The belts run over matching toothed pulleys or sprockets. It is designed to run over matching toothed pulleys or sprockets. Some motors have electronic rheostat-type speed controls, which obviates cone pulleys or gears. The electric motor had to overcome friction between the pulleys and their bearings. Simple machines such as pulleys and levers are widely used. Power transmission is achieved by purposely designed belts and pulleys. A block is a set of pulleys or sheaves mounted on a single frame. Between the two pulleys runs a drive belt. The distance depended on the weight of the shaft and the number of pulleys. An assembly of blocks with a rope threaded through the pulleys is called tackle. Pulleys and weights were supplied to regulate the gas pressure. More advanced rigs can include working with pulleys, steel cable or trusses. They may be opened and closed with a metal lever, pulleys, or through motorized operators. This changes the effective diameters of both pulleys, which changes the overall drive ratio. The entire robotic system was operated by a series of pulleys and cables.