Document 6BzD5J9RjbO26b0o90OGQdRa6

the hydraulic and the pneumatic-powered presses Each is driven by a gear tram which can be started or stopped by simply cutting the power supply All that has been said so far about mechan ical power presses (except for the source of power) applies to hydraulic or pneumatic powered presses These are used for the same kind of work and have similar hazards, such as sudden dropping of the slide because of power failure or breaking of a pressure line, defective dual controls, unexpected eneigizing of one or more electric circuits because of oil or dirt, defects in wiring which cause the press to close, leaking valves, residual or back pressure, and air pressure built up between dual controls Where air or hydraulic power is used either alone or in combination, there is the hazard of residual or back pressure Even though the control valve be turned off and locked in place, some pressure may be locked m the system past the valve point. This residual pressure has produced acci dental cycling of equipment even though per sons involved had thought all the necessary precautions had been taken by shutting off the power source valve As a result, when the tripping device for the machine was next touched or when the power source valve was again turned on, the equipment accidentally cycled, producing a dangerous situation Figs 32-2 and -3 show methods used to pre vent this Press brakes The press brake is classed as a power press because it utilizes the same construction and operational features that characterize the me chanical and hydraulic power presses It has a bed and a descending slide, called the ram, and can be powered either mechanically or hydraulically--but there the similarity ends The ram and bed are long and narrow, matched to each other, and usually set m front of the gapped frame (Fig 32-4) In some cases the bed and ram are located be tween upnghts, but this arrangement limits the length of material that can be processed In operation, both types of press brakes uti lize the part revolution clutch type of dnve, and thus can be started and stopped at will Most press brakes operate only m what Fic, 32-4a --Typical mechanical press brake Fic 32-4b --Typical hydraulic press brake From ANSI Standard Bll 3-1973 would be termed "low gear," but some special purpose brakes are equipped with two speeds which can be used either individually or in combination Most press brakes are used to form or bend laige or small pieces of sheet metal to a de sired shape The open end construction of the ram and bed even permits the continuous forming or bending of pieces that are much longer than the bed and ram, when appropri ate accessories are used 861