Improvements in brakes

Abstract

A brake caliper (100) for a vehicle, providing a novel piston bore geometry, is herein disclosed. The brake caliper (100) comprises a bore (130) for housing a caliper piston (120), the bore (130) having a central axis along its length and having a depth defined from an opening of the bore (130). The bore (130) comprises a first portion (210) adjacent the opening of the bore (130), wherein the diameter of the first portion (210) decreases with depth, a second portion (220) at a greater depth than the first portion (210), wherein the diameter of the second portion (220) increases with depth, and a seal groove (240) located between the first portion (210) and the second portion (220).

Description

[0001] Improvements in Brakes

[0002] The present invention relates to improvements in brakes, and particularly to improvements in braking systems for motor vehicles, and particularly electric vehicles.

[0003] Background

[0004] Modern cars typically have equipped caliper brakes. The brakes are activated by a brake pedal. Such brakes are known. They comprise pairs of pistons that, under braking, are forced towards a brake disc under pressure from brake fluid. A braking action is concluded when the brake pedal is released. Pressure seals, typically made of an elastic material, within the caliper are used to return the pistons back to their original position.

[0005] The pistons are located within respective bores formed in the caliper. Each such piston bore in a typical caliper is substantially cylindrical, and includes a groove for the pressure seal, which contacts the piston or is attached thereto, to be fitted in and act against. The diameter of the bore is substantially the same but slightly larger than that of the piston partially housed therein.

[0006] In conventional caliper designs, the hydraulic pressure causing the movements of the pistons and elastic deflection of the pressure seal tends to lead to deflection of parts of the caliper and ultimately tilting and lifting, which in turn results in operational inefficiencies and undesirable component wear in elements of the brake system. New features are herein proposed to address this problem.

[0007] Summary of the Invention

[0008] The present invention seeks to provide improvements to current braking arrangements, by providing a novel caliper piston bore geometry.

[0009] In accordance with an aspect of the invention, there is provided a brake caliper for a vehicle, comprising a bore for housing a caliper piston, the bore having a central axis along its length and having a depth defined from an opening of the bore, wherein the bore comprises a first portion adjacent the opening of the bore, wherein the diameter of the first portion decreases with depth, a second portion at a greater depth than the first portion, wherein the diameter of the second portion increases with depth, and a seal groove located between the first portion and the second portion.

[0010] Preferably, wherein the diameter of the first portion and / or the second portion decreases linearly with depth. Preferably, the decreasing diameter of the first portion defines a first angle between the side of the bore and the central axis, the first angle being between 0.5 and 3.5 degrees (more preferably below 3 degrees). Preferably, the increasing diameter of the second portion defines a second angle between the side of the bore and the central axis, the second angle being between 0,5 and 3.5 degrees (more preferably below 3 degrees). Preferably, the first angle is equal to the second angle in magnitude.

[0011] Preferably, the bore further comprises a third portion at a greater depth than the second portion, wherein the diameter of the third portion is constant.

[0012] Preferably, the bore further comprises a fourth portion, known as a land, between the first section and the seal groove, the fourth portion having a constant diameter. Preferably, the fourth portion has a length of between 0.05 and 0.3 mm depth wise.

[0013] Preferably, the diameter of the third portion is greater than the diameter of the fourth portion (land). Optionally, it is greater than or equal to a maximum diameter of the first and / or second portions.

[0014] Optionally, the second portion has an extent in the axial direction that is greater than that of the seal groove and / or the first portion. Optionally, the second portion has an extent in the axial direction that is less than that of the seal groove, the first portion and / or the third portion.

[0015] Preferably, the bore is substantially cylindrically symmetric about the central axis.

[0016] In order that the present invention be more readily understood, specific embodiments thereof will now be described in reference to the following drawings.

[0017] Brief Description of the Drawings

[0018] Fig. la is a cross-sectional view of a car brake caliper and two pistons in use.

[0019] Fig. lb is an illustration of tilting of a piston relative to the caliper (e.g. when under braking conditions).

[0020] Fig. 2a is a close-up view of a side of a piston bore.

[0021] Fig. 2b shows a piston parallel to the central axis of its associated bore.

[0022] Fig. 2c shows a piston in a tilted configuration relative to its bore (e.g. under braking conditions). Fig. 3 illustrates a close-up view of a piston bore with an alternative arrangement.

[0023] Description of Preferred Embodiments

[0024] In a typical caliper for a racing car or road car, the caliper body contains two sides, namely a mounting side (inboard side) which is fixed to the car, and the cover side (outboard side) which is opposite to the mounting side. On each side there is provided at least one piston each moving in and out of its bore, in response to brake fluid pressure.

[0025] An exemplary embodiment of a caliper assembly 100 is illustrated in Fig. la. For simplicity, this cross- sectional drawing shows only one pair of pistons 120. A caliper preferably has more than one piston on each of the mounting side 110a and the cover side 110b, such as two pistons or three pistons.

[0026] When pressurised by the brake fluid (e.g. under braking conditions), each side of the caliper 100 will try to deflect from the centre 170. This is especially notable for the cover side 110b which is furthest from the mounting point; it will want to move outward away from the centre of the caliper assembly. Seen from the viewpoint of Fig. la, the cover side of the caliper is urged to rotate clockwise 180. This effect is called "lifting", and as it does not apply to the pistons, has the consequence of increasing the angle between the axis of a piston and that of its bore 130, due to the structure of the caliper assembly.

[0027] This lifting or deflection of (chiefly) the cover side 110b may occur until the angle eventually exceeds a "critical angle", at which the relevant piston 120 contacts the bore i.e. touches the substantially cylindrical side wall of the bore 130. This causes the piston to "tilt" with the caliper body, thus changing the position of piston force on the pad away from the intended position in the caliper design. Fig. lb illustrates the likely contact points 140a / 140b between a piston 120 and its bore 130 located on the cover side of the caliper, when the critical angle is reached, at which point the piston starts to tilt with the rotation / lifting of the caliper cover-side half 110b. The tilting can be understood as a rotation of the piston relative to its corresponding bore about an axis (out of the page in Figs, la and lb) that is perpendicular to the axis of symmetry of the central piston and bore (roughly left-to- right in Figs, la and lb).

[0028] Tilting of the piston is undesirable and should be avoided. One reason is that it stops the piston from evenly loading the brake pad(s) it is responsible for, as the centre of pressure exerted by the piston would migrate towards the upper pad radius. This potentially creates radial pad taper wear, and leads to inefficiencies in brake caliper, pad and disc mechanical performance and increased variation in NVH (noise, vibration, harshness) response, for example, and other potential unpredictable consequences in terms of hardware maintenance.

[0029] Conventional bore geometry diametrically supports the piston either side of the pressure seal groove. Caliper stiffness is optimised by positioning material in the caliper body to reduce lift and prevent the contact between piston and bore that causes piston tilting.

[0030] In the present invention, in at least one of the piston bores defined in the caliper (for example, all of those on the outboard / cover-side of the caliper, but potentially the inboard / mounting-side half as well), the diameter of the bore increases away from the pressure seal groove on both sides. In cross¬ section (see Figs. 2a, 2b and 2c), this appears as inclined side walls. This arrangement increases the caliper critical angle before the piston touches the caliper body; this contact of piston and caliper body unevenly and incorrectly loads the brake pad, creating the undesirable effects described. In some cases, the critical angle is at least doubled compared to conventional design.

[0031] This allows the caliper body to be less stiff in the "lift" direction and reduce caliper weight, at least partly because other mitigation measures (e.g. caliper material mitigation measures) against lifting or tilting may no longer be necessary.

[0032] The novel features are herein described in further detail with reference to Figs. 2a, 2b and 2c. The bore is defined in the caliper body with a depth starting from its opening / mouth towards a bottom end.

[0033] The bore 200 has a substantially cylindrical shape with a central axis, but deviates from a simple cylinder, in that the diameter of the space varies with depth (defined along the axial direction of the bore). This is aside from having a substantially ring-shaped groove 240 to house the substantially ring-shaped seal which is attached to the piston. The pressure seal groove preferably has a varying diameter. In the example of Fig. 2b, the groove diameter decreases with depth.

[0034] In a first, outer section 210 of the bore, starting substantially from its mouth 202 until substantially the depth at which the seal groove 240 starts, the diameter of the bore decreases gradually (tapers), preferably linearly. In other words, the mouth is wider than the inside of this portion 210. This section of the bore can thus be described as having a frustoconical shape. The extent of this section 210 in the axial direction of the bore is preferably greater than the extent of the groove section 240 in the same direction, and is preferably 1 to 10 mm.

[0035] In a second, inner section 220 of the bore, starting substantially from the depth at which the seal groove 240 ends until a certain depth of the bore, the diameter of the bore increases gradually, preferably linearly. In other words, the bore widens out past the seal groove. This section of the bore can thus be described as having a frustoconical shape. The extent of this second section 220 in the axial direction of the bore does not need to be the same as that of first section 210. It is preferably greater than the extent of the groove section 240 in the same direction, and 1 to 10 mm.

[0036] After the caliper cover side begins to lift and the critical (i.e. maximum) angle between the cover and the piston is reached, that is at the point before tilting (rotation) of the piston happens, a substantial continuous part of the piston is in contact with the side of the bore of the second section 220. Alternatively or additionally, a substantial continuous part of the piston is in contact with the side of the bore of the first section. There is optionally a third, innermost section 230 of the bore in which the diameter is substantially constant as depth increases, i.e. the side wall is parallel to the central axis of the bore. The diameter of this portion smaller than the diameter of the seal groove 240. The edge of the bottom 204 of the bore is preferably rounded off, as seen in Figs. 2b and 2c.

[0037] Fig. 2a shows a close-up view of the piston in the bore viewed in cross-section. The angle Al is defined between the (linearly) inclined wall of the outer (pre-groove) first portion 210 of the bore and the axis of the bore. Al is preferably between 0.5 and 3.5 degrees, more preferably between 0.5 and approximately 3 degrees.

[0038] The angle A2 is defined between the (linearly) inclined wall of inner (post-groove) second portion 220 of the bore and the axis of the entire bore. A2 is preferably between 0.5 and 3.5 degrees, more preferably between 0.5 and approximately 3 degrees.

[0039] The magnitudes of the two angles Al and A2 may be the same, within manufacturing tolerances; this may help to provide stability to the movement of the piston. Alternatively, this need not be the case; the magnitudes and relative sizes of the angles are determined by the caliper geometry and associated specific needs. The exact angles may depend on other design aspects of the caliper, such as its material (e.g. aluminium, steel), and the brake torque and pressure the caliper is designed to deliver.

[0040] This arrangement enables a larger relative tilt of the piston to be accommodated within the space of the bore, before parts of the piston, likely its corners and edges, contacts the side of the bore. The inventors envision that this critical angle is increased by at least 0.5 degrees compared to a conventional bore with simple cylindrical sides. For example, the total angular range of allowable tilting may increase by a minimum factor of 2 x Al. The exact increase in critical angle is dependent on the type of caliper and of features therein. Fig, 2b shows an ideal situation where the piston axis and the direction of its movement are exactly in parallel with the axis of the bore. Fig. 2c shows how- much the caliper body is allowed to rotate (appearing as an anticlockwise shift in the piston's orientation in this perspective), and the bore can still accommodate the piston and its sliding motion, without causing tilting contact.

[0041] Optionally, there is provided a short fourth portion 250, known as a "land", just before the seal groove 240, i.e. where the first section 210 (with its continuously decreasing diameter) would meet the beginning of the seal groove 240. In this land section 250, the diameter of the bore remains constant, i.e. the wall viewed in cross-section is "flat" and parallel to the central axis of the bore. This can be seen in Fig. 2a. The extent (i.e. length along the axis of the bore) of the land portion may be as small as 0,05 mm, but can be up to approximately 0.3 mm. Optionally, there may be provided a similar short land (a fifth portion) just after the seal groove 240, i.e. where the other end of the seal groove would meet the beginning of the second section 220 (with its continuously increasing diameter). In this land section, the diameter of the bore remains constant, i.e. the wall viewed in cross-section is flat and parallel to the central axis of the bore. The length of this land may be approximately 0.1 mm, and preferably in the range of 0.05 and 0.3 mm. The length of this second land, if included, may or may not be the same as that of the first land.

[0042] Note that the diameter of the third, constant-diameter portion 230 is preferably greater than that at the fourth portion (land before the seal groove) 250, and / or greater than that at the fifth portion (land after the seal groove).

[0043] The lands 250, where included, are short sections in the bore, i.e. each has an extent in length in the direction of the central axis of the bore that is (significantly) smaller than the extent of any of the first section 210, the second section 220 or the third section 230, or any combination thereof. The lands advantageously prevent or attenuate sharp changes in the geometry of the side wall (reduce sharp angles / edges), and provide some stability during tilting of the piston relative to the bore and caliper body, but are preferably minimised or avoided altogether if not needed (see more below). Additionally, a flat land can provide a regular geometric surface to support the pressure seal if and when seal extrusion occurs under high brake pressure or piston rollback control.

[0044] These angled areas 210, 220 and flat areas / lands 250 on either side of the seal groove 240 may be "joined" by a curved profile with a radius between 1 and 3 mm (e.g. approximately 2 mm), to further aid the transition(s) between the areas. This is optional, and there may be designs where a curved profile with a radius between 0 and 1 mm are appropriate.

[0045] In this arrangement, the seal groove 240 can be seen as the turning point at which the diameter of the bore changes from decreasing to increasing with depth. This may not be the case in alternative embodiments (not illustrated).

[0046] The seal groove 240 defined on the side wall of the piston bore is not itself described in detail here. As a non-limiting example, the length of the seal groove in the axial direction is 2.5 to 5.5 mm, and / or the diameter of the piston bore at the seal groove is approximately 4 to 8 mm greater than the other parts of the bore. As shown in Figs. 2a, 2b and 2c, a groove may have straight edges (namely, a leading edge next to the first section of the bore and a trailing edge next to the second section) that are perpendicular to the axis of the bore, but may be otherwise e.g. disposed at an angle or even curved (not illustrated). At the mouth of the groove, there are optionally provided small chamfers 242 at both faces. These chamfers do not need to be of the same size (as seen in Figs. 2a). For the purpose of discussion in this present description, these chamfers are considered part of the seal groove i.e. they do not form a feature in the piston bore 200 outside of and independent of the seal groove 240.

[0047] The inventors note that different solutions to the problem of lifting are possible, in another arrangement, as illustrated in Fig. 3, in the piston bore beyond the seal groove 340 there is provided an undercut 332 in the bore side wall, causing the bore diameter to be larger than normal but constant, i.e. the side wall is flat and parallel to the central axis of the bore. Thus, this section 330 with the undercut feature 332 can be considered equivalent to the third portion (of substantially constant bore diameter as depth increases) mentioned above. There is a relatively small portion 320 separating the seal groove 340 and the undercut section 330 that increases in diameter, that can be considered equivalent to the second portion mentioned above, but here it is of significant smaller extent than the third / undercut portion 330. Preferably, there is also a land 350 immediately after the seal groove, which can be considered equivalent to the fifth portion mentioned above. Other features may be same or similar as corresponding ones in the arrangement previously described. It may be described that this alternative arrangement differs from the one earlier described in that the second portion 320 does not gradually and / or linearly increase in diameter; optionally, the second portion 320 may be entirely absent (i.e. the bore diameter abruptly increases between the land portion 350 and the undercut / third portion 330), but this is not preferable.

[0048] Compared to a traditional bore geometry, where the diameter of the bore is not "expanded" in this manner but remains uniform and very similar to (only slightly larger than) the diameter of the piston, a larger angle of tilting of the piston is indeed allowed. However, when tilting occurs, the piston will be pivoted around the land 350 which is shaped like a narrow protrusion; when the piston can rotate no more (the critical angle is reached and lifting tends to begin), it is only in contact with the bore at two points (at the land 350) and at the bottom end of the piston. The sharp change in diameter in the side wall (with potential sharp corners or edges) at the pivoting point means the piston is likely to suffer damage or faster wearing out as tilting repeatedly happens, especially if the land provided is small in extent.

[0049] In the favoured arrangement, as already described above, an entirely inner section 220 with a flat but angled side wall (relative to the bore axis) is provided after the seal groove 240, with or without a land, instead. When the maximum angle of piston rotation (i.e. critical angle) is reached, a substantial part of its length of the piston is in contact with the side of the bore wall, i.e. the piston benefits from a "soft" support at high angles of articulation, with no hard stop. In other words, the piston is supported over its length or a substantial part thereof. This arrangement provides a major improvement as it avoids "scoring" of the piston and damage, leading to significant improvements in stability and longevity of parts. The flat angled sidewall aspect can be used, where appropriate, in conjunction with an undercut feature in the same piston bore, both contributing to increasing the critical tilting angle before lifting occurs. However, it will be preferable that, of all the space in the bore that is behind (i.e. deeper than) the seal groove, a substantial portion of it has flat angled sides in cross-sectional view (e.g. Figs.

[0050] 2a, 2b and 2c) i.e. linearly expanding radius; said substantial portion is preferably at least 20% of said space behind the seal groove, preferably 30%, more preferably 50%, up to 100%. The remainder may have parallel walls.

[0051] It is to be understood that the above-described embodiments are done so for information purposes only, and that many modifications and variations are included within the scope of the attached claims.

Claims

CLAIMS1. A brake caliper for a vehicle, comprising:a bore for housing a caliper piston, the bore having a central axis along its length and having a depth defined from an opening of the bore;wherein the bore comprises:a first portion adjacent the opening of the bore, wherein the diameter of the first portion decreases with depth:a second portion at a greater depth than the first portion, wherein the diameter of the second portion increases with depth; anda seal groove located between the first portion and the second portion.

2. The brake caliper of claim 1, wherein the diameter of the first portion decreases linearly with depth.

3. The brake caliper of any previous claim, wherein the diameter of the second portion increases linearly with depth.

4. The brake caliper of any previous claim, wherein the decreasing diameter of the first portion defines a first angle between the side of the bore and the central axis, the first angle being between 0.5 and 3.5 degrees.

5. The brake caliper of any previous claim, wherein the increasing diameter of the second portion defines a second angle between the side of the bore and the central axis, the second angle being between 0.5 and 3.5 degrees.

6. The brake caliper of any previous claim, wherein the bore further comprises a third portion at a greater depth than the second portion, wherein the diameter of the third section is constant.

7. The brake caliper of any previous claim, wherein the bore further comprises a fourth portion between the first section and the seal groove, the fourth portion having a constant diameter.

8. The brake caliper of claim 6, wherein the bore further comprises a fourth portion between the first section and the seal groove, the fourth portion having a diameter that is smaller than the diameter of the third portion.

9. The brake caliper of any one of claims 7 to 8, wherein the fourth portion has an extent, in the direction of the central axis, of more than 0.05 mm but preferably less than 0.3 mm.

10. The brake caliper of any previous claim, wherein the bore is substantially cylindrically symmetric about the central axis.

11. The brake caliper of any previous claim, wherein an extent of the second portion is greater than extent of the seal groove and / or the first portion, in the direction of the central axis.

12. The brake caliper of any one of claims 1 to 10, wherein an extent of the second portion is smaller than the extent of the seal groove, the first portion and / or the third portion, in the direction of the central axis.

13. A brake caliper for a vehicle, comprising:a bore for housing a caliper piston, the bore having a central axis along its length and a depth defined from an opening of the bore, and comprising a seal groove at a first depth;wherein the bore is configured to accommodate, when in use, tilting of the caliper up to a maximum angle relative to the central axis, andwherein, when the caliper is disposed at the maximum angle, a substantial continuous part thereof contacts the side of the bore beyond the first depth.

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