a 10 Ways to Increase Hydraulic Circuit Cycle Speed
  By Bud Trinkel, Certified Fluid Power Engineer
a      It is frustrating when a hydraulic circuit does not operate as fast as calculated.  The pump is actually producing design flow, piping and valves are sized at or below rated flow, machine members are not binding, and the control circuit is not causing delays, but, cycle time is just short of that required.  Following are things to check before increasing pump or component size or just operating at a slower speed.
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     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
     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.
Fig. 2
     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.
Fig. 3
     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.
Fig. 4
     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 applications.
     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.
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The author, Bud Trinkel, has worked for 31+ years in the fluid power industry. He is currently a consultant to the industry, providing training, troubleshooting, and circuit design expertise. He ca be reached at budthyd@comsource.net
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