WO2005012757A1

WO2005012757A1 - Permanent magnet braking and coupling arrangements

Info

Publication number: WO2005012757A1
Application number: PCT/AU2004/001023
Authority: WO
Inventors: Franz Kocijan
Original assignee: Magswitch Technology Worldwide Pty Ltd
Priority date: 2003-07-31
Filing date: 2004-08-02
Publication date: 2005-02-10

Abstract

A braking device operatively adapted to provide a braking effect to and at least slow-down relative movement between two machine components, said braking device including a switchable permanent magnetic device that is arranged to be selectively switched to maintain directly or indirectly said components in forced frictional engagement with one another so as to achieve said braking effect.

Description

PERMANENT MAGNET BRAKING AND COUPLING ARRANGEMENTS Technical Field The present invention relates to friction braking arrangements and devices that serve to slow down moveable machine or vehicle components, as well as to coupling arrangements and devices by means of which movable machine or vehicle components can be selectively joined to move together (such as in the case of a friction clutch or locking clutch). Background of the Invention Many practical mechanical situations require the use of mechanisms and devices to either slow down or prevent movement of one component relative to another, or to selectively couple a moving part to another part for joint movement or retention in one stationary position. Brake and coupling mechanisms are required for many applications and are typically found in complex machines such as airplanes, automobiles, trains, manufacturing machinery, electric (and other) motors, as well as in simpler devices such as stationary exercise machines and simple wheeled devices like trolleys, bikes etc. Consequently, a plethora of types of braking (and coupling) devices have been conceived, mostly with one application in mind, but often subsequently adapted for other applications. One category of brake devices or arrangements is generally characterized by bringing an otherwise stationary part into contact with a moving part and exerting a force so as to press the parts to one another to maintain frictional engagement, thereby slowing relative motion and ultimately stopping one component relative to the other. Such types of brakes are often referred to as friction brakes, and many types thereof exist. A simple type of friction brake is embodied in Bowden cable operated calliper brakes found in many bicycles. A pair of brake blocks is carried at the ends of calliper arms that are responsive to Bowden cable actuation whereby the seizor-like movement of the calliper arms brings the brake blocks into contact with the rim of the bicycle wheel. In conventional arrangements, without force augmentation measures, the maximum braking force that can be exerted will depend directly on the amount of force that a bike rider is able to exert on the brake handle that pulls the Bowden cable and thus forces the brake blocks against the wheel. Another application is found in disc brake systems commonly used in vehicles. A brake disc is fixed to rotate with the wheel of the vehicle. A pair of brake pads is arranged within a stationary calliper housing (that itself is fixed to the chassis of the car) such that the pads can be displaced towards into and away from contact with the brake disc that is located to rotate between the pads. The force required for moving the brake pads into engagement and exert the required force to achieve frictional braking is provided through a hydraulic fluid circuit that incorporates at each wheel location at least two hydraulic cylinders with pistons that move the brake pads in response to hydraulic pressure generated in a master cylinder. The pressure in the latter is generated in response to mechanical force being exerted by a car driver on the braking pedal and onto a master cylinder piston. Such hydraulic breaking systems boost the maximum force which the vehicle driver is able to exert given that the pressure exerted on the hydraulic fluid in the master cylinder will be equally present in the smaller brake cylinders at the wheels. Whilst the braking force in such an arrangement is augmented by use of the hydraulic system, it still relies on the effective transfer of mechanical force exerted by the driver to the brake pads. There are also numerous types of braking and clutch systems reliant upon magnetic forces. Some of these braking (and coupling) systems employ electromagnets that cooperate with armature elements to form a brake or clutch unit, and others rely on eddy current magnetic field generation using driven permanent magnets and stationary or driven (ferro) magnetic switching plates. The latter type are 'contact-free' braking or retarder devices that do not employ frictional engagement between moving parts to achieve a braking or coupling action. For example, US Patent document US 6,581 ,731 B2 describes a dynamo- principle brake assembly for use with bicycles, having a hub rotor with permanent magnets in which is received a stator with multiple armature coils and magnetic induction coils that act on a second rotor received within the hub and located inside the stator thereby to generate eddy currents and achieve controlled braking (by torque transmission) of the bike wheel spindle which carries the assembly.

Whilst being indeed autonomous in its braking capacity, it requires the presence of a rectifier circuit to convert AC current generated by the armature coils upon rotation of the external rotor with its magnets, into DC current that is required to energize the magnetic coils that generate the eddy currents that lead to braking of the bike spindle, as well as a control circuit for the DC current to properly control braking torque. Overall, a complex and expensive device only suitable for high tech, expensive bikes. An example where magnetic forces are used to couple moving parts is provided in US Patent publication US 2004/0118656 A1 which discloses a magnet fan clutch device that combines an electromagnetic clutch at the input drive shaft end of the unit with a permanent magnet coupling on the output driven shaft end of the unit, a disc armature that is biased using a leaf spring against the permanent magnet rotor providing a frictional coupling member for both magnetic clutch components upon activation of the electromagnet. It will be immediately appreciated that such clutch devices require an external (or internal) current source for energizing the electromagnet. One drawback of eddy current braking devices is generally that they require the relative movement of their components for operation, i.e. to generate the eddy currents ultimately responsible for braking force, and they are adapted to gradually slow down but do not stop relative movement as achieved by conventional frictional brakes. With the above background art in mind, the present inventors set out to devise simple friction braking mechanisms that are able to provide a desired braking force without the need for exertion of an external mechanical or electromagnet-generated force to maintain a forced, frictional engagement between the otherwise stationary brake element and the component to be slowed down. A self-contained energy source would be desirable. It would hereby also be advantageous to provide friction-braking mechanisms where the force required to activate the brake mechanism is minimised. It would also be beneficial if it would be possible to create such braking mechanisms wherein the braking force, which is exerted by the otherwise stationary brake element onto the moving component that is to be slowed down, can be regulated selectively or set in advance. A further object of the inventors is to provide simple frictional clutch mechanisms that enable coupling of two working parts with reduced external force exertion required to engage the coupling. In the context of the latter, it would be advantageous to incorporate a structure that allows positive form-locking of the two working parts once contact between the parts has been effected.

Summary of the Invention In a broad form, the present invention utilises magnetic force provided by a switchable permanent magnetic device to apply, directly or indirectly, braking or retaining forces between at least two components that are moveable relative to one another and which can be brought into frictional engagement with one another using such magnetic force itself, or another force source. According to a first aspect, the present invention provides a braking device operatively adapted to provide a braking effect to and at least slow-down relative movement between two machine components, said braking device including a switchable permanent magnetic device as herein below identified that is arranged to be selectively switched to maintain directly or indirectly said components in forced frictional engagement with one another so as to achieve said braking effect. Preferably, the switchable permanent magnetic device is also used to cause relative displacement of at least one of said components from a position in which they are not in touching engagement into the frictional engagement position. In a second aspect, the present invention provides a brake arrangement that includes at least one switchable permanent magnetic device as herein below identified, and at least one ferromagnetic armature component, the armature component and the switchable permanent magnetic device being mounted to or incorporated into a respectively associated one of two components of a machine, which machine components are arranged for relative movement with respect to one another, wherein the permanent magnetic device has a switching state in which magnetic force exerted on the armature component causes it to maintain its associated machine component in frictional and forced abutment with the machine component associated with the permanent magnetic device thereby achieving a braking effect between the machine components when in relative movement and otherwise a holding effect against relative displacement when the components are stationary with respect to one another. Advantageously, the switchable permanent magnetic device is also used to cause relative displacement of the ferromagnetic armature component and thus of at least one of said components associated with the armature or switchable permanent magnetic device from a position in which the components are not in touching engagement into the frictional engagement position. The above two main or generic aspects of the invention will become clearer in the following and in particular from the specific embodiments described below. In essence, the invention was conceived upon the surprising realisation that so called switchable permanent magnetic devices of the type described in patent document WO 01/43147, compare also US Patent document US 6,707,360 B2, the whole contents and disclosure of these documents being hereby incorporated into the present document by way of short hand cross- reference, and in US Patent 4,055,824 (Baermann), the latter one having been conceived as a holding device for ferromagnetic work-pieces and objects on machining jigs and tables, can be adapted for incorporation into braking arrangements which hitherto utilized purely mechanical force input to achieve frictional braking and engagement between movable machine components or such machine components that are to be coupled for joint movement. There are commercially available permanent magnets magnetized to a high degree thereby being able to exert substantial magnetic forces when in a circuit with a ferromagnetic armature component. By way of example, the below table provides some typical values for switchable permanent magnet devices that can be used to implement the invention.

Notes: (^*) Breakaway Force is determined in accordance with Test Methods outlined by the US Magnet Distributors and Fabricators Association 1997 (#) Actuation Torque is the peak force requirement for magnetic switching. Figures do not include frictional forces caused by lubricants and mechanical losses. The figures indicate static forces only. Switching speed will greatly affect these results. (&) Actuation Energy is the energy required for one activation cycle. It is an indicative calculated value. Switching speed has been assumed to be low and additional losses due to lubricant properties have been neglected. The Actuation Energy for deactivation has not been included in this table. It constitutes an energy return and is the negative numeric equivalent to the activation energy less frictional losses. (%) Peak value measured at centre of outer edge at bottom of poles. As described above in the context of friction brakes, at least part of the force required to effect braking of wheels in conventional vehicle applications is provided by the human driver or operator through depressing of a brake pedal or handle. Whilst it is of course well known to incorporate into the force transfer chain between operator brake actuating element (eg pedal or handle) and the brake component at the wheels (eg hydraulic pistons in disc and drum brakes) additional 'force boosters' (eg hydraulic booster circuits), to increase available braking force at the machine element that requires slowing-down, the inventors here departed from such concept altogether. In contrast to 'conventional' braking arrangements of the type described at the outset, according to the invention the force required to achieve the braking effect between a movable machine part (eg a wheel) and machine component relatively stationary thereto (eg brake pad or block) is provided solely by an array of permanent magnets which are disposed in a switchable arrangement that allows controlled variation of the magnetic attractive force which the magnets exert onto a ferromagnetic component that forms the other main component of the braking arrangement. In a preferred embodiment of the present invention, the switchable permanent magnetic device includes any one of the different embodiments of the device disclosed and described in US patent document 6,707,360, to which reference should be made for further details and which is to be read concurrently with this document. However, in the context of the present invention, switchable permanent magnetic devices include also such that are able to be shifted only between a "fully on" and a "fully off" position, in comparison to the devices illustrated and described in US patent document 6,707,360, which allow gradual (vs stepped) variation of the external magnetic field of the device that provides the magnetic holding force. Herein below, all such switchable permanent magnetic devices will also be abbreviated to SPMD It will be noted that the broad aspects of the invention may be implemented in many different machines where it is required to slow-down and bring to full stop one machine component with respect to another component of the machine. Such applications include rotary and linear-reciprocating devices and it is believed that the broad aspects of the invention may be incorporated into the latter type of machines, although initial embodiments conceived by the inventors relate to applications in which a rotor (in the broadest sense of the word, such as a wheel, fan disk, etc) is mounted on an axle either such as to freely rotate on said axle or such as to be driven for rotation with or on the axle. The brake arrangement is provided for braking of the rotor, be it either to control its rotational speed using friction energy dissipation or to selectively stop and arrest rotational movement altogether. The device can equally be used to selectively couple two rotors for synchronous or slipped asynchronous rotation as is illustrated below in one embodiment of a torque defined clutch arrangement. Currently preferred implementations cover a vast number of self-propelled as well as non-motorised land vehicles and transport contrivances, including light motor scooters, motor bikes, baggage trolleys, wheel chairs, bikes, movable beds, baby strollers, etc. However, applications in stationary machinery to effect torque control such have also been examined, given that the invention allows implementation of simple and reliable torque controlled clutches that prevent damages to drive train components driven by say an electric motor, where jamming of any component may occur. Applications include generators for dredging pumps, mower decks, larger power tools, roller door drive devices, etc Accordingly, whilst not to be construed as limiting of the invention, in accordance with a more specific aspect of the present invention, there is provided a brake arrangement according to the first or second aspect of the invention, for a land vehicle having at least one wheel that is to be selectively slowed-down and/or arrested against rotational movement. The brake arrangement includes a ferromagnetic brake armature arranged for rotation with a wheel rim that runs freely on or rotates with an axle, and at least one switchable permanent magnetic device of the aforementioned type (SPMD) that is mounted on a carrier that is stationary with respect to the wheel rim. The SPMD is located in close proximity to the brake armature such that upon appropriate setting of the SPMD to have a strong external magnet field, either the brake armature or the SPMD is caused to move and bring into frictional and forced abutment frictional wear components or bodies present at least on the ferromagnetic armature and preferably also on the SPMD, respectively, for these to remain in direct contact thereby to effect said frictional braking effect. Advantageously, the brake armature may be embodied in a disc brake, wherein the brake disc may itself be made of a suitable ferromagnetic material and have as a frictional wear component an annular or disc-shaped friction pad or liner on the surface facing the SPMD. Alternatively, the brake disc may be a unitary body composed of ferromagnetic and abrasive particles moulded or sintered together, whereby the choice of abrasive material will cover those normally employed in brake pads and blocks according to wear and brake force transfer requirements. It should be within the normal skill of an engineer to determine appropriate material selection for a given application situation. In embodiments where braking forces are relatively low, it is possible to dispense with a wear component on the SPMD and only have a wear component on the armature, given that an external front face of the housing of the SPMD may be appropriately shaped to perform the counter ambos function when frictionally engaged and forced against the armature. In embodiments where a brake wear component at the SPDM is required, to ensure adequate frictional braking characteristics of the overall arrangements, a separate, replaceable thin brake pad or liner may be attached to either cover entirely or only partially the front face of the SPMD. Preferred, however, may be an arrangement whereby a replaceable wear component is carried at a support structure associated with the SPMD and which is devised such that the front face of the SPDM may be brought in very close or only slightly touching relationship with the armature whilst equally ensuring that the respective brake wear components of the armature and the SPMD come into forced and direct frictional contact when the SPMD is activated to exhibit a strong external magnet field (and thus generating a strong attraction force for the armature). Such arrangements will minimise wear of the front face of the SPMD itself, whilst ensuring that sufficient (attraction) force is generated across any small air gap that may be present between the SPMD and the brake armature to achieve the desired level of frictional braking. Advantageously, the materials used for the wear components can be selected to provide differential wearing at the armature and the SPMD, whereby a softer or more easily wearable material may be preferably provided at the armature, for ease of replacement. In another embodiment, the brake armature may be embodied in an annular band of ferromagnetic material embedded in a suitably formed channel either in the radially extending main face of the wheel rim, or the wheel rim bed that extends generally perpendicular from the wheel rim disc circumference. The wheel rim may be substituted for a brake drum body where the wheel rim is secured instead to such brake drum body. In such case, ie where the brake armature is held in fixed and unmovable position at the wheel rim or brake drum body, as the case may be, it is then necessary to provide for a mounting arrangement of the SPMD at its stationary carrier that enables the SPMD itself to be displaced towards and away from the brake armature whilst otherwise remaining fixed to the carrier in order to achieve forced frictional engagement of the relevant brake wear components at the rotating and stationary brake arrangement components. A simple solution is one wherein the stationary carrier includes a receptacle having a cross-section corresponding to that of the exterior (housing) of the SPMD and which provides therewith a clearance fit allowing end-restricted linear motion of the SPMD within its receptacle. In an embodiment where the SPMD is secured against any movement at its carrier and where the brake armature is provided at a brake disc that is axially movable along the axis of rotation of the wheel rim, but otherwise retained for rotation therewith, it is advantageous to provide a biasing structure that urges the disc into an out of contact position with regards the SPMD, such that upon the SPMD being switched into a state in which the outer magnetic field generated by the device is at a minimum after a braking event, the disc disengages automatically from and moves away from the SPMD into its normal rest position. Preferably, the rest position can be set (using adjustable stops) to maintain a desired gap between front face of the SPMD and the facing side of the brake disc that is small enough for the magnetic force that is present when the SPMD is switched into a predetermined magnetic flux state to achieve brake activation thereby overcoming the biasing force and displacing the disc towards engagement with the SPMD. As noted, preferred embodiments of the present inventive brake arrangements and devices utilize switchable permanent magnetic devices of the type disclosed in US patent 6,707,360. The most simple of these devices incorporate a first and a second cylindrical magnet, both of which are diametrically polarised. The magnets are aligned on a common axis of relative rotation and are received within a housing that includes a pair of passive ferromagnetic pole elements that are disposed about the periphery of the permanent magnets. The device further includes an actuator for causing rotation of one of the cylindrical permanent magnets relative to the other, whereby in a "fully off" position the magnets are aligned with the north and south pole sectors of the first and second magnets in superimposed aligned relationship such that a relatively weak external magnetic field is produced through the passive pole elements, while in a "fully on" position where the north and south pole sectors of both magnets align in axial direction of the device, thus generating a relatively strong external magnetic field through the passive poles. In such a way, as the magnets are rotated relative to each other, they move between an "on" position and an "off" position. The device may also be in an intermediate state, with a proportion of the maximum possible field. As already noted in US 6,707,630, actuation of the device to obtain "on" and "off" external magnetic activation states can be effected in many different ways, the simplest comprising a lever handle connected to the rotatable one of the magnets. In the context of the present invention, where remote actuation of the SPMD is likely to be the norm, there are many different actuation mechanisms that can be used to achieve rotation of the magnet within its housing, a simple one including Bowden cable operated levers, more complicated ones including hydraulic or pneumatic systems with relatively low pressurisation requirements, given that preferred SPMDs may incorporate the different mechanisms described in US 6,707,630 to overcome the natural tendency of the magnets to resist switching between the on and off states, in particular where no external load is applied to the device (by the ferromagnetic armature of the braking device). The present invention may be implemented also by modifying existing disc and drum brake devices. The usually employed hydraulic cylinders are in such case replaced by SPMDs located and oriented in the stationary brake support structures (eg calliper body of disc brakes and stator disc of drum brakes) such that the variable-strength external magnet field, which is generated by appropriately setting the relative rotational position of the co-axially arranged cylindrical permanent magnets about their common axis within the device housing, can interact magnetically with a ferromagnetic armature component at the brake disc or brake drum. An advantage of using such SPMDs instead of hydraulic pistons is that permanent magnets provide a magnetic field (and thus magnetic force) without requiring a dedicated hydraulic circuit. Essentially, the SPDMs are self-contained force generators that can be switched using simple mechanical or electric lines. This avoids the necessity of having to provide a constantly available source of hydraulic pressure thereby reducing complexity. The present invention shall now be illustrated by reference to some simple embodiments thereof that will enable the skilled reader to fully appreciate the invention. Brief description of the drawings Figures 1a and 1 b show in highly schematic and simplified form components of a brake arrangement embodying the invention; Figures 2a and 2b shows in highly schematic and simplified manner components of a second embodiment of the invention in form of a torque defined clutch; Figure 3 shows in longitudinal and simplified section a trolley wheel incorporating a disc brake arrangement according to a third embodiment of the invention; Figure 3a shows the detail marked Ilia in fig. 3 in front view; and Figure 4 shows in longitudinal and simplified partial section a wheel assembly for light vehicles incorporating a drum brake arrangement according to a fourth embodiment of the invention. Detailed Description Before turning to the more specific embodiments illustrated in figures 3 and 4, reference will be made to figures 1 and 2 which serve to illustrate the basic principles underlying the present invention. The drawings are schematic representations of the main components of a braking arrangement according to the invention, without any of the other components of a machine which may employ such braking and coupling devices. Figures 1a and 1b seek to illustrate a disc brake arrangement according to a first aspect of the invention. The brake 1 essentially is comprised of one switchable permanent magnet device (SPMD) 2 such as described in principle in US Patent 6,707,360, to which reference shall be had if required, and a ferromagnetic disc 4 that may advantageously incorporate on the surface facing the SPMD 2 a brake liner 5 of suitable material for frictional brakes. The SPMD 2 in this example is mounted on a fixed support 3 to remain stationary with respect to disc 4, while the ferromagnetic disc 4 is supported on an axle 6 for rotation (as indicated by arrow 7) therewith or thereon, but free axial movement (as indicated by arrow 8). In Fig. 1a, the switching state of the SPMD 2 is "off' and no (or a very weak) external magnet field is present at the face of the SPMD 2 that faces the disc 4, and a small clearance gap is maintained between the facing surfaces of SPMD 2 and disc 4 that permits free rotation of disc 4. The gap is chosen such that upon activation of the SPMD 2 into a 'fully on' position, as illustrated in Fig 1 b, which represents a fully activated state of the brake 1 , the axially displaceable ferromagnetic disc 4 can be attracted towards the SPMD 2 and friction braking takes place as consequence of the strong adhesive force 9 which is exerted by the magnets onto the disc. This force ultimately leads to stopping of rotation of disc 4. It will be appreciated that a suitable lining or surface could be provided also on the front face of the SPMD 2 that engages brake liner 5 to prevent wear of the SPMD housing. Any suitable device may be provided to ensure the ferromagnetic disc 4 returns into its gap maintaining non-contact position with the SPMD 2 once it is turned into a fully off switching state, such as a spring or a set of weaker magnets. Given the SPMD is arranged to allow steples adjustment of the external magnetic field it generates, it is possible to adjust the braking torque to a desired value by appropriately setting the position of the rotatable cylindrical, diametrically polarized, permanent magnets that are inside the SPMD; Operation between 'fully on' and 'fully off' is also possible, wherein on activation of the SPMD 2 it engages the disc with a given, non-variable braking force. It will be appreciated that more than one SPMD 2 could be provided, so that the braking effect is not at a single point. Further, the strength of magnet required will be proportional to the braking situation. The illustrated implementation is most suited to a relatively low speed system, and a specific example will be described below with reference to figure 3. Turning next to figures 2a and b, there is illustrated also in basic schematic manner a further embodiment of the invention, a torque defined clutch 1a. Given that the two mechanisms of figs 1 and 2 share the same basic common principle, similar parts are designated using the same reference numerals. Clutch 1a consists of one SPMD 2 that is fixed on an engagement plate 2a which in turn is resiliently pivoted for rotation about support 3a, and a ferromagnetic clutch disc 4a having a plurality of locking apertures 4b spaced equidistantly from rotation axis 6 along a circle on the surface facing the SPMD 2; an engagement member in form of a steel ball 2b is provided on the engagement plate 2a to cooperate with any one of said locking apertures 4b to achieve form-locking but releasable coupling between clutch plate 4a and the biased engagement plate 2a upon activation of SPMD 2. It will be noted also that the clutch disc 4a is mounted to an achsle or similar about axis 6 for rotation s per arrow 7 but such as to be freely displaceable in axial direction as illustrated by arrow 8. Fig. 2b illustrates the activated state of the clutch assembly lap. In this position, SPMD 2 is in the "fully on" position where a strong external magnet field exist. The magnetic circuit is closed by the ferromagnetic disc 4a that is attracted into forced abutment towards SPMD 2, and the locking ball 2b rests in one of the locking recesses 4b thereby creating a torque defined coupling: If the torque imparted on clutch disc 4a exceeds the retention force that the SPMD 2 is able to exert on the disc, and the biasing force exerted by engagement plate 2a that maintains its locking ball 2b in recess 4b, then the clutch allows slippage of the clutch disc 4a until such time as the torque decreases and the locking engagement is resumed. Controlled sliding forces between the engaging surfaces on the ferromagnetic disc and the SPMD front face (where the latter also rests on the facing surface of the disc, and in addition to any frictional contact which engagement plate 2a may exert onto disc 4a) determine the translation of pulling force to release torque, taking also into consideration the contact angle between the engagement plate 2a and SPMD. Figure 2a illustrates the deactivated state of the clutch 1a. In this case, the SPMD 2 is in the "fully off" position. No force transfer takes place, and the ferromagnetic clutch disc 4a is maintained in spaced apart relation ship from engagement plate 2a using appropriate biasing members, as described above in relation to Fig 1a. As is the case with the brake arrangement, the slipping torque can be set to a desired value by setting and/or varying the attracting force of the switchable permanent magnetic device 2. Turning next to Figure 3 there is illustrated in longitudinal section a wheel- brake assembly for use in luggage trolleys, shopping trolleys and the like where it is required to arrest motion of the wheel with respect to the trolley during operation. It will be understood that such constructions require only a relatively weak holding force to arrest wheel movement, so that a single SPMD of the type previously described can be used per wheel unit to achieve satisfactory braking. Reference numeral 10 identifies a free spinning castor wheel assembly which includes three main components, a wheel rim 12, a wheel rim mount 22 and an axle 32 by means of which the wheel rim 12 is rotatably secured to the wheel rim mount 22. The body of wheel rim 12 may be cast from a light metal alloy or hard duroplastic material in known manner, and carries on its wheel rim bed portion 14 a rubber tyre element 20. The disc-shaped face portion 16 of rim 12 has an inwardly extending hub portion 18 with a cylindrical dead end bore 29 in which is housed in known manner a suitable ball or pin bearing 30 that receive the terminal end of axle 32. The opposite terminal end of axle 32 is received within a similar ball or roller bearing 31 mounted into a dead end bore 28 of a central portion of wheel rim mount member 22. Wheel rim mount member 22 is preferably a metallic castling that comprises a disc shaped backing plate portion 24 whose diameter is chosen such as to cover the otherwise open inner cavity of wheel rim 12, and a strut or leg portion 26 which extends from the back face of plate portion 24 and which serves to attach the castor wheel structure to an appropriate mounting at the trolley. A suitable sealing ring 33 or similar structure is provided at the outer peripheral rim of backing plate 24 thereby to prevent ingress of contaminants into the wheel structure cavity. A brake arrangement is generally identified by reference numeral 40 and consists of a ferromagnetic brake disc member 42 which is received on axle 32 in slidable manner allowing axial displacement in either direction along the axis of rotation of axle 32. Ferromagnetic brake disc element 42 is provided with a plurality of retention grooves 46 spaced apart along its peripheral rim (see Figure 3A). These grooves 46 are disposed to be engaged with suitably shaped tenon or bar elements 48 integrally formed on the inside of wheel rim bed portion 14, thereby providing an axial mount for the brake disc 42 within the wheel rim that allows the brake disc 42 to rotate together with the wheel rim 12. Each tenon 48 is provided at its rear most inward end with a protrusion 49 that provides a defined stop which fixes the rear most axial position of the disc 42 within rim 12. In a preferred, non-illustrated embodiment, the stops 49 could be provided by displaceable elements, thereby to allow adjustment of the rear most position of axial displacement of disc 42. Reference numeral 50 identifies a saucer or disc spring appropriately secured against axial displacement on axle 32 and intended to weakly bias the brake disc 42 against the stops 49. The brake arrangement 40 further includes a switchable permanent magnet device (SPMD) 52 of type and construction as illustrated and described in US patent 6, 707,360, to which reference shall be had for further details. The SPMD 52 is secured in an appropriate opening provided in backing plate portion 24 so as to remain in a fixed axial position with regards the wheel rim 12, with a front end face 53 of SPMD extending into the wheel rim cavity so as to maintain a small air gap 55 with respect to the facing surface of brake disc 42 when the latter is in its un-biased but otherwise spring-retained axial rest position. It will be noted that brake disc 42 is provided at the surface which faces SPMD 52 with an annularly shaped wearable brake pad or lining 44. As will be explained below, given that the frictional force required to effect braking of the wheel assembly in such type of application will be relatively small, it is not necessary for the front face 53 of SPMD 52 to have a counterpart, wearable brake lining pad or similar, since the metallic housing of the SPMD 52 will provide sufficient surface area to ensure adequate frictional engagement and transfer of force to achieve braking. An actuator mechanism 54 of the SPMD 52 is illustrated only schematically and includes a Bowden cable 56 arranged to switch the SPMD between an "off state", in which an external magnetic force field that can be generated by the SPMD is at a minimum or absent, and an "on state" where the external magnetic force field is at maximum. In operation of the brake, when the SPMD is switched into its "on" position, the magnetic attraction force generated will be sufficient to overcome the air gap 55 in order to close the magnetic circuit and attract and axially displace the ferromagnetic brake disc 42 against the biasing force of spring 50 into abutting engagement with the front face 53 of SPMD 52. Frictional braking of the entire wheel rim 12 is thus achieved. It will be appreciated that the switch characteristics of the SPMD can be set to suit the application. Equally, the specific geometries, in particular the air gap distance and the spring characteristics can be chosen to suit the specific application and which will depend on the force required to achieve braking of the wheel assembly within a desired time frame upon actuation of the SPMD. Figure 4 shows yet a further embodiment of a brake arrangement in accordance with the present invention. The arrangement is in form of an otherwise known drum brake arrangement commonly found in self-propelled vehicles like small scooters, electric carts and the like. Cylindrical, hollow drum break housing 100 has a radially extending disc- shaped front portion 102 and a peripherally extending rim portion 101. Drum body 100 is secured to a rotatable axle 105, which carries at its terminal end a suitable connection flange 106, by means of a plurality of suitable fastening screws identified schematically at 107. A wheel rim 112 of conventional construction and having a disc-shaped radially extending face portion 116 and a peripherally located wheel rim bed portion 114 is secured in known manner to the front face 102 of drum brake housing 100, using a plurality of bolts 117, thereby fixing wheel rim 112 for rotation in conjunction with the wheel drum 100. Wheel rim 112 may carry an inflatable tyre (not illustrated) in known manner. Arranged within the drum body 100 is received a stationary stub-like housing body 130 comprised of a unitary rotation-symmetric light metal alloy castling, that serves to support and carry four switchable permanent magnet devices (SPMD) 152 as will be described in more detail below. Housing body 130 is supported/mounted to a carry arm 134 that attaches to the vehicle chassis (not shown) thereby to fix the housing body 130 against rotation within drum body 100, whereby axle 105 extends through a central through-bore of body 130, where it is additionally supported by means of suitable bearings 132, 133. The housing body 130 has a total of four circular dead-end bores 136 that are equi- radially and peripherally equi-distantly spaced on a radially outward ledge that faces the inside of drum rim portion 101. One switchable permanent magnetic device 152 is received in each of the dead-end bores 136, whereby the outer diameter of the cylindrical housing of the SPMDs 152 and the bore diameter are dimensioned such as to allow a slight fit enabling the SPMDs 152 to move in radial direction with respect to the axis of rotation 105a of the wheel assembly, as illustrated by arrow 154. Suitable biasing components, in this case tension spring elements 156, are provided between the bottom of each dead-end bore 136 and the lower end face of the SPMDs 152 in order to bias the SPMDs towards the axis of rotation 105a. That is, the SPMDs can only move in a radially outward direction against the biasing force of the spring elements 156, the spring constant of which is low but sufficient to keep the SPMDs in a predefined rest position at housing 130. Reference numeral 158 serves to identify the schematically illustrated actuator device required to effect switching of the SPMDs, ie to change the relative rotational position of the cylindrical permanent magnets housed within the SPMDs, as described above in the context of Figures 3 and the device disclosed in US Patent 6,707,360 B2. An actuator device control line 160 allows remote operation of the actuator device 158. Actuation may be effected using a simple hydraulic or pneumatic circuit, but preferred is a mechanic transmission train that includes a Bowden cable or similar actuation arrangement. The break arrangement 140 further includes a ferromagnetic annular inlay band 142 that is received and secured against movement within an appropriately shaped grove on the inwardly facing surface of drum skirt portion 101. The ferromagnetic annular inlay band 142 itself may be a composite body consisting of ferromagnetic particles and abrasive materials usually employed in brake lining manufacture. In operation of the breaking arrangement 140, in the "off state" of the break, SPMDs 152 will maintain a predetermined small gap 155 between their radially outward located end faces and facing ferromagnetic inlay 142 which rotates with the wheel assembly. The inlay 142 effectively acts as an armature element for the magnetic circuit required to effect engagement between the SPMDs and the brake liner provided by the inlay 142. Upon the SPMDs 152 being switched into the "on position", the magnetic force generated by the external magnetic field will tend to pull out the SPMDs 152 from their receptacle bores 136 in radial direction and into engagement with the annular break inlay 142. Movement of the SPMDs 152 is effected against the biasing force of spring elements 156, which serve to return the SPMDs 152 into their rest position on deactivation. The SPMDs are dimensioned, from the point of view of the magnetic field generation capabilities, such that they are able to generate in the "on position" a sufficiently great attraction force across the air gap 155 to lift the SPMDs 152 into forced contact with the armature component provided by the ferromagnetic annular inlay 142, the external field strength having to be of sufficient value to generate a desired frictional retarding force appropriate for the weight of the vehicle that is provided with such breaking arrangement. Here again, the specific geometric dimensions and other specifications of all components can be chosen by the skilled worker without undue experimentation. The invention may be implemented in a variety of ways, and the embodiments illustrated are only illustrative constructions.

Claims

CLAIMS:

1. A braking device utilising magnetic force provided by a switchable permanent magnetic device to apply, directly or indirectly, braking or retaining forces between at least two machine components that are moveable relative to one another.

2. A braking device operatively adapted to provide a braking effect to and at least slow-down relative movement between two machine components, said braking device including a switchable permanent magnetic device that is arranged to be selectively switched to maintain directly or indirectly said components in forced frictional engagement with one another so as to achieve said braking effect.

3. A braking device according to claim 1 or 2, where the switchable permanent magnetic device is also used to cause relative displacement of at least one of said machine components from a position in which they are not in touching engagement into the frictional engagement position.

4. A brake arrangement that includes at least one switchable permanent magnetic device and at least one ferromagnetic armature component, the armature component and the switchable permanent magnetic device being mounted to or incorporated into a respectively associated one of two components of a machine, which machine components are arranged for relative movement with respect to one another, wherein the permanent magnetic device has a switching state in which magnetic force exerted on the armature component causes it to maintain its associated machine component in frictional and forced abutment with the machine component associated with the permanent magnetic device thereby achieving a braking effect between the machine components when in relative movement and otherwise achieving a holding effect against relative displacement when the components are stationary with respect to one another.

5. A brake arrangement according to claim 4, wherein the switchable permanent magnetic device is used to cause relative displacement between the ferromagnetic armature component and thus of the at least one of said components associated with the armature or switchable permanent magnetic device from a position in which the components are not in touching engagement into the frictional engagement position.

6. A brake arrangement according to claim 4 or 5, for a land vehicle having at least one wheel that is to be selectively slowed-down and/or arrested against rotational movement, wherein the ferromagnetic brake armature is arranged for rotation with a wheel rim that runs freely on or rotates with an axle, and the at least one switchable permanent magnetic device is mounted on a carrier that is stationary with respect to the wheel rim, said switchable permanent magnetic device being located in close proximity to the brake armature such that upon appropriate setting of the switchable permanent magnetic device to have a specified external magnet field, either the brake armature or the switchable permanent magnetic device is caused to move and bring into frictional and forced abutment frictional wear components or bodies present at least on the ferromagnetic armature and preferably also on the switchable permanent magnetic device, respectively, for these to remain in direct contact thereby to effect said frictional braking effect.

7. A brake arrangement according to claim 6, wherein the brake armature is a brake disc.

8. A brake arrangement according to claim 7, wherein the brake disc and frictional wear component are a unitary body composed of ferromagnetic and abrasive particles moulded or sintered together, whereby the choice of abrasive material will cover those normally employed in brake pads and blocks according to wear and brake force transfer requirements.

9. A brake arrangement according to claims 6, 7 or 8, wherein the frictional wear component on the switchable permanent magnetic device is a separate, replaceable thin brake pad or liner attached to either cover entirely or only partially the front face of the switchable permanent magnetic device.

10. A brake arrangement according to claims 6, 7 or 8, wherein the frictional wear component on the switchable permanent magnetic device comprised of a replaceable wear component carried at a support structure associated with the switchable permanent magnetic device, the support structure is devised such that the front face of the switchable permanent magnetic device may be brought in very close or only slightly touching relationship with the armature whilst equally ensuring that the respective brake wear components of the armature and the switchable permanent magnetic device come into forced and direct frictional contact when the switchable permanent magnetic device is activated to exhibit a strong external magnet field.

11. A brake arrangement according to any one of claims 6 to 10, wherein the materials used for the wear components can be selected to provide differential wearing at the armature and the switchable permanent magnetic device.

12. A braking arrangement according to any one of claims 6 to 11 wherein a biasing structure is provided that urges the brake disc out of contact position with regards to the switchable permanent magnetic device, the biasing structure arranged such that when the outer magnetic field generated by the device is at a minimum, the disc disengages automatically from and moves away from the switchable permanent magnetic device into its normal rest position.

13. A brake arrangement according to claim 12 wherein adjustable stops are provided to set the rest position of the brake disc to maintain a desired gap between a front face of the switchable permanent magnetic device and the facing side of the brake disc that is small enough to allow the magnetic force that is present when the switchable permanent magnetic device is switched into a predetermined magnetic flux state to achieve brake activation thereby overcoming the biasing force and displacing the disc towards engagement with the switchable permanent magnetic device.

14. A brake arrangement according to claim 6, wherein the brake armature is an annular band of ferromagnetic material embedded in the wheel rim.

15. A brake arrangement according to claim 14, wherein a mounting arrangement is provided for the switchable permanent magnetic device at its stationary carrier that enables the switchable permanent magnetic device itself to be displaced towards and away from the brake armature whilst otherwise remaining fixed to the carrier in order to achieve forced frictional engagement of the relevant brake wear components at the rotating and stationary brake arrangement components.