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Gripping stuff

5th October 1995, Page 46
5th October 1995
Page 46
Page 47
Page 46, 5th October 1995 — Gripping stuff
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Which of the following most accurately describes the problem?

Talk of clutches and operators think of a simple device that lives between engine and gearbox and wears out too quickly. There was a time when that might have been true — but even if it was, it isn't any more. Technology has come to clutches, although the changes are not always obvious to the eye.

/4 ife has become more demanding for a clutch. Nobody wants to have to adjust clearances during use. Operators expect them to last longer despite specifying vehicles with higher engine power and torque; flywheels must not get damaged while synchromesh gearboxes cannot have any drag if they are to engage cleanly.

This has sent clutch manufacturers back to the drawing board to invent new designs and cook up new friction materials.

One of the first of the latest generation of clutches to break cover is Spicer's Solo which is undergoing trials in multi-drop vehicles, skip loaders and the like. It is a self-adjusting clutch where the traditional Spicer adjusting ring is replaced by two ceramic hoops with counter facing ramps.

Adjusting ring

One hoop is keyed on to the adjusting ring (which used to have a screw thread round the outside) and provides the pivot for the levers. The other is dropped into the cover and pre-tensioned against a spring. As the clutch plate wears, its reduced thickness shows up in a small amount of clearance between the hoops. This allows the spring to rotate the moving hoop so the ramps can take up the slack.

While ceramic clutches were designed to last a long time, the increase in engine torque may be causing problems with heat build up. In extreme cases, this has caused the ceramic button to warp, lifting either side of the rivets. To counter this Spicer has moved the rivets from the centre of the button to the outside.

While operators love the longevity of ceramic clutches, they dislike flywheel wear and drivers find they can have a very sharp take-up. To counter these problems, clutch manufacturers are looking to develop new materials in the same way as non-asbestos brakes.

And, roughly in line with brake lining manufacturers, Spicer's new friction materials is said to give 30% longer clutch life while reducing flywheel wear by 50%.

To improve the engagement/disengagement, Spicer is 'kevstoning' the ceramic buttons. This means that instead of the buttons being flat and parallel to the flywheel and pressure plate, there is a 0.4mm taper running from the outer to the inner edge.

The theory is that when engaged, conventional ceramic buttons can 'wring' to the flywheel and pressure plate. This would cause not only a sticking clutch but may also remove minute amounts of material from the flywheel on each disengagement.

By adding the keystoning, Spicer says the idea is to 'peel' the ceramic button off the fly wheel thereby giving a clean disengagement without the passibility of removing material from the flywheel.

As the clutch takes up, it first makes contact on the outer edge of one side of the friction plate, and the inner edge of the other. Once the clamp load is greater than about 13kg (301bs) the carrier plate flexes, allowing the buttons to lie flat on the flywheel. According to Spicer, the keystone does not wear off during use_ A problem that has arisen with the latest generation of engines is vibration—not of the engine itself but of the vehicle. As the latest engines' peak torque is now developed at around 1,200rpm, the normal working range has come down to suit.

Vibrations

At these engine speeds the flexing of the crankshaft as each cylinder fires causes small vibrations. In itself this is not a problem unless the vibrations harmonise with the chassis' resonant frequency when the whole vehicle can vibrate with unpleasant consequences.

According to Spicer the vibratory torque can be several times the peak torque of the engine and cause damage to the drivetrain as well as severely upsetting the driver.

To overcome the symptoms (although not the vibration itself) Spicer is introducing a clutch plate which allows 110 of movement

either side of a central neutral position. This is much larger amount of movement than is traditiorrally provided by damped clutch plates.

In effect the ceramic buttons can move round 11 before any vibration is transmitted to the splined hub and onto the gearbox. This is achieved by using just seven coaxial springs to keep the plate in the central position. The springs allow enough movement to absorb the shock from the small vibrations but do not transmit the engine torque.

According to one customer (a vehicle manufacturer's main dealer) the result can be dramatic. One driver refused to take a truck out until the vibration problem was fixed — a quick exchange of clutch plates did the job.

Spicer says his damming does not get rid of the vibration but has the effect of moving it to a lower frequency (about 600-800 rpm). This means that the engine will pass through the period where the vibrations harmonise into the smoother running rev range for normal working.

While most of this technology is (or can be) applied to both single and twin plate clutches, the latter has a particular problem when it comes to drag.

With the pedal pressed, there is about 1.5rnm clearance between each friction face. However, drag will be induced if the plates move out of parallel (ie one tilts sideways).

To prevent this, Spicer is introducing 'positive plate separation' which involves placing several roll pins around the circumference of the intermediate plate. This keeps the intermediate plate parallel to the flywheel and prevents any drag. As the clutch plates wear, the clamp load pushes the roll pins through the intermediate plate to maintain even clamping on both friction plates.

This is not the end of clutch development, the ever-present pressures for longer life and higher torque will ensure that. But one thing is certain; the humble clutch will never be the same again.

L. by Colin Sowman

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