US Patent 7926622 Lift cable slack monitoring device and method

A lift drive unit has at least one monitoring device for monitoring unallowed raising of a lift cage. The drive unit has a motor unit and a deflecting unit. If a counterweight supported by the deflecting unit, for example rests on a shaft pit buffer, the deflecting unit is unloaded and is raised by way of a spring element of the monitoring device. A sensor of the monitoring device detects the movement of the deflecting unit and switches off the motor of the drive unit via a safety circuit.