Document OnD4pxzwxmbL1pJyZeE4GJoj

Cold Forming of Metals Air friction clutch presses On a press with air friction clutch/brake, the control performs the following functions 1 Provides that the press operator must hold both run buttons during the down stroke until the slide has descended to a point where there is no longer a hazard Re lease of any button during "holding time" should stop the slide 2 Allows the operator to release buttons after completion of "holding time" and press will complete stroke 3 On "single cycle" operation, it stops the slide at end of a stroke even though op erator holds control buttons depressed 4 Actuates clutch/brake air valve(s) so that friction brake will stop machine at top of stroke 5 Provides anti-repeat protection and pre vents repeat operation with any buttons tied down Starters for main drive motor should be the magnetic, reversing type This feature per mits backing up of the press m the event of a jam without the need for an electrician to change wiring Selector switches should re quire keys so that backing up can be done by authorized personnel only The clutch control should be provided with a main drive motor "forward interlock" so that the press can be run only when the motor is turning m the forward direction "Back ward" operation of the press is unsafe be cause there is no run button "holding time " This interlock also prevents "run" actuation of the clutch air valve before the motor is started or when the flywheel is coasting Starting the motor with the clutch engaged is unsafe since it may result m unexpected downward movement of the slide Inch or jog operation should normally bypass the interlock to allow power-off "inching ' Pressure switches should be provided on the compressed air system downstream from pressure regulators and shut-off valves to as sure that air is present for proper actuation of clutch-brake mechanisms and counterbal ance cylinders These switches should be normally open and wired in senes into the control circuit ahead of the main-dnve-motor starter and clutch/brake control 1 The clutch pressure switch assures that the correct air pressure is available before the machine is started This assures proper movement of the slide and protects clutch plates from excessive wear resulting from slippage 2 The counterbalance pressure switch pre vents a sudden drop of the slide dunng the instant between release of the brake and engagement of the clutch Proper counter balance pressure is also required for quick stopping of the slide dunng the downstroke, or prevention of drop back if the slide is stopped dunng the up stroke Jogging or inching of the slide is often re quired when setting up or trying out dies The use of two-hand inch buttons is preferred for these operations so that both hands must be out of the danger area Where a single button is used, it should be guarded so that it cannot be operated unintentionally The clutch/brake solenoid valve(s) should not be energized directly across the line through the contacts of a single relay, in such a way that faulty operation of this single relay will result in unintended energization of the valve(s) Protection can be accomplished through the use of multiple relays and/or energization of the valve(s) through push but ton switches and rotary limit switches The previous paragraph makes stopping a machine independent of operation of the valve or run relay Normally closed contacts of this relay should be incorporated into the circuit to check actual dropout of the relay The use of a multiple clutch control valve is recommended This is a relatively inex pensive means for reducing the possibility of press repeats due to malfunctioning of a single valve Press clutch control circuits can be either of two types 1 Grounded. with all coils connected to one side of the isolated secondary of a control transformer, and this side grounded to the machine frame, or 2 Ungrounded (double ended) with ground detector lights connected to each side of the line and a double break arrangement On multiple operator presses, complete operating stations not required should be 728