Brake assembly

A hydraulic brake piston (11) is formed of a clay based ceramic or conventional ceramic material, such as a kaolin based ceramic with a low thermal conductivity. The piston (11) may be incorporated in a disc brake assembly comprising a caliper body (10) provided with a cylinder which receives the piston (11). Hydraulic brake fluid is introduced under pressure into the space (12) within the body (10) to displace the piston from the body and into contact with a disc pad (14). The ceramic piston retards heat transfer from the disc pad into the brake fluid thus reducing the cause for vapor lock.

BACKGROUND OF THE INVENTION 
This invention relates to a brake assembly for a wheel. 
A conventional disc brake assembly for a wheel of a vehicle comprises a 
cast iron or steel disc rotor (ventilated or solid) which is either joined 
or cast integrally with the wheel carrying hub and bearings to facilitate 
fitting to the axle stub, and when fitted the disc rotates with the wheel 
assembly. A brake caliper is then positioned such that it straddles each 
side of the disc rotor. This brake caliper assembly consists of a main 
body, usually either cast iron or cast alloy, which holds disc brake 
friction material pads in a position in relation to the disc such that the 
pads can be applied to each side of the disc rotor face. The brake caliper 
is manufactured such that it has the provision for one or more pistons 
which are slidably received into the caliper body. The caliper is 
hydraulically actuated from a master cylinder assembly which displaces 
brake fluid from a master cylinder through a brake line to the caliper. 
The brake fluid is introduced into the caliper under high pressure and 
results in displacement of the pistons in the caliper body out towards the 
disc face. The pistons act upon the friction material pads which result in 
the pads being forced against the disc. In practice these pistons have 
always been manufactured from steel or aluminium and the final finishing 
of them effected by either chrome plating or anodising. 
A problem with steel hydraulic pistons is that they are usually very 
conductive to heat transfer. Thus upon continued braking of a wheel, the 
friction pads generate extreme heat which is then conducted by the 
hydraulic pistons and the caliper body to the brake fluid add to the 
seals. In order to minimize the possibility of such heat transferral 
causing the hydraulic brake fluid to vapourize, it has been necessary to 
design a brake fluid with increasingly higher boiling points. The 
currently acceptable disc brake fluid carries a nominated boiling point of 
approximately 270.degree. C. This boiling point is only some 30.degree. C. 
below the degradation point of the synthetic rubber seals which 
hydraulically seal the piston from the caliper body. If the brake fluid is 
contaminated with small amounts of water the boiling point can decrease to 
a about 140.degree. C. It is a property of most brake fluids that they are 
hygroscropic and thus readily absorb moisture resulting in a significant 
decrease in the boiling point of the brake fluid. This can result in 
potentially dangerous brake failure known as "vapour lock" which occurs 
when the heat transferred from the disc pads to the hydraulic brake fluid 
results in the formation of small pockets of vapour. 
Chromium plated steel pistons are the most common is disc brake systems but 
aluminium and titanium may be used in specialised cases. The aluminium and 
titanium pistons have the advantage of light weight and less corrosion 
compared to steel, but have a much higher thermal conductivity. 
A further disadvantage with conventional pistons is that corrosion or 
pitting can occur on the surface of the piston or caliper due to 
environmental conditions or contaminated brake fluid. Such corrosion 
results in roughening of the surface of the piston which subsequently 
results in an accelerated degradation of the hydraulic seals by abrasion. 
Furthermore engineers make a considerable effort to reduce the unsprung 
weight of automotive suspensions. With light weight pistons, the inertia 
of the braking system is reduced, leading to quicker reaction, as well as 
lower unsprung weight. As mentioned above, aluminium or titanium may be 
used in the pistons of brake assemblies to reduce weight, but these 
pistons suffer from corrosion and heat transfer problems. 
The present invention has been devised with the general object of 
overcoming the above and other disadvantages by the use of nonmetallic 
pistons which have a low degree of thermal conductivity, are not generally 
subject to corrosion and can have the ability to absorb moisture from the 
brake fluid to reduce the possibility of vapour lock. 
SUMMARY OF THE INVENTION 
With the foregoing and other objects in view the invention in one form 
resides in a hydraulic brake piston formed of an unglazed clay based 
ceramic or conventional ceramic material wherein the ceramic material is 
formed and fixed to be permeable to water and/or water but not to 
hydraulic fluid. 
In another form the invention resides in a brake assembly comprising at 
least one friction pad and at least one piston actuating said friction pad 
wherein said piston comprises an unglazed clay based or conventional 
ceramic material such as a kaolin based ceramic with a low thermal 
conductivity wherein the ceramic material is formed and fixed to be 
permeable to water and/or water vapour but not to hydraulic fluid. 
It is preferred that the ceramic piston is porous to allow said material to 
absorb moisture from the brake fluid but whereby the piston will not 
absorb brake fluid. 
In another form the invention resides in a hydraulic brake piston formed of 
a clay based or conventional ceramic material, such as a kaolin based 
ceramic. 
BRIEF DESCRIPTION OF THE DRAWING 
The invention will be better understood by reference to the following 
description of one specific embodiment thereof as shown in the 
accompanying drawing which is a schematic side sectional view of a brake 
calliper comprising the ceramic piston of this invention. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION 
The embodiment is directed to a disc brake assembly for vehicles. The brake 
assembly comprises a caliper body 10 which is provided with a cylinder 
into which is received a ceramic piston 11. Hydraulic brake fluid is 
introduced under pressure into the space 12 within the caliper housing 
between the piston and the housing which results in displacement of the 
piston from the housing. A seal 13 is provided in the wall of the cylinder 
between the housing and the piston to prevent hydraulic brake fluid from 
flowing past the piston and out of the caliper assembly. A disc pad 14 is 
supported adjacent to the outer face of the piston and is optionally 
provided with anti-squeal shim 15 between the piston and disc pad. Upon 
introduction of hydraulic fluid into the caliper housing the piston is 
displaced out of the housing and presses up against the disc pad to press 
the disc pad onto a disc. 
The piston is cup shaped with its closed face forming one side of the space 
12. The walls of the piston are convergent towards the their outer edge to 
facilitate the removal of water that may have inadvertently entered the 
space between the piston and the pad 14. The ceramic from which the piston 
is formed comprises a clay based or conventional ceramic such as a kaolin 
based ceramic. 
A typical clay based ceramic which may be used with the embodiment may have 
a composition of the following form: 
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SiO.sub.2 50%-70% 
Al.sub.2 O.sub.3 
10%-%40% 
K.sub.2 O 0%-10% 
CaO 0%-5% 
Na.sub.2 O 0%-10% 
______________________________________ 
In addition fluxes such as feldspar, nephelene syenite, cornish stone, 
silica and/or talc. This can result in a composition of the following 
form: 
______________________________________ 
SiO.sub.2 
50-70% 
Al.sub.2 O.sub.3 
10-40% 
K.sub.2 O 
0-10% 
CaO 0-5% 
Na.sub.2 O 
0-10% 
Fe.sub.2 O.sub.3 
0-2% 
MgO 0-5% 
TiO.sub.2 
0-2% 
MnO 0-1% 
______________________________________ 
In a specific example of the embodiment the ceramic used in the 
construction of the piston is a standard tableware ceramic, based on 
Goomalling clay. The composition of the ceramic was designed to produce 
high grade vitrified tableware and is based on Goomalling clay with the 
addition of other materials to give a composition of: 
______________________________________ 
ScO.sub.2 
Al.sub.2 O.sub.3 
Fe.sub.2 O.sub.3 
CaO MgO Na2O K.sub.2 O 
TiO.sub.2 
LO1* 
______________________________________ 
62.4 27.7 0.33 0.14 0.84 0.60 1.90 0.63 6.00 
______________________________________ 
(*LO1 represents loss on ignition) 
This is then fired at a temperature of 1200.degree. C. to give a body which 
has controlled porosity which is determined by weighing the fired body, 
then placing body in water and boiling for 4 hours, allowing to cool in 
water, removing the surface water and re-weighing. 
The properties of the ceramic piston when compared with the metal pistons 
currently in use (the conventional piston material is chromium plated 
steel, but aluminium and titanium are used in special circumstances). 
______________________________________ 
COMATIVE COMPRESSIVE STRENGTHS IN 
KGS/SQ CM. 
CERAMIC STEEL ALUMINIUM TITANIUM 
4500 5600 2800 7000 
DENSITY IN GMS/CUBIC CM: 
CERAMIC STEEL ALUMINIUM TITANIUM 
2.5 7.85 2.77 9.5 
COMATIVE RUBBING FRICTION: 
Readings of ammeter from a dynamic system with the piston 
held against a rotating disc on a dynamometer. 
CHROME 
PLATED 
CERAMIC STEEL ALUMINIUM STEEL 
1.10 1.35 1.20 1.00 
THERMAL CONDUCTIVITY 
Units W/m/.degree.C. 
CERAMIC STEEL ALUMINIUM TITANIUM 
1.30 45.30 224.77 15.56 
THERMAL EXPANSION 
Linear .times. 10.sub.-5 
CERAMIC STEEL ALUMINIUM TITANIUM 
0.02 0.633 1.244 0.49 
______________________________________ 
The formation of the piston of the embodiment can be effected by a number 
of methods.

METHOD 1 
The clay plus other components are blunged together in an ark, screened, 
filter pressed, filter cake dried and ground. Dry powder is then pressed 
in steel die at a pressure of 13790 kilonewtons/sq. meter. The fragile 
pressed piston is then removed from the die and placed in a furnace. 
Firing temperature is 1200.degree. C. for about eight hours. The fired 
piston is then ground to finished size. 
METHOD 2 
As Method 1 but adding a small amount of organic binder such as gum 
tragacantb to increase the strength of the pressed body. 
METHOD 3 
Slip Casting 
A deflocculated slip containing approximately 60% solids of the same 
composition as Method 1, and 2, is deflocculated using sodium silicate and 
sodium carbonate. The slip is then aged for 12 to 24 hours and then poured 
into plaster moulds to give the appropriate external form. The wall 
thickness is controlled by time--the fluid slip is decanted after a wall 
thickness of 10-15 mm has been built up. The mould is then allowed to 
slowly dry until sufficient shrinkage has occurred to allow cast to be 
removed from the mould. The cast is then dried, fired and ground as for 
Method 1 and 2. 
METHOD 4 
A mixture as Method 1 or 2 of the same composition, blunging, screening, 
filter pressing the mixture then feeding the filter cake to de-airing pug 
mill at about 80% solids to improve plasticity and workability. An 
extrusion press extrudes a column that is cut into pre determined lengths 
to give sufficient material to make the piston. This slug is then passed 
to a roller head cup making machine and spun into the required shape. The 
cup is then dried and fired in a conventional tunnel kiln. 
Any other suitable forming technique can be used. In each method the 
porosity is controlled so that the piston is permeable to water vapour but 
not to conventional brake fluids. The permeability may be created by 
adding a component which burns out during firing, or by controlling the 
firing temperature so that the body is not completely vitrified. 
The ceramic body is formed in order that the permeability is carefully 
controlled so that the water vapour may be transported through the body 
without the loss of brake fluid, i.e. the piston is designed to act as a 
filter to separate the hydraulic fluid from any water which has collected 
in the brake system. Heat generated at the interface of the piston and 
brake pad would mean that the face of the piston in contact would be 
hotter in use at time of application of the brakes. When the brake is 
released the disc and pad cool more rapidly than the caliper body assembly 
causing a temperature gradient across the piston from inner to outer 
faces. This temperature gradient will drive water vapour from the ceramic 
body and due to capilliary forces the pores in the piston body will draw 
into the piston any water or water vapour in the hydraulic fluid in 
contact with the piston. As the outer surface of the piston is in contact 
with the atmosphere evaporation will take place. 
The advantages of the use of the ceramic piston according to this invention 
are that the ceramic piston can be in contact with a hot disc pad and will 
retard heat transfer into the brake fluid thus reducing the cause for 
vapour lock. Additionally a ceramic piston does not pit, corrode or rust 
and thus does not result in accelerated deterioration of the hydraulic 
seal surrounding the piston due to abrasion. An additional advantage of a 
ceramic piston with a low thermal conductivity is that thermal degradation 
of the hydraulic seal is also significantly reduced due to reduced 
temperatures of the portion of the piston in contact with the seal. The 
ceramic piston of this invention can also absorb moisture and dispose of 
such moisture without any operational damage to the ceramic piston. This 
particular property of the ceramic piston is extremely advantageous 
towards a minimisation of vapour lock. In addition the ceramic piston does 
not exhibit significant thermal expansion as do steel or aluminium pistons 
and thus will not seize into the caliper body. In addition the 
electrolytic neutrality of the ceramic material reduces the possibility of 
electrolytic attack of the cylinder. 
Various other changes and modifications may be made to the embodiments 
described without departing from the spirit or scope of the invention.