The defects in the gasoline engine, relative to its flexibility, have been previously mentioned. Among these is the inability of the motor to develop its full torque from a standstill. The crank shaft of the motor must rotate at a speed consistent with power requirements, while the road wheels must rotate consistent with road conditions, or as the operator wills. For this reason it becomes necessary to use a transmission. The motor must be started by a hand crank, or some starting device, which only produces enough torque to just turn the motor over against compression, so that it becomes necessary to disconnect the motor from the other driving units of the vehicle for starting and after the motor has attained its speed to connect it with the vehicle again.
For this purpose a device must l>e used, which will allow a certain amount of slippage until the motor speed has been reduced and the vehicle speed gradually accelerated to such a point that the two correspond, in this way preventing shock and jar to the driving mechanism.
This feature is accomplished by the clutch, which is most generally placed in close proximity to the motor. The most popular position is inside the flywheel. In commercial cars a single clutch is generally employed, which serves to connect the engine to the driving wheels through all of the different gear reductions. It is normally held in engagement by a single spring of large diameter, or by a number of smaller springs, and is controlled by a foot pedal to disconnect it from the motor by releasing the friction surfaces, thus disconnecting the power of the motor from the driving units. When it is desired to disconnect the engine in order to stop the car, or to change the gear, the clutch is first disengaged by foot pressure upon the pedal, which compresses the spring: the gear is then disengaged or changed and the clutch let in again.
There are quite a number of different types of clutches, all more or less extensively used, as follows: Conical clutches of the indirect or direct type, multiple disc clutches, dry plate clutches, band clutches, and combinations of cone and disc type. The construction of each type varies considerably in details of design and the materials used for the frictional surfaces.
In light commercial cars of 4,000-lbs. capacity, or under, there is a tendency to use the unit power plant, in which the motor, clutch and transmission are always held in alignment, while on the heavier types the transmission and jack shaft are combined in a unit or mounted amidships for shaft drive. With the latter types it is necessary to use a double universal joint between the clutch and transmission units. The universals take up any misalignment due to frame weaving. In some cases they are bolted to the clutch, spider or spigot, while in others they are built into the clutch center. This latter construction seems to be gaining favor with multiple disc clutches which operate in oil, a portion of which is distributed to the universal, causing it to be self-maintaining.
A variety of methods are resorted to for mounting the clutch on the spigot, plain, ball and roller bearings being used for this purpose.
There is also a tendency to provide clutch brakes so that the tendency of spinning caused by the inertia may be reduced to facilitate gear shifting.
Among the conical clutches we find two types in general use, direct and indirect types. Either type consists of a male and female member, the male member being forced into the female member by the pressure of the spring or springs. When one spring is used, it is attached to the clutch spigot and when a number of small springs are used they are attached to a spider, which is free to float on the clutch spigot. The action of the clutch members is similar to a wedge movement. It is the oldest type and also the simplest type in use at present. The flywheel generally forms the female member for the direct type, while the male member may either be made from aluminum or pressed steel and covered with a material such as Raybestos, leather, etc. In the indirect type it is necessary to bolt the female member to the flywheel. The clutch spigot may either be an extension of the crank shaft or it may be bolted to the flywheel.
Fig. G9 serves to illustrate the general construction of the direct type. The male member is provided with cork inserts to obtain a higher coefficiency of friction and is bolted to a cast-steel housing, which is mounted on the clutch spigot and surrounds the clutch spring. The spigot is formed by an extension of the crank shaft, and is provided with a thrust bearing.
The cone clutch, depicted in Fig. 70, differs from the above in that three small springs are used. These small springs are supported on studs, which are riveted to the clutch spider. The spider is provided with a die-cast babbitt bearing instead of a cast-bronze bearing. It depicts a type which is generally incorporated in the unit power plant, owing to its short length, which is a desirable feature.
An indirect type of cone clutch is shown in Fig. 71. The male member is made of aluminum and is provided with a frictional lining and cork inserts. Small pieces of rubber are placed.under the lining to obtain a smooth and gradual engagement.. The female member is made of gray iron and bolted to the flywheel. It also forms the retaining member.
The spigot is bolted to the flywheel and provided with a bronze bearing, while a roller bearing is used as a thrust bearing, and the disengaging collar is provided with a ball-thrust bearing. Cone clutches of both types are also provided with flat springs, or plungers, and coil springs, which are placed under the frictional facing, the object being to prevent the tendency to jerk when first engaged. The friction material is, in most cases, riveted to the male member, while in a few cases T-head bolts are used to facilitate its replacement, while one or two makers rivet it to the female member.
Fig. 69. General Construction of Direct Cone.
Fig. 70. Cone Clutch with Three Small Springs.