, lifts the bundle of crane with the aid of a "swingle-bar" or steel bar fitted with three hooks; the labourers on the truck attach the three free rings to these hooks. The crane lifts the bundle and deposits it on the heap of cane which is called a "stock-pile". The catches of the sling-hooks are then released, and the crane lifts the swingle-bar with the slings hanging (Fig. 2). For a 3-ton crane, each bundle is of the order of 3,000-5,000 lb. of cane, generally 4,000-5,000 lb. A 4- or 5-ton truck carries two such bundles. For cranes of 6 tons (the commonest size) and 10 tons capacity, an effort is generally made to work near the limit of the capacity of the crane, and the bundles average about 5 and 8 tons respectively. Tipping trucks are sometimes employed. These are trucks of which the body (or tray) is lifted by two hydraulic pistons and pivots about a hinge, placed either at the back, or along one side of the truck. This system permits of tipping the cane directly into the cane carrier or an auxiliary carrier, the driver operating alone or with the aid of one labourer. It also dispenses with slings, which are expensive. 2. Cane arriving in drays
Delivery from drays is generally made directly into the cane carrier, the driver backing his dray up to the carrier. For this purpose, the horizontal portion of the carrier should be furnished with protective wooden stops, which serve to block the wheels of the dray and prevent damage
1
CANE ARRIVING IN DRAYS
3
Fig. 2. Lifting a bundle of cane.
to the steel plates of the carrier. The driver then empties his drayload by hand into the carrier. It is only when there are too many drays that some of them are permitted to unload their cane onto the stock-pile. The foreman in charge of the cane yard takes care that there is no abuse of this practice, since cane unloaded on to the pile involves an additional handling, as it must be picked up again by the crane and "grab", to be deposited on the carrier (the grab is a device for picking up the canes, which replaces the swingle-bar used for unloading). If the supervision of the yard is poor, there is a risk that the crane will not be able to maintain a regular cane supply to the carrier. If about 50% of the cane supply arrives by drays, and 50% by lorries and rail, the drays may be utilised for the supply to the carrier during the day, while the trucks are unloaded by the crane to make provision for the overnight supply. At night, the swingle-bar is removed from the crane and replaced by the grab, by the aid of which it picks up the cane from the pile and dumps it on the carrier. This night duty of the cane is the more difficult, since (1) a "handful" of cane for the grab is much less than a load from
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DELIVERY, UNLOADING AND HANDLING OF CANE
1
the slings; (2) the crane alone is responsible for assuring the supply to the carrier, since there are no more drays, and it must handle a tonnage equal to the crushing rate of the mills. Time for unloading a dray. A dray carries about 2500 lb. of cane. One or two men unload it on the cane carrier in 10 or 15 minutes, arrival and departure included. Number of positions for unloading drays. One unloading position for drays therefore allows of a supply of 6 t.c.h. (tons cane per hour). In order to ensure full supply from drays, it is therefore necessary to provide a number of positions:
-=τ
(4)
C = crushing rate of the factory, in t.c.h. (tons cane per hour). Length of carrier required for drays. Reckoning 10 ft. per dray (including space between), it is then necessary to make available for drays a length of the horizontal portion of the carrier equal to: L
C 5C = - x l O = —
(5)
L = length of carrier reserved for drays, in ft. C = crushing rate of the factory, t.c.h. Length of the horizontal portion of the carrier, in order to determine, for these conditions, the length of the horizontal poition of the carrier, it will be a good precaution to increase this figure by 20%, in order to allow for interruptions to the working of the drays, and to have a comfortable margin. The length of the horizontal portion therefore will be: LH = LH
=
1.2
X
SC
3 1.2
X
5C
3 x 2
2C C
(unloading on one side only of the carrier) (unloading on both sides)
(6) (7)
say, for example, in the latter case, 50 ft. for 50 t.c.h. LH = length of the horizontal portion of the carrier, in ft. C = crushing rate of the factory, in t.c.h. (assuming it is maintained entirely by drays at certain hours). There will be an additional position at the end of the carrier, if the end is accessible. These equations assume that there is no obstruction to impede the access of the drays to the cane carrier. CANE-HANDLING EQUIPMENT
The principal types of equipment used at the factory cane yard are: 1. The cane crane, or "derrick", 2. The travelling crane, 3. The tipping platform or truck tip, 4. The cane rake.
1
CANE CRANE
5
1. Cane crane
This is by far the most-used device, and we shall consider it first. It is often called by its American name: "derrick". It consists (Fig. 3) of a mast of fabricated steel, mounted on a pivot, so that it can rotate through a full circle. At a suitable height this vertical mast carries a horizontal arm forming a track on which a trolley, carrying two pulleys, can run to and fro. A cable passes over these two pulleys and hangs between them, thus forming a loop which carries a snatch block, on which may be hung either the swingle-bar or the grab. The driver works in a cabin, attached to the base of the mast, which houses the driving motor and the winding drums for the various cables controlling the trolley. These cranes may be driven by steam, exhausting to atmosphere, or by electric motor. They are also classified according to their mode of support, into: (a) Guyed cranes, (b) Self-supporting cranes. (a) Guyed cranes. This is the lighter form, the stability of the crane being ensured by retaining cables or guy ropes attached to a ring at the top of the mast (Fig. 3). These guys must permit of rotation of the horizontal arm, and so must befixedin the ground at a considerable distance from the axis of the crane.
Fig. 3. Crane with guys (Fives-Lille).
Number of guys. If necessary 3 guy ropes, arranged at an angle of 120° to each other, would be sufficient, ^owever, the number of guys is generally chosen so that breakage of one of them does not cause the derrick to fall. This condition requires a minimum of 5, at 72°. For preference 6 or 8 should be used, if possible.
6
1
DELIVERY, UNLOADING AND HANDLING OF CANE
It would be necessary then to fix 5, 6, 7, or 8 anchor blocks, placed symmetrically on a circle of 180-240 ft. radius with the axis of the crane as centre, to which the guys are attached. The presence of factory and office buildings makes the solution of this problem very difficult. If it is necessary to modify the spacing of two cables to avoid in particular the factory buildings, it is necessary to ensure that a breakage of one of these widely spaced cables will not leave a sector of more than 150°, which is the limit permissible for stability of two adjacent cables. Obviously, in case of breakage, it will be necessary to stop the crane immediately until the damaged cable is replaced, since the remaining guys would not be able to ensure stability under the dynamic loading of the crane in operation. Tension of the cables. To obtain maximum rigidity, the cables must be tightened so that the mast describes only a very small cone as the derrick is rotated, but without increasing unduly the tension of the cables themselves. This tension should be checked frequently. The cables should be painted or tarred every two or three years to prevent corrosion. Thickness of the cables. For a crane of 3 tons capacity, 5 to 8 cables of 1 in. (25 mm) diameter are provided.
rOxixixi>ixl;¥;
wyf/*7E*fli*gr
Fig. 4. Self-supporting pivoting crane (Fives-Lille).
(b) Self-supporting cranes. These are obviously heavier and more massive (Fig. 4), but avoid the nuisance and danger of guys, which restrict movement at the cane yard and which are sometimes very difficult to locate conveniently.
1
CANE CRANE
7
In countries subject to cyclones, these cranes should be designed for a wind pressure of at least 60 lb./sq.ft. (300 kg/m2) (live load). Capacity of cranes. The interesting characteristics of an unloading crane are: (1) The lifting force, (2) The radius of operation, (3) The maximum height of lift of the grab. (1) Lifting force. This is the maximum weight which the crane can lift, at the end of the arm. It is the most important factor, which serves to specify its capacity. For instance, one speaks of a 3-ton or 5-ton crane. (2) Radius of operation. This is the horizontal distance between the pivot of the crane and the pulley of the snatch block (corresponding to the centre of the grab) when the trolley is at its extreme position at the end of the crane arm. Typical values range from 60 to 80 ft. (18-25 m). (3) Maximum height of lift. This is the height from the end of the tines of the grab above the platform level, when the grab is raised to its maximum height. To increase this height, and at the same time to enable the driver to oversee the operations and traffic of the platform, the base of the derrick is generally raised by placing it on a trun cated cone of masonry about 6 ft. in height. The height of lift is generally of the order of 20-25 ft. (6-8 m). As a general rule, for a factory having only one derrick at its unloading station, the charac teristics adopted are approximately as follows: (1) Lifting force: F = 0.1C
(8)
F = lifting force in tons C = crushing rate of the factory, in t.c.h. (2) Radius of operation: R = 9 ]/c ft.
(9)
R = radius of operation, in ft. (3) Maximum height of lift: # = 2 5 ft.
(10)
When two derricks (or two unloading devices) are available, the total tonnage C is divided between them by allotting the tonnages C\ and C% respectively, such that Ci + C2 = C Location of the crane. At what distance from the carrier is it advisable to locate the centre of the crane? This distance is evidently a function of its radius of operation. If the centre of the crane is placed too far from the carrier (Fig. 5), the mean angle of rotation of the crane and the travel of the carriage are both increased. This is readily seen by considering for example the point A, the "centre of travel" of the pile of cane, which is located at a distance from the centre of the crane equal to about 80% of the radius of operation. If, on the other hand, the crane is placed too close to the carrier, the angle of rotation is
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DELIVERY, UNLOADING AND HANDLING OF CANE
1
reduced, and little or no traversing is necessary; but the useful area served by the crane is appreciably reduced. The best compromise is to place the centre of the crane at a distance from the centre line of the carrier equal to half the radius of operation. The angle ω is then 120°. The grab may then
Mills
Fig. 5. Location of the crane.
be lowered on to two billets of timber placed above an inclined plane P, from which the cane tumbles in a tangled condition into the carrier, thus being distributed to some extent and helping to avoid blockages at the knives. This inclined plane is very useful, and should always be provided, when there is no auxiliary carrier or lateral feeding table. Bulk density of cane in the pile. This density is similar to that of cane loaded mechanically, or about 12 lb./cu.ft. (200 kg/m 3 ), if it refers to cane dumped at random by hand, or deposited by the grab. It increases to 20 lb./cu.ft. (300 kg/m3) if the pile is made up of bundles of parallel stalks, deposited by the swingle-bar. Volume for overnight supply. We have seen (eqn. 3) that the stock of cane should be built up by evening to: P = 15Ctons At 20 lb./cu.ft. (300 kg/m3) this represents a volume: ¥,
V=
15C x 2,240 — 20
i
^o_
r
= 1,680C cu.ft. Cane
(11;
carrier
Fig. 6. Ground area for overnight supply.
Now, the derrick covers an area equal to that of a circle with radius equal to its radius of operation, less the area of the small inner circle, which must be left free for traffic around the base of the crane.
1
CANE CRANE
9
A radius of about 16 ft. (5 m) must be allowed for this small space, or better, in order to take into account the dimensions of the crane and the installation, say one third of the radius of operation, or from (9): r=3)/Cft.
(12)
Let us consider on what fraction a of the whole circumference it will be necessary to store the cane in order to accumulate the required quantity. We have: V= n(R* — r*)H—
= l,680Ccu.ft.
(13)
Replacing R and r by their normal values, as functions of the crushing rate, and assuming a mean value for H of 20 ft. (6 m) we have: π(81 — 9)C X 20 — - = 1,680C
Whence: a ~ 134°
Fig. 7. Travelling crane for unloading cane (Fives-Lille).
(14)
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DELIVERY, UNLOADING AND HANDLING OF CANE
1
On account of the irregularity of the pile, and the sloping form assumed by its sides, it is far from forming a compact or geometrical mass; hence it is necessary to allow an angle of about 150°. 2. Transporter crane
This system is closely analogous to that of the derrick. Instead of a circle, it serves a rectangle, the length and breadth of which may be fixed at will. For this reason, it will be indicated for certain long and narrow cane yards which have not sufficient space for a crane to swing. The transporter crane must obviously span the cane carrier which it serves. Sometimes a gantry crane is considered sufficient; this has only the two movements, ele vation and traversing, in the one vertical plane, and accordingly serves only for discharging the cane from the lorries on to the carrier, without providing storage (Fig. 7). 3. Truck tip
When the cane arrives by railway trucks, it is more practical to empty the truck in one action, rather than to lift separately with the crane, the bundles of cane piled in the truck. This presents several advantages: (1) Saving in time. (2) Saving in slings (this is by no means negligible). (3) Less handling, because the cane is emptied directly into the carrier, as with drays. At the same time, the absence of slings eliminates the risk of pieces of chain or ratchet hooks passing through the mill. When using crane and slings, it is generally necessary to remove two or three such pieces from the mills in the course of a campaign which means severe damage to the roller grooving.
Fig. 8. Lateral tip for trucks (Fives-Lille).
Fig. 9. Small truck for end-tipping.
1
TRUCK TIP
li
There are two types of tip: (1) The lateral tip, for large waggons (Fig. 8). (2) The endwise tip, for small trucks. In the latter case, the trucks (Fig. 9) are provided with a U-shaped iron frame forming a cradle, in which the cane is placed lengthwise parallel to the track. The ends are free, and the cane, well compacted during the journey from the fields to the factory, falls out when the tip reaches the desired inclination. Tips are sometimes installed for lorries also. The lorry is generally tipped endwise, toward
Fig. 10. "Wicks" rake (Mirrlees-Watson).
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DELIVERY, UNLOADING AND HANDLING OF CANE
1
the rear. Certain models of lorry are not well suited to this operation, or allow oil to run out of the crankcase, beyond a certain inclination. 4. Rakes
This is an auxiliary unloading device, which is used mostly in Java and in some British coun tries. Fig. 10 gives a view of it which will make description superfluous. It will be noted that the rake has, not only a "picking" movement and a to-and-fro movement in a mean plane, but that it can also make the same movement in a great number of planes forming a dihedral angle of some 30-40° to one side or the other of this mean plane. Another type of rake, known as the Davids rake, is common in Queensland and uses rakes supported in an endless chain, and pivoted about the driving shaft so that the rake may be raised or lowered to suit the height of cane in the truck. The width of the rake is such as to cover the full length of a small truck (approx. 6 ft.). LATERAL FEEDING TABLES AND CARRIERS
When the number of drays is insufficient (which may occur at certain fixed times of day, or accidentally at any moment), the crane makes up the cane supply to the carrier with the aid of the grab. However, variations occur in the quantity of cane so deposited per unit length of the carrier; there are gaps which cannot be filled in time. On the other hand, a bundle of cane from a lorry, deposited almost intact on the carrier, can cause an overload at the knives, causing the driving engine to stall or, in the case of electric drive, opening the circuit breaker. When such dense bundles of parallel canes occur, it is necessary to slow down the carrier and practically stop it, otherwise there is a risk of choking the knives. Even if careful attention is paid to this, the attempt is not always successful. When there are no knives, such fluctuations in the cane blanket are detrimental to the crusher feed and reduce the tonnage. These disadvantages may be overcome by installing a "cross-conveyor". This may take two different forms: 1. That of a wide and relatively short platform or lateral table (Fig. 11). 2. That of a carrier identical with the main carrier, but at right angles to it. This is often called an "auxiliary carrier". 1. Lateral table
This is, in effect, a very wide and short carrier, driven by an independent motor. In plan it is rectangular or approximately square in shape. Its upper platform is at a level slightly higher than the raised edge of the side plates of the main carrier. The crane deposits the cane on this platform, and keeps up the supply to it as it discharges. An attendant is posted close to the driving motor of the lateral table and puts it in motion when he sees that the main carrier is not carrying a full feed. The cane falls from the platform on to the main carrier. The advantage of this arrangement is that it falls in a more or less tangled state, making the work of the knives much easier. As soon as the desired quantity of cane has fallen on to the carrier, the table is stopped. The motion of the latter then is jerky and interrupt ed. It often remains for a long time unused, and consequently immobile. The lateral table is particularly useful in countries where payments for cane is made according
LATERAL TABLE
Electric motor Chain drive
13
».
Fig. 11. Lateral feeding table (Fives-Lille).
to sugar content or recoverable sugar. The cane to be tested from a particular supplier may then be accumulated on a special lateral table, without interfering with the feeding of the mills. When the table is filled, its load of cane is fed to the mills without interrupting the movement of the main carrier; the beginning and end of the batch to be analysed are simply marked with lime, in such a way that the juice sample boy can see clearly the beginning and end of the batch. The lateral table is much more effective if fitted with a "tumbler". This is a horizontal shaft, placed above the axis of the front drum of the table, and which rotates owly in the reverse direction. It is provided with arms which ensure that the cane falls into the carrier in small lots, avoiding a heavy fall of large masses which would be liable to provoke chokes at the knives. Area of the lateral table. The dimensions of the table vary considerably according to individual cases. A good dimension, designing for ample capacity, is: S=6.5C S = area of the lateral table, in sq. ft. C = crushing rate of the factory, in t.c.h. For example, a table of 16 x 20 ft. for 50 t.c.h.
(15)
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DELIVERY, UNLOADING AND HANDLING OF CANE
Speed of the table. By reason of its width, the speed of the lateral table is made much lower than that of the main carrier. A value of 10-20 ft./min may be adopted. With a higher speed, there is a risk that too much cane will be dropped on the carrier at a time. Power required for the motor. A motor should be provided of power roughly equal to T= 0.055
(16)
T = h.p. of the driving motor S = area of the table, in sq. ft. This value represents about double the mean power absorbed by the motor in operation. 2. Auxiliary carrier (cross carrier)
This is a carrier of the same dimensions as the main carrier, but with its axis at right angles to that of the latter, and discharging to it. It supplies the main carrier in the manner of a regulating tributary which feeds and evens out the main stream. Speed. It will be driven at a speed about half that of the main carrier. Power. Since it is subject to somewhat more friction, we may provide, if the conveyor is horizontal: Γ-0.065
(17)
T = h.p. absorbed by the auxiliary carrier S = area loaded with can« on the auxiliary carrier, in sq. ft. Construction of cross conveyors
These tables or carriers should move between two lateral steel plates, designed to prevent cane from falling beside the carrier. These side plates can conveniently be given a 10° batter.
A cane sugar factory generally operates continuously from Monday morning till Saturday evening. It shuts down for about 36 hours, including Sunday, for cleaning the multiple effects and for minor repairs. It operates then for about 132 hours per week. During the day, cane transport is generally carried on for only 12 hours, from 6 a.m. to 6 p.m. So that the mill will not run out of cane, it is necessary that the factory should receive in 12 hours, during the day, the tonnage which it crushes in 24 hours. Towards 6 p.m. then there will have accumulated a stock at least equal to half the daily tonnage: Overnight provision = 12C -f a (1) C = quantity of cane crushed by the factory per hour a = margin of safety, which should always be maintained to avoid being obliged to stop the mill for lack of cane. This margin a is necessary to make provision for accidental fluctuations in supply: rain retarding the cutting and restricting transport, irregularities in supply by the farmers, etc. It should not be too small, nor too large. In the latter case, the cane yard will be uselessly encumbered, and the delay occurring between the cutting of the cane and its passage through the mills will be unnecessarily increased. A suitable quantity is that corresponding to three hours' crushing: a = 3C (2) If this value be adopted, the yard should be able to accommodate an overnight stock of: P = 12C + 3 C = 15C
(3)
2
DELIVERY, UNLOADING AND HANDLING OF CANE
1
UNLOADING OF CANE
From the handling point of view, cane arriving at the factory may be divided into two cate gories : 1. Cane transported by mechanical means: trailers, lorries and railway wagons. 2. Cane transported by carts or drays. 1. Cane arriving by lorries
This cane is generally loaded in "packets" or bundles, bound by three chains or "slings" (Fig. 1). At one end these slings slide in a special hook fitted with a pawl, while a ring is attached to the other end.
Π
Fig. 1. Sling with automatic hook.
The unloading device at the factory, generally a crane, lifts the bundle of crane with the aid of a "swingle-bar" or steel bar fitted with three hooks; the labourers on the truck attach the three free rings to these hooks. The crane lifts the bundle and deposits it on the heap of cane which is called a "stock-pile". The catches of the sling-hooks are then released, and the crane lifts the swingle-bar with the slings hanging (Fig. 2). For a 3-ton crane, each bundle is of the order of 3,000-5,000 lb. of cane, generally 4,000-5,000 lb. A 4- or 5-ton truck carries two such bundles. For cranes of 6 tons (the commonest size) and 10 tons capacity, an effort is generally made to work near the limit of the capacity of the crane, and the bundles average about 5 and 8 tons respectively. Tipping trucks are sometimes employed. These are trucks of which the body (or tray) is lifted by two hydraulic pistons and pivots about a hinge, placed either at the back, or along one side of the truck. This system permits of tipping the cane directly into the cane carrier or an auxiliary carrier, the driver operating alone or with the aid of one labourer. It also dispenses with slings, which are expensive. 2. Cane arriving in drays
Delivery from drays is generally made directly into the cane carrier, the driver backing his dray up to the carrier. For this purpose, the horizontal portion of the carrier should be furnished with protective wooden stops, which serve to block the wheels of the dray and prevent damage
1
CANE ARRIVING IN DRAYS
3
Fig. 2. Lifting a bundle of cane.
to the steel plates of the carrier. The driver then empties his drayload by hand into the carrier. It is only when there are too many drays that some of them are permitted to unload their cane onto the stock-pile. The foreman in charge of the cane yard takes care that there is no abuse of this practice, since cane unloaded on to the pile involves an additional handling, as it must be picked up again by the crane and "grab", to be deposited on the carrier (the grab is a device for picking up the canes, which replaces the swingle-bar used for unloading). If the supervision of the yard is poor, there is a risk that the crane will not be able to maintain a regular cane supply to the carrier. If about 50% of the cane supply arrives by drays, and 50% by lorries and rail, the drays may be utilised for the supply to the carrier during the day, while the trucks are unloaded by the crane to make provision for the overnight supply. At night, the swingle-bar is removed from the crane and replaced by the grab, by the aid of which it picks up the cane from the pile and dumps it on the carrier. This night duty of the cane is the more difficult, since (1) a "handful" of cane for the grab is much less than a load from
4
DELIVERY, UNLOADING AND HANDLING OF CANE
1
the slings; (2) the crane alone is responsible for assuring the supply to the carrier, since there are no more drays, and it must handle a tonnage equal to the crushing rate of the mills. Time for unloading a dray. A dray carries about 2500 lb. of cane. One or two men unload it on the cane carrier in 10 or 15 minutes, arrival and departure included. Number of positions for unloading drays. One unloading position for drays therefore allows of a supply of 6 t.c.h. (tons cane per hour). In order to ensure full supply from drays, it is therefore necessary to provide a number of positions:
-=τ
(4)
C = crushing rate of the factory, in t.c.h. (tons cane per hour). Length of carrier required for drays. Reckoning 10 ft. per dray (including space between), it is then necessary to make available for drays a length of the horizontal portion of the carrier equal to: L
C 5C = - x l O = —
(5)
L = length of carrier reserved for drays, in ft. C = crushing rate of the factory, t.c.h. Length of the horizontal portion of the carrier, in order to determine, for these conditions, the length of the horizontal poition of the carrier, it will be a good precaution to increase this figure by 20%, in order to allow for interruptions to the working of the drays, and to have a comfortable margin. The length of the horizontal portion therefore will be: LH = LH
=
1.2
X
SC
3 1.2
X
5C
3 x 2
2C C
(unloading on one side only of the carrier) (unloading on both sides)
(6) (7)
say, for example, in the latter case, 50 ft. for 50 t.c.h. LH = length of the horizontal portion of the carrier, in ft. C = crushing rate of the factory, in t.c.h. (assuming it is maintained entirely by drays at certain hours). There will be an additional position at the end of the carrier, if the end is accessible. These equations assume that there is no obstruction to impede the access of the drays to the cane carrier. CANE-HANDLING EQUIPMENT
The principal types of equipment used at the factory cane yard are: 1. The cane crane, or "derrick", 2. The travelling crane, 3. The tipping platform or truck tip, 4. The cane rake.
1
CANE CRANE
5
1. Cane crane
This is by far the most-used device, and we shall consider it first. It is often called by its American name: "derrick". It consists (Fig. 3) of a mast of fabricated steel, mounted on a pivot, so that it can rotate through a full circle. At a suitable height this vertical mast carries a horizontal arm forming a track on which a trolley, carrying two pulleys, can run to and fro. A cable passes over these two pulleys and hangs between them, thus forming a loop which carries a snatch block, on which may be hung either the swingle-bar or the grab. The driver works in a cabin, attached to the base of the mast, which houses the driving motor and the winding drums for the various cables controlling the trolley. These cranes may be driven by steam, exhausting to atmosphere, or by electric motor. They are also classified according to their mode of support, into: (a) Guyed cranes, (b) Self-supporting cranes. (a) Guyed cranes. This is the lighter form, the stability of the crane being ensured by retaining cables or guy ropes attached to a ring at the top of the mast (Fig. 3). These guys must permit of rotation of the horizontal arm, and so must befixedin the ground at a considerable distance from the axis of the crane.
Fig. 3. Crane with guys (Fives-Lille).
Number of guys. If necessary 3 guy ropes, arranged at an angle of 120° to each other, would be sufficient, ^owever, the number of guys is generally chosen so that breakage of one of them does not cause the derrick to fall. This condition requires a minimum of 5, at 72°. For preference 6 or 8 should be used, if possible.
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1
DELIVERY, UNLOADING AND HANDLING OF CANE
It would be necessary then to fix 5, 6, 7, or 8 anchor blocks, placed symmetrically on a circle of 180-240 ft. radius with the axis of the crane as centre, to which the guys are attached. The presence of factory and office buildings makes the solution of this problem very difficult. If it is necessary to modify the spacing of two cables to avoid in particular the factory buildings, it is necessary to ensure that a breakage of one of these widely spaced cables will not leave a sector of more than 150°, which is the limit permissible for stability of two adjacent cables. Obviously, in case of breakage, it will be necessary to stop the crane immediately until the damaged cable is replaced, since the remaining guys would not be able to ensure stability under the dynamic loading of the crane in operation. Tension of the cables. To obtain maximum rigidity, the cables must be tightened so that the mast describes only a very small cone as the derrick is rotated, but without increasing unduly the tension of the cables themselves. This tension should be checked frequently. The cables should be painted or tarred every two or three years to prevent corrosion. Thickness of the cables. For a crane of 3 tons capacity, 5 to 8 cables of 1 in. (25 mm) diameter are provided.
rOxixixi>ixl;¥;
wyf/*7E*fli*gr
Fig. 4. Self-supporting pivoting crane (Fives-Lille).
(b) Self-supporting cranes. These are obviously heavier and more massive (Fig. 4), but avoid the nuisance and danger of guys, which restrict movement at the cane yard and which are sometimes very difficult to locate conveniently.
1
CANE CRANE
7
In countries subject to cyclones, these cranes should be designed for a wind pressure of at least 60 lb./sq.ft. (300 kg/m2) (live load). Capacity of cranes. The interesting characteristics of an unloading crane are: (1) The lifting force, (2) The radius of operation, (3) The maximum height of lift of the grab. (1) Lifting force. This is the maximum weight which the crane can lift, at the end of the arm. It is the most important factor, which serves to specify its capacity. For instance, one speaks of a 3-ton or 5-ton crane. (2) Radius of operation. This is the horizontal distance between the pivot of the crane and the pulley of the snatch block (corresponding to the centre of the grab) when the trolley is at its extreme position at the end of the crane arm. Typical values range from 60 to 80 ft. (18-25 m). (3) Maximum height of lift. This is the height from the end of the tines of the grab above the platform level, when the grab is raised to its maximum height. To increase this height, and at the same time to enable the driver to oversee the operations and traffic of the platform, the base of the derrick is generally raised by placing it on a trun cated cone of masonry about 6 ft. in height. The height of lift is generally of the order of 20-25 ft. (6-8 m). As a general rule, for a factory having only one derrick at its unloading station, the charac teristics adopted are approximately as follows: (1) Lifting force: F = 0.1C
(8)
F = lifting force in tons C = crushing rate of the factory, in t.c.h. (2) Radius of operation: R = 9 ]/c ft.
(9)
R = radius of operation, in ft. (3) Maximum height of lift: # = 2 5 ft.
(10)
When two derricks (or two unloading devices) are available, the total tonnage C is divided between them by allotting the tonnages C\ and C% respectively, such that Ci + C2 = C Location of the crane. At what distance from the carrier is it advisable to locate the centre of the crane? This distance is evidently a function of its radius of operation. If the centre of the crane is placed too far from the carrier (Fig. 5), the mean angle of rotation of the crane and the travel of the carriage are both increased. This is readily seen by considering for example the point A, the "centre of travel" of the pile of cane, which is located at a distance from the centre of the crane equal to about 80% of the radius of operation. If, on the other hand, the crane is placed too close to the carrier, the angle of rotation is
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DELIVERY, UNLOADING AND HANDLING OF CANE
1
reduced, and little or no traversing is necessary; but the useful area served by the crane is appreciably reduced. The best compromise is to place the centre of the crane at a distance from the centre line of the carrier equal to half the radius of operation. The angle ω is then 120°. The grab may then
Mills
Fig. 5. Location of the crane.
be lowered on to two billets of timber placed above an inclined plane P, from which the cane tumbles in a tangled condition into the carrier, thus being distributed to some extent and helping to avoid blockages at the knives. This inclined plane is very useful, and should always be provided, when there is no auxiliary carrier or lateral feeding table. Bulk density of cane in the pile. This density is similar to that of cane loaded mechanically, or about 12 lb./cu.ft. (200 kg/m 3 ), if it refers to cane dumped at random by hand, or deposited by the grab. It increases to 20 lb./cu.ft. (300 kg/m3) if the pile is made up of bundles of parallel stalks, deposited by the swingle-bar. Volume for overnight supply. We have seen (eqn. 3) that the stock of cane should be built up by evening to: P = 15Ctons At 20 lb./cu.ft. (300 kg/m3) this represents a volume: ¥,
V=
15C x 2,240 — 20
i
^o_
r
= 1,680C cu.ft. Cane
(11;
carrier
Fig. 6. Ground area for overnight supply.
Now, the derrick covers an area equal to that of a circle with radius equal to its radius of operation, less the area of the small inner circle, which must be left free for traffic around the base of the crane.
1
CANE CRANE
9
A radius of about 16 ft. (5 m) must be allowed for this small space, or better, in order to take into account the dimensions of the crane and the installation, say one third of the radius of operation, or from (9): r=3)/Cft.
(12)
Let us consider on what fraction a of the whole circumference it will be necessary to store the cane in order to accumulate the required quantity. We have: V= n(R* — r*)H—
= l,680Ccu.ft.
(13)
Replacing R and r by their normal values, as functions of the crushing rate, and assuming a mean value for H of 20 ft. (6 m) we have: π(81 — 9)C X 20 — - = 1,680C
Whence: a ~ 134°
Fig. 7. Travelling crane for unloading cane (Fives-Lille).
(14)
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DELIVERY, UNLOADING AND HANDLING OF CANE
1
On account of the irregularity of the pile, and the sloping form assumed by its sides, it is far from forming a compact or geometrical mass; hence it is necessary to allow an angle of about 150°. 2. Transporter crane
This system is closely analogous to that of the derrick. Instead of a circle, it serves a rectangle, the length and breadth of which may be fixed at will. For this reason, it will be indicated for certain long and narrow cane yards which have not sufficient space for a crane to swing. The transporter crane must obviously span the cane carrier which it serves. Sometimes a gantry crane is considered sufficient; this has only the two movements, ele vation and traversing, in the one vertical plane, and accordingly serves only for discharging the cane from the lorries on to the carrier, without providing storage (Fig. 7). 3. Truck tip
When the cane arrives by railway trucks, it is more practical to empty the truck in one action, rather than to lift separately with the crane, the bundles of cane piled in the truck. This presents several advantages: (1) Saving in time. (2) Saving in slings (this is by no means negligible). (3) Less handling, because the cane is emptied directly into the carrier, as with drays. At the same time, the absence of slings eliminates the risk of pieces of chain or ratchet hooks passing through the mill. When using crane and slings, it is generally necessary to remove two or three such pieces from the mills in the course of a campaign which means severe damage to the roller grooving.
Fig. 8. Lateral tip for trucks (Fives-Lille).
Fig. 9. Small truck for end-tipping.
1
TRUCK TIP
li
There are two types of tip: (1) The lateral tip, for large waggons (Fig. 8). (2) The endwise tip, for small trucks. In the latter case, the trucks (Fig. 9) are provided with a U-shaped iron frame forming a cradle, in which the cane is placed lengthwise parallel to the track. The ends are free, and the cane, well compacted during the journey from the fields to the factory, falls out when the tip reaches the desired inclination. Tips are sometimes installed for lorries also. The lorry is generally tipped endwise, toward
Fig. 10. "Wicks" rake (Mirrlees-Watson).
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DELIVERY, UNLOADING AND HANDLING OF CANE
1
the rear. Certain models of lorry are not well suited to this operation, or allow oil to run out of the crankcase, beyond a certain inclination. 4. Rakes
This is an auxiliary unloading device, which is used mostly in Java and in some British coun tries. Fig. 10 gives a view of it which will make description superfluous. It will be noted that the rake has, not only a "picking" movement and a to-and-fro movement in a mean plane, but that it can also make the same movement in a great number of planes forming a dihedral angle of some 30-40° to one side or the other of this mean plane. Another type of rake, known as the Davids rake, is common in Queensland and uses rakes supported in an endless chain, and pivoted about the driving shaft so that the rake may be raised or lowered to suit the height of cane in the truck. The width of the rake is such as to cover the full length of a small truck (approx. 6 ft.). LATERAL FEEDING TABLES AND CARRIERS
When the number of drays is insufficient (which may occur at certain fixed times of day, or accidentally at any moment), the crane makes up the cane supply to the carrier with the aid of the grab. However, variations occur in the quantity of cane so deposited per unit length of the carrier; there are gaps which cannot be filled in time. On the other hand, a bundle of cane from a lorry, deposited almost intact on the carrier, can cause an overload at the knives, causing the driving engine to stall or, in the case of electric drive, opening the circuit breaker. When such dense bundles of parallel canes occur, it is necessary to slow down the carrier and practically stop it, otherwise there is a risk of choking the knives. Even if careful attention is paid to this, the attempt is not always successful. When there are no knives, such fluctuations in the cane blanket are detrimental to the crusher feed and reduce the tonnage. These disadvantages may be overcome by installing a "cross-conveyor". This may take two different forms: 1. That of a wide and relatively short platform or lateral table (Fig. 11). 2. That of a carrier identical with the main carrier, but at right angles to it. This is often called an "auxiliary carrier". 1. Lateral table
This is, in effect, a very wide and short carrier, driven by an independent motor. In plan it is rectangular or approximately square in shape. Its upper platform is at a level slightly higher than the raised edge of the side plates of the main carrier. The crane deposits the cane on this platform, and keeps up the supply to it as it discharges. An attendant is posted close to the driving motor of the lateral table and puts it in motion when he sees that the main carrier is not carrying a full feed. The cane falls from the platform on to the main carrier. The advantage of this arrangement is that it falls in a more or less tangled state, making the work of the knives much easier. As soon as the desired quantity of cane has fallen on to the carrier, the table is stopped. The motion of the latter then is jerky and interrupt ed. It often remains for a long time unused, and consequently immobile. The lateral table is particularly useful in countries where payments for cane is made according
LATERAL TABLE
Electric motor Chain drive
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».
Fig. 11. Lateral feeding table (Fives-Lille).
to sugar content or recoverable sugar. The cane to be tested from a particular supplier may then be accumulated on a special lateral table, without interfering with the feeding of the mills. When the table is filled, its load of cane is fed to the mills without interrupting the movement of the main carrier; the beginning and end of the batch to be analysed are simply marked with lime, in such a way that the juice sample boy can see clearly the beginning and end of the batch. The lateral table is much more effective if fitted with a "tumbler". This is a horizontal shaft, placed above the axis of the front drum of the table, and which rotates owly in the reverse direction. It is provided with arms which ensure that the cane falls into the carrier in small lots, avoiding a heavy fall of large masses which would be liable to provoke chokes at the knives. Area of the lateral table. The dimensions of the table vary considerably according to individual cases. A good dimension, designing for ample capacity, is: S=6.5C S = area of the lateral table, in sq. ft. C = crushing rate of the factory, in t.c.h. For example, a table of 16 x 20 ft. for 50 t.c.h.
(15)
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DELIVERY, UNLOADING AND HANDLING OF CANE
Speed of the table. By reason of its width, the speed of the lateral table is made much lower than that of the main carrier. A value of 10-20 ft./min may be adopted. With a higher speed, there is a risk that too much cane will be dropped on the carrier at a time. Power required for the motor. A motor should be provided of power roughly equal to T= 0.055
(16)
T = h.p. of the driving motor S = area of the table, in sq. ft. This value represents about double the mean power absorbed by the motor in operation. 2. Auxiliary carrier (cross carrier)
This is a carrier of the same dimensions as the main carrier, but with its axis at right angles to that of the latter, and discharging to it. It supplies the main carrier in the manner of a regulating tributary which feeds and evens out the main stream. Speed. It will be driven at a speed about half that of the main carrier. Power. Since it is subject to somewhat more friction, we may provide, if the conveyor is horizontal: Γ-0.065
(17)
T = h.p. absorbed by the auxiliary carrier S = area loaded with can« on the auxiliary carrier, in sq. ft. Construction of cross conveyors
These tables or carriers should move between two lateral steel plates, designed to prevent cane from falling beside the carrier. These side plates can conveniently be given a 10° batter.