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by pressure of a coil spring against a pressure plate, which presses against the outer driving plate and holds the entire disc assembly into firm engagement. Obviously as long as the discs are held together the engine power must be transmitted from the clutch case attached to the flywheel through the disc assembly to the central member carrying the driven plate and attached to the gearset drive shaft. To release the multiple disc clutch the spring is compressed enough to allow the driven plates to turn freely between the driving plates.

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Fig. 151.-Multiple Disc Clutch Parts Shown in the Order They are Removed from Flywheel in Taking Down Assembly.

Q. How many types of multiple disc clutch are used.

A. Multiple disc clutches may be of the all metal disc type, which are designed to run in a bath of lubricant, or of the dry plate pattern where one set of plates, usually the driven members, are faced with some material designed to have greater frictional qualities than metal surfaces when run without lubrication. One form of multiple disc clutch, that shown at Fig. 152, differs from the conventional form in that the clutch discs are stamped out so the surfaces in contact are in the form of a cone. In general construction this clutch follows conventional design.

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Fig. 152.-Sectional View of Hele-Shaw Multiple Disc Clutch and Efficient English Design.

Q. Describe advantages of multiple disc clutches.

A. The most important advantage of a multiple disc clutch is that it is small and compact, and its construction such that very gradual engagement is attained. Owing to the large number of friction surfaces and the presence of a film of oil between each pair of surfaces it is practically impossible for a clutch of this pattern to engage harshly, as the spring pressure must first squeeze out the film of oil between each pair of plates before the friction adhesion is great enough to drive positively. Owing to the small size and relatively light weight of the driven plate assembly this is not liable to spin as is the heavier cone or three plate clutches, but to offset this advantage there is a liability of dragging, due to the adhesive properties of the oil film present between the plates.

Q. What materials are used in multiple disc clutches?

A. The discs of multiple disc clutches designed to operate in an oil bath are usually thin stampings from saw steel sheets which are hardened and ground to a true surface. In some clutches the plates are all steel and in other forms the driving members are of steel while the driven plates are stamped from phosphor bronze sheets. In dry plate clutches the driven plates, which are faced with Raybestos or other suitable friction material, are steel stampings while the driving plates may be either steel stampings or cast iron plates. In some dry plate forms the driving members are cast iron while the driven plates are of bronze studded with cork inserts.

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Fig. 153. Construction and Application of Haynes Constricting Band

Clutch.

Q. What is a positive clutch?

A. The forms of clutches in which projecting members fit into depressions in the coacting part and which act as positive mechanical locks are termed "positive clutches."

Q. Are positive clutches ever used alone?

A. Positive clutches are always used in connection with a master clutch of the friction type because they would be much too harsh in action and impose considerable strain on the power plant and transmission parts if used alone.

Q. What types of band clutches have been used and how do they operate?

A. Band clutches have been made in two forms, the internal expanding and external constricting types. The former is seldom used at the present time because it is a very difficult type to balance properly and to keep in proper adjustment. The external band form outlined at Fig. 153 is used on the Haynes car and it consists of a metal clutch band adapted to constrict around a drive ring attached to the flywheel. The band is tightened by means of a constricting lever operated by a cam or inclined plane member pressed forward by a coil spring. In operation the external band, which is attached to the gearset drive shaft, clamps tightly around the drive ring while the internal band form spreads out against the internal periphery of a similar drive ring integral with the flywheel.

Q. What are the essential requirements of clutches and which type meets most of the requirements?

A. A clutch must be simple to construct, apply and operate and it must be capable of transmitting the maximum engine power in a positive manner. It must release promptly and yet it should not engage too harshly. It should be of such form that any wear occurring will be automatically taken up and it should be easy to restore the frictional materials when these deteriorate. Practically all of these requirements are met by the friction clutch in any of its forms and the cone form owing to its simplicity and strength of parts is the most favored of all clutch types.

LESSON SIXTEEN

THE FRICTION TRANSMISSION

Q. Why is change speed gearing necessary on gasoline automobiles?

A. The gasoline engine develops power in direct proportion to its speed up to a certain point and it will slow down whenever an overload is applied. More power is needed to climb hills than is used to propel the car on the level and if the gasoline engine was directly attached to the rear wheel with a definite unvariable ratio it would not be possible to climb hills unless this reduction of speed between engine shaft and rear wheels was very low. When conditions were favorable, such as when the resistance to motion was slight, as on smooth level highways, the low reduction provided would be too slow and the car speed would be limited to that permitted by the gear ratio. At the other hand if the gear ratio was higher to permit more speed under favorable conditions it would be too high for hill climbing or propelling the car over rough road surfaces. In order to enable the power plant to develop its full power it is customary to provide some form of change speed gearing to permit of altering the ratio of drive between the engine and the rear wheels to suit road conditions. The usual ratio on the direct drive or highest speed varies from three to one to five to one in pleasure cars and may go as high as ten to one on heavy vehicles. This means that the engine crankshaft will make from three to ten turns while the rear wheels are revolving once. As a gasoline engine is not reversible it is customary to incorporate gearing that will permit of reversing the car motion in the change speed gear set. Q. Why are electric and steam automobiles without change speed gearing?

A. It has been previously explained that steam engines or elec

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