Lawn mower

Abstract: A braking system for a rotary mower comprises a brake element (14) which is secured to an impeller (12) and is urged by a spring (26) into braking engagement with a fixed brake friction member (20). On start up of a motor (4) of the mower, rotation of the impeller (12) causes axial movement of the brake element (14) against the spring (26) out of braking engagement. Movement of the brake element (14) may be assisted by mechanical means, for example a ball and ramp arrangement.

Description: The present invention relates to an improved braking system which is particularly suitable for use with a lawn mower of the rotary type in which a blade and 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 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 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 horizontally 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 deactivating 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 lawn mower. 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. FR-A-2 411 503 describes and claims a braking system for a lawn mower, which system comprises i)an impeller 24 mounted for rotation about a shaft 3 and movable in an axial direction relative to the axis of that shaft; (ii)a brake element 23 mounted for rotation with the impeller; (iii)a brake friction element 12 which is restrained from rotation; and (iv) resilient means urging the brake element in an axial direction into braking contact with the friction element. It is an object of the present invention to provide a braking system suitable for a lawn mower, in which the above disadvantages are reduced or substantially obviated. The invention provides a braking system for a lawn mower, which system comprises (i)an impeller mounted for rotation about a shaft driven for rotation by a drive system and movable in an axial direction relative to the axis of that shaft; (ii)a brake element mounted for rotation with the impeller; (iii)a brake friction element which is restrained from rotation; and (iv) resilient means urging the brake element in an axial direction into braking contact with the friction element, characterised in that the impeller is arranged in such a way that the axial force produced by the pressure differential generated by the impeller opposes the resilient means and thereby acts to oppose contact between the brake element and the friction element. In a preferred embodiment of the braking system according to the invention, the braking contact between the brake element and the friction element is released by means of the axial force produced by the pressure differential generated by the impeller. In a preferred embodiment of the braking system according to the invention means are provided to lift the impeller on start-up of the motor, in order to release the braking contact between the brake element and the friction element. The means provided to lift the impeller on start-up of the motor may, for example, be a mechanical system such as an angled drive dog or ball and ramp arrangement, or may be an electromagnetic system such as the electromagnetically releasable spring actuated system described above in connection with the braking of horizontally mounted axles. 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 a first embodiment of a braking system according to the invention; Figure 2 is an enlarged section, showing the braking system of Figure 1 in more detail; Figure 3 is a side view, partially in section, of a second embodiment of a braking system according to the invention; and Figure 4 is a side view, partially in section, of a third embodiment. Figure 5 shows an adaptor used in the third embodiment. The rotary lawn mower 10 comprises a deck 2 on which a series electric motor 4 is mounted. The motor 4 is provided with an output shaft 6 on which is fitted a blade 8 and impeller 12. A brake disc 14 is attached by means of screws 16 to the impeller 12. The impeller 12 and disc 14 assembly is slidably mounted on the output shaft 6, by means of an adaptor 18, so that the assembly is free to move in an axial direction relative to the shaft 6 but rotates with the shaft 6. An annular brake friction plate 20 is secured to a housing 22 of the motor 4. Brake friction pads 24 are attached to an inner circular flange portion of the brake friction plate 20. A spring 26 is provided to urge the brake disc 14 downwardly into braking contact with the pads 24 on the brake friction plate 20. In operation, as the motor 4 is energised, the impeller 12 is driven by the output shaft 6 and begins to rotate. As the speed of rotation increases, a pressure differential is built up due to the rotation of the impeller 12, and the pressure on the motor side of the impeller 12 falls sharply, tending to draw the assembly comprising the impeller 12 and brake disc 14 towards the motor 4 against the action of the spring 26. This removes the braking contact between the brake disc 14 and brake friction plate 20 so that the impeller 12 and blade 8 are free to rotate. When the motor 4 is switched off, the impeller 12 is no longer driven by the output shaft 6 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 26 is no longer opposed by the suction force generated by the rotating impeller 12 and thus urges the brake disc 14 and brake friction plate 20 into braking contact, thus braking the system. In a second embodiment shown in Figure 3, the mower is provided with means to release the brake on initial start-up of the motor. The motor 34 comprises an armature 36 located within a stator 38. The armature 36 is mounted so that it is able to move axially with respect to the stator 38. Spaced steel discs 40 are mounted to rotate with the armature 36, and friction pads 42 are positioned in the spaces between the steel discs 40. The friction pads 42 are fixed in the motor housing 22 against rotation but are free to move axially. An impeller 44 is mounted on the output shaft 46 and is urged by a spring 48 away from the armature 36 and associated steel discs 40 and brake pads 42. Needle roller bearings 50 are provided, and allow the shaft to rotate and move freely in the axial direction. In the non-running position, the magnetic field of the armature 36 is offset relative to that of the stator 38. In operation, as the motor 34 is energised, there is an initial surge in current and hence in the magnetic field. The offset arrangement of the magnetic fields of the armature and stator causes the armature to lift relative to the stator and release the braking contact. The speed of rotation of the impeller increases and the pressure effects described with respect to the first embodiment of the braking system, come into force. A third embodiment is shown in Figures 4 and 5. Figure 4 shows a deck 62 on which a motor 64 is mounted. The motor 64 comprises an output shaft 66 on which is secured an adaptor 68. The adaptor 68 comprises a cylindrical body 70 from which extends a sleeve 72. In an upper surface 74 of the body 70 are four bearing recesses 76 which support ball bearings 77. Also secured to the upper surface 74 are four axially extending hardened steel pins 78. A blade 80 is secured to the shaft 66 for rotation therewith. An impeller 82 has a sleeve portion 84 which surrounds the body 70 of the adaptor 68, and is secured by screws 86 to a dished brake disc 88. The disc 88 comprises bosses 90 having bores therein which fit over the pins 78. The assembly of impeller 82 and brake disc 88 is thus constrained to rotate with the shaft 66 and adaptor 68, but can move axially relative to them. An annular brake friction plate 92 is secured to a housing of the motor 64, and a spring 94 urges the brake disc 88 downwardly into braking contact with brake pads 96 on the brake friction plate 92. The recesses 76 in the adaptor 68 each comprise a lower surface 79 and an upwardly inclined ramp surface 81. It can be seen that the operation of the third embodiment is essentially similar to that of the first embodiment, save that on the first rotary impulse given to the adaptor 68, the balls 77 are caused to ride up from the lower surfaces 79 up the ramp surfaces 81, whereby to assist in moving the assembly of the impeller 82 and brake disc 88 upwards to release the braking effect between the disc 82 and the friction plate 92.

Claims: A braking system for a lawn mower, which system comprises (i) an impeller (12) mounted for rotation about a shaft (6) driven for rotation by a drive system and movable in an axial direction relative to the axis of that shaft; (ii) a brake element (14) mounted for rotation with the impeller (12); (iii) a brake friction element (20) which is restrained from rotation; and (iv)resilient means (26) urging the brake element (14) in an axial direction into braking contact with the friction element (20), characterised in that the impeller (12) is arranged in such a way that the axial force produced by the pressure differential generated by the impeller (12) opposes the resilient means (26) and thereby acts to oppose contact between the brake element (14) and the friction element (20). A braking system for a lawn mower according to claim 1 characterised in that the braking contact between the brake element (14) and the friction element (20) is released by means of the axial force produced by the pressure differential generated by the impeller (12). A braking system according to claim 1 which further comprises means to lift the impeller on start-up of the motor, in order to release the braking contact between the brake element (14) and the friction element (20). A braking system according to claim 3 in which the means to lift the impeller comprises an electromagnetically releasable spring-actuated system. A braking system according to claim 3 in which the means to lift the impeller comprises an angled drive dog. A braking system according to claim 6 in which the means to lift the impeller comprises a ball and ramp arrangement.

Inventors: CARR VINCENT, THOMAS W ROGER, CARR, VINCENT, THOMAS, W. ROGER
Applicants: BLACK & DECKER INC, BLACK & DECKER INC.
Application year: 1992
Main classification: A01D34
IPC codes: A01D34, F16D59, H02K7, F04D27