WO2010112078A1

WO2010112078A1 - Piston retraction arrangement

Info

Publication number: WO2010112078A1
Application number: PCT/EP2009/053947
Authority: WO
Inventors: Hendrikus Jan Kapaan; Laurens Verhulst; Cornelius Petrus Antonius Vissers
Original assignee: SKF Industrial Trading and Development Co BV; SKF BV
Current assignee: SKF Industrial Trading and Development Co BV; SKF BV
Priority date: 2009-04-02
Filing date: 2009-04-02
Publication date: 2010-10-07
Anticipated expiration: 2011-10-02

Abstract

The invention relates to a retraction device for retracting a piston in a cylinder when a force acting on the piston is released, comprising a piston (2), a retraction member (5) having a first contact surface (6), a retraction body (3) having a second contact surface (7) 1 and a resilient element (4) which bears on the first contact surface (6) and the second contact surface (7), where the retraction member is fixedly attached to the piston, where the retraction body further comprises a friction member (8) adapted to bear on an inner cylinder wall with a force that is greater than a maximum spring force of the resilient element (4), and where the deflection length of the resilient element defines the stroke length of the retraction device. The invention further relates to a disc brake and a drum brake comprising such a retraction device, and to a vehicle comprising such a brake. The invention further relates to an inventive method for retracting a piston in a cylinder.

Description

PISTON RETRACTION ARRANGEMENT

TECHNICAL FIELD

The present invention relates to a piston retraction device that is adapted to improve the retraction of a piston or the like when the actuating pressure is released.

The invention further relates to a method for retracting a piston in a cylinder. The retraction device and method is especially advantageous to use in combination with components having an increased rotational lateral run out, e.g. on vehicles or other machinery.

BACKGROUND ART

On many modern vehicles, disc brakes are used. A disc brake may comprise either a single piston mounted in a floating calliper and acting on a fixed rotor or brake disc, or a single piston mounted in a fixed calliper acting on a floating rotor or brake disc. The brake piston is mounted in the brake cylinder with a deformable sealing. A moving brake piston will slightly deform/roll-up the seal in its sealing groove, thereby creating a spring-back force. This sealing deformation, causing the spring back force, is meant to retract the piston into the cylinder when the brake pressure is released. In a system where the brake disc is properly lined up and the piston is clean and without corrosion, the retraction of the piston using a deformable sealing may be sufficient.

There are situations when the brake piston does not retract enough, which will cause the brake pad to be in abrasive contact with the brake disc. One such situation may occur even when the brake disc is properly aligned, i.e. running completely parallel with the brake pads. In this situation, the brake piston does not retract into the cylinder due to excessive friction between the piston and the cylinder. This may be due to corrosion or contamination of the piston and/or the cylinder, or due to an improper function of the sealing ring, e.g. because of ageing or wear. In those cases, the brake pad(s) will be in contact with the brake disc when the brake pressure is released. This is often referred to as off-brake drag and will increase the wear of the brake pads and the brake disc and at the same time, due to the increased friction in the brake, will increase the energy consumption of the vehicle. In severe cases, the brake properties may even degrade due to excessive heating of the brake. Off-brake drag may also cause the brake to squeal, which is unpleasant.

Another situation when off-brake drag may occur is when the brake disc is not properly aligned, i.e. is not running completely parallel to the brake pads. This is referred to as lateral run out. In this situation, the brake disc will wobble somewhat back and forth during a rotation, which will also cause the brake pad to be in abrasive contact with the brake disc. These situations may occur alone or in combination. The brake disc may also deform due to heat which will increase the problem.

Lateral run out is difficult to remove completely, since all components in the wheel end are manufactured with some tolerances. The tolerances may add up during assembly, which means that each vehicle will have a different production-related lateral run out. For a typical wheel-end / brake disc assembly, lateral run out may be in the magnitude of 30 to 50 microns. The retraction of the brake piston should be larger than the lateral run out in order to retract the brake pad from the brake disc. A typical retraction by a deformable sealing is in the magnitude of 60 to 100 micron, which will normally suffice for a new vehicle.

Such a small lateral run out is however rather costly to achieve. The wheel end parts must be produced with very small tolerances, which require fine precision machining of the contact surfaces. Furthermore, the tolerances often become greater when a part is replaced, which emphasizes the need for a larger retraction stroke length. Another cause of concern is a phenomena referred to as knock-back. This occurs when the wheel, the hub and the wheel bearing deflect during e.g. cornering. Since the brake disc is attached to the wheel hub, it will follow and will thus deflect some as well. The brake calliper is attached to the chassis which is more rigid and will deflect less. From this follows that the brake disc may come in contact with the brake pads due to the deflection of the brake disc. The brake disc may even force the brake pads away from one another and if the deflection is relatively large, may push a brake piston back into the cylinder by a tiny amount. When the deflection stops, the piston may then remain in the pushed back position. This will have a negative impact on the brake response time when the brakes are applied the next time. Such a knock-back situation is more likely to happen when the piston is not suspended properly in the cylinder and when the gap between the piston and the cylinder wall is relatively large.

DE 4304616 A1 describes a retraction system for a brake piston that uses a spring washer as retracting element acting on the piston. This solution might provide a working retraction system, but will be difficult to implement on vehicles since the spring washer must be adapted for both the retraction and wear adjustment of the piston. The spring washer is also relatively rigid which means that the piston will have a rigid suspension point in the cylinder, on which it may tilt back and forth some due to the play between the piston and the cylinder.

EP 0140549 B1 describes a disc brake piston with an integrated retraction system comprising a coil spring. The retraction system is positioned in the middle of a divided piston. The retraction stroke is determined by the distance between an intermediate element and the front and rear part of the piston. This solution might work but is rather large which may mean that the complete brake cylinder has to be redesigned. In addition, splitting the piston in the mid section with a central connecting sliding interface may result in unstable behaviour and, in service, such an arrangement will probably be very sensitive to dynamic vibrations. This solution is thus not suitable for use in existing brake cylinders. The dimensions of the brake piston will also make the piston rather weak in the radial direction during use, which may cause the piston to oscillate or to experience knock-back. The shown solution will also be rather costly to manufacture, with several high-precision parts to assemble.

Even though these devices and methods may function for some applications, there is still room for improvements.

DISCLOSURE OF INVENTION

An object of the invention is therefore to provide a retraction device for retracting a piston in a cylinder, having an improved retraction means. A further object of the invention is to provide an improved retraction device that is cost-effective and easy to manufacture. A further object of the invention is to provide an improved retraction device that is possible to mount in existing cylinders. A further object of the invention is to provide an improved retraction device having an improved stroke length that is well defined and possible to adapt to the application. A further object of the invention is to provide an improved disc brake comprising a retraction device. A further object of the invention is to provide an improved drum brake comprising a retraction device. A further object of the invention is to provide a vehicle comprising an improved brake. A further object of the invention is to provide an improved method for retracting a piston in a cylinder.

The solution to this problem according to the invention is described in the characterizing part of claim 1 regarding the retraction device, claim 11 regarding a disc brake, claim 14 regarding a drum brake, claim 15 regarding a vehicle and in claim 16 regarding the method. The other claims contain advantageous embodiments and further developments of the retraction device and method according to the invention. With a retraction device for retracting a piston in a cylinder when a force acting on the piston is released, comprising a piston, a retraction member having a first contact surface, a retraction body having a second contact surface, and a resilient element which bears on the first contact surface and on the second contact surface, where the retraction member is fixedly attached to the piston, where the retraction body further comprises a friction member adapted to bear on an inner cylinder wall with a friction force requiring an axial moving force that is greater than a maximum spring force of the resilient element, the object of the invention is achieved in that the deflection length of the resilient element defines the stroke length of the retraction device.

With the retraction device according to the invention, a retraction device is provided, which allows for a more reliable and consistent retraction stroke of a piston in a cylinder, where the retraction stroke is larger than in existing solutions. This is especially advantageous in vehicle brake systems, where the pistons are subjected to different dynamic loads and also to different environmental conditions. Further advantages of the inventive retraction device are that it is low in weight and may be retrofitted in existing brake callipers.

In a first embodiment of the retraction device according to the invention, the resilient element is a spring washer. The advantage of using a spring washer is that the spring force compared with the stroke length is relatively large and that the spring force is nearly linear. This allows for a well- defined and consistent retraction stroke. Depending on the retraction stroke and force required, it is also possible to use one or more spring washers stacked on each other as the resilient element. In this way, the stroke length and/or the force of the resilient member can be defined for a large range.

In an advantageous development of the retraction device according to the invention, the retraction member is mounted to an inner surface of a cavity in the piston. This is advantageous in that the retraction cartridge may be manufactured separately and can than be mounted to the piston in an easy way. The retraction member may be mounted to the piston by means of a press fit. This allows for a cost-effective and simple assembly of the retraction device.

In an advantageous development of the retraction device according to the invention, the retraction member is disc shaped and has a solid bottom portion. The advantage of this is that the retraction member can seal a hollow piston from the pressure chamber.

In an advantageous development of the retraction device according to the invention, the retraction member is ring shaped and is mounted in a groove in the piston. The advantage of this is that the retraction member can be mounted in a cost-effective way.

In an advantageous development of the retraction device according to the invention, the holding means is an annular friction ring. This is advantageous in that the retraction device can compensate for wear in the object on which pressure is applied in a reliable way.

In an advantageous development of the retraction device according to the invention, the retraction body and/or the retraction member is manufactured from sheet metal. The advantage of this is that the manufacture of the retraction device can be made in a cost-effective way. A further advantage is that the retraction device can be light in weight.

In an advantageous development of the retraction device according to the invention, the stroke length of the retraction device is at least 100 micrometers. The advantage of this is that the stroke length of the retraction member can be larger than the lateral run out of a wheel end system. A further advantage of having a large stroke length is that the tolerances of the wheel end system for lateral run out can be made slightly larger, which will make the parts comprised in the wheel end cheaper without affecting the performance of the brake system of the vehicle.

In an inventive disc brake, the disc brake comprises at least one retraction device. The advantage of this is that an improved disc brake having an enlarged retraction stroke is obtained.

In an inventive drum brake, the drum brake comprises at least one retraction device. The advantage of this is that an improved drum brake having an enlarged retraction stroke is obtained.

In an inventive vehicle, at least one disc brake or drum brake having an inventive retraction device is comprised. The advantage of this is that the brake system of the vehicle will be less sensitive for lateral run out of the brake system.

In a method for retracting a piston in a cylinder by using a retraction device mounted to the piston, the steps of applying an actuating force on the piston such that the piston is pushed forwards in the cylinder, thereby exerting a force on an external object and at the same time compressing a resilient element in the retraction device, releasing the actuating force, and allowing the resilient element to expand, thereby retracting the piston into the cylinder with a stroke length corresponding to the deflection length of the resilient element are comprised.

By this first embodiment of the method according to the invention, a method is provided, which allows for an improved and well defined retraction of the piston. The advantage of this is that an enlarged retraction stroke is obtained, which will make e.g. a brake system less sensitive for lateral run out. A further advantage of the method is that is allows larger tolerances of the parts comprised in e.g. a brake system which will allow for a more cost-effective manufacture of the brake system. In an advantageous development of the inventive method, the method comprises the step of compensating for wear in the external object, in which the retraction device is allowed to slide forwards in the cylinder when the required actuation stroke of the piston is greater than the deflection length of the resilient member. This is of advantage in that the wear of an object can be compensated for in an easy and reliable way.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be described in greater detail in the following, with reference to the embodiments that are shown in the attached drawings, in which

Fig. 1 shows a first embodiment of a retraction device according to the invention,

Fig. 2 shows a view of an inactive retraction device according to the invention in a cylinder,

Fig. 3 shows a view of an active retraction device according to the invention in a cylinder,

Fig. 4 shows a second embodiment of a retraction device according to the invention,

Fig. 5 shows a development of the first embodiment of the retraction device according to the invention,

Fig. 6 shows an example of the retraction device according to the invention used for a drum brake, and

Fig. 7 shows an example with two retraction devices according to the invention used in one cylinder. MODES FOR CARRYING OUT THE INVENTION

The embodiments of the invention with further developments described in the following are to be regarded as examples only, and are in no way to limit the scope of the protection provided by the patent claims.

Fig. 1 shows a first embodiment of a retraction device according to the invention. The retraction device is adapted to retract a piston that runs in a cylinder. The piston may be used in different mechanical applications, where it is of advantage to retract a piston with a greater distance than is possible with a retraction based on the use of a deformable piston seal. Suitable applications include disc brake pistons, drum brake pistons, clutch pistons and pistons for other automotive applications, but also include industrial and other machinery applications where a piston will apply a pressure on an object and will thereafter retract from the object such that no contact remains.

The retraction device 1 comprises a piston 2. In the shown embodiment, the piston is provided with an inner cavity 15 at least in a rear end of the piston. The rear end of the piston is the end of the piston that is in contact with the pressure chamber used for the activation. The front end of the piston will thus be the end directed towards the object on which the piston will apply a force or a pressure. The piston may be rigid, hollow or partly hollow. In Fig. 1 , a hollow piston is shown. In Fig. 4, a piston having an intermediate wall 18 is shown. In the example shown in Fig. 1 , the inner cavity 15 in the rear end of the piston is adapted for the mounting of a retraction cartridge 13. The shown piston also comprises an external groove 17 at the front end, which is adapted for a protection sealing commonly used to protect the surfaces of the piston and cylinder from environmental influences.

In the cavity 15 of the piston, the retraction cartridge 13 is mounted. The retraction cartridge comprises a retraction body 3, a retraction member 5 and a resilient element 4. The retraction member 5 of the retraction cartridge is mounted in a fixed way to the piston, in the shown example the retraction member is mounted with a press fit to the inner surface 16 of the piston. The retraction body 3 of the retraction cartridge is movable in the axial direction in the cylinder with respect to the retraction member and the piston. The resilient element will force the retraction member and the retraction body towards each other when the retraction cartridge is in a first, inactive state.

The retraction member 5 is mounted in the cavity of the piston in a fixed way, e.g. by means of a press fit or by means of a joining technique such as gluing, soldering, welding or the like. The type of mounting will depend on the materials used for the piston and the retraction member, and also on the shape of the retraction member. It is also possible to mount the retraction member in an internal groove in the cavity or on a central stud or the like of the piston. It is important that the retraction member cannot move in relation to the piston at any time, not during use and not when the retraction cartridge is inactive.

The retraction member 5 is in a first embodiment of the inventive retraction device made from sheet metal. The retraction member comprises a bottom portion 20 that is perpendicular to the centre axis 19 of the retraction cartridge 13, i.e. the axis x. The bottom portion in the shown embodiment is continuous in order to seal off the interior of the piston. The bottom portion 20 may also comprise one or more cut-outs or holes, to allow for a free flow of the pressurized medium, e.g. a brake fluid. The retraction member further comprises a side portion 21 that is parallel with the centre axis and that is adapted to bear against the inner surface 16 of the piston in a fixed way. The length of the side portion is in one example dependent on the retraction stroke of the retraction cartridge. The retraction member further comprises a top portion 22 that is substantially perpendicular to the side portion. The side of the top portion directed towards the bottom portion 20 constitutes a first contact surface 6 of the retraction cartridge, against which the resilient element 4 bears. The top portion 22 extends towards the axis x such that a sufficiently large contact surface is provided, which will allow the resilient element to bear on it securely.

The retraction body 3 is in this embodiment also made from sheet metal. The retraction body comprises a cylindrical portion that is essentially parallel to the centre axis x, and a rim 23 that extends outwards from the cylindrical portion in the direction towards the top portion 22 of the retraction member and the inner surface of the piston. The rim 23 comprises a second surface 7 against which the resilient element 4 bears, the rim being formed by e.g. cold deformation of an edge of the cylindrical portion at the front-end side. In order to be able to assemble the retraction cartridge, the rim has an outside diameter that is smaller than an inside diameter of the top portion of the retraction body.

A cartridge of the type shown in Fig. 1 is assembled as follows. The resilient element 4 e.g. a spring washer is mounted over the cylindrical portion of the retraction body 3. Next, the rim 23 is formed in e.g. a pressing operation to provide a second axially oriented contact surface 7 for the spring washer. Then, the side portion 21 of the retraction member 3 is placed over the outside diameter of the spring washer and in a final step, the top portion 22 of the retraction member is formed by e.g. pressing part of side portion 21 in a radially inward direction, over the spring washer 4. Thus the first contact surface 6 is created for the spring washer, the first contact surface 6 facing in an opposite axial direction than the second contact surface 7. The final pressing operation locks the spring washer between the first and second contact surface and can also be used to apply a preload on the spring. The resilient element will then urge the first and the second contact surfaces apart in an axial direction when the retraction cartridge is inactive, forcing the retraction member and the retraction body towards each other.

The retraction body further comprises a sliding surface 24 with which the retraction body can slide on the inner surface of the piston or on a lining mounted on the piston. In the shown example, the sliding surface 24 slides on a lining 25 in the form of an elbow shaped washer fixed to the piston, but the sliding surface may also slide directly on the piston, depending on e.g. the design/manufacturing method used. The surfaces of the parts in sliding contact may be treated with a friction reducing coating, to reduce friction and wear. Furthermore, all the sheet metal parts may be treated with a corrosion protecting coating, to protect against corrosion when in contact with an aggressive pressure medium, e.g. a brake fluid.

The retraction body 3 further comprises a holding means 8 that is adapted to engage with the inner surface of the cylinder. The holding means is in the shown example an annular ring made from a friction material. The holding means is fixedly positioned in the retraction body, here in an annular groove 26. The holding means acts with a force towards the cylinder wall to position the piston in the cylinder. The force required to axially move the holding means is higher than the compression force of the resilient element 4. The holding means can nevertheless slide on the cylinder wall when it is exposed to a sufficiently large force, e.g. from a pressurized medium. The force that will press the piston towards an object is larger than the holding/friction force of the holding means. This allows the retraction cartridge to retract the piston when the actuation pressure is released, and will at the same time allow the retraction cartridge to compensate for the wear that may arise in the object that the piston presses against. An O-ring 27 provided in a groove 28 in the cylinder wall suitably functions as a seal to keep the pressurized fluid in the pressure chamber. The seal is preferably positioned at the front end of the cylinder, such that the front end of the piston is supported. This will allow some brake fluid to enter between the piston and the cylinder, which will lubricate the surfaces.

A further advantage of having an additional seal in the form of an O-ring 27 positioned at the front end of the cylinder is that the O-ring will provide an additional stabilisation to the piston. The piston will in this way be supported by the O-ring 27 at the front end of the piston and by the holding means 8 at the rear end of the piston. Due to this play-free suspension, the piston will be less sensitive for vibrations and the so- called knock-back phenomena.

The resilient element 4 is in the shown embodiment a cupped spring washer, also known as a Belleville washer. The advantage of using such a spring as the resilient element is the properties of the spring washer. The free length of the spring washer is relatively small, allowing for a compact retraction cartridge 13. The spring force, especially in relation to the compression stroke is relatively high and is nearly linear with the stroke. Since the compression length of the spring washer corresponds more or less to the free length of the washer, the retraction stroke of the retraction device can be defined by the used spring washer. Depending on the retraction stroke and force required, it is possible to use one or more spring washers stacked on each other as the resilient element. It is also possible to use other components with spring back characteristics, e.g. a wave spring.

In a typical example for a disc brake in a passenger car where an inventive retraction device is used, the retraction of the brake piston and thus the brake pads can be designed to be e.g. 500 microns (micrometers) or more. This is a relatively large retraction compared with the retraction possible with known techniques, which is in the region of 60 - 100 microns. A large retraction stroke makes sure that the brake pads will not be in abrasive contact with the brake disc even when the tolerances of the brake system are larger than normal or because of wear and/or corrosion problems. The stroke length of the retraction device can be set depending on the required tolerances in the manufacturing and service conditions of the complete wheel end system. With a possibility to handle larger tolerances, it is possible to reduce the cost of the components in the brake system, including the wheel end bearing on which the brake disc is mounted. A slightly larger lateral run out of the brake disc will in this case not affect the performance of the brake system, since the brake pad will be retracted from the brake disc. The retraction stroke can be adapted to the tolerances. It is therefore of advantage to design the retraction stroke in line with the allowable maximum lateral run out. A larger retraction stroke length will affect the reaction time of the system and it may therefore be necessary to adapt the actuating system to cope with the larger retraction stroke.

Depending on the system in which the retraction device is to be used, other types of resilient elements are also possible, such as rubber, elastic plastic or other compressible materials. The stroke length may also be up to several millimetres if allowed by the actuating system.

The mode of operation will now be described. In Fig. 2, a retraction device provided in a cylinder 14 is shown. The retraction device is suitable for all different kinds of cylinders. In the shown embodiment, a disc brake cylinder housed in a brake calliper 9 will be used as an example. The piston 2 with the assembled retraction cartridge 13 is thus placed in a brake cylinder 14. The disc brake may be either of the floating brake calliper or the fixed brake calliper type. Both types and their executions are well known to the skilled person and are not described further. The resilient element, here in the shape of a spring washer, retracts the piston into the cylinder so that the spring washer is more or less relaxed. In Fig. 2, the retraction device is in an inactive or released mode.

When hydraulic pressure is applied to the pressure chamber 11 , e.g. by a driver applying the service brakes which causes brake fluid to enter through the inlet opening 12, a force will act in the x-direction on the piston, either directly on the piston or through the retraction member, i.e. the piston will be forced out of the cylinder. This is shown in Fig. 3. The pressure in the pressure chamber will push the piston out of the cylinder and at the same time, the spring washer will be compressed, since the brake pressure required to apply the brakes is much higher than the compression force of the spring washer. The piston will apply a force to the brake pad 29 which in turn will press against the brake disc 34 with the brake pressure ordered by the driver.

If there has been no wear of the brake pad or brake disc since the last actuation of the brake or if the brake pressure is not extremely high, the stroke length y of the piston will be sufficient to apply the brakes when the spring washer is compressed fully. If the stroke length is not enough, i.e. if the required brake pressure is not applied to the brakes when the spring washer is compressed fully, the retraction cartridge will adjust its position so that the piston can apply the required brake pressure. The adjustment of the piston position takes place in the following manner.

The retraction cartridge is held in position in the cylinder by the holding means, in this example a friction ring. The holding force with which the friction ring engages the cylinder wall is higher than the compression force of the resilient element, but lower than the brake pressure applied to the pressure chamber. If the required piston stroke is greater than the compression length of the resilient element, the brake pad will not engage the brake disc with a sufficient force. The brake force will in this case continue to push the piston towards the brake disc, and since the resilient element is fully compressed, it cannot compress anymore. Instead, the brake force will cause the retraction cartridge to follow the piston forward, since the brake force is higher than the holding force. When the piston acts on the brake pad with the required pressure, the retraction device will stop at the new position, and will retract the piston with the retraction stroke when the brake pressure is released. The retraction device is now in a new, adjusted position from which the complete piston stroke can be used.

The holding means can be of different designs. The main purpose of the holding means is to hold the piston/retraction device in position during the compression of the resilient element, and to be able to adjust the position of the retraction cartridge in a forward direction when necessary. It will also be possible to manually push the piston/retraction device back into the cylinder when e.g. a brake pad is replaced. The holding means is in the example described above a friction ring made from a material having a predefined friction coefficient in contact with the material of the cylinder 14, e.g. steel. Such a friction ring material may be rubber or an elastic plastic material, e.g. formed as an O-ring. Such a ring shaped sealing is continuous. It is also possible to use spring loaded holding means that are divided with a slit in order to be resilient. Such a holding means may be a spring steel ring with or without a friction coating. The holding means may have a more or less continuous contact with the cylinder wall or may have a number of slots in the periphery of the holding means.

Since the force of the holding means acting sideways on the cylinder wall is larger than the spring force of the resilient element, the resilient element will first be compressed by the applied force such that the piston can move in the x-direction. In this way, the piston 2 will force the brake pad 29 towards a brake disc 34. This in turn will apply the brake and the brake disc and will slow down the vehicle.

The piston stroke length y is defined by the design of the resilient element, taking into account the distance between the contact surfaces. When the retraction cartridge is fully retracted, the resilient element is almost completely unloaded. It may be of advantage to pre-load the resilient element somewhat. The piston stroke y is preferably larger than the retraction stroke obtained for a conventional retraction design using a deformable seal, where such a retraction stroke is normally in the magnitude of 60-100 micron for a working system. If the piston is e.g. corroded and/or the seal is aged, the retraction stroke of a deformable seal may be lower than this value, or even none. For the inventive retraction device, the piston stroke y is at least 100 microns and can be designed for a magnitude up to 500 microns or more, depending on the wheel-end/brake rotor lateral run out requirements. With such an enlarged piston stroke, the brake pad can be retracted from the brake disc eliminating abrasive contacts even when there is some misalignment, e.g. in the form of a lateral run out of the disc or if the piston will not retract due to increased friction caused by corrosion or contamination. By using a resilient element in the form of a spring washer, an increased retraction force compared with the force of a deformable seal is possible to obtain. It is also easier to design both the required retraction force and the retraction stroke using a separate resilient element.

The material and the surface protection of the retraction cartridge is selected depending on where and with which pressure media the retraction cartridge is to be used. If the retraction cartridge is to be used in a brake system where the pressure media is a hydraulic brake fluid, either the material used should be corrosion resistant or the material should be coated such that the material will not be affected by the brake fluid. If possible, it is often better to use a corrosion resistant material instead of a corrosion protection coating, but material properties, manufacturing methods or cost may also be important for the selection of the material used. In an air actuated brake system, the force on the piston may be applied with a mechanical lever which in turn is activated with air pressure in an air brake cylinder. In such a system, the requirement on the material may be less demanding, e.g. regarding corrosion resistance. Moreover, electro-mechanical actuation can be applied with or without the use of liquid or air as pressure medium, e.g. for a parking brake. Materials used in the retraction cartridge may be ferrous or non-ferrous sheet metal, e.g. stainless steel, coated steel or aluminium. It is also possible to make the retraction body or the retraction member from a plastic material by moulding. Combinations of the above and other manufacturing methods are also possible.

In a second embodiment, shown in Fig. 4, the retraction cartridge comprises an alternative retraction member 5. In this embodiment, the retraction member is a ring shaped washer 10 that is inserted in a groove 30 in the inner side wall 16 of the piston. The ring washer 10 is provided with the first contact surface 6 for the resilient element. In this embodiment, the retraction member 5 in the form of the ring washer 10 is open and does not close the inside of the piston. The piston thus comprises an intermediate wall 18 that closes the pressure chamber. In the shown embodiment, an example is shown where the sliding surface 24 of the retraction body slides directly on the inner surface of the piston.

In a further development of the retraction device, shown in Fig. 5, a mechanical lever 31 acts on the piston 2 via the retraction member, but the lever may also act directly on an intermediate wall in the piston. The mechanical lever is powered by an external force and is used to actuate the brake. Such a lever can be used in air actuated brakes, where the energy of the compressed air is converted into mechanical movement by a brake cylinder.

A mechanical lever may also be used in combination with a hydraulic brake pressure system. When used in combination, the hydraulic pressure can be used for the service brake of a vehicle and the mechanical lever can be used for the parking brake. The parking brake is normally operated manually to actuate the parking brake in a mechanical way with a wire or the like. There are also electrically operated parking brakes, in which a power source, such as a motor, will actuate a parking brake lever for the parking brake. It is vital that the parking brake system uses a different actuation means than the service brake. For some brake systems, the parking brake should also function as an emergency brake, providing a continuous application of the brake. When the mechanical lever arm is used in a hydraulic combination, the lever 31 is sealed at the entrance opening in the housing 9 with an appropriate sealing ring 32, e.g. an O- ring, positioned in a groove 33.

In a third embodiment, shown in Fig. 6, the inventive retraction device is used for a drum brake. In a drum brake, two half circular brake shoes are pressed towards the inner surface of the brake drum encompassing the brake shoes and the actuating system. Normally, a drum brake is equipped with an actuating brake cylinder attached to the brake shoes at one end of the brake shoes, and an adjuster mechanism attached to the brake shoes at the other end. In the shown embodiment, two brake cylinders each comprising a retraction cartridge are used.

A first brake cylinder 40 is attached to the upper end of a second brake shoe 43 and the brake piston 44 of the first brake cylinder is attached to the upper end of a first brake shoe 42. A second brake cylinder 41 is attached to the lower end of the first brake shoe 42 and the brake piston 45 of the second brake cylinder is attached to the second brake shoe 43. In an advantageous example, both brake cylinders can axially float in the brake housing, suspended with one or more O-rings 39. The O-rings are positioned in a fixed structure of the drum brake. The holding force of the O-rings 39 is preferably larger than the force required to actuate the brake piston. In this way, each brake cylinder may move in a longitudinal direction. This will allow the brake cylinders to apply a symmetric force to the brake shoes. When the brake pressure is applied to the pressure chamber of the first brake cylinder 40, the piston 44 will be forced out of the cylinder in order to apply a force to the first brake shoe 42. At the same time, a force acting towards the second brake shoe 43 through the brake cylinder applies a force to the second brake shoe 43. In this way, the force will act in both directions, so that the force acting on the brake shoes is equal. The same will apply for the second brake cylinder 41 , so that the brake pressure applied to the second brake cylinder is also applied to the brake shoes in a symmetric way. This is one way of achieving full floating cylinders.

When the brake pressure is released, the first piston will retract into the first cylinder, thereby retracting the first brake shoe 42 from the brake drum. Since the second cylinder 41 attached to the lower end of the brake shoe 42 will not retract, the first brake shoe 42 will rotate around the first attachment point 46 when the brake shoe is retracted by the first piston 44. Depending on the retraction stroke of the retraction device attached to the piston, this rotation of the brake shoe will pull at least most of the brake shoe surface from the brake drum. There will thus be no force acting on the brake shoe when the brake pressure is released. The same situation applies to the second brake cylinder, where the second brake shoe rotates around the second attachment point 47 when the brake shoe is retracted by the second piston 45.

The brake cylinders may slide in the brake housing 38 in a somewhat restricted way due to the O-rings 39, so that they are held in place when the brake is released. This will prevent noise to emerge and will prevent excessive wear of the cylinders due to vibrations when used on a vehicle. It is also possible to include a mechanical parking brake actuator in the brake cylinders. The parking brake actuator may comprise a plurality of teeth that will lock the brake cylinder in an actuated state, thereby preserving a brake force acting on the brake pad.

In a further development of the inventive retraction device, two retraction devices are used in one cylinder, as shown in Fig. 7. In this example, a cylinder 50 comprises a first piston 51 having a retraction cartridge and a second piston 52 having a retraction cartridge. When pressure is applied to the central pressure chamber 49 in the cylinder, both pistons are pushed out of the cylinder until each piston applies a required force on an object. In this way, each piston will also adjust to the wear of the object on which each piston applies the force. Each piston will also retract with the predefined retraction stroke when the pressure is released. Such a double piston cylinder may e.g. be used for drum brakes, where two brake shoes are to be actuated at the same time. In a drum brake, it is possible to use two double cylinders as described fixedly attached to the brake housing, or one double cylinder in combination with a known adjuster mechanism.

In all the described embodiments and developments, it is advantageous to integrate a wear sensor in the retraction device. The wear sensor is adapted to measure the wear of the object on which the piston acts, e.g. the brake pad of a disc brake system. The sensor will measure the movement of the retraction body in the cylinder. This movement can be measured by e.g. measuring the distance between the rear of the pressure chamber and the retraction body. The measured distance gives a measure of the wear of the brake pad. The wear sensor is preferably connected to a control unit of the vehicle, which can give a warning when the brake pad is worn. The control unit can also use the measured wear signal to predict when a brake pad is to be replaced. A wear sensor may be integrated in the pressure chamber. It is also possible to let an extension of the retraction body extend out of the housing through a sealed opening. The extension is then coupled to the wear sensor such that the travel of the retraction body can be measured. Depending on the pressure media used in the pressure chamber and the type of wear sensor used, it may be easier to measure the wear outside of the pressure chamber. A wear sensor may be of the resistive, magnetic, capacitive or optical type. The wear sensor may measure the wear continuously or may measure a number of discrete positions. To monitor the brake force, sensors can also be integrated e.g. using load sensors or pressure sensors in combination with wired or wireless data transfer to a data logger. The retraction device may be assembled in a cylinder at a manufacturing plant. The complete cylinder is then delivered as a system component to a producer of vehicles, machinery or the like. It is also possible to adapt the properties of the retraction device such that they correspond to existing pistons on the market. In this way, a retraction device can be used as a spare part for an existing piston. Since the retraction device provides an improved retraction stroke, the retraction device can be used to replace an existing piston in a system which, e.g. due to age or wear, displays an enlarged lateral run out of e.g. a disc brake.

A further advantage of the inventive retraction device is that the improved retraction stroke will ensure that e.g. a brake pad is completely retracted from a brake disc when the brake is released. This in turn will reduce wear of the brake components and will also reduce the friction between the brake pad and the brake disc, which will reduce the energy consumption of the vehicle.

The described retraction device may be used in different types of applications, using one or more retraction devices at the same time. Above, a single piston use is described for a floating brake calliper with a fixed rotor. Two floating cylinders with a single piston are also described. Another combination is to use two single pistons in fixed callipers acting on a floating rotor from opposite sides of the rotor. It is also possible to use multiple pistons acting on the same object in order to enlarge the pressure surface. One example of this is to use two or more pistons acting on the same brake pad. Since the retraction stroke is relatively large and well defined, such a solution will not have any problems with the brake pad being partly engaged with the brake rotor when the brake is released.

The invention is not to be regarded as being limited to the embodiments described above, a number of additional variants and modifications being possible within the scope of the subsequent patent claims. REFERENCE SIGNS

1 : Retraction device

2: Piston

3: Retraction body

4: Resilient element

5: Retraction member

6: First contact surface

7: Second contact surface

8: Holding means

9: Housing

10: Ring shaped washer

11 : Pressure chamber

12: Inlet opening

13: Piston retraction cartridge

14: Cylinder

15: Inner cavity

16: Inner surface

17: Groove

18: Intermediate wall

19: Centre axis

20: Bottom portion

21 : Side portion

22: Top portion

23: Rim

24: Sliding surface

25: Lining

26: Annular groove

27: O-ring

28: Groove

29: Brake pad

30: Groove

31 : Lever

32: Sealing

33: Groove

34: Brake disc 38: Brake housing

39: O-rings

40: First brake cylinder

41 : Second brake cylinder

42: First brake shoe

43: Second brake shoe

44: First brake piston

45: Second brake piston

46: First attachment point

47: Second attachment point

49: Pressure chamber

50: Cylinder

51 : First piston

52: Second piston

Claims

1. Retraction device for retracting a piston in a cylinder when a force acting on the piston is released, comprising a piston (2), a retraction member (5) having a first contact surface (6), a retraction body (3) having a second contact surface (7), and a resilient element (4) which bears on the first contact surface (6) and the second contact surface (7), where the retraction member (5) is fixedly attached to the piston (2), where the retraction body (3) further comprises a friction member (8) adapted to bear on an inner cylinder wall with a force that is greater than a maximum spring force of the resilient element (4), characterized in that the deflection length of the resilient element (4) defines the stroke length of the retraction device.

2. Retraction device according to claim 1, characterized in that the resilient element (4) is a spring washer.

3. Retraction device according to claim 2, characterized in that the resilient element comprises a plurality of spring washers.

4. Retraction device according to any of the preceding claims, characterized in that the retraction member (5) is mounted to an inner surface (16) of a cavity (15) in the piston (2).

5. Retraction device according to any of the preceding claims, characterized in that the retraction member (5) is mounted to the piston by means of a press fit.

6. Retraction device according to any of the preceding claims, characterized in that the retraction member (5) is disc shaped and has a solid bottom portion (20).

7. Retraction device according to claim 4, characterized in that the retraction member (5) is ring shaped and is mounted in a groove (30) in the piston.

8. Retraction device according to any of the preceding claims, characterized in that the holding means (8) is an annular friction ring.

9. Retraction device according to any of the preceding claims, characterized in that the retraction body (3) and/or the retraction member (5) is manufactured from sheet metal.

10. Retraction device according to any of the preceding claims, characterized in that the stroke length of the retraction device is at least 100 micrometers.

11. Disc brake, comprising a calliper housing (9) and at least one retraction device according to any of claims 1 to 10.

12. Disc brake according to claim 11, characterized in that the retraction device is adapted to be actuated by a pressured medium.

13.Disc brake according to claim 12, characterized in that the disc brake further comprises mechanical or electromechanical actuation using a mechanical lever (31).

14. Drum brake comprising at least one retraction device according to any of claims 1 to 10.

15. Vehicle, comprising at least one brake according to any of claims 10to 14.

16. Method for retracting a piston in a cylinder by using a retraction device mounted to the piston, comprising the steps of: applying an actuating force on the piston such that the piston is pushed forwards in the cylinder, thereby exerting a force on an external object and at the same time compressing a resilient element in the retraction device,

- releasing the actuating force, and

allowing the resilient element to expand, thereby retracting the piston into the cylinder with a stroke length corresponding to the deflection length of the resilient element.

17. Method according to claim 16, further comprising a step of compensating for wear in the external object, in which the retraction device is allowed to slide forwards in the cylinder when the required actuation stroke of the piston is greater than the deflection length of the resilient member.