GB2324127A

GB2324127A - Brake assembly for a powered rotary tool e.g. a lawnmower or a hedge trimmer

Info

Publication number: GB2324127A
Application number: GB9807782A
Authority: GB
Inventors: Michael Bowden
Original assignee: Electrolux Outdoor Products Ltd
Current assignee: Husqvarna UK Ltd
Priority date: 1997-04-11
Filing date: 1998-04-09
Publication date: 1998-10-14
Anticipated expiration: 2018-04-09
Also published as: GB9807782D0; GB9707415D0; GB2324127B

Abstract

A powered rotary tool e.g.a lawnmower or a hedge trimmer comprises a transmission including a rotary drive shaft 3 and a rotary driven shaft 5, and a brake assembly which includes a first brake element 7c, which constitutes part of the transmission, and a second brake element 11. The brake element 7c is movable relative to a drive element 7a circumferentially and axially of the driven shaft 5 by means of spring and a camming arrangement (9a, 9b, 10, Figure 3). When drive to the drive shaft 3 is removed, overrun of the drive element 7a causes the brake element 7c to move into contact with the second brake element 11. The transmission may include a belt or intermeshing gears, and various camming arrangements are disclosed including, in an alternative embodiment, an arrangement in which intermeshing helical gears 24, 28 effect axial movement of gear 28 (first brake element) towards and away from the second brake element 29.

Description

BRAKE ASSEMBLY This invention relates to a brake assembly for use with a powered rotary tool such as a lawnmower or a hedge trimmer.

A powered rotary tool, and in particular one driven by an electric motor, is often required to stop quickly and safely when power to the motor is switched off. Known methods of achieving such a motor braking action utilise electromechanical or mechanical devices attached to the motor or its associated transmission system.

The invention is particularly concerned with a lawnmower driven by an electric motor via a belt or other indirect driven system. In this connection, it should be noted that legislation requires that a metal blade of a lawnmower driven by an electric motor must stop within three seconds of the motor switch being actuated to turn the motor off.

The present invention provides a brake assembly for a powered rotary tool having a rotary drive shaft, a rotary driven shaft, and drive means for transferring drive from the drive shaft to the driven shaft, a brake assembly comprising a first brake element mounted for rotation with the driven shaft and a second brake element which is restrained from rotation, wherein the drive means comprises a pair of drive members which are movable relative to one another in a direction parallel to the axis of the driven shaft, the first brake element being associated with one of the drive members, and the arrangement being such that, when drive to the drive shaft is removed, relative axial movement in a first direction of the drive members occurs, thereby moving the first brake element into engagement with the second brake element.

In a preferred embodiment, the assembly further comprises spring means for biassing the drive members axially relative to one another in said first direction, and interengageable drive devices associated with the drive members for biassing the drive members axially relative to one another in a second direction opposite to said first direction.

Preferably, the arrangement is such that, when drive is applied to the drive shaft, relative axial movement of the drive members in said second direction is caused by the interengagement of the drive devices to effect movement of the first brake element out of engagement with the second brake element, and such that, when drive to the drive shaft is removed, relative axial movement of the drive members in said first direction is caused by the spring means.

Alternatively, the arrangement is such that, when drive to the drive shaft is removed, the inertia of the driven shaft causes said relative axial movement of the drive members in said first direction..

Advantageously, inner and outer rings constitute the pair of drive members, the outer ring being mounted on the inner ring for limited rotational movement relative thereto.

Preferably, the brake assembly further comprises cam/cam follower means associated with the inner and outer rings for transforming relative rotation between the rings to relative movement therebetween in the direction parallel to the axis of the driven shaft. Advantageously the cam/cam follower means is constituted by a plurality of cam/cam follower devices. Conveniently, each of the cam/cam follower devices is constituted by a slotted ramp formed in one of the rings and a pin extending from the other ring.

Where relative axial movement in the first direction is effected by the spring means, an inner hub and an outer ring may constitute the drive members, the outer ring being mounted on the inner hub for limited rotational movement relative thereto.

Preferably, the drive means further comprises first and second pulley wheels and a drive belt, the first pulley wheel being associated with the drive shaft, and the second pulley wheel being associated with the driven shaft. In this case, the second pulley may be constituted by the outer ring.

Alternatively, the drive means further comprises first and second spur gears, the first spur gear being associated with the drive shaft and the second spur gear being associated with the driven shaft. In this case, the outer surface of the outer ring may be configured to form the second spur gear.

In another preferred embodiment, first and second intermeshing helical gears constitute the pair of drive members, the first helical gear being associated with the drive shaft, and the second helical gear being associated with the driven shaft.

The invention also provides a powered rotary tool comprising a mounting plate, a motor mounted on the mounting plate and having a rotary output drive shaft, a rotary driven shaft supported on the mounting plate, indirect drive means for transferring drive from the drive shaft to the driven shaft, and a brake assembly for applying a braking force to the driven shaft, wherein the brake assembly is as defined above.

Advantageously, the first brake element is an annular friction pad fixed to the mounting plate.

Four transmission systems, each of which incorporates a brake assembly constructed in accordance with the invention, will now be described in detail, by way of example, with reference to the drawings, in which: Figure 1 is a sectional view of the first transmission system; Figure 2 is a plan view of part of the first transmission system; Figure 3 is a cross-section taken on line 111-111 of Figure 2; Figure 4 is a part elevation, on the arrow 'A' of Figure 2, showing the brake in the "off' position; Figure 5 is a part elevation, on the arrow 'A' of Figure 2, showing the brake in the "on" position.

Figure 6 is a sectional view of the second transmission system, showing the brake in the "off' position; Figure 7 is a sectional view of the second transmission system showing the brake in the "on" position; Figure 8 is a cross-section taken on the line VIII-VIII of Figure 6; Figure 9 is a sectional view of the third transmission system; Figure 10 is a plan view of part of the third transmission system; Figure 11 is a cross-section taken on the line XI-XI of Figure 10; Figure 12 is a part elevation, on arrow "F" ofFigure 10 showing the brake in the "off' position; Figure 13 is a part elevation, on arrow "F" of Figure 10, showing the brake in the "on" position; Figure 14 is a sectional view ofthe fourth transmission system; Figure 15 is a plan view of an inner hub forming part of the fourth transmission system; and Figure 16 is a side elevation of the inner hub.

Referring to the drawings, Figure 1 shows a transmission system for a rotary lawnmower, the transmission including a motor 1 mounted on a mounting plate 2, the motor having an output shaft 3. A drive pulley 4 is mounted on the output shaft 3. A drive spindle 5 for the rotary blade (not shown) of the lawnmower is rotatably supported on the mounting plate 2 by means of bearings 6. The drive spindle 5 is fixed to a driven pulley assembly, indicated generally by the reference numeral 7. The pulley assembly 7 is constituted by a hub member 7a fixed to the spindle 5, and interengageable inner and outer rings 7b and 7c. The inner ring 7b is integrally formed with the hub member 7a.

The outer ring 7c constitutes a driven pulley, and is in driving engagement with the drive pulley 4 by means of a drive belt 8.

The outer ring 7c is mounted on the inner ring 7b for limited rotation relative thereto by three equispaced cam arrangements 9 (one of which can be seen in each of Figures 3 to 5). Each cam arrangement 9 includes a bayonet pin 9a fixed to, and extending radially outwardly of, the outer circumferential edge of the inner ring 7b, and an inclined slotted ramp 9b formed in the cylindrical surface of the outer ring 7c. A respective spring 10 (see Figure 2) is associated with each of the cam arrangements 9 for biasing the respective bayonet pin 9a towards the lower end of the corresponding slotted ramp 9b.

An annular friction pad 11 is fixed to the mounting plate 2 in a position immediately above the upper edge of the outer ring 7c.

In use, with the motor 1 tumed off, the outer ring 7c is biased such that the slotted ramps 9b are urged by the springs 10 along the bayonet pins 9a so that the bayonet pins are positioned towards the lower ends of the slotted ramps This biasing causes the outer ring 7c to rise until the top surface of the outer ring contacts the friction pad 11.

When power is supplied to the motor 1, the output shaft 3 and the drive pulley 4 begin to rotate, in this case in an anti-clockwise direction as viewed on the arrow B (see Figure 1). This rotation is transmitted by the drive belt 8 to the outer ring 7c. Because of the load on the transmission and its inertia, any initial attempt to rotate the driven pulley assembly 7, results in the outer ring 7c rotating relative to the inner ring 7b against the force of springs 10. The outer ring 7c is urged by the cam arrangements 9 away from the mounting plate 2 and the friction pad 11. As shown in Figure 4, the brake device is now de-activated; and, because of the continued torque load on the driven pulley assembly 7, the outer ring 7c is retained in its lower position relative to the inner ring 7b.

When the transmission system is operating normally and power to the motor 1 is removed, the output shaft 3 of the motor begins to decelerate quickly due to the inherent friction within the motor. The load driven through the transmission system, and attached to the driven pulley assembly 7, now begins free wheel or overrun due to its relatively high inertia. The transmission system is now effectively reversed, with the load trying to transfer torque and rotation through the driven pulley assembly 7 and the drive belt 8 to the drive pulley 4. When this change in power transmission begins to take place, the load which caused the outer ring 7c to rotate against the springs 10 and relative to the inner ring 7b is reversed, and the springs urge the outer ring to rotate in a clockwise direction as viewed on the arrow B (see Figure 1) relative to the inner ring. This relative rotation between the inner ring 7b and the outer ring 7c results in the slotted ramps 9b moving along the bayonet pins9a so that the bayonet pins are positioned towards the lower ends ofthe slotted ramps, causing the outer ring to be forced axially upwards towards, and into firm contact with, the friction pad 11 (see Figure 5). This causes the outer ring 7c to be braked, and causes further deceleration of the transmission load through the inner ring 7b and the cam arrangements 9. Furthermore, as the outer ring 7c is braked, the overrun effect ofthe rotating load transmitted from the inner ring 7b tries to urge the bayonet pins 9a towards the lower ends of the slotted ramps 9b, thereby causing further axial pressure to be applied by the top of the outer ring on the friction pad 11. This has a significant effect in enhancing the braking action.

Figures 6 and 7 show a different form of transmission system for a rotary hedge trimmer or a lawnmower; the transmission system including a motor 21 mounted on a gear case 22 constituted by upper and lower parts 22a and 22b. The motor 21 has an output shaft 23, a helical drive gear 24 being attached to the output shaft, and being located within the gear case 22. A drive spindle 25 for the rotary cutting tool (not shown) ofthe hedge trimmer is rotatably supported in the gear casing 22 by means of bearings 26.

A drive cage 27 is fixed to the spindle 25. A driven helical gear 28 is mounted on the spindle 25 in such a manner as to permit axial movement relative thereto. The driven helical gear 28 and the drive cage 27 interengage, as shown in Figure 8, to prevent relative rotation therebetween, so that any rotation of the driven helical gear will result in rotation of the spindle 25.

An annular friction pad 29 is fixed to the lower part 22b of the gear casing 22, and a spring 30 is provided for biasing the driven helical gear 28 towards the friction pad.

In use, with the motor 21 turned off, the driven helical gear 28 is urged into contact with the fiction pad 29 by the spring 30, thereby braking the spindle 25. When power is supplied to the motor 21, the output shaft 23 and the helical gear 24 begin to rotate. The engagement between the helical gears 24 and 28 is such that rotation of the drive helical gear 24 tends to urge it axially in the direction of the arrow C, and to urge the helical gear 28 axially in the direction of the arrow D. Because the output shaft 23 ofthe motor 21 is axially restrained, all of the axial movement is transferred to the driven helical gear 28, thereby separating this gear and the friction pad 29 against the force of the spring 30, and thereby allowing rotation of the spindle 25. Owing to the inherent inertia of the spindle 25 and the associated load, the driven helical gear 28 is constantly held clear of the friction pad 29 provided power is supplied to the motor 21.

When the power to the motor 21 is removed, the motor begins to decelerate.

Owing to the inherent friction within the motor 21, and the relative inertia within the motor, this decelerates more rapidly than the spindle 25 and its associated load. This results in the transmission system being effectively reversed, with the helical gear 28 trying to transfer torque and rotation to the helical gear 24. This also results in the reversing of the axial biasing of the two helical gears 24 and 28, such that the gear 28 is urged by the helical interengagement ofthe gears and the spring 30 towards the friction pad 29, thereby resulting in a braking force being applied to the helical gear 28, the spindle 25 and the associated rotary cutting tool.

Figures 9 to 13 show a modified version of the transmission system of Figures 1 to 5, and so like reference numerals will be used for like parts, and only the modifications will be described in detail. The main modification is to replace the pulley/drive belt indirect drive by an indirect drive constituted by a pair of spur gears. Thus, a drive spur gear 4 is fixed to the output shaft 3 of the motor 1, and the external surface of the outer ring 7c is provided with a spur gear 7d. Also, the bayonet pins 9a are provided on the outer ring 7c, with the slotted ramps 9b being provided by the inner ring 7b, and the springs 10 bias the bayonet pins 9a towards the upper ends of the ramps 9b.

The only other modification is that the slotted ramps 9b are open-ended at their upper ends. This facilitates assembly of the arrangement.

Figures 14 to 16 show another modified version of the transmission system of Figures 1 to 5, and so like reference numerals will be used for like parts, and only the modifications will be described in detail. The main modification is to the pulley assembly 7, which here includes an inner hub 7'a and an outer ring 7'c. The outer ring 7'c is biassed axially upwardly (as seen in Figure 14) relative to the inner hub 7'a by a spring 10'. The inner hub 7'a carries three equispaced, inclined ramps 9'a which, in use, co-operate with three equispaced internal flanges 9'b formed on the outer ring 7'c. The ramps 9'a and the flanges 9'b constitute the cam arrangements 9.

In use, when power is supplied to the motor 1, the output shaft 3 and the drive pulley 4 begin to rotate. This rotation is transmitted by the drive belt 8 to the outer ring 7'c. Because of the load on the transmission and its inertia, any initial attempt to rotate the driven pulley assembly 7 results in the outer ring 7'c rotating relative to the inner hub 7'a. The outer ring 7'c is urged by the cam arrangements 9 away from the mounting plate 2 and the fiction pad 11, against the force of the spring 10'. The brake device is now deactivated; and, because of the continued torque load on the driven pulley assembly 7, the outer ring 7'c is retained in its lower position relative to the inner hub 7'a.

When the transmission system is operating normally and power to the motor 1 is removed, the output shaft 3 of the motor begins to decelerate quickly due to the inherent friction within the motor. The load driven through the transmission system, and attached to the driven pulley assembly 7, now begins free wheel or overrun due to its relatively high inertia. The transmission system is now effectively reversed, with the load trying to transfer torque and rotation through the drive pulley assembly 7 and the drive belt 8 to the drive pulley 4. When this change in power transmission begins to take place, the load which caused the outer ring 7'c to rotate relative to the inner hub 7'a is reversed to move the ramps 9'a out of engagement with the flanges 9'b. The spring 10' then urges the outer ring 7'c axially upwards, towards and into firm contact with, the friction pad 11. This causes the outer ring 7'c to be braked, and causes further deceleration of the transmission load through the inner hub 7'a and the cam arrangements 9.

It should be noted that the rate ofthe spring 10' can be varied to suit different load requirements of the tool.

It will be apparent that modifications could be made to the transmission systems described above. For example, the slotted ramps 9b of the embodiment of Figures 1 to 5 could be modified to be open-ended at their upper end

Claims

CLAIMS 1. A brake assembly for a powered rotary tool having a rotary drive shaft, a rotary driven shaft, and drive means for transferring drive from the drive shaft to the driven shaft, a brake assembly comprising a first brake element mounted for rotation with the driven shaft and a second brake element which is restrained from rotation, wherein the drive means comprises a pair of drive members which are movable relative to one another in a direction parallel to the axis of the driven shaft, the first brake element being associated with one of the drive members, and the arrangement being such that, when drive to the drive shaft is removed, relative axial movement in a first direction of the drive members occurs, thereby moving the first brake element into engagement with the second brake element.
2. A brake assembly as claimed in claim 1, further comprising spring means for biassing the drive members axially relative to one another in said first direction, and interengageable drive devices associated with the drive members for biassing the drive members axially relative to one another in a second direction opposite to said first direction.
3. A brake assembly as claimed in claim 2, wherein the arrangement is such that, when drive is applied to the drive shaft, relative axial movement of the drive members in said second direction is caused by the interengagement of the drive devices to effect movement ofthe first brake element out of engagement with the second brake element, and such that, when drive to the drive shaft is removed, relative axial movement of the drive members in said first direction is caused by the spring means.
4. A brake assembly as claimed in claim 1, wherein the arrangement is such that, when drive to the drive shaft is removed, the inertia of the driven shaft causes said relative axial movement of the drive members in said first direction.
5. A brake assembly as claimed in claim 4, wherein inner and outer rings constitute the pair of drive members, the outer ring being mounted on the inner ring for limited rotational movement relative thereto.
6. A brake assembly as claimed in claim 5, further comprising cam/cam follower means associated with the inner and outer rings for transferring relative rotation between the rings to relative movement therebetween in the direction parallel to the axis of the driven shaft.
7. A brake assembly as claimed in claim 6, wherein the cam/cam follower means is constituted by a plurality of cam/cam follower devices.
8. A brake assembly as claimed in claim 7, wherein each of the cam/cam follower devices is constituted by a slotted ramp formed in one of the rings and a pin extending from the other ring.
9. A brake assembly as claimed in claim 2 or claim 3, wherein an inner hub and an outer ring constitute the drive members, the outer ring being mounted on the inner hub for limited rotational movement relative thereto.
10. A brake assembly as claimed in claim 9, wherein the spring means is constituted by a spring acting between the inner hub and the outer ring.
11. A brake assembly as claimed in any one of claims 1 to 10, wherein the drive means further comprises first and second pulley wheels and a drive belt, the first pulley wheel being associated with the drive shaft, and the second pulley wheel being associated with the driven shaft.
12. A brake assembly as claimed in claim 11 when appendant to any one of claims 5 to 8, wherein the second pulley wheel is constituted by the outer ring.
13. A brake assembly as claimed in any one of claims 1 to 8, wherein the drive means further comprises first and second spur gears, the first spur gear being associated with the drive shaft and the second spur gear being associated with the driven shaft.
14. A brake assembly as claimed in claim 13 when appendant to claim 3, wherein the outer surface of the outer ring is configured to form the second spur gear.
15. A brake assembly as claimed in claim 1, wherein first and second intermeshing helical gears constitute the pair of drive members, the first helical gear being associated with the drive shaft, and the second helical gear being associated with the driven shaft.
16. A powered rotary tool comprising a mounting plate, a motor mounted on the mounting plate and having a rotary output drive shaft, a rotary driven shaft supported on the mounting plate, drive means for transferring drive from the drive shaft to the driven shaft, and a brake assembly for applying a braking force to the driven shaft, wherein the brake assembly is as claimed in any one of claims 1 to 15.
17. A powered rotary tool as claimed in claim 16, wherein the second brake element is an annular friction pad fixed to the mounting plate.