CHAPTER VIII

Utility of Clutches and Gearsets Defined-Why These Vital Components of the Transmission System Are Needed on Gasoline Motor-driven Vehicles -Conventional Forms of Cone Clutch and their Practical Application--Characteristics of Three- and Five-plate and Multiple-disk ClutchesFunction of Gearset-Types of Speed Changing Mechanism-The Friction Transmission-Planetary and Individual Clutch Types-Progressive and Selective Sliding Gearsets-Typical Speed Changing Mechanisms Outlined.

ONE of the important functions making for efficient operation of the gasoline motor car is the method of power transmission employed. While power plant efficiency is an important factor and one that should be conserved to the utmost, it is well to remember that the actual power of the car is not the rated power of the engine but the amount of energy exerted at the point of contact between the traction members and the ground. A 60 H. P. car in which there is a 30% loss in power transmission is not as efficient as a vehicle of but 45 H. P. which delivers the power to the rear wheels with but 10% loss. Under the conditions stated the rear wheels of the lower-powered car would actually receive more useful effort than the driving members of the high-powered vehicle, and the energy is supplied with less stress on the various parts and with a lower fuel consumption.

The attention of mechanical engineers had been directed to the efficient transmission of power long before the motor car was commercially practical. As a result many ingenious systems which had been applied in standard mechanical work and for driving the machine tools of manufacturing establishments have been readapted for use in motor-car propulsion. One who has studied the subject can easily trace the evolution from the crude forms of a decade ago to the perfected types used in modern automobiles. The problem of power transmission in motor cars was one that was not easily solved,

and much experimenting was necessary before the perfected forms of the present day were evolved.

A typical power-transmission group such as employed in the modern gasoline automobile is depicted in Fig. 234. In this the power is applied to the crank shaft of the motor and from thence it is delivered to the motor fly wheel which forms the female member of a friction clutch. The male member of the clutch is coupled to the change-speed gearing and this in turn is joined to the driving pinion in the rear axle by a length of shaft. The driving pinion delivers its power to a bevel-driving gear which is carried by the differential casing in the rear axle housing. From the differential gear independent shafts or axles drive the rear wheel hubs.

The function of the clutch is to permit the engine to be run independently of the transmission gearing when desired. The engine can drive the car only when one of the sets of gears in the gearset and the clutch are engaged simultaneously. For example, if the clutch is out or released, even if the gears were in mesh in the change-speed device, the rear wheels would not be turned until the clutch cone was allowed to engage the female member formed in the fly-wheel rim. At the other hand, when the parts are as shown with the clutch in engagement and the speed gears out of mesh the engine can still be revolved without turning the rear wheels.

Why Clutch Is Necessary.-In order to secure a better understanding of the general requirements of clutching devices it will be well to consider the conditions which make their use imperative when a motor car is propelled by a hydrocarbon motor. If a steam engine or an electric motor are installed as prime movers it is not necessary to include any clutching device or gearset between them and the driving wheels, and these members may be driven directly from the power plant if desired. With either of the forms mentioned the power is obtained from a separate source which may be uncoupled from the motor by the simple movement of a throttle valve or switch lever. Steam and electric motors are also capable of delivering power in excess of their rating and are more flexible than internal combustion power plants.

If steam is the motive agent it is generated and contained in a special device known as a boiler, and the amount of power delivered

by the engine to which the boiler is connected will vary with the amount of steam admitted and its pressure. If the steam supply is interrupted entirely the engine and the car which it drives are brought to a stop. When it is desired to start again a simple movement of the throttle-valve lever will permit the steam to flow from the boiler to the engine cylinders again and the vehicle is easily set in motion. If it is desired to reverse the car the steam flow is reversed by a simple mechanical movement and the engine will run in the opposite direction to that which obtains when the car is driven in a forward direction.

If an electric motor drives a vehicle the electrical energy is secured from a group of storage batteries. When these are fully charged varying amounts of electric current may be drawn from them and allowed to flow through the windings of the field or armature of the motor and different ratios of power or speed obtained. The vehicle is easily started by completing the circuit between the motor and the source of current and stopped by interrupting the supply of electrical energy. As the flow of electricity can be reversed easily by a switch the car may be driven backward or forward at will, and as the speed may be easily varied by changing the value of the current strength there is no need of speed changing or reversing gears.

When a gasoline engine is fitted, conditions are radically different than with either a steam or electric power plant. The power developed depends upon the number of explosions per unit time and the energy augments directly as the number of explosions and revolutions of the crank shaft increase. It is not possible to start a gasoline engine when under load because the power is obtained by the combustion of fuel directly in the cylinders, and as there is no external source of power to draw from it is obvious that the energy derived depends upon the rapidity with which the explosions follow each other. It has been demonstrated that a certain cycle of operation is necessary to secure gasoline-engine action and it is imperative that the engine revolves freely until it attains sufficient speed to supply the torque or power needed to overcome the resistance that tends to prevent motion of the car before it can be employed in driving the vehicle.

Then, again, it is very desirable that the vehicle be started or

stopped independently of the engine. With a steam or electric motor the vehicle may be started just as soon as the driving power is admitted to the prime mover, but with a gasoline engine it is customary to interpose some device between the engine and driving wheels which make it possible to couple the engine to the wheels or driving gearing and disconnect it at will. The simplest method of doing this is by means of some form of clutching device which will lock the driving shaft to the crank shaft of the engine.

Clutch Forms and Their Requirements.-Clutch forms that have been applied to automobile propulsion are usually of the frictional type, though some have been devised which depend upon hydraulic, pneumatic, or magnetic energy. Those which utilize the driving properties of frictional adhesion are most common and have proven to be the most satisfactory in practical application. The most important requirement in considering clutch forms is that such devices. must be capable of transmitting the maximum power of the engines to which they are fitted without any power loss due to slipping. Such a clutch must be easy to operate and but minimum exertion should be required of the operator. When the clutch takes hold the engine power should be transmitted to the gearset and driving means in a gradual and uniform manner, or the resulting shock may seriously injure the mechanism. When released it is imperative that the two portions of the clutch disengage positively so that there will be no continued rotation of the parts after the clutch is disengaged.

The design should be carefully considered with a view of providing as much friction surface as possible to prevent excessive slipping and loss of power. It is very desirable to have a clutch that will be absolutely silent whether engaged or disengaged. If the clutch parts. are located in an accessible manner it may be easily removed for inspection, cleaning, or repairs. It is desirable that adjustment be provided, so a certain amount of wear can be compensated for without expensive replacement. A simple, substantial design with but few operating parts is more to be desired than a more complex device which may have a few minor advantages, but which is more likely to cause trouble.

The friction clutch in its various efficient types is the one that more nearly realizes the requirements of the ideal clutch. As a result