BRAKING SYSTEM FOR ELECTRIC MOTORS

Abstract: A braking system (5) for an electric motor (1) comprises a disc-shaped driver (6), which drives a brake disc (10) by means of the motor (1). The brake disc (10) in turn drives an assembly, such as the cutting assembly of a rotor lawnmower. Between the driver (6) and the brake disc (10) there is some play, thanks to which ramp surfaces (19), which are pressed together by a helical spring (20), can move the brake disc (10) away from the motor (1) out of engagement with a number of brake blocks (24).

Description: BRAKING SYSTEMFOR ELECTRIC MOTORS The present invention relates to a braking system for an electric motor, which is adapted to drive, in a predetermined direction of rotation, a rotatable motor shaft, which projects through a bearing plate of the motor for driving an assembly, such as a cutting assembly in a lawnmower, said system comprising a stationary brake element, which is nonrotatable in relation to the bearing plate, a rotatable brake element, which is rotatably and displaceably mounted on the motor shaft, a driving means, which is nonrotatably arranged on the motor shaft between the bearing plate and the rotatable brake element for driving the same, a spring means, which is adapted to axially push the rotatable brake element towards the stationary brake element, a first ramp means, which is fixedly arranged on the driving means on the side facing the rotatable brake element, a second ramp means, which is fixedly arranged on the rotatable brake element on the side facing the driving means, the two ramp means being adapted, when the electric motor drives the motor shaft in said direction of rotation, to axially push the rotatable brake element in the direction away from the stationary brake element, and an assembly part, which is adapted to be driven by the motor shaft via the driving means and the rotatable brake element. Braking systems of the above-mentioned type are known and are used especially in electric rotor lawn- mowers, in which the rotor, for safety reasons, is to be stopped immediately as the motor is switched off. Owing to the relatively difficult surroundings, in which these braking systems must operate, they must, besides being of robust design, also be little sensitive to dirt and easy to maintain. In these respects, the prior art braking systems suffer from obvious drawbacks, and an object of the present invention is to obviate these. A further object of the present invention is to provide a braking system which than the prior art braking systems which are often very complicated. These objects are achieved by a braking system which is of the type mentioned by way of introduction and which is characterised in that the driving means is a substan¬ tially circular disc, which has a central tubular hub, which is attached to the motor shaft and which at its end facing away from the motor has a radially projecting collar, and a plurality of recesses, which are dis¬ tributed along the circumference of the disc and which each extend over a certain part of the circumference, that the stationary brake element projects radially beyond the disc, and that the rotatable brake element is a substantially circular brake disc, which has a central hub surrounding the hub of the driving disc and which, for the purpose of being driven by means of the recesses of the driving disc, has a plurality of projections, which project axially towards the driving disc and which extend over a smaller extent in the circumferential direction than the recesses of the driving disc, with which recesses they engage with play, the collar on the hub of the driving disc being adapted to retain the brake disc on this hub, and the spring means being adapted, when the electric motor stops to drive the motor shaft in said direction of rotation, to press the brake disc axially in the direction of the stationary brake element for braking the assembly. By designing both the driving means and the rotat¬ able brake element as simple discs with engaging portions (the recesses of the driving disc and the projections of the brake disc) which are arranged far away from the centre of the discs and, thus, subjected but to a small load, disc materials can be selected that are both in¬ expensive to purchase and easy to work and, consequently, the manufacturing cost of the braking system is reduced to a considerable extent. Moreover, the braking system is very easy to mount thanks to the possibility of designing the components included such that, in mounting as a pre- assembled unit, they are pushed onto the motor shaft. In a preferred embodiment of the invention, the driving disc and the brake disc are made of metal sheet, and the recesses and projections are made by punching of the metal sheet. Metal sheet is a very inexpensive mate¬ rial which is also very easy to work. Thus, the cost of this embodiment is reduced to an absolute minimum, especially if the above-mentioned ramp means are also made by punching of the metal sheet. In a preferred embodiment, at least one of the ramp means, however, is a ring made of plastic, which at its one side has pins for engaging in the corresponding apertures in the driving disc and the brake disc and, at its other side, has uniformly distributed ramp surfaces. It is easy to understand that this embodiment is prefer¬ red since it permits the use of comparatively tough and strong plastic materials for the ramp means. The employed spring means preferably is a helical spring, which surrounds the hub of the driving disc, and the hub of the brake disc preferably has an axial abut¬ ment bore with a narrower portion for the hub of the driving disc and a wider portion for the helical spring. In this manner, the helical spring will be easy to mount and besides is protected by the surrounding hub from contact with the surroundings. A tubular bush is suitably arranged between the hub of the driving disc and the narrower portion of the bore, the bush having a radially projecting collar, which is placed between the helical spring and the abutment of the bore. The object of the tubular bush, which preferably is made of plastic, is to reduce the friction between on the one hand the hub of the driving disc and the helical spring and, on the other hand, the hub of the brake disc. Preferably, the helical spring is fixed between the collar on the hub of the driving disc and the abutment of the bore. In combination with the other components mounted on the hub of the driving disc, this embodiment facili- tates the mounting operation quite significantly. In an application of the invention, the hub of the brake disc is cylindrical and has external circum¬ ferential key grooves for guiding a driving belt, which constitutes a part of the assembly driven by the motor. The driving belt is used to drive, for instance, a cutting assembly, whose belt pulley intended for the driving belt suitably has a larger diameter than the hub of the brake disc in order to accomplish a gear change which both permits the use of an electric motor which is smaller but has a higher speed, and reduces the load to which the braking system is subjected. In a preferred embodiment of the invention, the stationary brake element comprises a plurality of brake blocks, and a plurality of recesses are formed in the bearing plate of the motor, in which recesses the brake blocks are mounted. The advantages of this embodiment are obvious. Finally, the driving disc preferably has a central hole for its hub, whose end opposite the end with the collar has a shoulder, against which the driving disc rests, and an upset portion, which nonrotatably connects the driving disc with its hub. In this manner, a unit of the above-mentioned type, which can be preassembled, is accomplished in a very easy way. The invention will now be described in more detail with reference to the accompanying drawings, in which: Fig. 1 is an exploded view of an electric motor with a braking system and a driving belt associated therewith; Fig. 2 is a sectional view of a braking system in its neutral position; and Fig. 3 is a corresponding sectional view of the braking system in its braking position. Fig. 1 shows an electric motor 1, which has a motor shaft 2 projecting through a bearing plate 3 of the motor 1 in order to drive, via a driving belt 4, a cut¬ ting assembly (not shown) in a lawnmower. With a view to immediately stopping the cutting assembly of the lawnmower when the electric motor 1 is switched off, i.e. when the motor shaft 2 ceases to drive, an inventive braking system, which is generally designated 5, is connected between the motor 1 and the driving belt 4. The braking system 1 comprises a plate-shaped, substantially circular driver 6, which is made of metal sheet and has a central hub 7 for nonrotatable mounting of the driver 6 on the motor shaft 2. The driver 6 has, along its circumference, six uniformly distributed re¬ cesses 8, each extending over about 1/12 of the circum¬ ference of the driver 6. The driver also has four punched apertures 9, which are uniformly distributed around the central hub 7 of the driver 6. The braking system 5 also comprises a brake disc 10, which is also plate-shaped and substantially circular. The brake disc 10, which is made of metal sheet, has a central hub 11, which is adapted to be mounted on the hub 7 of the driver by means of a bush 17 in an axially and rotatably displaceable manner. Along its circum¬ ference, the brake disc 10 has a flat brake surface 13, in which six projections 14 are formed by punching. The projections 14 are adapted to engage, with play, in the recesses 8 of the driver 6 and extend in the circumferen- tial direction over a smaller part of the circumference of the driver 6 (about 1/24) . Between the flat brake surface 13 and the central hub 11, the brake disc 10 has apertures 15 corresponding to the apertures 9 in the driver 6. Between the driver 6 and the brake disc 10, two plastic rings 16, 17 are arranged. The rings 16, 17 are identical and have on their one side four pins 18, which are adapted to engage in the apertures 9 and 15 in the driver 6 and the brake disc 10, and on their opposite side four ramp surfaces 19, which are symmetrically arranged along the circumference of the rings 16, 17. Furthermore, the braking system 5 comprises a helical spring 20, which is adapted to press the brake disc 10 in the direction of the electric motor 1 and, by means of a collar 21 on the driver hub 7, which is adapted to be fixed to the free end of the motor shaft 2, to hold the bush 12, the brake disc 10 and the rings 16, 17 in place. Finally, the braking system 5 comprises three brake blocks 24, which are fixedly mounted in suitable recesses 25 in the bearing plate 3 of the motor and are adapted to brake, in cooperation with the brake surface 13 of the brake disc 10, said cutting assembly when the motor shaft 2 of the motor 1 ceases to drive. Some further details of the braking system and the function thereof will be described in more detail below with reference to Figs 2 and 3. Figs 2 and 3 illustrate some details more clearly than in the exploded view in Fig. 1. One of these details concerns the hub 11 of the brake disc 10. As is obvious, the hub 11 has an axial abutment bore 26 with a narrower portion 27 and a wider portion 28. In the narrower por- tion 27, the bush 12 is inserted, resting with a circum¬ ferential collar 29 against the abutment 36 of the bore 28. In the wider portion 28, the helical spring 20 is accommodated, one end thereof resting against the collar 21 of the hub 7 and the other end resting against the collar 29 of the bush 12. Another one of these details concerns the attachment of the tubular hub 7 to the driver 6. As will be seen, the driver 6 has a central hole 22 and the hub 7 has at the end opposite its collar 21 a shoulder 23, against which the driver 6 rests. The end of the hub 7 above the shoulder 23 extends through the hole 22 and is, for non- rotatable connection of the driver and the hub, upset at 36, the upset portion being provided only after mounting of the brake disc 10, the bush 12, the rings 16, 17 and the spring 20 on the hub 7. It is also obvious from Figs 2 and 3 that the hub 11 of the brake disc 10 has circumferential key grooves 30 for engagement with corresponding portions 31 (Fig. 1) of the driving belt 4. The hub 11 thus serves as a belt pulley for driving the cutting assembly. Fig. 2 shows the braking system 5 in its neutral position when the electric motor 1 drives the motor shaft 2. This condition is illustrated by means of the arrow 32, which also indicates the direction of rotation. When the motor shaft 2 drives in the direction 32, the ramp surfaces 19 of the rings 16, 17 climb on to each other and the brake disc 10 is moved in the direction of the arrow 33 away from the brake blocks 24. The motor shaft 2 can thus rotate freely and unimpededly drive the cutting assembly via the driving belt 4. Fig. 3 shows the braking system 5 in its braking position when the brake surface 13 of the brake disc 10 engages the brake blocks 24. In the braking position, the motor 1 has stopped to drive its motor shaft 2, which is illustrated in Fig. 3 by the arrow 32 being cancelled. In this position, the brake disc 10 is pressed upwards by the helical spring 20 in the direction of arrow 34 to¬ wards the motor, and the ramp surfaces 19 of the rings 16, 17 slide down on each other. A requirement for the braking system 5 to be able to take its two different positions is the play existing between the recesses 8 of the driving driver 6 and the projections 14 of the driven brake disc 10. A further requirement is, of course, that the projections remain in the recesses 8 also when the brake disc 10 is moved away from the electric motor 1. To prevent the projections 14 from being too long in spite of this requirement, the two rings 16, 17 are suitably contained in a recessed central portion 35 in the brake disc 10, which is clearly shown in the sectional views in Figs 2 and 3. Figs 2 and 3 also show that the helical spring 20 is well protected inside the hub 11 of the brake disc 10 when the braking system 5 takes its neutral position and the cutting assembly of the lawnmower thus operates and throws up dust and the like. It will be appreciated that the above-described embodiment can be varied in different ways within the scope of the claims, and that the driver 6 as well as the brake disc 10, which in the embodiment shown are metal- sheet constructions, can also be constructions of some other type, such as components turned in a lathe.

Claims: CLAIMS 1. A braking system for an electric motor (1) , which is adapted to drive, in a predetermined direction of ro¬ tation (32), a rotatable motor shaft (2), which projects through a bearing plate (3) of the motor (1) for driving an assembly, such as a cutting assembly in a lawnmower, said system comprising a stationary brake element (24), which is nonrotatable in relation to the bearing plate (3), a rotatable brake element (10), which is rotatably and displaceably mounted on the motor shaft (2) , a driving means (6) , which is nonrotatably arranged on the motor shaft (2) between the bearing plate (3) and the rotatable brake element (10) for driving the same, a spring means (20) , which is adapted to axially push the rotatable brake element (10) towards the stationary brake element (24) , a first ramp means (16), which is fixedly arranged on the driving means (6) on the side facing the rotatable brake element (10), a second ramp means (17), which is fixedly arranged on the rotatable brake element (10) on the side facing the driving means (6), the two ramp means (16, 17) being adapted, when the electric motor (1) drives the motor shaft (2) in said direction of rotation (32) , to axially push the rotatable brake element (10) in the direction (33) away from the stationary brake element (24), and an assembly part (4), which is adapted to be driven by the motor shaft (2) via the driving means (6) and the rotatable brake element (10), c h a r a c t e r - i s e d in that the driving means (6) is a substantially circular disc, which has a central tubular hub (7), which is attached to the motor shaft (2) and which at its end facing away from the motor (1) has a radially projecting collar (21), and a plurality of recesses (8), which are distributed along the circumference of the disc (6) and which each extend over a certain part of the circumfer¬ ence, that the stationary brake element (24) projects radially beyond the disc, and that the rotatable brake element (10) is a substantially circular brake disc, which has a central hub (11) surrounding the hub (7) of the driving disc and which, for the purpose of being driven by means of the recesses (8) of the driving disc (6), has a plurality of projections (14), which project axially to¬ wards the driving disc (6) and which extend over a smaller extent in the circumferential direction than the recesses (8) of the driving disc (6), with which recesses they engage with play, the collar (21) on the hub (7) of the driving disc being adapted to retain the brake disc (10) on this hub (7) , and the spring means (20) being adapted, when the electric motor (1) stops to drive the motor shaft (2) in said direction of rotation (32), to press the brake disc (10) axially in the direction (34) of the stationary brake element (24) for braking the assembly. 2. A braking system as claimed in claim 1, c h a r a c t e r i s e d in that the driving disc (6) and the brake disc (10) are made of metal sheet, and that the recesses (8) and the projections (14) are made by punching of the metal sheet. 3. A braking system as claimed in claim 1 or 2, c h a r a c t e r i s e d in that at least one of the ramp means (16, 17) is a ring made of plastic, which at its one side has pins (18) for engaging in corresponding apertures (9, 15) in the driving disc (6) and the brake disc (10) and, at its other side, has uniformly distrib¬ uted ramp surfaces (19) . 4. A braking system as claimed in any one of claims 1-3, c h a r a c t e r i s e d in that the spring means (20) is a helical spring, which surrounds the hub (7) of the driving disc, and that the hub (11) of the brake disc (10) has an axial abutment bore (26) with a narrower por¬ tion (27) for the hub (7) of the driving disc and a wider portion (28) for the helical spring. 5. A braking system as claimed in claim 4, c h a r a c t e r i s e d in that a tubular bush (12) is arranged between the hub (7) of the driving disc and the narrower portion (27) of the bore (26), the bush (12) having a radially projecting collar (26), which is placed between the helical spring (20) and the abutment (36) of the bore (26) . 6. A braking system as claimed in claim 4 or 5, c h a r a c t e r i s e d in that the helical spring (20) is fixed between the collar (21) on the hub (7) of the driving disc and the abutment (36) of the bore (26) . 7. A braking system as claimed in any one of claims 1-6, c h a r a c t e r i s e d in that the hub (11) of the brake disc (10) is cylindrical, and that it has external circumferential key grooves (30) for guiding a driving belt (4), which constitutes a part of the assem- bly driven by the motor (1) . 8. A braking system as claimed in any one of claims 1-7, c h a r a c t e r i s e d in that the stationary brake element (24) comprises a plurality of brake blocks, and that a plurality of recesses (25) are formed in the bearing plate (3) of the engine (1), in which recesses (25) the brake blocks are mounted. 9. A braking system as claimed in any one of claims 1-8, c h a r a c t e r i s e d in that the driving disc (6) has a central hole (22) for its hub (7), whose end opposite the end with the collar (21) has a shoulder (23), against which the driving disc (6) rests, and an upset portion (36) , which nonrotatably connects the driving disc (6) with its hub.

Inventors: LAHTI ERKKI, LAHTI, ERKKI
Applicants: FHP ELMOTOR, LAHTI ERKKI, LAHTI, ERKKI
Application year: 1997
Main classification: H02P3
IPC codes: F16D55, F16D65, H02K7