Designing Slow Feed Circuits - Part 2, Air Cylinders
FLUID POWER - Design Data Sheet 38
Part 1 of this series, covering hydraulic cylinders, is in
Design Data Sheet 28. The purpose of the present issue
is to suggest circuit arrangements which will minimize the tendency
of an air cylinder to "chatter" or move erratically when operated
at a slow rate of travel.
Air cylinders are essentially power, not feed, devices. In
general they should not be used to move metal cutting tools at a
slow feed rate. An exception is for "crowding" a drill bit into the
work; for this, the work resistance provides the stabilizing force.
For slow feeding of milling machine tables, grinder tables, shapers
or planers, hydraulics must be used.
Air cylinders perform best when operated at moderate to high
travel speed. But where air power must be used at moderate to slow
speeds, the ideas presented in this issue should prove helpful.
Figure 1. Speed Control Method. Experience
shows that an air cylinder is less likely to chatter at low speed
when "meter-out" rather than "meter-in" speed control is used. This
puts a substantial pressure on both sides of the piston, thus
increasing stability. The higher the back pressure, against the
forward face of the piston, the greater the stability.
In order to have a high back pressure for stability, the
cylinder bore should be large enough to give much more force than
needed to just move the load. The greater the cylinder is
overpowered with respect to the load, the higher the back pressure
and the more stable the action will be.
For example, in Figure 1, if inlet pressure to
the blind end is 100 PSI, and if pressure actually required to move
the load is 30 PSI, this will give approximately 70 PSI back
pressure. If the load should require 80 PSI just to move it, this
would leave only 20 PSI back pressure and the cylinder would have a
greater tendency to move erratically.
On existing systems, if the inlet pressure can be raised, this
will give a higher back pressure under the same load and speed
conditions. On new designs, the air cylinder should be oversized by
200 to 300% over the load resistance.
Figure 1. A
high back pressure against the forward
surface of the piston increases stability at slow
Figure 2. Pressure Differential Switching.
While "meter-out" speed control as described above gives more
stable operation, it presents a problem when a pressure sensitive
device such as a pressure switch or sequence valve is to be
installed at the blind end cylinder port for sequencing to a second
A standard pressure switch, for example, teed into the cylinder
blind port will not work well with meter-out speed control because
it may be prematurely activated. The solution is to use a pressure
switch of the "differential" type having two pressure sensing
ports. These ports are teed into the two cylinder ports. The switch
is then sensitive to the pressure difference across the
While the cylinder is moving the load, the switch senses only
the load pressure, and this is insufficient to trip its contacts.
But when the cylinder stops against a positive stop, the
differential pressure rises to full system pressure and closes the
switch contacts. The pressure switch should be adjusted to trip
very close to full system pressure so it will not momentarily trip
when the load is first started.
A sequence valve can be connected to have an action similar to
above. See Design
Data Sheet 15.
Figure 2. For
reliable action in a sequencing circuit,
a differential type pressure switch should be used
the speed control valve is connected meter-out.
Figure 3. Air-Over-Oil System. This is a
conventional, single-tank, air-over-oil system as described in
Womack textbook "Industrial Fluid Power - Volume
The cylinder speed is metered in the forward direction as oil,
pushed out ahead of the advancing piston, is forced through the
flow control valve and into the air/oil tank. When the 4-way valve
is reversed to retract the cylinder, air pressure on top of the oil
in the tank forces the oil back into the rod end of the
The 4-way valve is a standard air valve. The cylinder can be an
air cylinder or low pressure hydraulic cylinder, but must have
leak-tight piston seals.
The flow control valve controls cylinder speed only while it is
extending. A double tank system must be used for speed control both
directions. Refer to above source for more information.
Conventional single-tank, air-over-oil system.
Figure 4. Metering Cylinder. An improved
arrangement for oil metering of the speed of an air cylinder is
shown. This method can be added without too much difficulty to an
existing air system which does not operate smoothly at slow feed
rates. This system has several important advantages over the
straight air/oil system shown in Figure 3
- The compressed air does not come into intimate contact with the
metering oil. Therefore, there is no oil mist discharged into the
atmosphere. In an air/oil system, some of the oil vaporizes into
the air every cycle and is carried out the exhaust of the 4-way air
- Since oil is not discharged into the atmosphere, it seldom has
to be replenished. This reduces maintenance cost.
- If desired, an oil metering cylinder of very large volume can
be used. The large flow through the flow control valve permits more
accurate metering of speed.
- All components are catalog standard. No specially constructed
air/oil tank is required.
- Only the small make-up tank has to be mounted at an elevation
higher than the metering cylinder.
A second cylinder, rated for low pressure oil, is added for the
sole purpose of metering an oil flow to control machine feed rate.
Its placement depends on the physical layout of the machine. For
simplicity of illustration it is shown attached to the load,
although it could be attached to any moving part of the machine or
even to the air cylinder. Its bore can be smaller or larger than
the air cylinder bore, but the greater its volumetric capacity, the
higher will be the oil flow during metering, and the more accurate
will be the speed control.
During the metering stroke, oil is pushed from the blind end of
the oil cylinder and metered through the flow control valve to the
rod end. Surplus oil which cannot enter the rod end is stored in
the make-up tank. On the return stroke this surplus oil again
enters the system and flows freely to the cylinder blind end
through the check valve.
Obviously, the make-up tank must have a greater volume than the
volume of the cylinder piston rod. A compressed air line filter
with plastic bowl and bowl guard makes an economical and
satisfactory make-up tank.
The oil used in the metering cylinder should be very low
viscosity and compatible with seals and the plastic in the make-up
tank. ATF transmission fluid, Type A or Type F is a good fluid to
The oil cylinder is shown in Figure 4 to meter
when the piston is pushed in. To make it meter when the piston is
pulled out, move the make-up tank to the blind end port and turn
the flow control valve around.
Note: Self-contained "hydraulic checking
cylinders'' can be purchased with self-contained flow control valve
and reservoir. These units can be installed and adjusted so there
is some fast free travel before the slow metering action
Figure 4. An
air powered system with oil metering for smooth travel at low
Download a PDF of
Fluid Power Design Data Sheet 38 - Designing Slow Feed Circuits -
Part 2, Air Cylinders.
© 1990 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