GB2588787A

GB2588787A - Static brake assembly

Info

Publication number: GB2588787A
Application number: GB1916153.8A
Authority: GB
Inventors: Turner Jim
Original assignee: Safran Electrical and Power SAS
Current assignee: Safran Electrical and Power SAS
Priority date: 2019-11-06
Filing date: 2019-11-06
Publication date: 2021-05-12
Also published as: GB201916153D0

Abstract

An integrated brake for a motor has a rotor disc 100 fixedly mounted to a motor shaft 20 at an inner region 150, and an outer region 160 deflectable relative to the inner region by a moveable braking member 300 into contact with a static braking member 200. Actuation may be by a solenoid. The rotor disc 100 may be cast/cut/rolled/stamped metal (e.g. stainless steel or Aluminium), and have apertures (163, fig 2) for flexibility. The rotor disc 100 can be attached at a shoulder of the motor shaft 20 by spot welding, a threaded connection, or an interference fit, and have attachment means 60 with two pieces 61,62. The motor can be used in aircraft landing gear or brake system.

Description

Static Brake Assembly The invention relates to a brake assembly for a motor of an aircraft, in particular a static brake assembly for a motor of an aircraft.

Background

Motors can be used in aircraft for various functions. Motors can act as primary actuators to activate a part of an aeroplane, or may act as a secondary interlocking device to lock a primary actuator in a chosen position. Specific uses of motors as primary actuators are described in W02019097137, which describes the use of a motor arrangement to drive one or more wheels of an aircraft landing gear. Another application of a motor in an aircraft is in a brake system of an aeroplane. The motor itself may have a brake assembly to prevent rotation of a rotor of the motor.

There is a need for improvements in aircraft brake assemblies for motors of aircraft.

Summary of the Invention

The inventor has identified that improvements can be made to known aircraft brake assemblies for motors of aircraft. These improvements may be best understood with reference to a known static braking system, in which there is a motor shaft, a rotor disc fixed to the motor shaft, and to each side of the rotor disc there are braking members. One of these braking members is fixed in position, and the other is moveable towards the fixed braking member. The rotor disc is fixed to the motor shaft by a spline attachment, such that rotation of the motor shaft causes rotation of the rotor disc (i.e. it has torque resistance capability), while the rotor disc remains moveable in an axial direction of the motor shaft (i.e. it has axial translation capability). When the brake is applied in this known system, the moveable braking member moves towards the rotor disc, and pushes the rotor disc along the spline connection towards and into contact with the static braking member.

In this state the static and moveable braking members act as a clamp to fix the rotor disc between the static and moveable braking members, to prevent the rotor disc from rotating. In a motor, this clamping force on the rotor disc would prevent the motor from being operated.

The inventor has identified several issues with this known system, which include the complexity of component parts, and the associated complexity of their manufacture. Complexity of the component parts and their manufacturing processes also has associated negative effects on the cost of production, and the degree of accuracy to which each part must be made in order for the brake to function well.

Use of spline attachments are well known as a connection which can restrict relative rotational movement between components, while allowing axial relative movement between the same components. Using a spline attachment in this context has been a "go-to" solution when this type of connection is required. However, the inventor has identified that replacing a spline attachment with the creation of the inventor as described herein, provides a simpler, cheaper, and therefore improved aircraft brake assembly for a motor of an aircraft.

According to the invention, there is provided a motor having an integrated brake assembly, the motor comprising: a motor frame; a motor shaft which is rotatably mounted relative to the frame; a rotor disc which is fixedly mounted to the motor shaft; a static braking member which is mounted in fixed relation to the frame, and a moveable braking member which is mounted in moveable relation to the static braking member; wherein the rotor disc comprises an inner region which is fixedly attached to the motor shaft, and an outer region which is configured to be deflectable relative to the inner region, such that by movement of the moveable braking member into contact with the outer region of the rotor disc, the outer region is deflected towards and contacts the static braking member so as to fix the rotational position of the rotor disc relative to the frame.

The combination of a rotor disc which has a deflectable outer portion and a fixed attachment of the rotor disc to the motor shaft achieves the same functionality as the known rotor disc and spline attachment, but without the associated complexity. Contrary to the known spline attachment, provision of a fixed attachment provides an assembly which is quicker, simpler, and easier to manufacture, while remaining reliable in use. The motor described herein may be particularly advantageous when the static torque capacity of the brake assembly is only required to be in the order of 1 Nm.

As a skilled person will understand from the teaching provided herein, the inner region of the rotor disc is a region which may be configured or adapted to be fixedly attached to the motor shaft, while the outer region is a region which may be configured or adapted to freely deflect relative to the inner region, for example in an axial direction of the motor shaft, and/or toward the static braking member. The inner region may be a radially inner region, and/or the outer region may be a radially outer region of the rotor disc. The inner region may be an innermost region, and/or the outer region may be an outermost region. The inner region may be an inner end or edge, and the outer region may be an outer end or edge. The outer region may be disposed further away from the motor shaft than the inner region. There may be provided a bridging region, which may be disposed between the inner and outer regions.

The dimensions of the inner, outer and bridging regions may depend on the dimensions of the overall motor and integrated brake assembly, and in particular the dimensions and/or type of any attachment means provided to fixedly attach the rotor disc to the motor shaft.

For example, the attachment means may comprise a threaded connection and/or an interference fit and/or a clamping means, and/or a weld, such as a spot weld, and/or a press fit. The inner region of the rotor disc may be, with reference to these attachment examples, a portion of the rotor disc which is prevented from displacing relative to the shaft by the attachment means. With reference to these examples, bridging and outer regions of the rotor disc may be a portion of the rotor disc which is not completely prevented from deflecting by the attachment means. The inner region may be larger if a clamping means is used than if the rotor disc is only attached to the motor shaft by means of a welded joint. Preferably, the rotor disc is fixedly attached to the motor shaft by a clamping means, such as the clamping means described in relation to the figures. Use of a clamping means in combination with a spot weld has the advantage of easy assembly, and an associated reduction in cost and manufacturing errors in the brake assembly.

There may be provided a bridging region, which may be disposed between the inner and outer regions. In terms of the boundaries for the inner and outer regions, the inner region of the rotor disc may extend to an innermost line of the rotor disc, and/or extend up to the bridging region. The bridging region may extend from the inner region to the outer region. The rotor disc may be composed of the inner region, bridging region, and outer region. The inner region may be fixedly attached to the motor shaft by a clamping means.

The clamping means may be configured to substantially or wholly cover the inner region of the rotor disc. The inner region may cover less than 20%, preferably less than 25%, further preferably less than 10% of the radius of the rotor disc. The outer region may cover at least 50%, further preferably at least 60°/o of the radius of the rotor disc. The rotor disc may be circularly shaped, and the radius of the rotor disc may be measured from a central point of the rotor disc to an outermost edge of the rotor disc.

The motor may be configured such that the outer region of the rotor disc can deflect relative to the inner region, toward the static braking member, by a maximum distance in the range of 0.5 mm to 10 mm. More specifically, this deflection may be by a maximum distance in the range of 1 mm to 2 mm. This has the advantage of providing a reliable brake assembly. Deflection by this maximum distance may be deflection during which the rotor disc does not undergo plastic deformation, or undergoes an amount of plastic deformation which would be negligible, or tolerable, over the expected lifetime of the rotor disc. Deflection by this maximum distance may be deflection to an extent that the rotor disc will not break during its expected lifetime, and/or such that work hardening will not substantially occur, or occur at all.

The rotor disc may comprise a material which does not undergo plastic deformation during deflection by the moveable braking member. This has the advantage of providing a particularly effective brake assembly, in which the rotor disc may return to its original shape and position once the moveable braking member is brought out of contact therewith. This also has the advantage of providing a rotor disc which does not undergo substantial, and preferably it does not undergo any, work hardening in use.

The rotor disc may comprise one or more apertures. This has the advantage of providing a particularly effective brake assembly, in which the flexibility of the rotor disc in the axial direction is increased. The one or more apertures may be provided between the inner and outer regions of the rotor disc. The one or more apertures may be provided in a bridging region of the rotor disc. The one or more apertures may be arranged so that the rotor disc has rotational symmetry. This has the advantage of providing a rotor disc which is balanced when rotated, and as such provides a reliable component with predictable performance.

The rotor disc may be substantially composed of a metal and/or alloy. This has the advantage of providing a particularly effective brake assembly, in which the rotor disc has suitable mechanical properties for its intended use under repeated applications of the brake. This also has the advantage of improved ease of manufacture, in which the rotor disc can be easily cast and/or cut and/or rolled and/or stamped to its desired shape. The rotor disc may be substantially composed of steel, preferably stainless steel and/or aluminium. This has the advantage of providing a particularly effective brake assembly, in which the rotor disc has particularly suitable material properties.

The rotor disc may comprise a microstructure indicative of having been stamped from a sheet of material. As a skilled person will appreciate, the rotor disc's microstructure may be identified using visual tests, specifically taking a cross-section, grinding, polishing and etching, magnifying and examining the cross-section of the material. Use of stamping to form the rotor disc has the advantage of ease of manufacture, as the required thickness may be realised without the use of machining or other complex and/or expensive processes.

The rotor disc may comprise a material configured for corrosion resistance. This has the advantage of providing a particularly effective brake assembly, in which the rotor disc can endure conditions within the brake assembly.

The motor may further comprise an actuating means. The actuating means may be configured to move the moveable braking member into contact with the rotor disc. The actuating means may also be configured to move the rotor disc into contact with the static braking member. This may be by causing an outer region of the rotor disc to deflect into contact with the static braking member.

The motor may further comprise a mounting means. The mounting means may be fixedly attached to the motor shaft. The mounting means may be configured to grip and/or clamp the inner region of the rotor disc so as to fixedly attach the inner region of the rotor disc relative to the motor shaft. The motor shaft may comprise at least one shoulder. The mounting means may be configured to abut against the at least one shoulder. The rotor disc and/or mounting means may be attached to the motor shaft. The attachment may be by means of spot welding and/or an interference fit and/or a threadedly connected joint. This has the advantage of providing a particularly effective brake assembly, in which the rotor disc is fixed relative to the motor shaft in a durable manner.

The rotor disc may have a thickness in an axial direction in the range of 0.3 to 0.7 mm.

This has the advantage of providing a robust rotor disc which is suitable for repeated applications of the brake. Preferably, the rotor disc has a thickness in an axial direction of 0.5 mm. The rotor disc may have a stiffness sufficient to resist rotational torque in use, while remaining flexible in an axial direction.

The rotor disc may substantially comprise a material which undergoes only elastic deformation during deflection by the moveable braking member. This has the advantage of providing a particularly effective brake assembly, in which the rotor disc may return to its original shape and position once the moveable braking member is brought out of contact therewith.

The static braking member and/or the moveable braking member may have a contact surface for contacting the rotor disc. The actuating means may comprise a spring.

The motor may comprise a releasing means. The releasing means may be configured to move the moveable braking member out of contact with the rotor disc. This movement may allow the outer region of the rotor disc to deflect away from the static braking member. The releasing means may comprise a solenoid.

The rotor disc may comprise and/or be composed of a homogenous material. This has the advantage of ease of manufacture and allows the performance of the brake assembly to be predicted reliably.

Brief Description of the Drawings

By way of example only, the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a cross-section through a motor with an integrated brake assembly; and Figure 2 is an enlarged perspective view of a part of the integrated brake assembly of figure 1.

A typical static brake includes a solenoid assembly, which attracts an armature to disengage the brake when energised. This is referred to as a "power off" brake, as it is engaged when power is not provided, and when there is no current passed through the solenoid. When the solenoid is not energised, springs in the assembly push an armature into contact with a rotatable disc, which keeps the disc stationary. The motor 1 of the present invention may be a "power off" brake.

With reference to the figures, there is provided a motor 1 having an integrated brake assembly 2. The motor 1 comprises: a motor frame 10; a motor shaft 20; a rotor disc 100; a static braking member 200; and a moveable braking member 300.

Since a skilled person will appreciate how a motor functions, only the components of the motor which help to explain the invention are elaborated on herein.

The motor shaft 20 is rotatably mounted relative to the frame 10. The motor shaft 20 may have a motor shaft axis, which may be a longitudinal, and/or central axis 22 about which the motor shaft 20 is configured to rotate. The motor shaft 20 may be configured to transmit rotational motion to one or more driven components.

The rotor disc 100 is fixedly attached to the motor shaft 20. The rotor disc 100 comprises an inner region 150 which is fixedly attached to the motor shaft 20, and an outer region 160 which is configured to be deflectable relative to the inner region 150, such that by movement of the moveable braking member 300 into contact with the outer region 160 of the rotor disc 100, the outer region 160 is deflected towards and contacts the static braking member 200 so as to fix the rotational position of the rotor disc 100 relative to the motor frame 10. This deflection may be caused by the moveable braking member 300 directly or indirectly contacting the rotor disc 100, and exerting a force on the outer region 160 of the rotor disc 100. The outer region 160 may be disposed further away from the motor shaft 20 than the inner region 150.

The rotor disc 100 may comprise a braking surface which is configured to be engaged by the fixed and the moveable braking members 200, 300, which in turn brakes the motor shaft 20. The rotor disc 100 may be provided as an extension of the motor shaft 20, so that the motor shaft 20 and the rotor disc 100 are an integral piece, or may be provided as a separate piece of material which is attachable to the motor shaft 20. The rotor disc 100 may comprise a material which does not undergo plastic deformation during deflection by the moveable braking member. The rotor disc 100 may have a microstructure indicative of having been stamped from a sheet of material. The rotor disc 100 may comprise a material configured for corrosion resistance. The rotor disc 100 may be substantially composed of a metal and/or alloy.

The rotor disc 100 may be one or more of: disc shaped; planar; circular; symmetrical about the central axis 22 of the motor shaft 20. The rotor disc 100 may comprise one or more apertures 163 between its inner 150 and outer 160 regions. The one or more apertures 163 may be arranged so that the rotor disc 100 has rotational symmetry, and/or may be identical to each other in shape, size and/or radial position. The one or more apertures 163 may be one or more of: square shaped, circular, oval, rectangular and tapered. The one or more apertures 163 may have a width in the plane of the rotor disc which is larger at or proximate to the outer region 160 of the rotor disc 100 than the width in the plane of the rotor disc 100 at or proximate to the inner region 150 of the rotor disc 100. The one or more apertures may (each) have an outer end 165 and an inner end 166. The or each aperture 163 may be substantially circularly shaped at its outer end 165.

The or each aperture 163 may be shaped so as to align with the shape of the motor shaft and/or the attachment means 60 at the inner end 166. The rotor disc 100 may comprise a microstructure indicative of having one or more apertures 163 pierced or punched from a sheet of material.

The motor 1 may be configured such that the outer region 160 of the rotor disc 100 can deflect relative to the inner region 150 by a maximum distance in the range of 0.5 mm to 10 mm. The motor 1 may be configured such that the outer region 160 of the rotor disc 100 can deflect relative to the inner region 150 by a maximum distance in the range of 1 mm to 2 mm. This deflection of the outer region 160 may be measured at an outermost point of the outer region, or in line with a point at which the rotor disc 100 contacts the static braking member 200. The rotor disc 100 may have a thickness in an axial direction in the range of 0.3 to 0.7 mm.

The static braking member 200 is mounted in fixed relation to the frame 10. The static braking member 200 may comprise a contact region 210 configured to contact the rotor disc 100 when in a braking condition.

The moveable braking member 300 is mounted in moveable relation to the static braking member 200. The moveable braking member 300 may comprise a contact region 310 configured to contact the rotor disc 100 when in a braking condition. The moveable braking member 300 may have a main part and the contact region 310 may extend from the main part. The moveable braking member 300 may be configured to be moveable in a direction substantially or wholly parallel to a central axis 22 of the motor shaft 20.

There are differences between the performance requirements of static brake assemblies and dynamic brake assemblies and the inventor has identified how to use this to advantageously create more weight-and cost-efficient braking arrangements for static brake assemblies. Firstly, the materials used in static brake assemblies need not be as wear resistant as those used in dynamic brake assemblies. In particular, the static braking member 200 and/or the moveable braking member 300 need not be as wear resistant as corresponding components in a conventional dynamic brake. Secondly, in a static brake, less heat is generated through friction than in a dynamic brake, so it is possible to use less heat-resistant materials, and materials having a lower melting point. The static braking member 200 and/or the moveable braking member 300 may therefore comprise materials having less heat resistant materials, such as materials having a lower melting-point than corresponding components in a conventional dynamic brake arrangement.

The frame 10 may be configured to act as a housing for at least some of the components of the integrated brake assembly 2. The frame 10 may comprise or be composed of a durable and rigid material such as a suitable metal or alloy.

The motor 1 may also comprise: a rotatable attachment means 30, an actuation means 40, a biasing means 50, and/or a rotor disc attachment means 60.

The rotatable attachment means 30 may be configured to enable rotational movement of the motor shaft 20 relative to the frame 10. The rotatable attachment means 30 may be provided around the motor shaft 20. The rotatable attachment means 30 may be a bearing, such as a rolling element bearing.

The biasing means 50 may be a biasing member, and may be configured to bias the moveable braking member 300 away from and/or out of contact with the rotor disc 100.

The biasing means 50 may be configured to maintain the moveable braking member 300 out of contact with the rotor disc 100 upon actuation. The biasing means 50 may be a spring. Actuation of the biasing means 50 may involve permitting the spring to move into an elastically relaxed, wholly or substantially un-stressed, state.

The actuation means 40 may be configured to move the moveable braking member 300 into contact with the rotor disc 100 upon actuation. The actuation means 40 may be configured to move the moveable braking member 300 into contact with the rotor disc 100, and the rotor disc 100 into contact with the static braking member 200, by causing an outer region 160 of the rotor disc 100 to deflect into contact with the static braking member 200. The actuation means 40 may be configured to move the moveable braking member 300 by overcoming the biasing force exerted by the biasing means 50. The actuation means 40 may be a solenoid, although a skilled person will appreciate that any suitable actuation means may be used. Provision of a biasing means 50 and an actuation means 40 as described herein and shown in the figures can provide a "power off" brake.

The rotor disc attachment means 60 may be configured to fixedly attach the rotor disc 100 to the motor shaft 20, which may be by gripping or clamping the rotor disc 100. The rotor disc attachment means 60 may provide a mechanical attachment between the rotor disc 100 and the motor shaft 20. The attachment means 60 may be attached to the motor shaft 20 by an interference fit. The rotor disc attachment means 60 may comprise a first piece 61 and a second piece 62. The first piece 61 may be configured for attachment to the motor shaft 20, for example by being fixed or fixable to the motor shaft 20 by means of an interference fit, and/or any other suitable attachment. The motor shaft 20 may comprise at least one shoulder. The first piece 61 may be configured to abut against the at least one shoulder. The first and/or second piece 61, 62 may be cylindrically shaped, and/or define a central aperture in which the motor shaft 20 is disposed. The first piece 61 may have a substantially L-shaped cross-section in a radial direction of the motor shaft 20, and/or the second piece 62 may have a substantially square or rectangular shaped cross-section. The second piece 62 and/or the inner region 150 of the rotor disc 100 may be configured to fit into the first piece 61. The second piece 62 may be configured for attachment to the first piece 62, to clamp an inner region 150 of the rotor disc 100 between the first and second pieces 61, 62. This attachment of the first and second pieces 61, 62 to each other may be achieved by use of a threaded portion and a nut. A skilled person will appreciate that although one possible attachment means is described here and shown in figure 1, various possible attachment means could be used with the present invention.

With reference to figure 1, when the motor 1 is in operation, the motor shaft 20 rotates about the motor shaft central axis 22. Given that the rotor disc 100 is fixedly attached to the motor shaft 20, the rotor disc 100 rotates along with the motor shaft 20 about the motor shaft central axis 22. The motor shaft 20 may be brought to a stop by removing any actuating force causing it to rotate, and/or applying a dynamic braking means (not shown). The dynamic braking means may be friction-based, or may be implemented by electronic motor control to slow the rotation of the motor.

When the motor shaft 20 and rotor disc 100 are in a stationary position relative to the frame 10, the integrated brake assembly 2 may be applied. The integrated brake assembly 2 may be applied by actuating the actuation means 40. Actuation of the actuation means may cause the moveable braking member 300 to move towards the rotor disc 100, specifically in a substantially or wholly axial direction, which may be parallel to or aligned with the central axis 22 of the motor shaft 20, towards an outer region 160 of the rotor disc 100, and towards the static braking member 200. The actuation means 40 may continue to move the moveable braking member 300 towards the rotor disc 100 until an outermost part of the contact region 310 is in contact with the rotor disc 100. The actuation means 40 may then continue to move the moveable braking member 300 towards the rotor disc 100, deflecting the outer region 160 of the rotor disc 100 relative to the inner region 150, until a side of the rotor disc 100 closest to the static braking member 200 contacts an outermost part of the contact region 210 of the static braking member 200.

In this position, referred to as the static braking position, the rotor disc 100, and as a result the motor shaft 20, are held in place between the braking members 200, 300, with the rotor disc 100 in a deflected, and elastically stressed, position. The static braking position of the integrated brake assembly may be facilitated by the mechanical properties, material properties, and shape and configuration of the rotor disc 100.

The or each contact region 210, 310 may be disposed at or proximate the outer region of the rotor disc 100. A skilled person will appreciate that the or each contact region 210, 310 may equally be disposed at or proximate the inner region 150 of the rotor disc 100. Locating the or each contact region 210, 310 at or proximate the outer region 160 of the rotor disc 100 has the advantage of providing a contact region 210, 310 at a location where movement of the rotor disc 100 is relatively fast, and as such, torque exerted on the rotor disc 100 by the or each contact region 210, 310 may be maximised. The or each contact region 210, 310 may be disposed proximate an outer half, preferably an outer third, preferably an outer quarter of the radius of the rotor disc 100.

As a skilled person will appreciate, a feature described herein in relation to only one of the braking members 200, 300, may apply to the other braking member 200, 300, or to both braking members 200, 300.

Features of the present invention are defined in the appended claims. While particular combinations of features have been presented in the claims, it will be appreciated that other combinations, such as those provided above, may be used.

The above examples describe one way of implementing the present invention. It will be appreciated that modifications of the features of the above examples are possible within the scope of the independent claims and that any and all compatible features of any embodiments described separately above, can be combined within a single embodiment of a device in accordance with the invention.

Claims

Claims 1. A motor having an integrated brake assembly, the motor comprising: a motor frame; a motor shaft which is rotatably mounted relative to the frame; a rotor disc which is fixedly mounted to the motor shaft; a static braking member is mounted in fixed relation to the frame, and a moveable braking member which is mounted in moveable relation to the static braking member; wherein the rotor disc comprises an inner region which is fixedly attached to the motor shaft, and an outer region which is configured to be deflectable relative to the inner region, such that by movement of the moveable braking member into contact with the outer region of the rotor disc, the outer region is deflected towards and contacts the static braking member so as to fix the rotational position of the rotor disc relative to the frame.
2. A motor according to claim 1, wherein the motor is configured such that the outer region of the rotor disc can deflect relative to the inner region, toward the static braking member, by a maximum distance in the range of 0.5 mm to 10 mm.
3. A motor according to claim 1 or claim 2, wherein the motor is configured such that the outer region of the rotor disc can deflect relative to the inner region, toward the static braking member, by a maximum distance in the range of 1 mm to 2 mm.
4. A motor according to any of the preceding claims, wherein the rotor disc comprises a material which does not undergo plastic deformation during deflection by the moveable braking member.
5. A motor according to any of the preceding claims, wherein the rotor disc comprises one or more apertures between its inner and outer regions.
6. A motor according to claim 5, wherein the one or more apertures are arranged so that the rotor disc has rotational symmetry.
7. A motor according to any of the preceding claims, wherein the rotor disc is substantially composed of a metal and/or alloy.
8. A motor according to any of the preceding claims, wherein the rotor disc comprises a microstructure indicative of having been stamped from a sheet of material.
9. A motor according to any of the preceding claims, wherein the rotor disc comprises a material configured for corrosion resistance.
10. A motor according to any of the preceding claims, further comprising an actuating means configured to move the moveable braking member into contact with the rotor disc, and the rotor disc into contact with the static braking member, by causing an outer region of the rotor disc to deflect into contact with the static braking member.
11. A motor according to any of the preceding claims, further comprising a mounting means which is fixedly attached to the motor shaft and configured to grip the inner region of the rotor disc so as to fixedly attach the inner region of the rotor disc relative to the motor shaft.
12. A motor according to claim 11, wherein the motor shaft comprises at least one shoulder, and the mounting means is configured to abut against the at least one shoulder.
13. A motor according to claim 11 or claim 12, wherein the rotor disc and/or mounting means are attached to the motor shaft by means of spot welding and/or an interference fit and/or a threadedly connected joint and/or a press fit.
14. A motor according to any of the preceding claims, wherein the rotor disc has a thickness in an axial direction in the range of 0.3 to 0.7 mm.