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SCIENCE FRICTION

13th October 1994, Page 104
13th October 1994
Page 104
Page 105
Page 104, 13th October 1994 — SCIENCE FRICTION
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Which of the following most accurately describes the problem?

Chris Graham reviews the clutch business and the options available when it comes to replacement units.

The service life of a vehicle's clutch is an important consideration for all operators. Much depends upon the nature of the business and the manner in which the vehicle is driven, but the type of clutch fitted also plays a part.

Usually it's the "organic" clutch which you'll find hard at work behind the majority of engines. This description is derived from the type of friction material used as a facing on the clutch plate. Historically this would have been asbestos—but fears about health-related sideeffects led to its replacement with woven glass or carbon fibre alternatives.

For more specialised applications there is the option of a sintered-metallic or phosphorinstead of having an bronze clutch which, organic friction material, makes use of metallic pads on the clutch plate to provide the necessary grip. This type is also sometimes referred to as "cerametallic" or "ceramic".

Irrespective of which is chosen, the purpose of the friction material is to grip the flywheel so the engine's power can be transmitted through the gearbox and on to the driving wheels. The ability to apply and release this grip, so that the gears can be changed smoothly, is a clutch's primary role.

In practice there are important differences between the two types. An organic clutch will run more simiothly and its operating comfort will be far greater. On the downside, though, its life is relatively short. A metallic-type clutch is much more durable, making it better for hauling heavy loads and it will operate more readily in harsh environments. However, it will be noticeably more snatchy because it has no cushioning effect. This aggressiveness also applies to its wear characteristics. The metal-to-metal action of the friction blocks on the flywheel causes the latter to become scored and so re-skimming (or even replacement) may be needed when the clutch is changed. In addition it can place extra strain on the rest of the drivetrain because of its severity.

In some cases a worn flywheel can be made serviceable again with a flywheel saver from Lipe (Reader reply card 60). This clever idea is only suitable for the Cummins 13 range of engines with an Eaton gearbox but it can save the need for expensive flywheel re-skimming.

While in theory there are assorted clutch configurations available—single-plate, twinplate, "push", "pull" and so on—this really does give a false impression of the real life options. The interchangeability between different designs is pretty limited, on a purely "one in, one out" basis. In 99% of cases you will be governed by the OE specification requirements and so the decision about replacement will have been made for you.

It is, nevertheless, worth having a basic understanding of the more important factors. First remember that the torque transmission of a clutch is proportional to its surface area. In other words, the larger the clutch, the larger its area and the greater its torque capacity. Twin-plate clutches are chosen to maximise this in cases where the torque capacity of an engine is too great for a single-plate to handle. Some manufacturers overcome the problem with a much larger single-plate design with a bigger plate diameter (typically 430mm).

Another important consideration is "clamp load". This is the pressure which forces the plate into contact with the flywheel and is the one which must be overcome (via the pedal in the cab) to disengage the clutch.

The clamp load becomes progressively greater as clutch diameter increases.

Smaller clutches are released with a levered pushing action but most large modern designs now incorporate "pull technology" which is better.

This can be 20% more efficient because higher spring capacities are possible on a larger overall diameter. The result of this is a much greater torque efficiency and simpler servicing too.

Another point to note concerns the clutch casing. This can either be cast or pressed and the choice between the two is governed primarily by demand. The tooling required for pressing is expensive so it tends just to be the volume manufacturers who offer this. Pressed casings are lighter than the cast ones so inertia effects are lessened although they don't have the ultimate strength.

THE FUTURE The increase in load weight, especially after the advent of air suspension and multiple axles, means that clutches have a harder time now than ever before. Not so much on the biggest vehicles which live on the motorways because gearchanging is relatively infrequent. With lighter vehicles, however, where cement mixers, multi-drop delivery trucks, skip wagons and so on have to endure a stop/start type of existence with increasingly heavy loads to carry is a different matter. The result is that clutches fail frequently To overcome clutch burn-out both AP Borg & Beck and Sachs are working on electronically controlled clutch systems which will greatly improve service life. AP's Anthony Lewis says: "Our ACTS gearbox is effectively a standard manual transmission but without a clutch pedal. It's not an automatic but is a stick-shift system in which the clutch is automatically controlled so it cannot be burnt, over-revved or under-braked."

Commenting on the Sachs system which seems to be based along similar lines, Richard Ledger adds: "This type of system will significantly reduce abuse of the clutch and make the vehicle easier to drive. It will certainly prolong the life of the clutch." Neither company is sure about when this sort of semi-automatic control will be introduced but both are confident that it will arrive.

The other big advance in terms of clutch life will be the introduction of bonded friction material. At present most friction material is riveted On to the clutch plate but this has its inherent problems.

With rivets you can only ever use about half the available thickness before exposing the head. Bonding, however, does away with this problem and makes the whole thickness avail able, giving an enormous improvement in service life.

AP Borg & Beck says it will have this sort of technology ready for the CV market in about three years, while Sachs says it already makes use of such methods on some linings which are jointly bonded and riveted.

The one aspect we haven't really touched on, in relation to clutch problems, is the performance of the driver's left foot! Misuse of the clutch can be the simple cause of an expensive problem so always bear this in mind before jumping to any more fundamental conclusions.


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