| During circuit design, give consideration
to acceleration and deceleration time of the actuators as well as the volume
needed to cycle them. Using actuator swept volume to size the pump
does not move it fast enough to meet the desired cycle time. When
actuators have just enough force to move the load, acceleration, time is
long. After acceleration the actuator must move faster than figured
to maintain cycle time. Also, heavy loads usually need some deceleration
time, so reduced speed during deceleration causes more delay. With
a light actuator load, acceleration and deceleration is fast, but still
adds to overall cycle time.
| Fig. 1 shows a typical plot for a double rod
end cylinder that has 10.16 square inch area and a stroke of 20? in one
direction. At a cycle time of 6 seconds it would need 17.6 GPM to
move it both ways if it ran full speed from start to stop. GPM =
Piston Area (sq. In.) X Stroke (in.) X 60 sec./Cycle Time (sec.) X 231
(cu. In./gal.). Actually the cylinder must accelerate twice and decelerate
twice each cycle and is only half speed during this time.
| When acceleration and deceleration take 1?
of stroke at each end it means the cylinder must travel at a faster speed
for the 18? between each acceleration and deceleration. In effect
the stroke increases to 22? for the GPM formula. This means it will
take a pump with 19.4 GPM flow to meet the required cycle time of 6 seconds.
| Another circuit problem that is often over
looked is cylinders with large rods. Over size rods are some times
necessary for column strength. Other uses for over size rods are
regeneration circuits and fast retract speed during the low force portion
of the stroke. In either case, flow from the cap end of a cylinder
with an over size rod can be twice as much as pump flow or more.
Also many regeneration circuits send rod end flow through the directional
valve during fast extend. Size valves and piping for this extra flow
so increased pressure, to force fluid to regenerate and/or return oil to
tank, is not necessary. When pressure raises to overcome extra flow
resistance, a relief valve may bypass or a pressure compensated pump might
start compensating. Now a cylinder that should extend and retract
in 1.5 seconds takes 1.6 seconds.
| There is always a relief valve in a circuit
with a fixed volume pump. Often during maximum flow periods, pressure
rises above relief setting and bypasses some oil to tank. When relief
valve bypass is suspect, pipe its tank line for visual inspection.
Another way to check for reduced system flow is to put a flow meter in
the line between the pump and relief valve and another flow meter downstream
of the relief valve.
| If the relief valve is a direct acting type
and it is bypassing, replace it with a pilot operated one. Direct
acting relief valves usually start bypassing some fluid 15-20% below their
set pressure. If the relief valve is already a pilot operated type,
raise pressure until bypassing stops. If the increased pressure is
too high for safety or system components, circuit changes may be necessary.
| A solenoid operated relief valve used to unload
a fixed volume pump between cycles has an adverse effect on cycle time.
A solenoid operated relief valve can take several milliseconds to close
after it receives an electrical signal. First there is the response
time of the solenoid control valve, then, oil flowing through the relief
valve control orifice must reseat the poppet or piston to stop tank flow.
This slow response is more noticeable while unloading the pump several
times during a cycle.
| According to the machine function, energize
the solenoid operated relief valve before the cycle starts and/or leave
it on during the entire operation.
| When a pressure compensated pump, Fig. 2,
is the prime mover, cycle starts always lag. Pressure compensated
pumps, at rest, hold full pressure but no flow. When a valve shifts
to start the cycle, pressure begins dropping. Until pressure drops
about 2-10%, the pump is still at zero flow. The pump's mechanism
finally starts to shift several milliseconds after a cycle start signal
goes to the directional valve. Soon after this the actuator starts
moving. Some pumps have longer response time than others. Generally
speaking, pressure compensated piston pumps start shifting at less pressure
drop and shift faster than vane pumps.
| With an accumulator added, Fig. 3, to a pressure
compensated pump circuit, pump response time does not change but actuator
start is greatly enhanced. Oil from the accumulator starts feeding
the actuator when the directional valve shifts. Pressure still drops
and the pump starts responding as before, but now it has time to catch
up with little or no affect on cycle time.
| Any hydraulic circuit can have trapped air
in the lines and actuators. These voids or empty spaces must be filled
with oil. At cycle start, the actuator sets still until the air pockets
reach a high enough pressure to move it. At work contact, compressing
the trapped air to working pressure adds cycle time. Air pockets
add volume and volume adds cycle time.
| Most of the time, air in a hydraulic circuit
quickly dissipates. If the air does not clear it will affect cycle
time. Air bleed ports at all high points in the piping and at each
end of all cylinders make bleeding fast and easy. Also, further bleeding
is easy through these bleed ports anytime the machine slows again.
| Spool type directional valves with on-off
solenoids have overlap of spool land to body lands, Fig. 4. Overlap
minimizes leakage through the valve when it is pressurized.
Overlap may only be .06-.12? but it takes time for the spool to move across
it to open valve ports. After a solenoid operated spool valve receives
a signal to start an actuator, there is no flow until the spool shifts
through its overlap. In the case of a solenoid pilot operated valve
the slave spool also has to move through overlap before the actuator can
start to move. This time is only milliseconds but adds to the overall
cycle each valve shift. It is possible to add .1-.3 seconds to the
cycle when several valves shift both directions of travel.
| Spool type directional valves with on-off
solenoids also shift completely when cycled. To keep pressure drop
low, these directional valves are often over sized, so complete shifting
may not be necessary. Extra spool travel past the point of maximum
system flow does not bother actuator start time but can add to the cycle
time when the spool returns to center or shifts to the opposite side.
Decreasing spool shifting distance can shorten cycle time when the directional
valve is oversized. Spool stroke limiters, Fig. 4, are the usual
method to shorten spool travel. Spool stroke limiters are screws
in the ends of a solenoid pilot operated valves main spool that can adjust
maximum shifting travel. Set them so the actuator speed is maximum
while over travel is minimum.
| Spool stroke limiters also reduce response
time as the spool of a solenoid pilot operated valve spring returns or
shifts to the opposite side. The times here are in milliseconds,
but over shift can increase cycle time greatly when using several valves.
| Direct operated solenoid valve spools should
not have their travel stroke limited since this might cause over heating
of the coils. Most valve manufacturers, though, offer a spool stroke
limiter option on the pilot operated spool of their solenoid pilot operated
valves. Spool stroke limiters can also replace flow controls in some
| Low pilot pressure at a solenoid pilot operated
directional valve is another cause for sluggish response. Most valves
need at least 50 PSI to shift against the springs and back pressure.
Higher pilot pressures up to 500 PSI make the valves shift much quicker
in all cases. Another possibility for most of this type valve is
a larger or removed orifice plug in the pilot circuit. When
pilot pressure changes throughout the cycle, a separate constant pressure
pilot circuit is advisable. Set the pressure on this external pilot
circuit at 250 to 500 PSI.
| Proportional solenoid valves work well in
a fast cycle situation. They usually shift only enough to get the
desired speed and most have minimum overlap in center condition so the
actuator starts quickly.
| Slip in cartridge valves are a great way of
getting fast response in high flow circuits. Slip in cartridges are
normally used on flows in excess of 60-100 GPM. Since this type of
valve is essentially a pilot to close check valve, it gives flow the instant
it moves and never opens wider than necessary while flowing. One
manufacturer now offers a D08 and a D10 size valve with slip in cartridges
to replace the standard slave spool.
| When using solenoid valves, another
option that can gain time is to replace the normal AC solenoids with DC
solenoids. A DC signal operates a solenoid when it reaches the coil.
An AC solenoid may have to wait for the alternating current to reach at
or near its peak to shift. Again the delay is in milliseconds but
does add to overall cycle time.
| Some of the above may seem a little unnecessary,
but every little bit counts with fast cycle times.