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October, 1944

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The introduction of cage guides, steam winders, and steel ropes, made for increased safety in spite of the greater winding speeds that they permitted, although increased speeds in themselves were conducive to accidents. Safety devices, however, kept pace with the other developments, and in particular much ingenuity was spent in providing adequate brakes on the winding equipment. It was early realised that drum brakes must be positive, easily controlled, and capable of smooth progressive application, as well as being powerful enough to hold the maximum rope load with an ample margin, or the maximum effort of which the winding engine is capable. It was also appreciated that the maximum intensity of pressure between the brake blocks and brake path must not exceed certain limits, otherwise there was the possible danger of brake blocks firing when the brakes were fully applied.

The first type of brake consisted of straps or bands encircling the circumference of the brake rim, but these were soon superseded by the so-called "post" brakes. In the earlier brakes of the post type it was impossible to obtain uniform pressure along the whole length of the brake block, but this disadvantage was overcome in the curved suspended type, in which, with the usual embracement of 900, the maximum pressure at the centre of the block was just over 10 per cent, greater than the average over the whole block. The caliper type (Fig. 1) is a development of the curved suspended type, giving a slightly increased percentage at the centre as compared with the remaining portion of the periphery; it has also the further advantage that the blocks can be made to subtend 105° without difficulty, thus giving 10 per cent, more holding power for the same maximum pressure per square inch, provided other dimensions are the same. The straight post brake was much inferior in both these respects.

It is apparent that in an emergency powerful brakes are essential, and this was early obtained by connecting the brake strap through a lever to the piston rod of a small steam engine. In the early days, such a brake acted on the "all or none" principle, since it could be operated only to its maximum or minimum extent. The power applied to the brake was a product of the area of the piston and the steam pressure, and since neither of these could be altered the power exerted was always constant. The winding equipment was thus subject to very severe shocks, and such brakes were only applied in an emergency. Later, the application of the steam brake was controlled by a floating lever through a cataract. Eventually, winding engines were provided with two brakes, one applied by the engineman's foot, thereby providing a comparatively light load for stopping the engine at the end of each wind, and the other by steam, this latter being used only on rare occasions.

Burn's brake

Several devices were introduced to increase the power and leverage of foot brakes in order to do away with the use of steam.

Burn's brake, Fig. 2, consisted of a brake strap which did not encircle the drum, but which consisted of a block of wood B, about 24 in. long by 6 in. broad, in which a series of holes was bored and filled with sand. The block of wood was placed at one end of a long lever, the other end being moved up and down by a rod connected to the arm of another lever controlled by the engineman. The block being small in area, and fitted to the drum rim, required a very small movement to free itself; and the length of the lower lever being nearly equal to the length of the engine, it is apparent that a large amount of power could be applied. Leverage to the extent of 200 to 1 was possible. The action of the sand in the holes was to keep the brake rim free from grease. By the assistance of an adjusting screw A, the brake could be tightened in a few minutes and any wear taken up in the blocks, which had to be renewed about every four months.

Another device was to fit a strap brake activated by a toggle-joint lever, the engineman exerting pressure through its foot treadle in the direction of the arrow W, Fig. 3, which pulled down the bell crank lever A, B, C; this moved the toggle joint D, E, F, and gave motion to the lever F, G, H, which worked about the centre G, the end H being attached to the strap going half round the brake ring on the drum. The arrows indicate the motion of each part; in the event of the brake being applied, the toggle joint D, E, F formed a straight line, and at the instant this took place the pressure exerted became infinitely large. B, D and G were the only fixed points.

Pasfield's brake

Pasfield gave a description of his brake* in which an endeavour was made to maintain the power of the steam brake and yet remove the difficulties previously referred to. This device, Fig. 4, consisted of an arrangement whereby the introduction of a special valve and gearing enabled any variation of power to be applied, as the pressure of steam in the brake cylinder could be altered in proportion to the amount of force exerted by the engineman on the controlling pedal. The passage L leads into the cylinder of the steam brake which was fitted with a piston and piston rod, the latter being connected by a link to the lever applying the brake. Steam was admitted on one side of the piston only, and entered or left the cylinder by the same passage. The valve box A was fitted with two valves, B and C, both connected by gearing to the lever to which the foot pedal F was attached, and controlled by the link E. The valve B, which admitted steam through the passage G, was kept closed by the spring D, which first balanced it against the steam in the boiler. The relief valve C, unless the brake was in action, was in equilibrium, free to open or close.

When the pedal was pressed down, the steam valve was relieved of some of its pressure (which kept it closed) and steam entered the cylinder until its pressure was sufficient again to close the valve against the force exerted by the pedal, so that the greater the force exerted on the pedal, the greater the pressure must the steam reach before it closed the steam valve. At the same time, as the pedal relieved the steam valve of some of the spring pressure, which tended to close it, it brought an equal force to bear on the relief valve C to keep it closed; any increase of steam pressure in the cylinder beyond that intended at once escaped through the relief valve. Thus, if the force exerted by the pedal to relieve the steam valve amounts to, say, 1 or 2 lb. per sq. in., the steam valve admitted steam into the cylinder until the pressure present was 1 or 2 lb. per sq. in., as the case may be, and then closed. The pedal at the same time exerted sufficient force to keep the relief valve closed until the steam in the cylinder had reached the 1 or 2 lb. pressure, and then allowed anything beyond that to escape.

It is obvious that the action of a steam brake is dependent upon the maintenance of steam pressure, and is useless if this pressure is not available on account of a steam pipe bursting or any other event which arrests the supply of steam to the brake engine. It was therefore found necessary to fit an additional appliance worked by weights as a safeguard for cases of emergency. The piston rod of the steam brake was continued through the back cover, fitted with a collar, and connected through a slotted link a, Fig. 5, with a bell crank lever b, the longer arm of which carried a heavy weight c. This weight, under normal conditions, was held up by a small winch d, locked by a ratchet and pawl, which could be released and the weight allowed to drop by the engineman. The piston rod of the brake cylinder could move to and fro in the link quite freely so long as the weight was suspended, and as steam was only admitted to the front end of the brake piston, when steam was shut off, the block on the back end of the piston rod engaged the front end of the link. Consequently, if the engineman released the weight, the brakes would be put on without shock, but any such probability was guarded against by inserting a friction arrangement which did not allow the weight to fall at more than a moderate speed.

Modern braking systems

Modern types of brake engines are shown in Figs. 6 and 7. Fig. 6 indicates a steam or compressed-air brake engine designed by Fraser & Chalmers Engineering Works (G.E.C.). The upper part consists of the working cylinder and the lower the cataract cylinder which is connected to a permanent water supply through the non-return valve N in order that leakage may always be automatically made up. The needle valve V gives control over the maximum speed at which the pistons move. Further control over this speed is generally provided by a form of "slow braking" valve. The piston rod is extended through the top cover so that the engineman can easily see the position of the pistons in the cylinders and ensure that they are not too near the bottom when the brake is full on.

Fig. 7 shows an oil brake engine manufactured by the same firm. This is shown without the side covers. The inverted ram design facilitates the piping back to the oil sump of any leakage. At the top of the stroke the top of the ram enters a recess which forms an air buffer to prevent bumping. A cataract cylinder is unnecessary, as the oil friction in the piping prevents a jerky and too-rapid motion.

The modern brake system is designed to enable the engineman to apply at will any pressure from zero up to a maximum of which the gear is capable, and this feature is obtained whereby as the hand lever is moved to apply the brakes a gradually increasing proportion of the deadweight is transferred to the brake lever, the remainder being supported by the brake engine. This function is obtainable in two ways, firstly by the Whitmore type of brake system, which will be described later, the principle involved being the interposition of a spring between the weight and lever, thus providing the engine with a linear follow-up gear so that the motion of the hand lever is reproduced by the piston. It is possible, by this means, to compress the spring to any extent, the brake pressure corresponding to the amount of spring compression. The second method is the adoption of the Iverson principle, which although having the value of simplicity is in practice less sensitive than the Whitmore type; it substitutes a pressure follow-up for the linear follow-up, so that the pressure under the brake-engine piston, and consequently the proportion of the deadweight it supports, gradually decreases as the hand lever is moved to the "on" position. No springs are required between the weight and brake lever, as the natural spring on the brakegear is sufficient to let the gear accommodate itself to the varying pressure.

The modern brake system should provide for automatic brake application in good time to prevent an accident independently of the engineman, if he has failed to apply the brakes. This feature is met, as regards the brake control gear, by the provision of two alternative means of brake application, one by the engineman's hand lever, and the other by the overwinder and other safety appliances. In an electric winder this is in the form of a floating lever; one end held by a solenoid and released in emergency, the other connected to the hand lever. When either end of this falls the brakes are applied. This arrangement is shown in Fig. 8. S indicates the brake control levers, arranged to be used separately or both together, controlling the brakes through the main brake valves T and a system of floating levers U. The brake control is on the linear follow-up principle, graduated braking resulting from the interposition of the springs V in the brake operating gear. By virtue of the follow-up levers W, which are connected through the linkwork shown to the brake levers X, the position of the brake-engine piston on its stroke is governed by the position of levers S in their quadrant.

For emergency brake operation the solenoid Y comes into action and operates on the valves through the levers S, and further the solenoid cannot be re-set unless these levers have been put to full-on position, thus closing the brake interlock switches AA. A similar arrangement is adopted for steam winders, a lever operated by the overwinder taking the place of the solenoid.

The design of the modern brake system should be such as to prevent the application of the full braking force when it is unnecessary to prevent an accident, and where such application will subject the winding system to excessive stresses. This function is met by the provision of slow braking gear which takes the form of a throttle valve in the brake-engine exhaust operated by a cam driven from the winding drum, or drums, so that it is closed to an adjustable extent during the full-speed part of the wind, but fully opened when the cage approaches either end of the wind. The introduction of this valve makes it impossible for the rapid application of the brakes when the winder is running at speed, and by suitably adjusting the rate of application it can generally be arranged that the winder is normally brought to rest, in emergency braking, before the full braking force is applied. The danger existing of an overwind being caused by such delayed action is prevented by the cam gear. Such an arrangement is noted for its simplicity, but it is inadequate for a double-drum winder operating at high speed from a great depth. Separate drums may have brakes sufficiently powerful to hold against a rope pull considerably greater than that due to the rope and the loaded cage, and if, when winding in balance, both brakes are fully applied when the rope pulls are more or less equal, very excessive decelerations can result. It is therefore usual to install a semi-automatic brake governor which prevents the deceleration in these circumstances from exceeding a predetermined value, except when the cages approach the end of a wind, or after the failure of the normal means to set the brakes. It is important, however, that near the end of the wind it should not hinder the application of the braking force necessary to prevent an overwind.

The essential parts. of the deceleration governor are shown in Fig. 9, and an auxiliary brake control valve, with the mechanism for its operation by the governor, in Fig. 10. This valve is connected in the pipe between the main brake control valve and the brake engine, and it has its own connections also to the pressure-fluid supply pipe and to atmosphere or oil sump.

A, in Fig. 9, is the driving shaft which is driven from the winder drum or pinion shaft through suitable gearing, and B is the valve-operating shaft. These shafts are inter-connected by a differential gear similar to that used in the rear axle of an automobile, the planet wheels C being carried in a flywheel D.

When the winder is running at constant speed there is no appreciable turning action on shaft B, but as soon as deceleration occurs the flywheel tends to over-run the driving shaft, and this produces a turning action on shaft B, which is proportioned to the deceleration. The motion of shaft B is spring-controlled and its displacement is therefore a measure of the deceleration.

In Fig. 10, E is the auxiliary brake control valve and F its casing with pipe connections as indicated. B is the governor shaft mentioned above, to which is fixed a two-armed lever G with pins engaging with slots on two links H acting through bell-crank lever J and valve rod K, its motion being controlled by spring N and adjustable dashpot M.

Valve E is shown in its normal undisplaced position in which it gives a free connection between the main brake valve and the brake engine. Deceleration causes the valve to rise, independent of the direction of rotation of the winder, and when it reaches the value for which the springs have been adjusted connection between the main brake valve and brake engine is interrupted and a further increase in the braking force is prevented. Further movement of the valve causes pressure fluid to be admitted to raise the brake weights a small amount, if necessary, in order to reduce the braking force to the required value. Cams driven from the winder drums come into action near the end of the wind, and acting through bell-crank lever N and pin O, restore the valve to its initial position, in which the brake is entirely controlled by the main valve. Interference by the governor with this action is prevented by springs P on links H. These springs are initially loaded so that they do not compress under the load necessary to operate the valve.

The failure of any part of the deceleration governor or its driving gear will have one of two results. Either the governor will cease to rotate or it will seize up and rotate as a single unit. In the former case valve E remains unaffected and the control of the brakes by the main valve is unimpaired. In the latter case, shaft B will be driven at the same rate as shaft A. The slots in link H are of such a length that when the valve has made its maximum travel the pins in lever G come to opposite ends of the slots and further movement of the lever can only take place by compressing the springs P. It will be seen that the effect of such further movement is to move the valve in the opposite direction, that is, to make it fall. This movement first restores connection with the main brake valve and then opens a by-pass connection to the brake engine cylinder, allowing the fluid to escape and causing the brakes to be applied at a rate determined by the setting of needle valve S. This valve is fully opened by the cams near the end of the wind by the means shown. Any danger of damage to the valves and valve operating gear is prevented by the provision of a spring-loaded slipping clutch T (Fig. 9) between shaft B and lever G, whilst slot O in the valve rods prevents any interference by the cam gear. It will thus be seen that the effect of any seizure of the governor parts is to apply the brakes independently of the main control valve; slowly if the mishap occurs in the middle of the wind, but rapidly if near the end. This is accomplished by the action of the auxiliary valve not in its primary function of deceleration limiter, but in its secondary function as the emergency-brake control valve. This valve can be brought into action as an emergency valve at any time by a second brake magnet or trip lever which controls a friction clutch U between the driving shaft A and flywheel D. When this operates, the clutch is engaged and causes positive rotation of shaft B and so applies the brakes as described above. This result is brought about by the operation of an overspeed governor set well in excess of the maximum speed. If for any reason the main brake control valve fails to act and to apply the brakes, the secondary means is thus brought into action before any excessive overspeed occurs. In an electric winder a second push button protected by a glass cover to prevent its use except in extreme urgency is also often fitted.

Whitmore brake

One of the earliest successful types of winding brake was that designed by Mr. L. F. Whitmore. His brake combined both the weight and steam application, and introduced the further advantage of providing for the automatic taking up of wear on the brake blocks, with the result that a certain definite pressure is always exerted by the engineman when his pedal is in a given position. The brake is shown in Fig. 11(a) and consists of the engine A made up of steam and cataract cylinders, fitted with a floating-lever gear by means of which the position corresponds exactly with the position of the foot pedal. A series of weights B is suspended by the rod C, attached to the crosshead of the brake engine. These weights put on the brake through the levers E and H, and steam has to be admitted under the piston to raise the weights to take off the brake. Consequently, should any accident occur to the steam main, the brake would go on. As a rule there are not enough weights provided to stop the engine against full steam pressure, but only enough to hold it against the ordinary winding load, because under working conditions the full load is applied partly by the weights and partly by steam on the top of the piston. The rod C is threaded through a spring box D, which is free to slip up or down. This bears upon the brake lever E and the springs F contained in the box are compressed between the bottom of the box and the plate G. The brake lever E is connected to the top ends of the brake posts by a cranked lever H, and tie rods, and draws them together. The varying load is applied by the compression of the springs, due to the lowering of the weight B bringing down the top plate G and the spring box, the position of each being controlled by the foot pedal. The further this goes down the more the springs are compressed owing to the downward movement of the weights. The maximum load is applied when the plate G is touching the sleeve distance piece in the spring box, and the minimum load is applied when the collar J is touching the underside of the brake lever E, as the springs are then fully extended. This happens when the foot pedal is about position 2 and in bringing this further back to position 1 the piston travels still higher, carrying with it the weights and fixed collar J and also lever E sufficiently far to give clearance between the brake blocks and the drum rim.

One of the great advantages of the Whitmore brake is the method adopted in taking up the wear of the brake blocks. This part of the installation, enlarged in Fig. 11(b), consists of a ratchet nut mounted on the cross-head at the end of the tie rod K. The ratchet nut L is fitted with a pawl N mounted in the lever O, which is connected by the rod P to an extended arm of the brake lever E. This take-up gear operates as follows; on the downward stroke of lever E, the rod P is moved upwards, consequently when the lever and rod E are forced down to a certain distance the pawl will take up another tooth. On the return or upward stroke the rod P is brought downwards by the collar at the end of the lever E, striking on the bottom of the rod P which screws up the nuts on the tie rods K to the same extent. It will be seen that the lever is not forced down to the position before mentioned by the same load being applied until the brakes are worn to that amount. The adjustment is small at each operation, and there is only a very slight difference in the position of the brake piston immediately before and after each automatic adjustment.

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* Transactions Nth. Staffs. Inst. Min. Engs., ii, p. 112.

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NOTE. — In this article much valuable information has been obtained from Bulletin No 17 on Winding Plant issued by the G.E.C. (Fraser & Chalmers).

(To be continued.)

Drawings and Photographs accompanying the article