HYDAC SB330 Sizing to Reduce Hydraulic Pump Start-Stop Cycling

An accumulator on an intermittent clamping machine supplies oil while the pump is stopped. Add the movement demand and leakage during that period, then compare the total with the accumulator's usable oil volume.

Nitrogen always occupies part of a HYDAC SB330. Only part of its oil is released between pump-stop and pump-start pressures. A nominal 5 L accumulator therefore does not supply 5 L to the machine.

The following calculation concerns a circuit that starts the motor at the lower pressure and stops it at the upper pressure. A continuously running motor with pump destroking or unloading has loading cycles rather than the motor start-stop cycles discussed here.

Oil demand for one movement every 20 seconds

Consider an intermittent clamping machine with these values:

The 0.15 L includes all oil supplied by the accumulator during a complete cycle. If extension and retraction both draw from this branch, include both demands.

This example selects a 90 bar precharge for a 100 bar minimum working pressure. HYDAC energy-storage sizing also accounts for temperature. A changed minimum pressure or temperature requires recalculating the precharge.

A 5 L unit delivers about 0.98 L over this pressure range

Estimate delivery using the polytropic gas relation:

ΔV = V₀ × [(p₀ abs / p₁ abs)^(1/n) − (p₀ abs / p₂ abs)^(1/n)].

V₀ is effective gas volume, p₀ precharge, and p₁/p₂ the start and stop pressures. Use absolute pressure: 90, 100 and 140 bar gauge become approximately 91, 101 and 141 bar absolute.

Using n = 1.4 as an adiabatic approximation for short delivery gives:

At these pressures, doubling gas volume doubles usable delivery. Dividing nominal volume directly by oil per cycle overestimates the number of movements.

Slow delivery with sufficient heat transfer is closer to an isothermal process, n = 1. This gives about 1.28 L for the 5 L unit. Cycle timing and thermal behaviour affect delivery; the remaining comparison continues with n = 1.4.

Eight cycles: 5 L or 10 L?

Eight cycles consume:

8 × 0.15 = 1.20 L.

At 20 seconds per cycle, the supply period is approximately 160 seconds. An additional continuous leakage of 0.05 L/min consumes:

0.05 × 160 ÷ 60 ≈ 0.13 L.

Total demand is about 1.33 L. The 5 L accumulator delivers only about 0.98 L under these conditions and cannot complete eight cycles. The 10 L unit provides about 1.97 L and meets the demand.

Reversing the same calculation gives a minimum effective gas volume of about 6.77 L. Adding a 20% allowance for demand variation increases this to about 8.13 L. Between the 5 L and 10 L options, choose 10 L.

The allowance covers modest demand changes. Repair a progressively increasing leakage path rather than repeatedly enlarging the accumulator to preserve pump-off time.

How long does the 10 L unit take to recharge?

Three cycles per minute consume an average 0.15 × 3 = 0.45 L/min. With 0.05 L/min leakage, total average demand is 0.50 L/min.

If the pump supplies 6 L/min throughout the 100–140 bar charging range, average net charging flow is approximately 5.50 L/min. Replacing about 1.97 L in the 10 L accumulator takes roughly 22 seconds.

Larger capacity extends pump-off delivery time but also increases oil replenished on each start. The pump must supply machine demand and still have flow available for charging.

If simultaneous movements consume nearly all pump output, pressure may fail to reach 140 bar promptly. Address supply capacity and movement scheduling; increasing accumulator size alone will not make the pump reach its stop pressure faster.

Controls must also respect the motor's permitted starting frequency and minimum running time, with appropriate delay to prevent rapid repeated starts.

Where to look when cycling suddenly becomes frequent

If the machine's operating rhythm is unchanged but cycling has recently increased, inspect leakage, precharge and controls before buying a larger accumulator.

Pressure falls quickly with no movement. Check the actuator circuit, check valves and unloading paths for continuing oil loss. A substantial reduction in previously stable holding time calls for finding new demand or leakage. See shorter SB330 pressure-holding time.

Small pressure changes repeatedly start the motor. Check start/stop thresholds and signal fluctuations. A narrow pressure window reduces delivery per cycle. Any threshold adjustment must maintain minimum machine pressure and stay within component pressure limits.

Charging reaches pressure quickly, but one movement causes a large drop. Check accumulator connection, isolation and safety-block operating positions, precharge and bladder condition. Isolate and release hydraulic-side pressure before measuring precharge.

Pressure drops just after pump stop, then stabilises. Rapid charging heats the gas; subsequent cooling also reduces pressure. Compare changes after temperature stabilises before attributing them to continuing leakage.

Conclusion

Eight movements plus leakage require about 1.33 L in this example. At 100–140 bar with n = 1.4, 5 L is insufficient and 10 L is suitable.

Size from movement demand, leakage and the pressure window. When an existing machine suddenly cycles more often, investigate new leakage, precharge, connection and controls. Insufficient capacity and equipment faults need different remedies.