MASCOT September - October 2003

How Does it Work - No 5 - The Clutch

The Tool Chest

My Oxford Dictionary has three definitions for "Clutch" - First, 'Set of eggs, Brood of chickens'; Second, 'Seize eagerly, Grasp tightly, Snatch at'; and Third,' An arrangement for throwing mechanical working parts into and out of action'.

I think we can discount the first, but most of us will probably have some experience of the second if we have released the clutch pedal too quickly! However it is the third that comes closest to the real function of the Clutch on a motor vehicle. Internal combustion engines need to be running at around 1000rev/min to develop enough power to get a car moving, and much higher when starting on a hill. This power needs to be transferred to the wheels gradually and smoothly to avoid "seizing eagerly" or "snatching" and the damage this can cause to the transmission components, apart from the discomfort to the vehicle's occupants We also need to be able to disconnect the wheels from the engine to change gear, and as we stop the car, so that the engine does not stall. The most common method of achieving these requirements is the single plate friction clutch, the fundamentals and basic operation of which is shown in Fig 1.

The driven plate (1) is splined to the primary shaft (2) of the gearbox. The friction linings or facings (3), which are made of a heat resistant material are riveted either one to each side of this plate, or one to each of the flywheel and pressure plate surfaces. The pressure plate (4) is located by a number of studs (5), and is forced towards the flywheel (6) by means of springs (7). When the pedal (9) is depressed, the clutch forks (10) act on the withdrawal sleeve (8) and pull the pressure plate away from the flywheel, thereby disengaging the drive. When the pedal is released, the springs force the pressure plate back into contact with the friction linings, clamping the driven plate to the flywheel, thus re-engaging the drive. At the centre of the flywheel is a spigot bearing (12), which locates the front end of the gearbox primary shaft and allows for the difference in speed between the engine and gearbox shafts. An adjustable link (11) is set to give a small clearance between the clutch fork and the withdrawal sleeve to ensure that the full force of the springs is applied to the driven plate. It also prevents undue wear on the forks, which is why the driver should not rest (ride) his foot on the pedal except to operate the clutch. This clearance is known as free pedal movement since it is measured at the pad of the clutch pedal, and is normally in the region of 1 inch.

Such an arrangement would require a very heavy pedal pressure, so the pressure plate is now operated by three or four release levers, which increase the mechanical advantage of the pedal. A typical clutch withdrawal linkage is at Fig 2. When the clutch pedal is depressed, the Release Fork (3) presses the Release Bearing (2) against the Release Pad (1), which is held onto the Release Levers (4) by retainer springs. The bearing shown is a graphite block, but can also be a ball-thrust bearing, which may operate directly on the levers, replacing the release pad. The release levers pivot on Floating Pins (6) which pass through adjustable Eye-Bolts (5), and the load is thus transferred to the Outer Casing (9), which is bolted to the flywheel. The outer ends of the release levers have a groove in which are located Knife-edged Struts (7) which connect them to the Pressure Plate (8). On further movement of the clutch pedal, the Strut pushes the Pressure Plate away from the flywheel and driven plate, thereby disengaging the drive.

With the solid driven plate used on early vehicles, very little pedal movement was required to engage/disengage the clutch, so the clutch was either "in" or "out". This type of plate was used on most pre-war Singers which are notorious for their fierce clutches. If you have this problem it is probably worth a try starting off in second gear rather than first, as this can give a much smoother take-up.

However, solid plates are also prone to distortion due to heat, which can cause the clutch to drag and make it difficult to engage gear. To overcome these problems, the plate is slotted, or set in such a way as to produce a 'flexing' action. This is generally known as crimping, and the inset at Fig 3 shows the Borg & Beck solution. The rim of the plate is cut into segments, each of which is dished a small amount so that the linings tend to spring apart when the clutch is disengaged. On re-engagement, the axial movement of the pressure plate is spread over a greater range of pedal travel, which makes for a smoother engagement. Also on Fig 3, another improvement to absorb the torsional vibrations inherent in most engines, and the shocks which can occur during clutch take-up, is to replace the rigid hub by a spring hub mounted independently of the main plate, the two elements being connected by a series of radial springs which absorb the vibrations and shock loads.

A common clutch problem which occurs particularly after long periods of winter storage, is that the resin bonding material in the clutch plate linings reacts with the moisture due to 'sweating' of the steel flywheel. This causes the linings to stick to the flywheel surface, thus preventing the clutch from disengaging. Bill Haverly described some methods of freeing a sticking clutch, together with some options for preventing the problem, in the July 2002 Mascot"`. Prevention is obviously the best cure, but whilst starting the engine and engaging the drive on a regular basis during storage is highly recommended, my preferred option is to prop the pedal down to hold the clutch in the disengaged position.

('If you would like a copy of Bill's article, let me know and 1 will send one to you - Ed)