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Colliery Engineering

February, 1945

Grimmitt's overwinder

In many of the overwinders objection was sometimes raised to their use on account of the winding engine receiving a sudden and intensive shock by the abrupt application of the brake. Grimmitt overcame this difficulty by employing a pneumatic arrangement which buoyed up the brake lever for a few seconds after it had been cast loose by the apparatus. As air escaped from a regulating tap, the air vessel and lever sank lower and lower, gradually increasing the pressure of the curbs on the brake wheel. Several designs of such appliances were used on the Continent. At the Lens Collieries the winding engines were fitted with an apparatus which automatically cut off the steam at a determined point at every wind. A tappet on the indicator opened a valve when the cage reached a distance of some 35 yards from the top, which shut off the steam and applied the brake.

Slow-banking devices

It was found in practice that perfect control over the winder was impossible even with the use of the overwinder. Overwinders, generally, are dependent upon the action of a centrifugal governor for their speed control; the governor, however, lacks power and precision at low speeds and cannot be relied upon to ensure safe landing speeds.

It has been ascertained that men riding in a descending cage are liable to suffer injury if the cage lands on the pit bottom bearers at a speed exceeding 5 ft. per sec. An overwinder device is generally incapable of guaranteeing this rate of cage travel and it has therefore become necessary to incorporate a slow-banking device in the winding system. A satisfactory slow banker, if it is to maintain continuous control of the winding speed throughout the wind, must be sensitive down to less than 5 ft. per sec., although the maximum speed at which the cage may travel in one part of the wind may be equal to or in excess of 60 ft. per sec.

It was at first considered that the application of multiple governors would control the speed from its maximum to minimum value, but consideration will show that many governors which would be required would introduce complications, and would further render undesirable any catch point tripping arrangement because of the discontinuity of control inherent in all such devices.

In view of the difficulties which would ensue with numerous governors controlling the slow banker, attention was given to their hydraulic operation, the main principle involved being the use of a dashpot containing a fluid which is forced in or out at a rate governed by the speed of the descending cage, and the rise or fall in the pressure in the dashpot cylinder is used as a measure of the danger of an excessive landing speed. The gearing is arranged that in the event of a rise or fall at any moment during the deceleration period exceeding a predetermined amount a trip out is caused and the cages brought to rest in a minimum of time.

The change in pressure depends upon two factors :–

(1) the speed of the dashpot piston, and

(2) the size of the orifice through which the fluid passes.

It will be obvious that if the dashpot piston speed is directly proportional to the cage speed, a constant pressure during deceleration can only be maintained by decreasing the size of the orifice as the piston speed decreases. Considerable difficulty is incurred in providing this without serious complications and without introducing discontinuity of action. Frequently, therefore, the opposite principle is utilised, namely, that the orifice remains constant and the piston is so operated that its speed is constant if the cage speed towards the end of the wind decreases at the correct rate to ensure the cage landing without shock. In the event of the cage speed at any time exceeding the safe maximum the predetermined pressure change is exceeded and the overwind gear is tripped.

Many efficient "slow bankers" have been designed to ensure safety in landing, the descriptions of which are beyond the scope of this article. A description of the Fraser & Chalmers "Ray-Cam" slow banker is indicative of the progress made to ensure safety.

The "Ray-Cam" slow banker is shown in Fig. 3, a sectional view and side elevation, being shown in Figs. 4 and 5 respectively.

The crank pin (1) engages with a hole on the end of rod (4), which is elongated so that for the main part of its motion the pin travels idly without moving the rod in the direction of its length. On the rod (4) there is a collar (5) held against a shoulder on the rod by the compression spring (6), which is sufficiently powerful to prevent the collar from moving away from the shoulder in normal operation. In the event of an overwind taking place the spring compresses and damage to the slow-banking gear is prevented. This collar has pins engaging with levers (7), the motion of which is transmitted to the rolling levers (8) by the rod (9), whose length can be easily adjusted. The other ends of the rolling levers are pivoted to a dashpot at (10). Towards the end of each wind the crank pin (1) which is driven by suitable means from the winder drum conies to the end of the slot in the rod (4) and causes the dashpot (10) to be raised through the linkages described. The levers (8) roll on the cams (11), with the resut that the leverage between the rod (9) and dashpot (10) gradually increases, so that the speed of the dashpot relative to that of the drum becomes increasingly greater as the end of the wind is approached. By this means the constant dashpot speed is derived from the decreasing drum speed, and by suitably shaping the cams any desired relation between drum speed and the position of the cages in the shaft can be associated with a constant speed of rise of the dashpot. A by-pass valve (20) is so adjusted that at this speed the piston just floats on its supporting spring. If the drum speed at any time during the controlled part of the wind exceeds the predetermined value, a switch of the sealed mercury type causes an electric alarm bell or buzzer to ring, warning the engineman to reduce speed, and if this is not done the further rise of the piston trips the overwinder, closing the throttle and fully applying the brakes.

In the event of the winding engine being used for a dual purpose, that is, winding both men and coal, the time expected for winding coal is less than that for winding men. This is obtained by fitting separate sets of cams for the two conditions, either of which can be brought into the operative position as required. As shown in Fig. 5, these comprise a coal-winding cam (11) fixed to the frame of the device, and a men-riding cam (12), which is supported at one end by a pivot (13) and can be raised to the operative position by means of the lever (14). The operating plunger (24), Fig. 5, is smaller in diameter than the dashpot and operates the trip toggles (28) and (29), Fig. 5, through collar (26) and lever (27).

The dashpot is filled with a special oil whose viscosity varies so little with temperature that no adjustment of the orifice is required on that account. A cover is fitted over the adjusting device and can be padlocked, if desired, to prevent unauthorised alteration. By adjusting the orifice (20) the pressure in the dashpot cylinder resulting from the steady upward motion of the dashpot corresponding to correct deceleration of the winding engine can be made sufficient to bring the toggles quite near the breaking position and very sensitive control of the winder speed during the deceleration period is thus obtained. Moreover, owing to the difference in diameter, a very small movement of the dashpot is sufficient to give enough travel to the operating piston to bring about tripping, and this is of importance in securing proper protection when it is necessary to complete a wind after the cage has been brought to rest quite near is final position.

When the winder is reversed after completing a wind and the next wind commences, the dashpot falls rapidly under its own weight assisted by the counterweight (35), no resistance being offered by the fluid in it owing to the non-return valve (19), and as it approaches its lowest position the adjustable bridle (38) attached to it forces the trip lever (27) back into the full "set" position, thus rendering impossible an accidental trip during the high-speed part of the wind when the sudden application of the brakes might have serious effects. The bridle acts on the trip lever quite near its fulcrum, so that only a very small upward movement of the dashpot suffices to release the trip lever. A dust sleeve (39) embracing an extension of the main piston shank (21) is fixed to the operating piston rod (24), and serves to exclude grit which might interfere with correct operation. The motion of all other dashpot parts is positive and therefore unaffected by accidental grit.

Electric winders

The Whitmore or other types of overwinders can be applied to electric winders for protection against overwind and excessive landing speeds. This protection is obtained by operation of the sliding catch sleeve principle previously described, although the sleeves are controlled not by a centrifugal governor, but by a gravity dashpot. During the wind the sleeves are held by a spring stirrup in a position well below that corresponding to decking level. Near the end of the wind the stirrup is released by the rising nut and the catch sleeve is then raised by the falling dashpot piston. This piston as it falls closes by-pass holes in the cylinder, so that its speed decreases at a controlled rate designed to represent the deceleration necessary to give shockless landing. If the overwinder nut overtakes the receding catch sleeve it means that the deceleration is insufficient, the overwinder is tripped, power cut off and the brakes applied in sufficient time to bring the winder to rest before landing is reached. Protection against overspeed during the main part of the wind is afforded by a centrifugal switch driven from the winder motor and connected in series with the overwind limit switches, so that when it operates the power is cut off and the brakes applied at a rate governed by the slow-banking device.

Protection against overwind and overspeed at any part of the wind in the case of large double-drum electric winders winding from considerable depth is obtained generally by the well-known Lilly controllers. These devices function electrically by means of overwind and overspeed limit switches operated by cams in co-operation with a centrifugal governor. When accurate control of the landing speed is required, the single governor is unsatisfactory for the reasons already given, and a second governor is then fitted to take over the control at low speed. When fitted to high-speed cages a separate slow-banking gear of the type previously described is necessary. For electric winders, electrical devices have virtues of compactness, sensitiveness and rapid operation which are difficult to reproduce in a purely mechanically operated overwinder.

(To be continued.)

Drawings and Photographs accompanying the article