How to Select a Hydraulic Power Pack

The method at a glance

Work through these seven steps in order. Each one produces a number that the next step needs, so resist the temptation to jump ahead to "which pump" before the application is nailed down.

1. Define the application & motion

Before any calculation, describe the job in plain terms. What does the machine physically do — clamp, press, lift, tilt, bend, feed, hold? A power pack that opens and closes a clamp for a few seconds every cycle is a very different animal from one that holds a press under load all shift.

Pin down these points:

• Function: clamp / press / lift / tilt / feed — and whether the load is pushing, pulling or holding.
• Number of actuators: how many cylinders or motors, and whether they move together or in sequence. Simultaneous motion adds up the flow; sequential motion does not.
• Stroke and travel time: how far each actuator moves and how quickly it must complete the move.
• Holding requirement: does a load need to be held under pressure (a clamp, a press) — this drives valve choice and heat.

Everything downstream depends on this description. A machine with three clamps that fire one after another needs enough flow for one clamp; the same three clamping at once needs three times the flow.

2. Working pressure — from required force

Pressure is set by the force you need and the piston area it acts on. A cylinder develops force according to:

Rearranged, the pressure needed to make a given force on a chosen bore is P = F ÷ A. Keep the units straight: with force in newtons and area in mm², pressure in bar is P (bar) = 10 × F (N) ÷ A (mm²), because 1 bar = 0.1 N/mm².

Worked example — a 5 tonne clamp on an 80 mm bore

Suppose the application from step 1 needs a clamping force of 5 tonne and you have chosen an 80 mm bore cylinder.

• Force: 5 tonne ≈ 5 × 9,807 = 49,033 N (≈ 49 kN).
• Bore area: A = π/4 × 80² = 5,027 mm².
• Required pressure: P = 10 × 49,033 ÷ 5,027 ≈ 98 bar.

That is the pressure to just make the force — you never design to it. Add a working margin of roughly 20% for seal friction, pressure drop in the lines and a little reserve, giving about 117 bar, then round up to a standard figure. A 120 bar working pressure is the sensible specification here. Choosing a larger bore would lower the required pressure; a smaller bore would raise it. This is the trade-off behind every cylinder-and-pressure pairing.

3. Flow & speed

Pressure makes the force; flow makes the speed. The rate at which oil enters the cylinder sets how fast the piston moves:

The first equation gives cylinder speed v from flow Q and bore area A. The second gives the flow a fixed-displacement pump delivers, from its displacement V_g (cc/rev) and drive speed n (rpm) — the same relationship used in our flow calculator (which also applies volumetric efficiency).

Worked example — setting the flow

Say the clamp must close at about 50 mm/s. Using the 80 mm bore (A = 5,027 mm²) from step 2:

• Required flow: Q = v × A = 50 mm/s × 5,027 mm² = 251,300 mm³/s.
• Convert to litres per minute: × 60 ÷ 1,000,000 ≈ 15 L/min.

Now pick a pump to deliver 15 L/min. At a typical 1,440 rpm motor speed, the displacement needed is roughly V_g = Q × 1000 ÷ n = 15 × 1000 ÷ 1440 ≈ 10.4 cc/rev. An 11 cc/rev gear pump is the nearest standard size and delivers about 15 L/min after volumetric losses. If several actuators move at once, add their individual flows before selecting the pump.

4. Motor power

With working pressure and flow both known, the motor follows directly. The drive power is:

The efficiency η covers pump and drive losses; 0.85 is a reasonable working figure. This is the same expression behind our motor power calculator.

Worked example — the motor for 120 bar / 15 L/min

Carrying the running numbers through:

• Motor kW = (120 × 15) ÷ (600 × 0.85) = 1,800 ÷ 510 ≈ 3.53 kW.

Always round up to the next standard motor size — the common frame ratings are 0.75, 1.1, 1.5, 2.2, 3.7, 5.5 and 7.5 kW. So 3.53 kW selects a 3.7 kW motor. Size the motor for the highest simultaneous pressure-and-flow demand in the cycle, not the average, or the motor will trip on the peak.

5. Reservoir / tank

The reservoir does more than hold oil: it lets air and contamination settle out, sheds heat, and gives the pump a settled supply. The long-standing rule of thumb sizes it at 3 to 5 times the pump flow per minute:

For our 15 L/min pump that is a 45 to 75 litre tank. Use the lower end (3×) for intermittent, space-constrained duty; use the upper end (5×) for continuous running, where the extra volume keeps the oil cooler and gives entrained air time to release. When duty is heavy and space is tight, add a cooler rather than starving the tank — an undersized reservoir is one of the most common causes of an HPU that overheats in service. See Hydraulic Tank Sizing for the full treatment.

6. Valves & control

The manifold is where the unit becomes a controllable machine rather than just a pump on a tank. At minimum, specify:

• Directional control valve (DCV) per actuator: one DCV controls the direction (and stop) of each cylinder or motor. Match the spool centre condition to the job — a closed centre to hold a load, an open or tandem centre to let a cylinder float or to unload the pump.
• Relief valve: the pressure ceiling that protects the whole system. Set it at the working pressure + 15–20%. For a 120 bar working pressure that is roughly a 140 bar relief setting — high enough not to nuisance-trip at working load, low enough to protect the components.
• Filtration: a return-line or suction filter to the cleanliness class the components demand. Clean oil is the single biggest lever on service life; servo and proportional valves in particular need tighter filtration.
• Pressure gauge: so the setting can be seen and verified, ideally with an isolator to protect it from pulsation.
• Cooling: if the calculated heat load or a continuous duty cycle pushes oil temperature above roughly 55–60 °C, add an air or water cooler. Check load-holding and throttling circuits especially, as they turn pressure straight into heat.

7. Duty cycle, supply & mounting

The last step covers the practical constraints that decide whether a sound calculation survives in the real installation.

• Duty cycle: how much of each hour the pump actually runs under load. Continuous duty generates far more heat than a clamp that fires briefly each cycle — it drives reservoir size, cooling and even motor rating. Estimate it honestly.
• Single vs three-phase: below ~2.2 kW a single-phase supply can serve light or intermittent duty and small stand-alone units. From ~2.2 kW upward, and for any continuous industrial duty, three-phase is strongly preferred — it runs cooler and starts reliably under load. Confirm the available supply before fixing the motor.
• Mounting & space: horizontal tank, vertical (submersible pump) or overhead? Check the footprint, the fill and drain access, and the head available to the pump. A compact 3× tank may be the only option that fits — if so, plan cooling accordingly.
• Environment: ambient temperature, dust, washdown or hazardous area all change component and seal choices.

Common mistakes to avoid

• Undersizing the tank. A reservoir below 3× flow cannot shed heat or release air — the oil runs hot and ages fast. When space forces a small tank, add cooling.
• Ignoring the duty cycle. Sizing on peak load while forgetting that the pump runs continuously leads to overheating in service, even when every static number checks out.
• No safety margin on pressure. Specifying the bare calculated pressure leaves nothing for friction, line losses or wear — always add ~20% and round up.
• Wrong relief-valve setting. Set too high it fails to protect; set below working pressure it trips constantly. Aim for working + 15–20%.
• Dirty oil / weak filtration. Contamination is the leading cause of premature valve and pump failure. Specify filtration to the cleanliness class the components need, and keep it maintained.

Frequently asked questions

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