Braking system

Abstract: A braking system for a powered rotary tool of the type in which a shaft is rotated about an axis by a motor, comprises (i) a first brake element (114) mounted for rotation with the shaft (106); (ii) a second brake element (136) which is restrained from rotation; (iii) resilient means (144) urging the first brake element (114) in an axial direction into braking contact with the second brake element (136) and (iv) means to release the braking contact between the first brake element (114) and the second brake element (136) on start-up of the motor. On start-up of the motor (104), the shaft (106) is free to rotate through a small angle of rotation before rotation of the first brake element (114) is initiated, and the means for releasing the braking contact on start-up of the motor (104) comprises at least one pair of co-operating ramp surfaces (126, 132), one (126) of the or each pair of ramp surfaces being arranged for rotation with the shaft (106) and the co-operating ramp surface (132) being arranged for rotation with the first brake element (114). The braking system is particularly suitable for use on a lawn mower of the rotary type.

Description: The present invention relates to an improved braking system which is particularly suitable for use with a powered rotary tool of the type in which a shaft is rotated about an axis by a motor, for example a lawn mower of the rotary type in which a blade and optionally an impeller are rotated about a common axis by a motor, which may be an electric motor or a petrol motor. For safety reasons, it is necessary to incorporate a braking system within the lawn mower, so that the blade and, where provided, the impeller are brought to a stop within a specified time. With the increased concern for safety, safety requirements are continually becoming more stringent, and it is now generally accepted that a stop time of less than 3 seconds is particularly desirable, and indeed is increasingly being required by statutory regulations in many countries. Braking systems which are currently available for use on mowers of this type include electromechanical systems which are spring loaded brake systems comprising a brake pad and a brake disc, the spring urging the pad and disc into braking contact and being opposed by the action of a solenoid. For mowers of the type driven by an induction motor, brakes of the type in which the magnetic field created in the rotor is used to hold a movable brake disc in an OFF position against the spring force while the motor is running have been proposed. In these systems, the magnetic field is energised only when the motor is energised. Braking systems for electric motors are also known, which comprise an electromagnetically releasable spring actuated brake with a braking surface, a brake disc driven in rotation, a release device non-rotatably secured to the braking surface, and an armature mounted non-rotatably and so as to be capable of axial displacement between the brake disc and the release device, and biased by springs against the brake disc. Such braking systems have been disclosed in connection with motors having substantially horizontal mounted axles to be stopped. In such braking systems, on energising the motor the axle moves to separate the brake disc from its corresponding braking surface. On de-activating the motor, the spring urges the brake disc back into contact with the braking surface. It is a disadvantage of the known electromechanical braking systems that they are expensive to manufacture and add significantly to the cost of each unit to which they are fitted. Where attempts have been made to reduce the cost of these systems by using part of the motor housing to provide part of the housing for the solenoid coil and spring, these systems only achieve a stop time in the order of 5 seconds, which is not sufficiently short to satisfy the more stringent safety requirements which require a stop time of less than 3 seconds. It would be possible to achieve stop times of less than 3 seconds by increasing the size of the brake disc, but this would lose part of the cost saving benefit. Known electromagnetically releasable spring actuated braking systems would not be suitable for use with a substantially vertical shaft, since the force generated by the magnetic field is not sufficient to hold the armature in its raised position under normal running conditions, after the initial surge observed when the motor is energised. A further braking system having the features contained in the preambles of Claims 1, 2 of the present patent is disclosed in EP-A-0 288 348 which relies on the inertia of the mower blade on starting and stopping of the motor to release and apply a brake. However, the construction of the braking system is relatively complex and thus relatively expensive to manufacture. It is an object of the present invention to provide a braking system suitable for a powered rotary tool, in which the above disadvantages are reduced or substantially obviated. The present invention provides braking system for a powered rotary tool comprising; a shaft which is adapted to be rotatably driven about an axis by a motor, a first brake element, engagement means between the shaft and the first brake element for rotatably driving the first brake element which engagement means allow a small amount of relative rotation between the shaft and the first braking element on start up of the motor, a second brake element which is restrained from rotation, resilient means which urge the first brake element in an axial direction into braking contact with the second brake element, and at least one pair of co-operating ramp surfaces, a first one of the or each pair of ramp surfaces being arranged for rotation with the shaft and the second one of the or each pair of ramp surfaces being arranged for rotation with the first brake element such that the first and second ramp surfaces co-operate to release the braking contact between the first brake element and the second brake element upon the relative rotation between the shaft and the first braking element on start up of the motor, characterised in that the or each first ramp surface is located on an adaptor which is mounted for rotation with the shaft and the or each second ramp surface is located on a drive insert which is mounted for rotation with the first brake element and which is slideably mounted on the shaft by means of the adaptor. The means for releasing the braking contact on start-up of the motor preferably comprises four equiangularly spaced pairs of co-operating ramp surfaces. The braking system according to the invention is particularly suitable for use with a lawn mower of the rotary type in which a blade and an impeller are rotated about an axis by a motor. The invention will now be further described with reference to the accompanying drawings, of which Figure 1 is a general side elevation, partially in section, of a lawn mower comprising an embodiment of a braking system according to the invention; Figure 2 is an exploded view on an enlarged scale, of the braking system of Figure 1; Figure 3 is a side elevation, partially in section, of the braking system of Figures 1 and 2; Figure 4 is a side elevation, partially in section, of an alternative embodiment of a braking system according to the invention, in the braked position; and Figure 5 is a similar view to Figure 4, in the non-braked position. The rotary lawn mower 110 comprises a deck 102 on which a series electric motor 104 is mounted. The motor 104 is provided with an output shaft 106 on which is fitted a blade 108 and impeller 112. A brake disc 114 is attached by means of screws 116 to the impeller 112. An adaptor 118 is securely fixed to the output shaft 106 and an assembly comprising the impeller 112 and disc 114, together with a drive insert 120 is slidably mounted on the output shaft 106, by means of the adaptor 118, so that the assembly is free to move in an axial direction relative to the shaft 106. As can be seen from Figure 2, the adaptor 118 is generally cylindrical in shape, with an annular flange 124 at the end remote from the output shaft 106. Four equiangularly spaced ramps 126 project from the upper surface of the annular flange 124, the height of the ramps increasing in the direction of rotation of the shaft 106. Associated with each of the ramps 126, a spline 128 projects from cylindrical wall of the adaptor 118, extending from the ramp 126 to the end of the adaptor remote from the flange 124. As can also be seen from Figure 2, the drive insert 120 is also generally cylindrical in shape and is similarly provided with an annular flange 130 at the end remote from the output shaft 106. Four equiangularly spaced ramp surfaces 132 are formed as recesses in the lower surface of the annular flange 130, the depth of the recesses increasing in the direction of rotation of the shaft 106. Associated with each of the ramp surfaces 132, a groove 134 in the internal wall of the drive insert extends from the ramp surface 132 for the full length of the insert 120. The internal diameter of the drive insert 120 and the external diameter of the adaptor 118 are selected so that the adaptor 118 is received within the insert 120 as a sliding fit. The width of the grooves 134 in the insert 120 is greater than the width of the splines 128 in the adaptor 118, so that a limited amount of angular rotation of the adaptor 118 with respect to the insert 120 is possible. A brake friction housing 136 is secured to a mounting plate 138 which is itself secured to a housing 140 of the motor 104. Brake friction pads 142 are attached to an inner surface of the brake friction housing 136. A spring 144 is provided to urge the brake disc 114 downwardly into braking contact with the pads 142 on the brake friction housing 136. A cup washer 146 is mounted on the shaft 106 and clamped against the bearing of the housing 104 by the adaptor 118. The cup washer 146 rotates with the shaft, and has the function of compressing the spring 144 against the brake disc 114. In operation, as the motor 104 is energised, the adaptor 118 is driven by the output shaft 106 and begins to rotate. As the adaptor 118 begins to rotate, the ramp surfaces 126 engage with the ramp surfaces 132, and the drive insert 120 is pushed axially upwards against the action of the spring 144. The drive insert 120 lifts the brake disc 114 away from braking contact with the brake pads 124 on the brake friction housing 120. After the initial rotation, the splines 128 engage with the corresponding side walls of the grooves 134 and the drive insert 120, and hence the impeller 112 begins to rotate. As the speed of rotation increases, a pressure differential is built up due to the rotation of the impeller 112, and the pressure on the motor side of the impeller 112 falls sharply, tending to draw the assembly comprising the impeller 112 and brake disc 114 towards the motor 104 against the action of the spring 144. The impeller 112 and blade 108 are free to rotate. When the motor 104 is switched off, the impeller 112 is no longer driven by the output shaft 106 and the speed of rotation reduces sharply. This has the effect of reducing the pressure differential between the two surfaces of the impeller, equalising the rotational forces on the impeller. The force of the spring 144 is no longer opposed by the suction force generated by the rotating impeller 112 and thus urges the brake disc 114 and brake friction plate 136 into braking contact, thus braking the system. An alternative embodiment of a braking system according to the invention is shown in Figures 4 and 5. The braking system shown in Figures 4 and 5 is similar in many respects to that of Figures 1 to 3, but differs therefrom in the design and detailed operation of the first and second brake elements. As is shown in Figure 4, a brake housing 136 is secured to a mounting plate 138 which is itself secured to a housing 140 of the motor 104. A spring 144 is provided to urge a brake disc 114 downwardly into braking contact with the brake housing 136. A ring shaped friction element 150 is attached to, and movable with, the brake disc 114. In Figure 4, the braking system is shown in the engaged position. The spring 144 is urging the brake disc 114, with attached friction element 150, into braking contact with the brake housing 136. The operation of the braking system is as described above in connection with the embodiment shown in Figures 1 to 3. As the motor 104 is energised, the adaptor 118 is driven by the output shaft 106 and begins to rotate. As the adaptor 118 begins to rotate, the ramp surfaces 126 engage with the ramp surfaces 132, and the drive insert 120 is pushed axially upwards against the action of the spring 144. The drive insert 120 lifts the brake disc 114, with friction element 150, away from braking contact with the brake housing 136. The brake friction element 150 which is in the form of a ring is attached to the brake disc 114 by a suitable method of attachment, for example an interference fit, bonding or a screw thread. In an alternative embodiment of a braking system according to the invention, the brake housing 136 or, preferably the disc 114, may comprise a moulded thermoset plastics part, in particular moulded from a thermoset plastics material which is based on a phenolic resin, and is filled with a suitable filler, for example glass fibres, in order to improve its strength. Where the disc is moulded from a thermoset plastics material, and the housing is in metal, the disc can then act as heat insulator to reduce the transfer of heat from the brake contact area to the plastic impeller.

Claims: A braking system for a powered rotary tool comprising; a shaft (106) which is adapted to be rotatably driven about an axis by a motor (104), a first brake element (114), engagement means (128,134) between the shaft and the first brake element for rotatably driving the first brake element which engagement means allow a small amount of relative rotation between the shaft and the first braking element on start up of the motor a second brake element (136) which is restrained from rotation, resilient means (144) which urge the first brake element in an axial direction into braking contact with the second brake element, and at least one pair of co-operating ramp surfaces (126, 132) a first one (126) of the or each pair of ramp surfaces being arranged for rotation with the shaft and the second one (132) of the or each pair of ramp surfaces being arranged for rotation with the first brake element (114) such that the first and second ramp surfaces co-operate to release the braking contact between the first brake element and the second brake element upon the relative rotation between the shaft and the first braking element on start up of the motor characterised in that the or each first ramp surface (126) is located on an adaptor (118) which is mounted for rotation with the shaft (106), and the or each second ramp surface (132) is located on a drive insert (120) which is mounted for rotation with the first brake element and which is slideably mounted on the shaft (106) by means of the adaptor (118). A braking system for a lawn mower of the rotary type, comprising; a shaft (106) which is adapted to be rotatably driven about an axis by a motor (104), an impeller (112) which carries a first brake element (114), engagement means (128,134) between the shaft and the impeller for rotatably driving the impeller which engagement means allow a small amount of relative rotation between the shaft and the impeller on start up of the motor, a second brake element (136) which is restrained from rotation, resilient means (144) which urge the first brake element in an axial direction into braking contact with the second brake element, and at least one pair of co-operating ramp surfaces (126, 132) a first one (126) of the or each pair of ramp surfaces being arranged for rotation with the shaft and the second one (132) of the or each pair of ramp surfaces being arranged for rotation with the impeller such that the first and second ramp surfaces co-operate to axially displace the impeller on start up of the motor in order to release the braking contact between the first brake element and the second brake element upon the relative rotation between the shaft and the impeller on start up of the motor, characterised in that the or each first ramp surface (126) is located on an adaptor (118) which is mounted for rotation with the shaft (106), and the or each second ramp surface (132) is located on a drive insert (120) which is mounted for rotation with the impeller and which is slideably mounted on the output shaft by means of the adaptor (118). A braking system according to claim 1 characterised in that the adaptor (118) and the drive insert (120) form the engagement means between the shaft and the first brake element which allow a small angle of relative rotation between the shaft and the first brake element on start up of the motor. A braking system according to claim 2 characterised in that the adaptor (118) and the drive insert (120) form the engagement means between the shaft and the impeller which allow a small angle of relative rotation between the shaft and the impeller on start up of the motor. A braking system according to any one of the preceding claims, characterised in that the first brake element (114) comprises a friction element (150). A braking system according to any one of claims 1 or claim 4, characterised in that the second brake element (136) comprises a friction element (142). A braking system according to any one of the preceding claims, characterised in that the adaptor and drive insert each carry four equiangularly spaced pairs of co-operating ramp surfaces (126, 132). A braking system according to any one of claims 1 to 7 characterised in that the first brake element (114) is a moulded thermoset plastic part. A braking system according to any one of claims 1 to 7 characterised in that the second brake element (136) comprises a moulded thermoset plastics part. A braking system according to any one of claims 1 to 7 characterised in that the first brake element (114) or the second brake element comprises a filled phenolic resin part. A braking system according to claim 10, characterised in that the phenolic resin is filled with glass fibre.

Inventors: TEMPLE MELVIN, THOMAS W ROGER, TEMPLE, MELVIN, THOMAS, W. ROGER
Applicants: BLACK & DECKER INC, BLACK & DECKER INC.
Application year: 1993
Main classification: A01D34
IPC codes: A01D34, F16D67