July/August 2020

44 n BRAKES AND CLUTCHES July/August 2020 www.drivesncontrols.com Hybrid transmissions: it’s horses for courses H ybrid mechanical transmission systems can use centrifugal, one- way or electromagnetic clutches, or combinations of these technologies. To determine the most suitable technology for an application you need to take into account a variety of factors including: relative speeds; noise levels; duty cycles; the type of driving motor being used; torsional vibrations; the environment; service aspects; and, of course, cost. There are several common types of clutches and brakes: n Centrifugal clutches These engage to drive as the speed is increased, via spring- controlled flyweights. Their operation is automatic. n Centrifugal brake s These engage as a predetermined speed is reached, and thereafter apply more retarding torque, as speed increases. They too are automatic. n One-way clutches These engage immediately in one direction but run free in the opposite direction. Their operation is automatic. n One-way brakes These allow free rotation in one direction but prevent reversal. They have an inner or outer ring locked against rotation. n Electromagnetic clutches These operate freely in both directions. They can be engaged to drive while stationary, or running in either direction. Their operation is not automatic, and they need an external electrical supply. n Electromagnetic brakes These can be applied using electrical supplies and released when switched off, or applied with spring pressure and held off by electrical supply. They are not automatic, and need an external supply. In addition, there are several ways that brake designs can be combined, including: n Electrically controlled centrifugal brakes During normal operation at high speed, the brake is free because a DC supply to a solenoid releases the brake housing via brake disc. Electrical failure results in the spring locking the brake housing and allows auto-controlled descent at low speed. n Dead stop centrifugal brake with manual reset Exceeding a preset speed trips a spring-loaded brake, which brings the system to a standstill. The brake can be released manually after remedial attention. n Dead stop centrifugal brake with auto reset Exceeding a preset speed generates enough heat to apply a disc brake via a temperature-reactive element. After a cooling period, the brake is released automatically. An example of where this might be applied is a small wind turbine where it could prevent damage caused by high winds. One example of how centrifugal clutches can be applied comes from Niftylift, the Milton Keynes based manufacturer of access platforms. It uses an engine-driven centrifugal clutch combined with a flexible coupling, connected to a combined electric motor/generator which, in turn, drives a hydraulic pump to operate the machine. The diesel engine can be directed to drive the hydraulic pump and charge the battery via the clutch as required. When inside a building, for example, where fumes/noise would be a problem, the battery-powered electric motor is used to drive the hydraulic pump, but does not back-drive the engine because only the clutch housing is rotating freely, without engagement. While, for this hybrid transmission, Niftylift could have used a one-way clutch or an electromagnetic clutch driven by the engine, a centrifugal clutch offers several advantages, including: n automatic load-free starting conditions for the engine, followed by progressive clutch engagement as the speed increases; n the selection of a clutch design that resists the potentially damaging effects of torsional vibrations from the diesel engine, especially at low driving speeds; and n fully automatic operation without needing external controls. Other examples of where hybrid transmission systems are being applied include children’s roundabouts, emergency descent control on service platforms, and toboggan rides, where overspeed brakes and one-way clutches are used to prevent running back. n Hybrid mechanical transmission systems that provide brake and clutch functions offer many potential attractions. Edwin Jones, managing director of Combidrive, explains the various technologies and combinations available, and looks at a practical example of a hybrid transmission application. This personal descent device, made by the German manufacturer Skylotec, incorporates amanual braking system Niftylift uses hybridmechanical transmission systems incorporating centrifugal clutches in its access platforms