GB2082728A - Brake Piston - Google Patents

Abstract

A fluid dynamic piston comprises a cylindrical member 45 having a closed end 46, reinforcing means 50 extending axially from the closed end through at least a portion of the cylindrical member's axial length, the reinforcing means terminating with abutment means 37 whereby axial forces are transmitted from the cylindrical member's closed end to the abutment means. <IMAGE>

Description

SPECIFICATION Improvements In and Relating to Pistons and to Brakes The invention reiates to a hydraulic piston for use in automotive hydraulic brake systems and composed of a synthetic material.

Although an automotive hydraulic drum brake embodiment of the invention is disclosed, the invention may be applied to other types of hydraulic or air-actuated brake systems.

Conventional hydraulic drum brakes for vehicles typically include two arcuate brake shoes slidably mounted end-to-end on a stationary backing plate affixed to the vehicle. The brake shoes have friction material affixed to their outer surfaces for engaging the inner surface of a rotatable drum upon which the vehicle wheel is generally mounted. The brake shoes have upper ends abutting opposed pistons of a hydraulic cylinder mounted on the backing plate. Typically, return springs keep the brake shoes in constant abutment with the pistons.

When hydraulic pressure is applied to the wheel cylinder, the pistons extend in opposite directions to urge the brake shoes outwardly against the inner surface of the brake drum. The return springs retract the brake shoes when the hydraulic pressure is released, thereby freeing the drum to rotate.

Conventional wheel cylinders generally have one-piece metal pistons or, alternatively, twopiece pistons having a piston body with an abutment plug inserted into the outboard end (see Fig. 1 of the accompanying drawings). The abutment plug, which is generally of hardened metal, serves as a bearing surface for engaging the upper ends of the brake shoes. Metal pistons are relatively heavy, expensive to manufacture and subject to high rates of wear between the pistons and their mating surfaces. The need has thus arisen for a hydraulic brake piston that can be inexpensively manufactured from a lightweight material having a low coefficient of friction, that is compatible with commonly-used brake fluids, and that is capable of withstanding the high stresses and temperatures of brake operation while maintaining its dimensional integrity.

In accordance with the present invention, a new and improved hydraulic brake piston composed of a high temperature-resistant thermoplastic resin reinforced with an organic or inorganic material is disclosed. The piston preferably comprises an outer sleeve or cylinder having a closed rearward wall with a coaxial load carrying member extending from said wall through said cylinder.

A preferred embodiment of the piston includes reinforcing ribs circumferentially-spaced and extending through the annular space between the outer sleeve our cylinder and the load carrying member. The load carrying member protrudes from the rearward wall and preferably extends beyond the open end of the outer sleeve. A hardened metal abutment surface is preferably affixed to the forward end of the load carrying member for engagement with elements of the brake shoe.

One form of previously known vehicle drum brake assembly and several forms of vehicle drum brake assembly constructed in accordance with the invention will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 is a side view of a typical known vehicle drum brake assembly with the wheel cylinder partially cut away to show a typical hydraulic piston; Fig. 2 is an axial cross-sectional view of a typical hydraulic drum brake wheel cylinder assembly having a piston embodying the present invention; Fig. 3 is an exploded cross-sectional view of one of the hydraulic pistons shown in Fig. 2, the section being taken along the line 3-3 of Fig. 4; Fig. 4 is a forward end view of the hydraulic piston shown in Fig. 3: and Figs. 5 to 10 are transverse cross-sectional views of six different forms of hydraulic piston embodying the present invention.

Referring to the accompanying drawings, where like elements are designated by like reference numerals, Fig. 1 illustrates a typical known vehicle drum brake assembly 9 for a vehicle wheel normally rotating in the direction indicated by the arrow 10. The drum brake assembly 9 includes a backing plate 11 on which leading and trailing arcuate brake shoes 1 2L and 12T, respectively, are slidably mounted. The brake shoes 12L and 12T have webs 1 3L and 13T supporting arcuate rims 1 4L and 1 T respectively, to which friction material sections 1 5L and 1 so, respectively, are affixed by any known means.

A known form of hydraulic wheel cylinder assembly 1 8 is shown in Fig. 1 secured to the backing plate 11 between upper brake shoe ends 1 9L and 1 9T. A return spring 20 includes hooks 21 L and 21 T which engage upper web apertures 22L and 22T, respectively, and bias the brake shoes 1 2L and 1 2T towards the wheel cylinder assembly 18. An anchor block 25 is rigidly attached to the backing plate 11 between lower brake shoe ends 26L and 26T. A retaining spring 27 includes hooks 28L and 28T, and engages lower shoe apertures 29L and 29T, respectively, to urge the lower brake shoe ends 26L and 26T towards each other in constant abutment with opposite sides of the anchor block 25.

In operation, as hydraulic pressure is applied to the wheel cylinder assembly 18, a piston 1 6 moves to urge the leading brake shoe 1 2L outwardly. The lower shoe end 26L rotates against the anchor block 25, and the friction material section 1 5L frictionally engages the inner surface of a brake drum 30, thereby retarding the rotation of the brake drum 30 and hence of the vehicle wheel. The trailing brake shoe assembly operates similarly and simultaneously. Upon release of the hydraulic pressure, the pistons 16 retract under the inwardly-directed biasing force of the return spring 20 which also urges the brake shoes 1 2L and 1 2T to their original positions, thereby freeing the brake drum 30, and thus the vehicle wheel, to rotate.

Referring to Fig. 2, a first form of assembly 1 8, incorporates two pistons 32. The pistons 32 are mounted for opposed sliding movement within a cylinder 31, forming a hydraulic chamber 33 therebetween. Elastomeric cups 34 act as hydraulic seals and abut the rearward surfaces of the pistons 32, and cup expanders 35 abut the rearward sides of the cups 34 and receive a biasing spring 36. Dust boots 38 engage external grooves 39 in the wheel cylinder assembly 18 and extend into the cylinder 21 to form a seal with the actuating pistons 32.

Each piston 32 includes a cylindrical outer sleeve 45 having a closed rearward wall 46, an open forward end 47, an inner surface 48, and an outer surface 49. A post 50 is coaxially disposed within the outer sleeve 45 and includes an outer cylindrical surface 51, an internal cavity 52, a cavity bottom 53, a cavity opening 54, which is preferably chamfered, and a cavity inner surface 55. The post 50 protrudes in a forward axial direction from the rearward wall 46 and may, depending upon the particular brake mechanism design, extend beyond the forward end 47 of the outer sleeve 45. The post 50 has an external diameter that is smaller than the internal diameter of the outer sleeve 45 thereby forming an annular space 56 therebetween.

The outer surface 51 of the post 50 diverges at an angle B, and the cavity inner surface 55 converges at angle C, to form a frustoconical annular base portion 70 adjacent to the rearward wall 46. The base portion 70 allows the axial forces encountered during brake operation to be more evenly distributed over the rearward wall 46. The angle B lies within the range of 5 to 250, and is preferably 150. The angle C lies within the range of 300 to 900 and is preferably 600.

Ribs 57 are circumferentially spaced within the annular space 56 and extend between the inner surface 48 of the outer sleeve 45 and the outer surface 51 of the post 50. The ribs 57 preferably extend from the piston rearward wall 46 along at least a portion of the axial length of the post 50 to a point rearward of the outer sleeve forward end 47. The forward portions 59 of the ribs 57 preferably slope forwardly and outwardly from the post 50 to the forward end 47 of the outer sleeve 45, thus leaving a portion of annular space 56 open to form an expansion space 58 between the post 50 and the outer sleeve.

Since existing synthetic resins of the day do not exhibit adequate surface load bearing characteristics, a metal abutment plug 37 or a metal abutment cap is preferably included in the piston assembly to act as a bearing surface for the brake shoe ends 1 9L and 19T, respectively.

Should resins be developed in the future that have such characteristics, the abutment inserts or caps may be omitted.

The abutment plug 37 includes a flange 61 and a pin 62. The pin 62 diverges in the rearward direction at an angle A so as to provide a tight fit when inserted into the post cavity 52. The angle A normally lies within the range of 0.50 to 20, and is preferably 1 . The rearward end 63 of the pin 62 may be provided with a chamfer 64 for ease of insertion into the cavity opening 54. Preferably, the abutment plug 37 is pressed into the post cavity 52 during assembly until its abutment flange 61 seats against the forward end 66 of title post 50. The pin 62 projects into the cavity 52 only to an axial depth equal to, or less than, the axial length of the open expansion space 58.Thus any diametric expansion of post 50 caused by the forc8-fitting of the abutment plug 37 into the cavity 52 is not imparted to the outer sleeve 45.

The ribs 57 therefore provide radial structural reinforcement between the post 50 and the cylindrical outer sleeve 45 without causing an increase in the outside diameter of the piston 32.

The expansion space 58 also serves as an area where the associated dust boot 38 (see Fig. 2) can form a seal with the post 50 and the sleeve 45.

Instead of the abutment plug 37, there may be provided an abutment plug having a pin of uniform diameter that does not cause expansion of the inner sleeve. The plug may be secured to the piston with any known adhesive to keep it in place during brake maintenance. Since such an abutment plug pin causes no expansion of the inner sleeve, the strengthening ribs may extend the full axial length of the post without resulting in expansion of the outer sleeve.

A further variation has an abutment cap that fits over the forward end of the post rather than an abutment plug that fits into a post cavity. Such a cap may be secured to the post with an adhesive as discussed above.

The piston 32 may be composed of any of several known thermoplastic or thermosetting resins that are compatible with commonly used hydraulic brake fluids and that exhibit suitable dimensional stability under the temperatures expected in the piston environment during brake operation. Such temperatures may range from ambient to as high as 2500 F. The resin may be reinforced with glass fibres, glass spheres, talc, carbon, or any other suitable organic or inorganic materials for added strength and dimensional stability. Examples of a suitable material is glassfilled 66 nylon (such as FIBERFILL G 10/40) or glass-filled 612 nylon (such as FIBERFILL G 4/34 or G 4/45).

The wheel cylinder assembly shown in Figs. 2 to 4 has been built and tested, the cylinder having a nominal bore size of 1 7 millimeters. The piston is composed of glass-filled nylon and is compatible with the glycol-based hydraulic brake fluid currently used in the United States. After 250,000 hydraulic pressure cycles simulating braking applications, the pistons and cylinders showed appreciably less scuffing, abrasion and wear than did aluminium pistons of the prior art.

Such reduced wear is especially advantageous because such pistons may be used in aluminium wheel cylinders without costly anodizing of the cylinders.

Other synthetic materials, such as phenolic or reinforced epoxy resins, which are thermosetting resins, also provide the necessary compatibility with glycol-based brake fluid and may be expected to have adequate mechanical properties to meet the above-mentioned criteria. The use of silicone or mineral based brake fluids common in Europe may require the use of other synthetic resins that are compatible with those fluids.

Since pistons embodying the present invention can be injection moulded, there is an almost unlimited choice of piston configurations and the design is not bound by the cost and process limitations associated with the machining and fabrication of other materials such as metals.

The remaining forms of wheel cylinder assembly in accordance with the invention to be described with reference to the accompanying drawings are substantially the same as that shown in Figs. 2 to 4 except for the form of the piston. Accordingly, in respect of those other forms of wheel cylinder assembly, on the piston is described and shown in the drawings.

Fig. 5 shows a piston 71 having an inner member 72 that is coaxial with the outer sleeve 45. In contrast with the cylindrical post 50 of the form shown in Figs. 2 to 4, the inner member 72 is star-shaped and so includes a plurality of apices or corners 73 circumferentially spaced about its periphery. The apices 73 extend to join the outer sleeve inner surface 48 and provide radial structural reinforcement between the inner member 72 and the outer sleeve 45.

The piston 71 is shown in Fig. 5 as having a coaxial cavity 74 for receiving an abutment plug.

However, as with the form shown in Figs. 2 to 4, the piston 71 and any of the other forms of piston shown in Figs. 6 to 10 may have solid inner members or posts. In such cases, an abutment cap may be provided to fit over the forward end of the inner member or post, or alternatively an abutment disc may be mounted on the end of the inner member or post. Such an abutment disc may be recessed in an opening in the forward end or may be so mounted that its thickness axially protrudes from the forward end.

Fig. 6 shows a piston 76 which includes an inner post 77 having a rectangular cross-section.

The inner post 77 intersects with, and is attached to, the outer sleeve inner surface 48 at its corners for radial reinforcement of the outer sleeve 45.

Fig. 7 shows a piston 79 which has an inner post 80 that is also rectangular in cross-section.

However, unlike that of piston 76 shown in Fig. 6, the inner post 80 intersects with; and is attached to, the outer sleeve inner surface 48 along its entire width (w) to provide radial reinforcement for the outer sleeve 45.

Fig. 8 shows a piston 82 which has a cruciform post member 83. The post 83 has integral radial reinforcement extensions 84 which intersect and join with the outer sleeve inner surface 48 along their entire width (w). The piston 82 is shown in Fig. 8 as having adjacent radial extensions 84 preferably oriented substantially perpendicularly to each other. However, adjacent radial extensions 84 may be oriented at angles other than 900 if such other angles are desirable or advantageous.

Figs. 9 and 10 show pistons 86 and 86', respectively, having posts 87 and 87', respectively. The posts 87 and 87' are both hexagonal in cross-section and have radial reinforcement spokes 88 and 88', respectively, for radially reinforcing the outer sleeve 45. The spokes 88 and 88' are circumferentially spaced about the peripheries of the posts 87 and 87', respectively, and extend radially to intersect and join the outer sleeve inner surfaces 48 of the pistons 86 and 86', respectively. In the case of the piston 86 shown in Fig. 9, the spokes extend radially from the intersections of the sides of the hexagonal cross-section of the inner member 87, whiie the spokes 88' of the piston 86' shown in Fig. 10 extend radially from the flat portions of the sides of the inner member 87'. Although Figs. 9 and 10 show pistons with posts having a hexagonal cross-section, the piston may have posts of other cross-sectional shapes if desirable or advantageous.

Claims (11)

Claims

1. A fluid dynamic piston comprising a cylindrical member having a closed end, reinforcing means extending axially from the closed end through at least a portion of the cylindrical member's axial length, the reinforcing means terminating with abutment means whereby axial forces are transmitted from the cylindrical member's closed end to the abutment means.

2. A piston as claimed in claim 1, wherein the reinforcing means includes a protrusion defining an annular space between the cylindrical member cylinder and the protrusion, and a plurality of ribs extending between the cylindrical member and the protrusion.

3. Afluid dynamic piston as claimed in claim 2, wherein the cylindrical member has an open forward end and a closed rearward end, and the protrusion comprises a post situated coaxially within the cylindrical member and having an axial cavity in its forward end, the post extending axially in a rearward direction from the closed end of the cylindrical member through at least a part of the cylindrical member's axial length, thereby forming the said annular space between the post and the cylindrical member, and wherein the ribs are circumferentially spaced and extend, in a radial direction, between the post and the cylindrical member and, in an axial direction, from the cylindrical member's closed rearward end over at least a portion of the axial length of the said annular space, the ribs sloping rearwardly away from the outer cylinder's open forward end and intersecting the post to the rear of the cylindrical member's open forward end, the abutment means being attached to the post's open forward end.

4. A fluid dynamic piston as claimed in claim 3, wherein the post further includes a frustoconical annular base portion axially adjacent to the cylindrical member's closed rearward end, the said base portion diverging rearwardly to intersect the cylindrical member's closed rearward end.

5. A fluid dynamic piston as claimed in claim 4, wherein the abutment means comprises a pin having forward and rearward ends and a flange on the pin's forward end, the rearward end of the pin extending into the axial cavity in the forward end of the post to a point no further rearward than the intersection of the ribs and the post.

6. A fluid dynamic piston substantially as hereinbefore described with reference to, and as shown in, Figs. 2 to 4 of the accompanying drawings.

7. Afluid dynamic piston substantially as hereinbefore described with reference to, and as shown in, any one of Figs. 5 to 10 of the accompanying drawings.

8. A fluid dynamic piston as claimed in any one of claims 1 to 7, wherein the piston, apart from the abutment means, is made of a substance comprising a plastics material.

9. A fluid dynamic piston as claimed in claim 8, which has been formed by injection moulding.

10. A piston-actuated vehicle brake, which comprises a piston slidably mounted in a cylinder and movable in response to fluid dynamic forces to activate the brake, wherein the piston is as claimed in any one of claims 1 to 9.

11. A piston-actuated vehicle brake, substantially as hereinbefore described with reference to, and as shown in, Figs. 2 to 4 of the accompanying drawings.