Most overhead trams require haulage or tow cable to travel up and down steep inclines. A 22-passenger, 12 000 pound (around 5000 kg) hydraulically powered and controlled tram is shown in Figure 9.2.
It is self-propelled and travels on a stationary cable. Since the tram moves instead of the cables, the operator can easily start, stop and reverse a particular car completely independent of any other car in the tram system.
Integral to the design of the sky tram drive is a pump (driven by a standard 8 cylinder gasoline engine) which supplies pressurized fluid to four hydraulic motors. Each of the four motors drives two friction drive wheels. Eight drive wheels on top of the cables support and propel the tramcar. On steep inclines, while a higher driving torque is required for ascending, a higher braking torque is required during descent. Dual compensation of the four hydraulic motors provides efficient proportioning of the available horsepower to meet the variable torque demands.