Patent Application
20130186726
This invention relates generally to electromagnetic brakes and clutches and to their method of operation.
In a known form of electromagnetic brake, the braking is achieved by frictional contact between confronting faces of a rotor disc and a stator disc. The rotor disc and stator disc are biased towards each other by a compression spring and the braking force is provided by the frictional contact of the two discs pressed together by the spring. To disengage the brake a stator coil is energised which moves the stator disc away from the rotor disc towards the stator coil against the bias of the compression spring. Once the stator disc is out of contact with the rotor disc the braking force is removed.
In the form of brake just described, the force exerted on the stator disc by the stator coil, when it is in contact with the rotor disc must be greater than the force exerted by the compression spring; furthermore the force is required when the stator disc is in contact with the rotor disc and therefore furthest from the stator coil. This limits the amount of force that the compression spring can be allowed to exert and therefore the amount of braking force that can be applied.
A further problem arises if the brake is subject to vibration; in that case the rotor disc may start to move in small increments relative to the stator disc even while the discs remain in contact, because the vibration may generate transient forces sufficient to overcome the friction forces.
Similar issues arise if the stator disc, instead of being held against rotation as in the case of a brake, is allowed to be drivingly rotated by the rotor disc, with the assembly then acting as a clutch rather than as a brake.
It is an object of the invention to provide an improved electromagnetic brake or clutch and an improved method of operating an electromagnetic brake or clutch.
According to the invention there is provided an electromagnetic brake or clutch comprising:
a rotatably mounted shaft;
a motor for rotating the shaft;
a rotor disc mounted on the shaft for rotation therewith;
a brake or clutch disc disposed around the shaft, a first face of the brake or clutch disc confronting a first face of the rotor disc and one of the discs being mounted for movement towards and away from the other disc;
a stator coil which when energised urges the first face of the brake or clutch disc away from the first face of the rotor disc;
the confronting first faces of the discs being provided with circumferentially spaced magnetic regions of alternating polarity, disposed such that, in a first set of rotational positions of the shaft, magnetic regions of one polarity on the first face of the brake or clutch disc confront magnetic regions of the opposite polarity on the first face of the rotor disc, and, in a second set of rotational positions of the shaft interposed between respective ones of the first set of rotational positions, magnetic regions of one polarity on the first face of the brake or clutch disc confront magnetic regions of the same polarity on the first face of the rotor disc.
By using magnetic forces to attract the discs together, the force of attraction can readily be made to increase as the discs get closer together and reduce as they move further apart. Also, the resistance to relative rotation of the discs can readily be arranged to reduce as soon as any significant rotation away from a static fully engaged position occurs. Thus, after just a small amount of rotation the friction force can be eliminated entirely; similarly when returning to a fully engaged position friction force may be applied only close to the very end of the relative movement of the discs. Furthermore, the brake may be more able to maintain the discs in their braked positions even if they are subjected to vibration.
Whilst reference is made to a rotor “disc” and to a brake or clutch “disc”, it should be understood that the “disc” need not be of circular shape; whilst the preferred shape of disc is a ring shape, it is within the scope of the invention for a “disc” to comprise only a sector of such a ring and/or to be of non-circular shape. Also, the “disc” may be of conical or frusto-conical shape. The face of the disc that makes frictional contact in operation may be grooved or otherwise shaped to increase the frictional contact area and/or improve the frictional contact in other respects.
Preferably the first set of rotational positions are equiangularly spaced around the axis of rotation of the shaft. The second set of rotational positions are preferably equiangularly spaced midway between respective ones of the first set of rotational positions. The magnetic regions are preferably all disposed at approximately the same radial distance from the axis of rotation of the shaft. Further magnetic regions may be provided on the discs at smaller or greater radial distances from the axis of rotation of the shaft, if desired, to increase the force of attraction between the discs.
The first face of the rotor disc is preferably provided with a high friction surface. Similarly, the first face of the brake or clutch disc is preferably provided with a high friction surface.
The brake or clutch preferably further includes a housing in which the shaft, the motor, the stator and the rotor and brake and clutch discs are at least partly disposed. In an embodiment of the invention described below, the shaft projects outwardly from the housing through an opening and the other parts are disposed within the housing.
In the case where the brake or clutch is a brake, the brake disc is preferably non-rotatably mounted. Preferably a peripheral portion of the brake disc non-rotatably engages an interior wall of the housing. In the case where the brake or clutch is a clutch, the clutch disc is preferably rotatably mounted. Preferably a peripheral portion of the clutch disc non-rotatably engages a member mounted for rotation within the housing on an output drive shaft of the clutch.
Whilst it is within the scope of the invention for the rotor disc to be mounted for axial movement towards and away from the brake or clutch disc which may then be axially fixed and, in the case of a brake, entirely fixed, it is preferred that the brake or clutch disc is mounted for movement towards and away from the rotor disc. This enables a simpler construction to be achieved.
The motor and the stator coil are each preferably disposed around the shaft and are axially spaced from one another with the brake or clutch disc and the rotor disc disposed between them. The brake or clutch disc is preferably disposed adjacent to the stator coil and between the rotor disc and the stator coil. Thus the brake or clutch disc is preferably disposed adjacent to the stator coil and the rotor disc is preferably disposed adjacent to the motor. The stator coil is preferably held in a fixed position relative to the brake or clutch disc. In an embodiment of a brake described below, the stator coil and the brake disc are fixed parts of the brake and in an embodiment of a clutch described below, the stator coil and the clutch disc are mounted for rotation together.
The motor may be of any kind but usually is preferably an electric motor including a rotor fixed to the shaft and a stator surrounding the rotor.
The magnetic regions in the rotor disc and the brake or clutch disc may be provided by discrete magnets in the disc. Each magnet is preferably a rare earth magnet, for example a neodymium magnet. Such a magnet may be made from an alloy comprising mainly, by weight, an alloy of neodymium, iron and boron. In that way an especially strong and lightweight magnet may be provided. The magnets may themselves be disc-shaped with opposite poles on opposite faces of the magnets. Instead of there being discrete magnets, the magnetic regions may be provided by integral magnetic regions of the discs.
The present invention further provides a method of operating an electromagnetic brake or clutch acting on a rotatably mounted shaft, the method comprising:
providing a rotor disc mounted on the shaft for rotation therewith, a brake or clutch disc disposed around the shaft, a first face of the brake or clutch disc confronting a first face of the rotor disc and one of the discs being mounted for movement towards and away from the other disc;
a stator coil which when energised urges the first face of the brake or clutch disc away from the first face of the rotor disc;
the confronting first faces of the rotor and stator discs being provided with circumferentially spaced magnetic regions of alternating polarity, disposed such that magnetic regions of one polarity on the first face of the brake or clutch disc confront magnetic regions of the opposite polarity on the first face of the rotor disc to hold the rotor disc against rotation relative to the brake or clutch disc and urge the rotor disc and brake or clutch disc together in frictional engagement with one another, the stator coil being deenergised;
energising the stator coil to urge the brake or clutch disc away from the rotor disc and energising a motor to rotate the rotor disc to move said magnetic regions of the opposite polarity on the first face of the rotor disc away from said magnetic regions of one polarity on the first face of the brake or clutch disc and to bring magnetic regions of said one polarity on the first face of the rotor disc to positions confronting said magnetic regions of said one polarity on the first face of the brake or clutch disc, whereupon the rotor disc is urged axially away from the brake or clutch disc and is free to rotate relative to the brake or clutch disc. The steps of energising the stator coil and of energising a motor may or may not be carried out simultaneously or in the order in which they are recited above. In the case of a clutch, it is as likely that the step of energising the stator coil will be carried out after the step of energising the motor.
The method may include the step of further rotating the shaft with the stator coil energised and magnetic regions of the rotor disc travelling past magnetic regions on the brake or clutch disc.
The method may include the subsequent steps of deenergising the stator coil and ceasing to drivingly rotate the shaft with the motor, whereupon the rotor disc and the brake or clutch disc move back into a position in which they are urged together in frictional engagement with one another, with magnetic regions of one polarity on the first face of the brake or clutch disc confronting magnetic regions of the opposite polarity on the first face of the rotor disc. The steps of deenergising the stator coil and of ceasing to drivingly rotate the shaft with the motor may or may not be carried out simultaneously or in the order in which they are recited above. In the case of a clutch, the step of deenergising the stator coil is likely to be carried out while the motor is still energised and drivingly rotating the motor shaft.
The method of the invention may be carried out using an electromagnetic brake or clutch of any of the kinds described above.
It will be appreciated that the brake or clutch of the invention and the method of the invention as described herein are closely related and that therefore essential or preferred features of one may, unless indicated otherwise or clearly inappropriate, be incorporated into the other. Thus, features described above in respect of the brake or clutch of the invention may be incorporated into the method of the invention and vice versa.
By way of example embodiments of the invention will now be described with reference to the accompanying schematic drawings, of which:
The housing 1 is in two parts having an end cap 1A closing off one end of the housing 1 and mounting a bearing 8 for one end of the shaft 7 and a main housing 1B in which the motor 2 is received and which mounts a bearing 9 through which the shaft 7 passes, providing the output drive shaft of the assembly, the output drive shaft passing through an opening 10 in the end of the main housing 1B.
The rotor disc 4 is fixed to the shaft 7 and therefore rotates with the shaft 7.
The brake stator coil 6 is mounted in a housing 6A that is fixed to the end cap 1A and in particular non-rotatably fixed to the end cap 1A, by keys 6B. The peripheral edge of the stator disc 5 is mounted in splined engagement with a peripheral edge of the stator coil housing 6A. The disc 5 is held against rotation by virtue of its splined engagement with the housing 6A but is free to slide axially relative to the housing 6A between a position shown in
The rotor disc 4 has a first, high friction, face confronting the stator disc 5 and carries a ring of permanent magnets 11 equiangularly spaced around the axis of rotation of the shaft 7. The stator disc 5 similarly has a first, high friction, face confronting the rotor disc 4 and carrying a ring of permanent magnets 12 equiangularly spaced around the axis of rotation of the shaft 7. Each ring of magnets 11, 12 consist of magnets whose first faces are of alternating polarity around the ring. The locations of the magnets 11 on the rotor disc 4 match the locations of the magnets 12 on the stator disc 5 so that in one position of the discs, shown in
The operation of the brake assembly will now be described referring first to
When it is desired to rotate the shaft, the motor 2 is actuated and the stator coil 6 is also energised. Energising the stator coil 6 is arranged to attract the stator disc 5 towards the coil. In a typical application the driving force of the rotor will be comparable to the braking force applied in the braking condition shown in
As the drive shaft 7 begins to rotate, the force of magnetic attraction between the magnets 11, 12 reduces because their poles move further apart; also the force with which the discs 4 and 5 are pressed together further reduces because the magnetic force pushing them together reduces, again because the poles of the magnets 11, 12 move further apart. After a rotation of the rotor drive by a distance of half the spacing of the magnets 11, 12 the poles of the magnets 11 are equispaced from the same poles and the opposite poles of the magnets 12 and at that stage there is no force pressing the discs 4 and 5 together and no friction or magnetic braking force; thus, before that position is reached the stator disc 5 is attracted by the stator coil 6 to move axially towards it (to the left as seen in
As the motor continues to operate, the drive shaft 7 continues to rotate and the magnets 11, 12 arrive at a position where the same poles of the magnets confront one another providing a further magnetic force to urge the stator disc 5 away from the rotor disc 4 and therefore towards the stator coil 6. In that position of the stator disc 5, shown in
When it is desired to brake the motor, the stator coil 6 and the motor 2 are de-energised. The disc 5 is then attracted towards the disc 4 and the disc 4 driven by magnetic forces into a rotational position in which poles of the magnets 11 confront opposite poles of the magnets 12, namely a position of the kind shown in
It may be noted that when the output drive shaft 7 is being drivingly rotated by the motor, the stator coil 6 is energised and the parts are in the position shown in
In the example of the invention described above, the assembly is a brake assembly but it should also be understood that a very similar device may be provided as a clutch assembly, as will now be described with reference to
The housing 1 is in two parts having an end cap 101A closing off one end of the housing 1 and mounting a bearing 108 for an output drive shaft 107B and a main housing 1B in which the motor 2 is received and which mounts a pair of bearings 109 in which the shaft 107A is journalled. The output drive shaft 107B passes through an open end of the end cap 101A.
The drive disc 104 is fixed to the shaft 107A and therefore rotates with the shaft 107A.
The clutch coil 6 is mounted in a housing 106A that is fixed to the output drive shaft 107B. The peripheral edge of the clutch disc 105 is mounted in splined engagement with a peripheral edge of the housing 106A. The clutch disc 105 is held against rotation relative to the housing 106A by virtue of its splined engagement with the housing 106A but is free to slide axially relative to the housing 106A between a position shown in
The drive disc 104 and the clutch disc 105 have confronting faces that are the same as described above for the discs 4 and 5 in
As will now be understood, the assembly of
In the description above the clutch is engaged and disengaged while the shaft 107A is rotating, but of course it can also be engaged or disengaged when the shaft 107A is stationary.
In the example of the clutch described above, the clutch coil 6 rotates with the clutch housing 106A when the drive shaft 107B rotates, but it remains in fixed relationship to the clutch disc 105 and is therefore referred to herein as both a “clutch coil” and a “stator coil”.
An advantage of the brake or clutch described above is that it can operate at a significant relative speed of rotation of the rotor disc 4 relative to the stator disc 5, without damage. This is in contrast, for example, to a dog clutch.
Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1013576.2 | Aug 2010 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB2011/051526 | 8/12/2011 | WO | 00 | 2/28/2013 |
