Discs vdrums: fewer parts, more relines
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iham Montgomerie sums the current brake pineering thinking. And, he says, our drivers n't all soon become disc keys. Drums will be und for a long time yet JM BRAKES will be around a long time in spite of the eh publicised but gradual adce of the disc brakes, accordto Lucas Girling. In order to I out what is happening to the king scene, I have been ing to Bob Burton, who is the sional chief engineer (heavyy brakes) and Stan Ankrett, opean marketing manager truck brakes. The subject of : brakes seemed an obvious ling point ...
he hardest pressure applied Lucas Girling for disc brakes rom people making generalpose buses, which are in Jble due to excessive heat. ile the heat dissipation can improved with discs, Girling verthcless has trouble vincing such operators that y might be swopping a heat blem for a disc and pad-life blern.
Jith a 221/2in wheel, the maxim diameter of disc which can squeezed inside is about mm {15in). Because no room a caliper is required around outside, a drum brake can go to 420mm (161/21n) in the le wheel but, and this is even re important, it can also be of lost any width. This means t the disc brake immediately as out on available frictional a.
he drum has a thermal adrtage as you move up the ght scale and this problem the disc will not be solved by ter friction materials for the Is. As Bob Burton explained: ley might improve the life, where does the heat go?"
Lt the recent European Truck intenance Conference, a nber of the American particiits were rather bemused at lack of acceptance of the disc 3ke for European heavy vehicles. Lucas Girling, however, argues that the operating conditions are not sufficiently similar for a direct corn parison to be made. In the USA the trend is to run bigger wheels for a given weight. Classes 5 and 6, for example (which cover the seven to 12 tonnes gvw range) tend to run on 20in wheels compared with the more usual European sizes of the 16 and 17in.
The USA vehicles do not have to cope with the European braking stability requirements. This has resulted in most US chassis having a maximum of around 35 per cent braking effort on the front axle. Girling contrasts this with the 50 per cent average for a European vehicle. Stan Ankreft claimed that it is easy to package a disc brake on a lightly loaded front axle with big wheels.
"Ford, GM and IH have done it, but it is with what is essentially an overgrown car caliper."
The big advantage with the disc brake is the much smaller number of parts when compared with the drum. Lucas Girling claims that the potential parts reduction is likely to be in the region of 260 to 160. One of the main design aims is to be able to change the pads quickly as it is almost inevitable that the pads will need to be changed more frequently than a drum brake lining.
On the disc braked Concept Cargo which Ford showed earlier this year, a 50 per cent reduction in reline time was claimed over a drum-brake layout from the point of taking the wheels off. Girling thinks it unreasonable to ask the operator to reline more than once a year except in the case of a very ,arduous operation.
Comparing production costs is something that the manufacturers have been reluctant to do in public and it is difficult not to sympathise with them up to a point.
Drums have been in production for a long time and thus to start up with an all-new braking system requires a high investment. Stan Ankrett argues that if the disc brake comes in as an option, taking up, say, five to 10 per cent, then the cost will of necessity be high. Accordingly, Lucas Girling is concentrating on those vehicle manufacturers who can see the benefit of discs in high volume.
In this case, the company argues that a disc brake for the six to 10-tonne-gvw category should be no more expensive than a drum.
The disc will be lighter than the equivalent drum layout because of the sheer mass of metal of the latter, but, on the production side, the disc needs to be machined on two faces. On the Concept Cargo, for example, a weight saving of 19kg (421b) was claimed for the disc-brake system which was used in both axles.
Renault Vehicules lndustriels already has a disc brake in production on a 13-tonner and Iveco is scheduled to announce a similar vehicle at the Scottish Motor Show. Lucas Girling anticipates that this will be just the beginning and predicts a sharp increase in disc brake fitment in 1985/86.
By and large, the disc and the drum are made from the same material — spheroidal graphite cast iron. Bob Burton commented: "We have some slightly more sophisticated cast iron up our sleeve". This is the result of a feedback from the railways where disc brakes are used in the high-speed passenger train.
With cast iron there has to be a compromise between thermal stress and thermal conductivity. Grey cast iron is good for conductivity but not for stress whereas SG iron is the other way round. Lucas Girling argues that there is no need to go to exotic materials as the cast irons are perfectly satisfactory. As Bob Burton put it: "If you need an exotic material then you must have a serious heat problem."
In the 11/12-ton-weight Gate-. gory the advent of the 191/2in wheel had a big impact on brake design, so much so that Girling developed a new range of drums specifically for this size of wheel and going up to 14 tons gross. Talking of full-air actuation conveniently brings us to the subject of compressors. There is a trend both in Europe and the USA away from twins to bigsingle cylinder units again in the interests of simplicity and fewer parts. Girling is looking at ways of improving compressor efficiency and is investigating the use of reed valves as opposed to the spring-loaded plate, which, it is claimed, will minimise the fall off in the free air delivery output curve.
The interest in the big single appears to Lucas Girling to be coming from the original equipment manufacturers rather than from the operator, as manufacturers try to reduce the service inventory. At the moment at least three oems are interested in moving to big-single cylinder compressors with reed valves.
On a motorway, there is no load on the compressor so, in effect, the pistons are thrashing up and down for nothing. Girling is now receiving requests to consider better ways of bringing in the demand by the use of clutches, for example.
It is feasible to save 41 (51/2bhp) for around 80 per ci of compressor operating time many cases. This will give 1
advantage of extending t mechanical life as, once I rings start wearing, the oil pumped up to where it c bonises and blocks the valv thus diminishing the perfor ance.
Girling is looking at varic ways of declutching the co pressor, including oil-immer. clutches. One very important pect of which Girling is vi much aware is that the serv life of the compressor clu should be compatible with • rest of the engine.
No change in chassis air cal city is anticipated by Lucas C ling. The size of the reserve has been set by the need to co ply with EEC legislation for least six stops, the last one c ing residual performance, afte compressor failure.
Some chassis like Scania a Volvo, for example, tend to ov specify their air capacity any to cope with options like air s pension.
As a matter of interest I worst consumer of air is manually adjusted S-cam brE compared with most economi user, which is the disc brake w automatic adjustment.
For the future (and in spite all the talk about disc brak Lucas Girling strongly belie\ that there is still a lot which c be done with drums, especiE at heavier gross weights.
Stan Ankrett explained: "It wrong to think that the dri brake business is going to • apart. Over 10 tons it will around for a long time."