Womack Data Sheet 10: Two-Hand Safety Control of Air and Hydraulic Circuits
Where cylinders are operating mechanisms which might present a safety hazard to the operator, dual controls may be provided. These require the use of both hands to start the cylinder and to keep it in motion. Dual controls are also used on large presses which require two operators. Each operator has a control; neither can start the press until the other is ready. Usually only one direction of cylinder motion is hazardous, so dual control circuits usually work in only one direction. In some applications the circuit does not need to be "unbeatable" as long as it offers some degree of protection. Where an "unbeatable" circuit is necessary, the final circuit on the opposite side of this sheet is offered.
Manually-Operated Hydraulic Cylinder
Figure 1. The operator must shift both Valves 1 and 2 to get the cylinder to move. If only one of them is shifted, the pump oil will simply by-pass to tank through the spool of the other one. This circuit gives 2-hand control in both forward and reverse. The lock valve prevents the cylinder from drifting or sagging if only one of the control valves is shifted.
Figure 2. For high volume circuits, over 30 GPM, this circuit is less fatiguing to the operator. Control valves 1 and 2 may be 1/4" miniature size with open center spools. Valve 3 is the main directional control and may be as large as needed for the circuit.
Pilot oil for operation of Valve 3 comes from the RC (remote control) port of the pilot-operated relief valve. With control valves 1 and 2 in center position, vent oil from the relief goes to tank and the pump remains unloaded. If both manual valves are shifted, vent oil from the relief flows into one of the pilot caps on Valve 3, shifting its spool to a side position. The orifice is optional and may be omitted. Its purpose is to increase the shifting speed of Valve 3 by adding a small amount of supplemental pilot oil taken directly from the pump line. Normally it should be very small, perhaps 1/32 to 1/16" diameter.
This circuit gives almost shockless shifting because the pump pressure does not build up until the 4-way valve has been shifted to a side position.
Manually-Operated Air Cylinder
Figure 3. The preferred method of 2-hand control on an air cylinder is to use a large 4-way valve, pilot-operated, for directional control of the cylinder. Two small 3-way air valves, 1 and 2, control pilot pressure to the main valve.
Because of the compressibility of air, the circuit of Figure 3 is considered to be better and safer than other circuits which place Valves 1 and 2 directly in the cylinder lines, one connected to each cylinder port. In the latter case, if only one valve should be actuated, a reactionary load might cause the cylinder to move out of position, or a vertically mounted cylinder might drop under force of gravity.
Electrically Operated (Solenoid) Valves
Figure 4. On electrically operated machines using solenoid valves, It is relatively simple to wire two pushbuttons in series. The operator must hold both buttons down before the valve solenoid can be energized. In this circuit, if the operator should release either button while the cylinder is traveling forward, the cylinder will immediately reverse.
To make it more difficult for an operator to tie or weight down one button, recessed pushbuttons should be used, and should be mounted on a vertical surface. Where "unbeatable" controls are required, use the electrical circuit of Figure 5.
Series push buttons may be used on both air and hydraulic cylinders. On hydraulic circuits using double solenoid valves, the pushbuttons are usually wired to de-energize both solenoids and allow the 4-way valve to center, thus stopping the cylinder motion rather than retracting it.
Non-Tie-Down Electrical Circuit
Figure 5. This is the standard circuit used on solenoid valves where maximum protection is needed to prevent the operator from tying down one pushbutton. Both pushbuttons, 1-PB and 2-PB must lie pressed at the same time within a time difference of 1 second, in order to energize the valve solenoid coil.
Two time delay relays, 1-TDR and 2-TDR, each with one set of normally closed contacts, are used. They are adjusted for the contacts to open 1 second after the coil is energized. If one pushbutton should be tied down, or if both buttons are not pressed within a 1-second interval, relay coil, 1-CR, although it may momentarily pull in and energize the solenoid valve coil, it will drop out again. After false operation, to restore the circuit, both pushbuttons must be released. When either or both pushbuttons are released, Relay 1-CR will drop out, de-energizing the solenoid valve coil through the opening of Contacts 1-CR-2.
The time delay relays are industrial relays with dashpot to delay contact action when the coil is energized. Your regular electrical supply house can furnish them. Relay 1-CR is a standard industrial relay with 2 sets of normally open contacts. Push buttons have two sets of normally open contacts.
© 1988 by Womack Machine Supply Co. This company assumes no liability for errors in data nor in safe and/or satisfactory operation of equipment designed from this information.