ON IMPROVING THE SAFETY BRAKE OF THE PM WINCH

I. K. Khrustalev

The braking device of the PM hoisting winch consists of a service brake and a safety brake; they are installed separately and have an independent actuating element and drive.

The safety brake is of the shoe type and acts on brake rims fastened to the drum. The drive of this brake, load-operated with manual release (see Fig. 1), consists of rod 1, fixed in hinge 2 and loaded at the other end by weight 3. Attached to rod 1 is pull rod 4 for connection with the shoes. To hold weight 3 in the raised position, which corresponds to the released machine, there is

Fig. 1. Safety brake of the PM winch

a hinge 2 and loaded at the other end by weight 3. Attached to rod 1 is pull rod 4 for connection with the shoes. To hold weight 3 in the raised position, which corresponds to the released machine, there is

a release mechanism with free travel, consisting of column 5, screw 6, handwheel 7 with threaded bushing 8. In the lower part of screw 6 there are two plates 9, which by means of projections 10 hold the head of rod 11 and the lever 1 connected with it. Between plates 9 is placed a double-sided cam 12, which is fastened on shaft 13 of lever 14; the latter is provided with counterweight 15 and is held by the retracted core of electromagnet 16. When the current is switched off, the core of the electromagnet drops, lever 14 turns under the action of counterweight 15, and by means of cam 12 spreads plates 9 apart; these release the head of rod 11 and the lever 1 connected with it. The latter drops under the action of load 3, and braking of the machine takes place.

To soften the impact from the falling load 3, an oil damper is used, consisting of cylinder 17 with oil and piston 18 with through holes, threaded for plugs. Piston 18 is connected through rod 19 with lever 1. The speed of descent of the piston is regulated by screwing plugs into or out of the holes of piston 18.

Fig. 2. Characteristic of the brake of the PM-20 winch, recorded with an indicator.

Free-travel time 0.42 sec; average deceleration upon meeting the vessel, 2.01 m/sec².

Engagement of the safety brake can be carried out either automatically, when the circuit of electromagnet 16 is opened by safety devices, or by the operator, by means of a button in the circuit of electromagnet 16.

With the aid of an indicator of the design of Prof.-Doctor I. A. Balashev, we recorded the characteristics of several such brakes on hoisting installations in the Kuzbass. Two of these characteristics are shown in Figures 2 and 3.

From the characteristics it is evident that the safety brake of PM winches satisfies the requirements of the PTB with respect to the magnitude of the free-travel time and the smoothness of the rise of the braking torque. All tests of similar brakes carried out by us on hoisting installations in the Kuzbass give a free-travel time of less than 0.5 sec. The lowering of the load takes place smoothly, without noticeable elastic oscillations.

However, the tests carried out on the safety brakes of PM winches and numerous observations give grounds to consider that this brake is not reliable in operation and does not meet the basic requirement of safety regulations in coal and shale mines: under all possible damages, reliable braking of the hoisting machine must occur with simultaneous switching off of the hoisting motor; i.e., when the protection circuit is opened by one or another safety device, the safety brake must operate without failure and reliably brake the hoisting machine. Thus, for example, when testing the braking device of the type described on the hoisting installation of one of the mines of the Kuz—

the basin, the protection circuit was broken three times, and all three times the safety brake did not come into action. The reason for this was the jamming of the head of rod 11 between the projections 10 of plates 9.

Such cases, cases of “failure” of the safety brake of PM winches in the practice of their operation, are not isolated. Jamming of the rod head between the projections of the plates, or its being strongly pressed against the latter under an excessively large load 3, often prevents the machine from being braked when the circuit of electromagnet 16 is broken.

In addition, when projections 10 or the head of rod 11 operate, cases are often observed in which the indicated projections do not hold the rod head, which makes it impossible to release the machine from the brake. In this case the rod head is usually brought behind projections 10 of plates 9, and the latter are tied together with wire. In this case the safety brake is put completely out of action.

Fig. 3. Characteristic of the safety brake PM-30, recorded with an indicator.

Idle-running time: 0.22 sec. Time for stopping the cages from the start of braking: 1.85 sec. (the protection circuit was broken when the cages met). Average deceleration: 2.5 m/sec².

These frequently occurring faults in the safety braking device of PM winches make the use of the indicated winches on hoisting installations, and especially on man-and-material hoists, unsafe. Hence follows the entirely understandable desire of mine operating personnel to improve the described braking devices so that they operate reliably and without failure. Such improvements were carried out on two hoisting installations of Kuzbass. On these hoisting installations the mechanism for manually releasing the machine from the brake and holding the load in the “released” position was removed, i.e., parts 5–19 were removed, with the exception of electromagnet 16, and an oil drive was installed instead. This made it possible to make the safety brake reliable in operation and convenient to control.

Instead of a simple weight with an oil damper for the safety brake, the PM hoisting winches of the indicated installations came to have an oil brake with a weight drive.

We carried out tests of such a brake, with its characteristic recorded; it is shown in Fig. 4.

The characteristic of safety braking and observations of the operation of the improved brakes at the indicated mines show that they are quite reliable in operation and fully meet the requirements of the PTB.

Summarizing the steps taken in improving the braking devices described, we propose the simplest and, in our opinion, the most perfect scheme of such an improved brake, which is shown in Fig. 5.

Safety braking by means of such a brake is carried out as follows.

When the circuit of electromagnet 8 is broken, the valve spool of cock 9, under the action of load 10, descends and opens access for the oil from the lower cavity of cylinder 4 through pipeline 7 into the upper cavity. In connection with this, load 3 descends—the machine is braked. The path of the oil during braking is shown by the dashed arrow. To regulate the lowering time of the load during braking, i.e., to regulate the smoothness of braking,

Fig. 4. Characteristic of the improved PM-30 brake, recorded by an indicator.

Idle-travel time 0.29 sec. Average deceleration when the vessels meet: 2.3 m/sec²

a gate valve 12 is provided. For releasing the machine, oil pump 11 is used, which pumps oil from the upper cavity of cylinder 4 into the lower one. The pump motor is switched on by the operator by means of a pushbutton in its starting circuit. Stopping is by means of button 13 (contacts a) when load 3 is fully raised. The path of the oil during release of the machine is shown by the solid arrow.

Fig. 5. Improved brake of the PM winch.

In the event of oil leakage through looseness in the connections and the consequent lowering of load 3, oil pump 11 is arranged to be switched on by means of button 13 (contacts b). Periodic refilling of cylinder 4 with oil is performed by the operator from tank 15 by means of gate valve 16.

In order to prevent oil pump 11 from being switched on during safety braking (when the circuit of electromagnet 8 is broken), there is button 14 in the starting circuit of the oil-pump motor, which breaks this circuit when the armature of electromagnet 8 drops. The circuit diagram for connecting the buttons in the starting circuit of pump motor 11 is shown in Fig. 6.

Operating experience with improved brakes at two mines in the Kuzbass shows that the diameter of cylinder 4 may be taken as 200–250 mm,

its height depends on the place of installation (on the stroke of piston 5); the diameters of the pipelines are 7–25 mm.

The safety brake of the PM winch, shown in Fig. 5, is simple in design (all improvements can be carried out directly at the mine), does not require enlargement of the existing basement room for its installation, and is undoubtedly more reliable than the safety brakes adopted on the indicated winches; therefore its introduction into industrial use can be begun immediately.

Fig. 6. Diagram for connecting the buttons into the starting circuit of the pump motor.
“Start”—the operator’s button for starting the oil pump. 13a—button for stopping the pump when load 3 has been raised to its limit. 14—“stop” button, which breaks the starting circuit during safety braking. 13b—button for starting the pump when lowering load 3 in the event of oil leakage.

Here it is also necessary to point out to the manufacturing plants of PM winches the imperfection of the safety brakes of the indicated winches and the need to improve them directly under factory conditions.

REFERENCES

1. German A. P., Shklyarskii F. N. Mine hoisting installations. Ugletekhizdat, 1947.
2. Elanchik G. M. Mine hoisting installations. Gostoptekhizdat, 1941.
3. Prof. Il’ichev A. S. Mine hoisting machines. ONTI-NKTP—USSR, 1933.
4. Umanskii V. B. Mine hoisting installations. Ugletekhizdat, 1949.
5. Safety regulations in coal and shale mines. Ugletekhizdat, 1951.
6. Khadzhikov R. N. Mine hoisting installations. Ugletekhizdat, 1950.
7. Balashev I. A. Experimental investigation of brakes of hoisting machines. Journal Ugol’, No. 11 (179), 1940.