All the methods of engaging and disengaging the engine at will, as discussed before, have been of a mechanical nature. The hydraulic clutch, on the other hand, partakes more of the fluid nature, although semi-mechanical, i.e., operated by mechanical means. Ordinarily it is in the nature of a pump with a by-pass, the pump working at ordinary speeds to force the heavy liquid, usually glycerine, through the by-pass. To clutch up tightly, however, the by-pass is closed and, the liquid being unable to circulate while the pump continues to operate, the whole device is rotated as a unit. In this case it operates just as any other clutch, but, due to the sluggish action of the fluid, it is slower to respond. Then, too, there is always present the grave question of leakage, since the smallest leak puts the clutch entirely out of use. These disadvantages, together with the necessary complications, have retarded the development of the hydraulic form so that there are few of them in use today. In the clutch shown in Fig. 63, a spider Fgt. 63. Stilson Hydraulic Clutch is fast to the engine shaft and carries two spur gears B B. These are constantly in mesh with C, which is fast to the driven shaft. The space surrounding these gears is filled with oil, which is pumped around by the rotation of the gears. Several valves, one of which is shown at D, allow the oil to circulate freely. Under these circumstances, the pinions on the crankshaft run around the gear on the driven shaft and transmit no motion to the latter. These valves are held open by springs, but by the operation of the foot pedal, a cone is brought forward which presses against the heads of the valves, closing them. Partly closed, the pinions drive the gear slowly, because of the resistance which the liquid offers, and when wholly closed, the transmission shaft is driven at full speed with little loss, since the fluid is not very compressible. This clutch was formerly made by the Stilson Motor Car Company, Pittsfield, Massachusetts, which firm has since gone out of business.

Another clutch designed with the same idea, but worked out in a far different manner, is that of the North Chicago Machine Company, North Chicago, Illinois, and shown in Fig. 64. This clutch is more complicated and has many more parts than the Stilson clutch, but has the advantage of having seen more severe trials and more actual service. It acts as does the Stilson, oil normally passing through passages when the clutch is out, but when prevented from passing, the whole mechanism turns as a unit. The prevention of the liquid passing is accomplished by an eccentric device not very different from a vane type of water pump, the method of throwing it into and out of engagement being very complicated. Fig. 64 shows all the parts, and a study of it will reveal the complete action.

Fig. 64. Pambla Hydraulic Clutch Courtesy of North Chicago Machine Works, North Chicago.