Band clutches are practically the same in general principle of operation, as band brakes, and are of the same general types, internal expanding and external contracting. This type of clutch is not very popular, and but few can be found on commercial cars at present.
There is a tendency on the part of manufacturers to use ball bearings for supporting clutches of all types, as the plain bearing is hard to lubricate effectively and the friction of same tends to produce dragging. The ball bearing can be more easily lubricated, requires less attention and eliminates this dragging evil.
The advantages of the cone clutch are that it may be engaged and disengaged with very small axial motion, axial pressure may be low because the normal pressure between frictional surfaces is multiplied by the angularity of the cone, its weight is not very great as the male member may be made of aluminum or pressed steel, its engagement is entirely independent of speed and centrifugal force, no liquid lubricant is needed with attending viscosity, drag and change due to wear and temperature. Disengagement may, therefore, be made perfect. The chief disadvantage of this clutch is its size, it being more bulky than the other types with the possible exception of the dry-plate type. Inertia is also a disadvantage, as this must be as small as possible, in order to make gear shifting easy and to avoid gear clattering.
These objections led to the introduction of the multiple-disc clutch, in which the frictional surface can be made larger and the frictional force smaller per unit surface. The chief disadvantage of multiple-disc clutch is its tendency to drag if the oil in the clutch housing is not suitable for the purpose. Most makers recommend a light machine or cylinder oil and kerosene. It can readily be understood that the thinner the lubricant, the better the clutch will hold, while the more viscous lubricant will permit it to pick up its load more gradually.
To overcome the dragging evil, the dry disc type was introduced. The surfaces are not lubricated but are most generally provided with frictional facings in order to increase the coefficient of friction. This clutch has its disadvantages of inertia, similar to the cone type.
The combination of dry plate and cone has the features of the dry plate, its tendency to gradually pick up its load and the holding power of the cone after it has assumed its load. It is also simple in construction. However,it requires frequent adjustment.
At the present time, the cone, multiple disc and plate clutches are by far the most popular and seem to be holding their own, with all the new types which are being experimented with*
In every gasoline engine it is absolutely necessary that some method be used for changing the relation between the speed and power of the car. When a gasoline engine is loaded above a certain limit it slows down, and the intervals between the explosion in each cylinder become so far apart as to cause the engine to labor and finally stop altogether, unless some means is used to increase the speed of the engine by decreasing the load upon it. In considering this subject it must be remembered that, when a car is using its maximum power, it may be divided either into considerable pulling power with slow speed, or high speed with low pulling power. Consequently, when a car is going at high speed and a considerable grade or a stretch of heavy road is encountered, the car will begin to slow down until the speed reaches such a point that the engine begins to knock and labor. When this point is reached, it becomes necessary to change to a slower gear, which, for the same speed of the vehicle, gives a considerably greater number of revolutions of the engine, with a consequently larger pulling power.
This pulling power is termed "torque," and if gasoline engines could be designed as to afford an increasing torque, with decreasing speed, all would be well and the transmission could be eliminated. As it is taking into account the power of the motor at several speeds, nothing of this sort can be considered. At very low speed torque becomes of greatest importance, and this is especially true in vehicle operation.
The use of the transmission is also necessary in starting the vehicle, because until the vehicle reaches a certain momentum, there is considerable load on the engine, so that a slow speed which allows a high number of revolutions of the motor must be used.
It is generally understood that to reverse the motion of the commercial car engine is to labor under disadvantages in numerous ways. Power will be lost, owing to the inferior valve timing relation which must follow if the cam shaft was designed to suit reversing condition. Unless certain complications were introduced in the valve action, and since in any case it would be necessary to add to the flexibility of the motor by the use of a transmission, it would seem unneeessary to add to the valve motion anything by the way of complicated devices. An addition to the gear set is less complicated and the end is adequately served.