Braking system for motors

Abstract: The present invention provides a brake for a motor wherein a resilient member urges a braking member into a braking position to apply the brake, the resilient member rotating with the motor and being deformable from a first configuration to a second configuration as the centrifugal force on the member increases from zero to a predetermined level whereby the biasing force exerted by the resilient member on the braking member decreases to release the brake as the speed of the motor increases, the brake being re-applied whenever the centrifugal force on the resilient member falls below the said predetermined level. The invention also provides an electric motor having a rotatable armature spindle (11) extending outwardly from the body of the motor and extending through a centrally located aperture in a brake disc (15) carried by and rotatable with the armature spindle, the spindle further carrying a spring (20) deformable from a first configuration when the spindle is static to a second configuration when the spindle is rotating at a predetermined speed, the spring in its first configuration urging the brake disc against a friction surface to apply a braking torque and in its second configuration allowing the brake disc to move axially away from the friction surface to release the brake.

Description: This invention relates to a brake for a motor, particularly a small motor such as is used in an electric lawnmower. Powered lawnmowers operate by providing one or more rapidly spinning or rotating blades to cut the vegetation. Even when the source of power is removed, the blade(s) continue to rotate or spin for some seconds. To reduce the risk of injury to the operator of the mower, there is a need to arrest the motion of the cutting blade(s) as quickly as possible. Many braking systems have been proposed for electric motors. One known device is described in German utility model number 8618334.6. An electric motor with a ventable spring-loaded brake is described comprising a brake disc which is non-rotatably connected to the motor shaft but is axially movably arranged and may be pressed against the brake friction surface rigid with the casing, Wherein the brake disc is movable at least in one axial direction by an adjusting element operating as a function of centrifugal force, wherein the adjusting element comprises at least two pivotally mounted double-armed centrifugal force levers which are coupled to one another by at least one tension spring and are each articulated by their internal lever arm to the brake disc in such a way that the brake disc is axially movable in both directions by the adjusting element. However, there is a need for a simple but effective braking mechanism that can be used on domestic lawnmowers. Accordingly, in one aspect of the present invention there is provided a brake for a motor wherein a resilient member is rotatable with the motor and is deformable from a first conf guration to a second configuration as the centrifugal force on the resilient member increases from zero to a predetermined level, the biasing force of the resilient member thereby decreasing as the speed of the motor increases, the reduced biasing force releasing the brake and the brake being re-applied whenever the centrifugal force on the resilient member falls below the said predetermined level; characterised in that the resilient member is of a concave shape and comprises a plurality of members extending radially from the centre thereof such that, with the motor at rest, a first braking surface rotatable with the motor is urged by the resilient member into frictional contact with a non-rotatable second braking surface to apply a braking torque and, as the speed of the motor increases, the radially extending members are splayed outwards by centrifugal force thereby reducing the braking torque and releasing the brake. The resilient member can be progressively deformable from its first configuration to its second configuration as the centrifugal force on the member increases, or it may deform abruptly when the force reaches a predetermined level and return to its original configuration when the force next falls below the predetermined level or a lower level. Preferably the tips of the radially extending members define the first braking surface and act directly upon the second braking surface to apply the brake. Alternatively, the first braking surface may comprise a brake disc carried by and rotatable with an armature spindle of the motor. In such an arrangement the resilient member is preferably held on the armature spindle and urges the brake disc against a friction surface on the body of the motor. In another aspect of the present invention there is provided an electric motor including a brake as described above. An embodiment of the present invention will now be described in greater detail with reference to the accompanying drawings in which: Figure 1 shows in cross-section an electric motor with a centrifugal brake embodying present invention; and Figure 2 shows an end view of the electric motor of Figure 1. Figure 1 shows a motor body (10) having an armature spindle (11) extending outwardly therefrom, and an annular friction pad (12) surrounding the armature spindle and adhesively bonded or otherwise secured to one end of the motor (10). In one alternative embodiment (not shown), the friction pad 12 is merely held against the end of the motor body 10 by gravity while co-operating ribs on the body 10 and pad 12 interlock with one another to prevent rotation of the pad. In a further alternative embodiment, the friction pad 12 may comprise a plurality of discrete friction elements secured to the body (10) of the motor. A non-circular (shown as hexagonal) nut (14) is located along the armature spindle and rotates with the spindle. A brake disc (15) having at the centre thereof an aperture matching the shape of the nut (14) is located over the nut such that rotation of the spindle (11) also causes rotation of the brake disc (15). A spring (20) is located on the distal end of the armature spindle (11) and is secured in position by a clip (21) formed integrally with spring (20). In an alternative embodiment, not shown, the clip may be provided as a separate item. As the spindle rotates so does the spring (20). The spring (20) is substantially concave and comprises a plurality of members in the form of legs (23) extending radially outwards from a planar central portion (22) (as can be seen in Figure 2), each leg being angled relative to the plane of the central portion (22), thereby extending towards the brake disc (15) and, in the stationary state, making contact therewith. At rest, the tips (24) of the legs (23) of the spring (20) urge the brake disc (15) into face-to-face contact with the friction pad (12) to apply the brake. When the power supply to the motor is switched on, the motor armature rotates, thereby causing rotation of the nut (14), the brake disc (15), and the spring (20). The motor has sufficient torque to overcome the braking torque provided by the contact between the brake disc (15) and friction pad (12) and, as the rotational speed of the armature increases, the centrifugal force on each of the legs (23) causes corresponding movement of the legs from a rest position la to a splayed position 1b (Fig. 1). The force exerted by the spring (20) on the brake disc (15) is consequently reduced and produces a corresponding reduction of the braking torque. Although the brake disc is constrained to rotate with the nut, it is axially displaceable along the nut. Thus, as the legs (23) of the spring splay outwards, the brake disc (15) is able to move axially away from the friction pad (12), the-eby releasing the brake completely. When the power to the motor is switched off the rotational speed of the motor armature is initially reduced by aerodynamic and internal frictional losses. This continues until the centrifugal force acting on the spring (20) is reduced sufficiently for the legs (23) of the spring to once again exert a force on the brake disc (15), forcing it back into contact with the friction pad (12) and thereby producing rapid braking of the motor. The motor (10) has particular application for rotating or reciprocating the cuttirg blade(s) in a lawnmower, hedgecut er or vegetation trimmer. In this case the opposite end of the armatire spindle 11 drives the cutting blades, and the centrifugal brake ensures that the blades are rapidly brought to rest after the motor has been switched off. In an alternative embodiment (not shown) the nut (14) and brake disc (15) may be omitted. The legs (23) of spring (20) then act directly upon the friction surface (12) to apply the brake.

Claims: A brake for a motor (10) wherein a resilient member (20) is rotatable with the motor and is deformable from a first configuration (1a) to a second configuration (1b) as the centrifugal force on the resilient member (20) increases from zero to a predetermined level, the biasing force of the resilient member (20) thereby decreasing as the speed of the motor increases, the reduced biasing force releasing the brake and the brake being re-applied whenever the centrifugal force on the resilient member (20) falls below the said predetermined level; characterised in that the resilient member (20) is of a concave shape and comprises a plurality of members (23) extending radially from the centre (22) thereof such that, with the motor it rest, a first braking surface (15, 24) rotatable with the motor (10) is urged by the resilient member (20) into frictional contact with a non-rotatable second braking surface (12) to apply a braking torque and, as the speed of the motor increases, the radially extending members (23) are splayed outwards by centrifugal force thereby reducing the braking torque and releasing the brake. A brake according to Claim 1 whereir the tips (24) of the radially extending members (23) define the first braking surface (15) and act directly on the second braking surface (12) to apply the brake. A brake according to claim 1 in which the first braking surface comprises a brake disc (15) carried by and rotatable with an armature spindle (11) of the motor. A brake according to claim 3 in which the resilient member (20) is held on the armature spindle and urges the brake disc (15) against a friction surface (12) on the body of the member (10). A brake according to any one of claims 1 to 4 wherein the resilient member (20) is progressively deformable from its first configuration to its second configuration as the centrifugal force on the member increases. A brake according to any one of claims 1 to 4 wherein the resilient member (20) deforms abruptly from its first configuration to its second configuration when the centrifugal force reaches a predetermined level and returns to its original configuration when the force next falls below the predetermined level or a lower level. An electric motor (10) comprising a rotatable armature spindle (11) and including a brake as claimed in any one of Claims 1 to 6, wherein the rotatable armature spindle carries the resilient member of the brake.

Inventors: QUIGLEY ANDREW, QUIGLEY, ANDREW
Applicants: ATCO QUALCAST LTD, ATCO-QUALCAST LIMITED
Application year: 1993
Main classification: H02K7
IPC codes: F16D59, F16D65, H02K7, F16D55