Sizing a hydraulic system correctly helps prevent overheating, slow movement, and unnecessary equipment wear. Selecting a HYD power pack involves calculating the required pressure and flow rate based on the machine’s workload. Understanding these requirements helps engineers select compatible components, improve operating efficiency, and maintain reliable performance across industrial applications.
Here’s a simple, field-friendly method.
Step 1: Start With the Load, Not the Catalog
Before looking at any product, write down what the machine actually needs to do:
- Force required (in kN or tons).
- Stroke length of each cylinder.
- Speed for extend and retract.
- Cycle time and how often it repeats.
- Operating environment: temperature, dust, humidity.
If you can’t answer these, stop here. Every later number depends on them.
Step 2: Work Out Operating Pressure
Pressure comes from force and area:
Pressure = Force ÷ Piston Area
Add a safety margin for friction, seal drag and acceleration, usually 10 to 20 per cent. Then set your system relief valve a little above the working pressure, commonly 10 to 15 per cent higher.
Quick tip: Higher pressure means smaller cylinders and pumps, but it also means stricter component ratings and more heat. Most industrial systems stay in the 100 to 250 bar range, and you can go higher when the application demands it.
Step 3: Calculate Flow Rate
Flow decides how fast your cylinder moves:
Flow (L/min) = Piston Area (cm²) × Speed (m/min) × 0.1
Example: A cylinder with 50 cm² area moving at 6 m/min needs:
50 × 6 × 0.1 = 30 L/min
Two things to remember:
- For double-acting cylinders, check both directions. The rod side has less area, so it retracts faster for the same flow.
- Size for the highest flow demand. If several cylinders move together, add their flows.
Step 4: Find the Power and Motor Size
Now combine pressure and flow:
Hydraulic power (kW) = Flow (L/min) × Pressure (bar) ÷ 600
Using our example at 200 bar:
30 × 200 ÷ 600 = 10 kW
That’s the power at the pump output. Real systems lose some to friction and leakage, so divide by overall efficiency, typically 0.80 to 0.90:
10 ÷ 0.85 ≈ 11.8 kW
Then round up to the next standard motor size, here 15 kW. Don’t round down. A motor that’s always at its limit will run hot and wear out early.
Step 5: Choose the Pump Displacement
For a standard 4-pole motor at roughly 1,450 rpm (50 Hz), the pump displacement is:
Displacement (cc/rev) = Flow × 1000 ÷ (rpm × volumetric efficiency)
For 30 L/min at about 95 per cent efficiency:
30,000 ÷ (1,450 × 0.95) ≈ 22 cc/rev
Choose the nearest standard pump, such as 22 or 25 cc. At this stage, a well-matched hydraulic power pack unit starts to take shape: the motor, pump, reservoir and valves all need to agree with each other, and the weakest link sets the limit for the whole system.
Step 6: Think About Duty Cycle
Does your machine work continuously or in short bursts? This changes everything.
- Continuous duty: Size the pump for peak flow and plan for heavy cooling.
- Intermittent duty: You may be able to use a smaller pump and an accumulator to supply short, high-flow demands.
An accumulator stores energy during idle moments and releases it when needed. That can cut motor size and energy use for applications with brief peaks.
Step 7: Size the Reservoir
A simple rule for stationary industrial units is a reservoir holding three to five times the pump flow per minute. For our 30 L/min pump, that’s around 90 to 150 liters.
Why it matters:
- Gives air time to escape from the oil.
- Lets contaminants settle.
- Provides some natural cooling.
Too small, and you’ll see foaming and overheating. Too large wastes space and costs.
Step 8: Don’t Ignore Heat
Any power the system doesn’t turn into useful work becomes heat. In the UAE, where ambient temperatures can climb past 45°C, this deserves extra attention.
Keep these in mind:
- Aim to keep oil temperature in the range of roughly 40 to 60°C.
- Estimate heat load from your system losses.
- Add an air-cooled or water-cooled heat exchanger if the reservoir can’t dissipate it alone.
- Check oil viscosity at both cold start and hot running conditions.
Overheated oil thins out, loses lubricating strength and shortens component life fast.
Step 9: Select Valves, Filters and Piping
Match these to your flow and pressure:
- Directional control valves rated above your peak flow.
- Return-line or pressure filtration to protect pumps and valves.
- Pipe and hose sizes that keep velocity under control, roughly 1 m/s on suction lines, up to about 5 m/s on pressure lines and 2 to 3 m/s on return lines.
Undersized lines create pressure drops, noise and extra heat.
Step 10: Leave Sensible Headroom
Allow 10 to 20 per cent room for future changes, such as an extra cylinder or higher cycle rates. But resist the urge to oversize everything. Bigger isn’t safer if it leads to wasted energy, excess heat and higher cost.
Common Sizing Mistakes
- Sizing only for average load instead of peak.
- Ignoring retract flow on double-acting cylinders.
- Forgetting heat in hot climates.
- Choosing a reservoir that’s too small.
- Skipping efficiency losses in the motor calculation.
- Not sharing duty cycle data with the supplier.
Suppliers who work closely with engineers on the application, such as Dynamic Industries, can help catch these gaps before commissioning, but the best defense is a clear specification from the start.
Conclusion
Sizing a power pack comes down to a steady sequence: define the load, calculate pressure and flow, work out power, select the pump and motor, size the reservoir and cooling, then add valves and filtration with sensible headroom. Before asking for a quote, collect your force, speed, stroke, duty cycle and ambient temperature. If the unit will join a larger line, involve your industrial automation systems integrators early, so control signals, sensors and safety interlocks match the hydraulic design from day one.
Dynamic Industries is a UAE-based manufacturer and supplier of hydraulic power packs, cylinders, control valves, accumulators, and heat exchangers. Their engineers work closely with clients to match each system to the load, pressure, and operating conditions, delivering custom configurations, reliable performance, and responsive after-sales support for demanding industrial applications.