GB2138519A - Power actuated drum brake - Google Patents

Abstract

A power actuated drum brake includes a pair of brake shoe assemblies 1 pivotally mounted on respective brake arms 3 and free to move through a limited distance with the rotation of the brake drum. Braking force is applied by spring means between the brake arms and the brake is disengaged by an electrically controlled actuator 8 moving the brake arms apart. An electric switch 19 (Fig. 2, not shown) is connected in the actuator control circuit and is operated by movement of one of the brake shoe assemblies with the brake drum. When the brake drum is braked and subject to reverse torque, the brake shoe assemblies move to a position in which the electric switch is opened to disable the actuator. The switch is not closed until the drive to the brake drum overcomes the reverse torque. The invention also provides brake shoe assemblies for converting existing drum brakes. <IMAGE>

Description

SPECIFICATION Improvements in or relating to power actuated drum brakes This invention relates to power actuated drum brakes for use in situations in which a drive shaft to a load is to be braked and the load, when stationary, will apply a reverse torque to the drive shaft. By "power actuated drum brake" is meant a drum brake operated by an electrically energised or controlled actuator; such as an electro-magnet, a thrustor, or an electrically controlled pneumatic or hydraulic cylinder.

A typical situation is that of a mining elevator conveyor driven through a fluid coupling by an electric motor, an electro-magnetic drum brake being located between the coupling and the conveyor for braking the drive shaft to the conveyor. In the braked condition the load of the conveyor applies a reverse torque to the drive shaft. When the brake is released, the conveyor will slip back until sufficient torque has been developed at the output of the coupling to overcome the reverse torque. It is therefore necessary to provide means whereby brake release does not occur until the aforesaid sufficient torque has been developed.

It has been proposed previously to include an auxiliary switch in the brake energisations circuit and to provide switch control means operable in dependence upon the torque applied to the drive shaft.

However, such previous proposals rely on mounting the whole brake mechenism for a rocking motion under the influence of reverse torque, which is geometrically incorrect.

Furthermore, one such proposal provides for the rocking motion against the force of a leaf spring, which is liable to fracture.

One object of the present invention is to provide a power actuated drum brake which avoids the aforementioned disadvantages.

Another object is to provide a power actuated drum brake in which an electric switch for controlling the brake actuator is operated by a brake shoe assembly.

A further object of the invention is to provide a novel brake shoe assembly enabling existing power actuated drum brakes to be readily modified to a brake in accordance with the present invention.

According to one feature of the invention, there is provided a power actuated drum brake for cooperation with an associated brake drum, comprising a pair of brake shoe assemblies each pivotally mounted on a respective brake arm, means for applying a force between said arms to move said brake shoe assemblies towards each other and into a drum-braking position, an electrically operated actuator operable to overcome said applied force and move said brake shoe assemblies away from each other and release the brake, an electric switch mounted on one of said brake shoe assemblies and connected in the operating circuit of said actuator, said one brake shoe assembly including means for operating said switch so that said actuator may not be operated to release the brake when said assembly is in the drum-braking position and the associated brake drum is subject to a reversetorque load.

According to another feature of the invention, there is provided a brake shoe assembly for a power actuated drum brake including an electrically-controlled actuator for releasing the brake; comprising a brake shoe, means for pivotably mounting said brake shoe on a fulcrum pin with freedom for substantially-radial movement relative thereto between two limiting positions, means for urging said brake shoe towards one of said positions, and means adapted for operation of an electric switch when the brake shoe is moved to the other of said positions.

One embodiment of the present invention will now be described with reference to the accompanying drawings, in which: Figures 1 a and 1 b are diagrammatic side and end views respectively of a known form of power actuated drum brake to which the present invention may be applied; Figure 2 is a perspective view of a brake shoe assembly according to the present invention, and Figures 3a and 3b are diagrammatic side views of a power actuated drum brake according to the present invention in different operative conditions.

Referring to figures 1 a and 1 b, a known power actuated drum brake comprises a pair of brake shoes 1 each pivotally attached by a fulcrum pin 2 to a brake arm 3. The brake shoes co-operate with a brake drum 4 secured to the shaft to be braked.

The brake arms 3 are pivotally attached at one end, at 5, to a base 6 and, at the other end, to a spring-loaded linkage 7 operated by a thrustor 8.

When energised the thrustor applies pressure to the linkage 7 forcing the brake shoes off the brake drum against the spring. When the thrustor is deenergised, the brake shoes are forced onto the drum.

In one embodiment of the present invention, the aforementioned brake shoes are modified as shown in Figure 2 in which a brake shoe assembly 10 has flanges 1 Oa each provided with two rigidly-attached (as by welding) blocks 11 having bores axially aligned with each other. Located in spaced relationship between the blocks 11 is a fulcrum block 12 having a bore 1 2a for receiving the aforementioned fulcrum pin 2, the latter being pivotally mounted on the brake arm 3 (Figure 1).

The block 12 carries pins 13, shown in broken line which are slidably located in the bores of the respective blocks 11.

In operation, when the fulcrum block 12 is mounted on the fulcrum pin 2 it is stationary relative to the brake arm (apart from pivotal movement) and the brake shoe is movable relative to the block 12 to the extent that the blocks 11 may slide on the pins 13, the limited degree of travel being indicated by arrows at 14. The fulcrum pin 2 passes through a hole (not shown) in the flange 1 Oa of the brake shoe behind the block 12 and this hole is appropriately elongated to permit the shoe to move axially relative to the fulcrum pin between the limiting positions of travel indicated by arrows 14. One pin 13 extends through and beyond the related block 11 and carries a spring 17 acting between the face of the block 11 and a locked nut 18 on the pin.This spring holds the brake shoe in a rest position relative to the block 12 (as shown in Figure 2) and applies sufficient force to overcome the weight of the brake shoe.

It is to be understood that both flanges 1 Oa are provided with the described arrangement including blocks 11 and fulcrum block 12.

Furthermore, in a pair of brake shoe assemblies in accordance with the invention, one electric switch is provided although, of course, additional switches could be provided should the need arise.

Such a switch, referenced 19, is shown in Figure 2 mounted on a flange 1 Oa so that the switch actuator 20 is located in alignment with the end of the adjacent pin 13. When, in operation the related block 11 is moved along the pin 13 (downwardly as seen in Figure 2) to abut the block 12, the actuator 20 is depressed by the end of the pin to open the switch contacts.

A power actuated drum brake in accordance with the invention may be produced by taking a known form of brake, such as shown in Figures 1 a and 1 b, and replacing each brake shoe 1 by a brake shoe assembly 10, one of the assemblies 10 including a switch as described. Figure 3a shows the brake shoe assemblies 10 of such an arrangement located about the brake drum 4, the brake being in the disengaged condition (i.e. with the thrustor 8 energised) and the brake drum being rotated by a driven shaft (not shown) in a clockwise direction. It is assumed that the driven shaft is driving a load which will apply a reverse torque when the shaft is stationary, as hereinbefore described.The switch 1 9 is connected in the energisation circuit of the thrustor 8 so that the thrustor can be energised only when the switch is in the non-operated condition as shown, with its contacts closed.

It will be noted that the brake shoe of the lefthand assembly 10 is held against gravity by its spring 17, in its uppermost or most-clockwise position relative to the fulcrum block 12 and fulcrum pin 2, as in Figure 2. Similarly the brake shoe of the right-hand assembly 10 is held in its most-clockwise position, but because the assembly is inverted, this is due mainly to gravity and the spring 1 7 serves to resist relative movement and as a return spring. Thus, the brake shoes are held in position ready for braking so that, due to the force of the springs 1 7, when the brake is applied there will be no relative movement causing undesirable hammer blows between the blocks 11 and 13.

When the drive is removed and the brake engaged by de-energising the thrustor 8, the brake shoes are held in the positions shown in Figure 3a until the brake drum comes to a halt. Then reverse torque applied by the load causes the drum to run back anti-clockwise and carry the brake shoes to their most anti-clockwise positions, as seen in Figure 3b, where they are held. Thus, the switch 19 is operated to open the thrustor energisation circuit and prevent the thrustor 8 being reenergised.

When the drive is re-applied, the energisation circuit of the thrustor is completed except for the switch 19. The brake therefore remains engaged and holding the load until the drive torque is sufficient to overcome the reverse torque and rotate the brake drum. Thereupon, the drum moves the brake shoes to their most clockwise positions (Figure 3a), the actuator of the switch 1 9 is released to close the switch contacts, and the thrustor is energised to disengage the brake.

Although the invention has been described with reference to a brake drum driven in a clock-wise direction, it will be obvious that the invention can be applied in relation to a drum driven in an anticlockwise direction by inter-changing the described positions of the brake shoe assemblies.

As will be apparent from the appended claims, it is to be understood that the present invention provides not only a novel power actuated drum brake but also a novel brake shoe assembly whereby known drum brakes may be converted, by replacing the brake shoe assemblies, to a drum brake according to the invention.

Claims (10)

1. A power actuated drum brake for cooperation with an associated brake drum, comprising a pair of brake shoe assemblies each pivotally mounted on a respective brake arm, means for applying a force between said arms to move said brake shoe assemblies towards each other and into a drum-braking position, an electrically operated actuator operable to overcome said applied force and move said brake shoe assemblies away from each other and release the brake, an electric switch mounted on one of said brake shoe assemblies and connected in the operating circuit of said actuator, said one brake shoe assembly including means for operating said switch so that said actuator may not be operated to release the brake when said assembly is in the drum-braking position and the associated brake drum is subject to a reverse-torque load.

2. A power actuated drum brake comprising a brake drum adapted to be mounted on a shaft to be braked, a pair of brake shoe assemblies mounted for releasable braking co-operation with said brake drum, means for urging said brake shoe assemblies into braking contact with said brake drum, an electrically-controlled actuator connected to release said brake shoe assemblies from said brake drum; wherein each brake shoe assembly comprises a brake shoe pivotally mounted on a fulcrum pin with freedom for substantially-radial movement relative thereto between first and second limiting positions, at least one of said brake shoe assemblies including an electric switch connected in the control circuit of said actuator and means for operating said switch to a closed condition when said brake shoe is in said first limiting position and to an open condition when said brake shoe is in said second limiting position; whereby reverse torque applied to said brake drum during braking will cause said brake shoe to move to said second limiting position thereby disabling said actuator until the reverse torque is removed.

3. A brake shoe assembly for a power actuated drum brake including an electrically-controlled actuator for releasing the brake; comprising a brake shoe, means for pivotably mounting said brake shoe on a fulcrum pin with freedom for substantially-radiai movement relative thereto between two limiting positions, means for urging said brake shoe towards one of said positions, and means adapted for operation of an electric switch when the brake shoe is moved to the other of said positions.

4. An assembly as claimed in claim 3, wherein the brake shoe includes flanges extending rearwardly from the braking surface, each flange having a spaced pair of fixed blocks rigidly secured to the outer face thereof and a fulcrum block mounted for sliding movement between said fixed blocks, each fulcrum block including a bore for receiving a fulcrum pin extending through both fulcrum blocks, each flange including an aperture to permit insertion of the fulcrum pin and elongated to permit said sliding movement of the fulcrum block when the fulcrum pin is inserted, and means urging each fulcrum block towards a fixed block.

5. An assembly as claimed in claim 4, wherein said fulcrum block includes mounting pins each slidably located in a bore in the adjacent fixed block.

6. An assembly as claimed in claim 5, wherein said means urging each fulcrum block towards a fixed block is a coil spring located on an extension of a mounting pin.

7. An assembly as claimed in any of claims 3 to 5, including said electric switch.

8. A power actuated drum brake including a brake shoe assembly as claimed in any of claims 3 to 7.

9. A power actauted drum brake arranged, constructed and adapted to operate as herein described with reference to Figures 2, 3a and 3b taken in conjunction with Figures 1 a and 1 b of the accompanying drawings.

10. A brake shoe assembly arranged, constructed and adapted to operate substantially as herein described with reference to Figures 2, 3a and 3b of the accompanying drawings.