Two-Hand Safety Control of Air and Hydraulic Circuits
FLUID POWER - Design Data Sheet 10
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
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
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
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 1s 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.
Download a PDF of
Fluid Power Design Data Sheet 10 - Two-Hand Safety Control of Air
& Hydraulic Circuits.
© 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.