Reviewing briefly the history of the automotive clutch and summarizing the most interesting achievements in clutch design during recent years, the author discusses friction facings and says that the development of the asbestos-base friction-bearing has made possible the multiple-disc dry-plate and the single-plate types. For severe service, the qualifications of a satisfactory friction-facing are density of structure, together with a reasonably high tensile-strength; the coefficient of friction should be high and fairly constant over a wide range of temperature; the facing must be able to withstand high temperature without deterioration; the impregnating compound must not bleed out at high temperature; and the permeation of the impregnating solution must be complete so that the wear resistance is constant throughout the thickness of the facing. The molded and the woven types of facing are treated at length.
Engagement methods, cooling and thermal efficiency, adjustment, methods of control, the lubricating of release sleeves and balancing are described. Since the clutch virtually converts engine torque into heat during the period of slippage just previous to full engagement, and since the heat must be dissipated through the clutch mechanism, the thermal efficiency of a clutch is of great importance. The severe service required of motorbuses has emphasized the importance of having a clutch rid itself of the large quantity of heat generated as a result of its frequent use and, from his experience, the author concludes that:
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(1)
If careful consideration is given to the subject of thermal efficiency, the unit pressure of the facings is unimportant over a wide range of pressures
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(2)
As a result of conclusion (1), it is believed that a single-plate or a two-plate clutch is the logical design for heavy-duty service. This is largely because it is much easier to provide for the necessary masses of absorption metal in these clutches than in the multiple-disc type
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(3)
The masses of absorption metal should be carried as a part of the flywheel weight
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(4)
Cast iron is the best metal to use as a friction surface for engaging the facing. The free graphitic content of cast iron provides a slight lubricating effect and permits the surfaces to attain a smooth high polish
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(5)
With their present knowledge of the subject, the engineers of the Long Mfg. Co. attempt to provide two elements for increasing the thermal efficiency of the clutch. A considerable mass of metal is provided in the driving discs, and this mass is designed to provide a large exposed area for a surface radiation. The mass serves as a reservoir that absorbs a large number of heat units without raising the temperature of the driving disc too quickly