Stock material or miscellaneous articles – Circular sheet or circular blank – Frictional
Reexamination Certificate
1998-11-23
2003-02-04
Ahmad, Nasser (Department: 1772)
Stock material or miscellaneous articles
Circular sheet or circular blank
Frictional
C188S073100, C188S073200, C188S25100R, C188S25100R, C192S10700R, C428S408000, C428S542800, C428S698000
Reexamination Certificate
active
06514592
ABSTRACT:
The present invention relates to a friction engaging device, such as, for example, a braking disc for a friction brake or friction clutch.
More particularly the present invention relates to a high performance friction engaging device, such as, for example, a braking disc for use in a guided trackway vehicle.
BACKGROUND OF INVENTION
The choice of materials used for making friction engaging devices is determined by the conditions under which a vehicle will operate and other surrounding circumstances.
By way of example, trains or their rolling stock conventionally have disc brakes which utilise steel or cast iron braking discs. Using such steel braking discs, the current maximum performance for a conventional train brake is that proposed for a new high speed passenger train where the maximum energy per steel braking disc in one stop is 22 MJ, with an average deceleration calculated from 350 km/h to stationary of about 0.7 ms
−2.
For trains or their rolling stock, which can have multiple brakes comprising at least four axle mounted braking discs per axle, a low margin between recovered fares and operating costs means the market price of a braking disc is relatively low and the life time requirement for the braking disc is high.
Also at around 100 kg for a single braking disc, each axle may have 400 kg of braking discs. Thus a greater number of braking discs adds to the weight of an axle and the greater the weight of the axle the greater the resulting track damage. Furthermore, since a maximum of four steel braking discs, due to their bulk, can be mounted on an axle, it is not very easy to increase the vehicle speeds much beyond 350 km/h using steel braking discs.
It would therefore be a clear advantage to reduce the number and/or weight and/or the bulk of these braking discs and/or improve their performance.
For trains, emergency stops at maximum rating have to be achievable with subsequent normal braking of the vehicle continuing after without any re-fitting of service parts. Consequently, good wear characteristics are very important.
In contrast, the braking discs for an aircraft's brakes, are in part determined by the type of flights they undergo. A jumbo jet may have as many as 9×16 clutch braking discs which are made of carbon-carbon fibre ie. a carbon fibre reinforced carbon composite. The current maximum performance for such a carbon-carbon fibre clutch brake is 71 MJ with an average deceleration from about 290-320 km/h to zero of about 4.2 ms
−2
, this being the performance required to cater with an aborted take-off. In contrast to a train, where subsequent normal braking with the same braking discs is required, the whole of the undercarriage of the aircraft, including the braking discs, has to be replaced after such an aborted take-off. Consequently, with an aircraft it is less important that the braking discs wear characteristics are poorer than those provided by steel braking-discs.
In aircraft where journey times are short and stops are frequent eg. a domestic shuttle operation, steel braking discs may be preferred as steel, unlike carbon-carbon fibre, does not give rise to judder and gives better wear characteristics thus making steel braking discs more economical despite the need for increased fuel load, due to heavier brakes.
The decreased weight of carbon-carbon fibre braking discs and their ability to operate at relatively high temperatures makes them attractive for use on other vehicles. So why not use carbon-carbon fibre for the braking discs to be used on train brakes? This has indeed been tried with little success due to a lack of frictional stability and a poor wear life under operational conditions.
To explain, the dry friction behaviour of carbon-carbon fibre is complex. It exhibits unstable friction at low temperatures (less than 300° C.) and the wet friction can be very low (&mgr;=0.05) at ambient temperatures. Wear behaviour is poor at low speeds and low temperatures (called snub stops) due to high friction and also at high temperatures (greater than 600° C.) due to oxidation. The above currently makes carbon-carbon fibre braking discs unsuitable for use on trains. In aircraft snub stops cause a lot of judder, primarily experienced when an aircraft is taxiing. The wear caused by such snub stops is often large compared to wear caused by landings at much higher energy levels. Overcoming this problem would therefore be of benefit to the aircraft industry. Also very high energy stops cause excessive wear owing to oxidation of the carbon which begins at 500° C. and rapidly accelerates as the temperature increases. Despite these observations carbon-carbon fibre is a useful material for braking discs where the majority of braking is done with the braking discs at middle range temperatures (say, 250° C. to 600° C.), which is the case for aircraft landings and Formula One racing cars. Also, these vehicles have disc brakes which are enclosed so that the braking discs do not get intermittently wet in service.
All of these factors means that generally different materials are preferred for the braking discs of trains and aircraft due to the different operational conditions they experience.
SUMMARY OF THE INVENTION
It is an aim of the present invention to develop a friction engaging device which overcomes at least some of the abovementioned problems and/or disadvantages of the prior art devices.
According to a first aspect of the present invention there is provided a friction engaging device in the form of a carbon-ceramic composite comprising a carbon fibre network and a filler comprising silicon carbide.
Such a composition can be made utilizing a number of known processes.
In one embodiment the carbon-ceramic composite comprises, by volume, 10 to 60% of a carbon fibre network and up to 90% by volume of a filler comprising silicon carbide.
The filler may comprise a volume of air (porosity).
According to a second aspect of the present invention there is provided a friction engaging device in the form of a carbon-ceramic composite consisting a carbon fibre network and a filler.
Preferably the filler consists essentially of silicon carbide. Alternatively the filler may consist of silicon carbide, silicon oxide, silicon and free carbon.
In another embodiment, the carbon-ceramic composite comprises, by volume, 10 to 60% of a carbon fibre network and 40 to 90% of a filler more preferably 30% of a carbon fibre network and 70% of a filler.
The overall composition, by end weight percent, comprises:
Component
Range
Carbon fibre
3-53
Free carbon
4-76
Silicon carbide
7-37
silicon
3-19
silicon oxide
3-19
The preferred ranges and actual weights will depend upon the carbon fibre content and the degree of impregnation. The porosity has no effect on the weight.
For a material produced from a starting material with a carbon fibre content of 30% by weight of the carbon-carbon composite, the carbon-ceramic composite will comprise (end weight percent):
Component
Range
Preferred Range %
Preferred %
Carbon Fibre
11-28
16-25
22,0
Free Carbon
15-59
30-55
46,8
Silicon Carbide
7-37
10-28
15,6
Silicon
3-19
5-14
7,8
Silicon oxide
3-19
5-14
7,8
For a material produced from a starting material, with a carbon fibre content of 10% by weight of the carbon-carbon composite after impregnation the carbon-ceramic composite will comprise (end weight percent);
Component
Range %
Preferred Range %
Carbon Fibre
3-9
5-9
Free Carbon
23-76
41-71
Silicon Carbide
7-37
10-28
Silicon
3-19
5-14
Silicon oxide
3-19
5-14
and, for a material produced from a starting material with a carbon fibre content of 60% by weight of carbon-carbon composite after impregnation the carbon-ceramic composite will comprise (end weight percent).
Component
Range %
Preferred Range %
Carbon Fibre
22-53
32-50
Free Carbon
4-33
14-30
Silicon Carbide
7-37
10-28
Silicon
3-19
5-14
Silicon oxide
3-19
5-14
Of course, the relative end weight percentages of free carbon to silicon carbide in these embodiments could be decreased if more silicon were to be i
Hubbard David A.
Taylor Alfred J.
Watson John C.
Ahmad Nasser
Pearne & Gordon LLP
Sab Wabco Products Limited
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