A clutch is a device employed to engage the engine to a manual transmission. Its purpose is to allow the engine to continue running when the tractor is at rest by disconnecting the flywheel from the transmission’s input shaft.
The function of a clutch depends on enough friction being developed between the contact surfaces of two or more members to transmit the engine torque without slipping. Before full engagement, the ability to slip is required to create a shock-free connection between the engine and transmission. Heat is generated during any slipping of the clutch friction material. Still, it is quickly dissipated due to the short exposure time.
A critical design element of a clutch is that the driven member connected to the transmission should not act as a flywheel. Instead, it should be lightweight and possess a minimum of inertia to ensure that it can be rapidly slowed or accelerated. The ability of a plate clutch to transmit torque depends on a few factors:
- The effective radius of the friction surfaces
- Coefficient of friction acting between the friction surfaces
- Number of friction surfaces
- The clamping force holding the friction surfaces together.
The modern clutch, employed for nearly sixty years now, can be considered a disk or plate design. Within this primary classification, subcategories identify the method used to create the clamping force against the flywheel.
The diaphragm spring clutch, for example, has long superseded the multi-coil spring type that was popular in the early 1900s. The diaphragm spring design was invented by General Motors in Detroit, Michigan, in the late 1930s. It differs from a multi-coil spring-type clutch by applying the spring loading and withdrawing from the pressure plate. Most clutch designs share the same essential components but function slightly differently from each manufacturer.
The components of a clutch are:
Flywheel: Attached to the engine’s crankshaft, it stores energy through mass and inertia while providing a surface for the friction material to grab onto. The surface of the flywheel must be accurate and allow the friction material to slip slightly as the clutch is being released but hold full engine torque under load when engaged.
Pressure plate: This component rotates permanently with the flywheel while supplying the other unlined friction contact surface. The pressure plate conveys the clamping force of the clutch springs to the driven member. The pressure plate must be provided with a certain amount of axial freedom to accommodate the friction-lined driven member’s wear and allow release.
The pressure plate is usually produced from stamped steel and is attached to the flywheel with either bolts or studs. It is essential that the pressure plate does not distort under the load of the spring engagement while also carrying away any heat generated.
Friction disk: The friction plate is the driven member of the plate-friction clutch because it is sandwiched between the friction contact surfaces of the flywheel and the pressure plate. The friction plate usually incorporates torsional springs between the center steel plate and the friction material. The springs are used to minimize and reduce shock loading on the transmission in the event of a sudden engagement and isolate fluctuations in crankshaft torque inherent to a four-stroke engine.
Throw-out or release bearing: The pressure plate incorporates release levers or fingers, based on the style. A unit called a throw-out or release bearing then rides on the levers/fingers. The release bearing is connected mechanically or hydraulically to the operator’s clutch pedal. When they depress the clutch pedal, the release bearing is evoked to push against the levers/fingers and disconnect the friction material from contact with the flywheel.
There are many different styles and designs of release bearings. The most common is either a metalized carbon ring or a ball-bearing race style.
The most straightforward part of a clutch is the friction material. It does not move, is usually riveted to the clutch plate, and, at first glance, seems to have enjoyed little engineering effort. Unfortunately, that cannot be farther from the truth. Early clutch disks were fitted with leather linings and were known not to last long.
In England, London city busses would require the friction leather to be replaced twice in one day if the driver was just a little aggressive with slipping the clutch. In like fashion, many early motorcars would need to have the clutch friction material replaced every 150 miles. A material that provided a long life, steady friction during the wear period, and smooth operation needed to be found. The leather linings were first replaced with impregnated cotton around 1905. The cotton-based lining dramatically improved friction plate life but was still a long way from being commercially viable.
Many started to test a woven asbestos fabric spun with brass wire to increase its strength. An Englishman, Herman Frood, invented this process and discovered a new bonding agent with high-temperature resistance. The clutch material was known as the Frodo Bonded Asbestos Lining. The new clutch material eventually evolved into a fiber asbestos base impregnated with copper or bronze strands to serve as thermal transfer agents and lubricate the lining under full load to prevent excessive wear.
Sintered clutch linings or segments were generally used in heavy-duty applications. The sintering process can be considered the squeezing together and heating of very fine particles of metal, such that extreme adhesion occurs between them. Many sintered linings employed a bronze base metal friction material. Although such materials were initially more expensive than a traditional asbestos base, many advantages existed. The sintered friction material would provide:
- Cooler operation and longer life with much higher thermal conductivity.
- More stable friction characteristics under difficult operating conditions such as full power or lugged.
- Can withstand high unit pressure without their performance being affected.
- Possess an extremely low rate of wear and minimize the need for in-field manual clutch adjustment.
- Thinner linings can be used to provide a more compact clutch unit.
The attachment of the friction material to the disk also has changed. This was in response to the higher engine output, reduced manufacturing costs, and the availability of new materials. The earliest friction materials were attached with rivets, and later, a bonding agent was tried with a thermosetting adhesive.
Bonding provided advantages over riveting. The useful life of the disk was increased since there was no rivet to consume an area of friction material. The rivet would encounter the flywheel before the friction material expired. Also, the bonding method provided more freedom in the number and position of the tangential ventilation grooves used as dust extractors. The downside of the bonded friction material is the increased effort it would take to remove an expired lining over drilling out a rivet.
It used to be expected that clutch material on the disk was replaced much like brake shoes were relined. This lowered the cost to the farmer for a repair since only the friction material was being changed – not the metal disk with its machining and springs. With the life of the friction material now increased to an acceptable level when the operator knew how to drive correctly, new designs and more engineering were required to improve the other characteristics. It was realized that the friction material was always a moving target since the behavior of one composition would perform well in an application but may have undesirable characteristics in another.
Furthermore, it was recognized that one-liner material would not serve every use. Thus, a set of standards were created but not officially published.
It was decided that a suitable material would have an amount of axial cushioning that would permit a smooth, gradual engagement that was shudder-free. It would need to reduce the initial pedal load required to disengage the clutch due to excessive spring force for good drive traction.
During the engagement process, the clutch pedal was vital for farmer satisfaction and needed to impart a linear action without grabbing too quickly or slipping excessively. And though life was still improved over the cotton material, the farmer demanded even less maintenance as the number of acres worked increased.
During the 1980s, the growing government pressure to reduce asbestos fiber and replace it with a more environmentally and healthy material forced the industry to reopen friction material development. For this reason, the friction material on the clutch you install on an antique tractor has little DNA with that used when it was built.