Sizing pumps and motors: matching flow, pressure, and horsepower for reliable systems

Hydraulic systems live or die by sizing. Too small, and you get sluggish motion, hot oil, and short component life. Too big, and you waste money and still risk heat from throttling and bypassing. The fix is simple in principle: match the pump and motor using flow, pressure, and horsepower based on the work you need to do.

This guide walks maintenance leads and procurement through the steps. You will learn how flow sets speed, how displacement connects RPM to litres per minute, how pressure creates torque, and how efficiency changes the math. We include quick selection checklists, simple examples, and the pitfalls that most often lead to overheating.

If you need parts to complete a build or refresh a circuit, Island Hydrostatics supplies pumps, motors, valves, gauges, and filtration across Canada with knowledgeable support.

Pump vs motor basics

• Are hydraulic pumps and motors the same? They share similar internal groups, but they are not the same.

• What is the main difference between a pump and a motor? A pump converts mechanical input power into hydraulic power (flow at pressure). A hydraulic motor converts hydraulic power back into mechanical power (torque and speed). Porting, case drains, timing plates, and sealing are optimized differently, so do not substitute one for the other unless the manufacturer certifies it as bi-directional and reversible.

The most commonly used hydraulic pump in mobile equipment is the gear pump for cost and robustness. In higher performance and industrial systems, axial piston pumps are very common thanks to higher efficiency and pressure capability. When speed or load varies, a variable displacement piston pump is often preferred for energy savings and heat control.

Flow, displacement, and speed

Flow determines speed. Displacement links RPM to flow.

• Pump flow (L/min) = Pump displacement (cc/rev) × Pump RPM ÷ 1000

• Motor speed (RPM) ≈ Pump flow (L/min) × 1000 ÷ Motor displacement (cc/rev) × volumetric efficiency

 

Volumetric efficiency accounts for internal leakage. For quick planning, use 85 to 92 percent for good piston units and 80 to 88 percent for gear or gerotor units. Always confirm with the data sheet.

Example 1: You need a shaft to turn 300 RPM with a 25 cc/rev motor at 85 percent volumetric efficiency.
Required flow ≈ 300 × 25 ÷ 1000 ÷ 0.85 ≈ 8.8 L/min.
If your prime mover runs a pump at 1800 RPM, the pump displacement for 8.8 L/min is ≈ 8.8 × 1000 ÷ 1800 ≈ 4.9 cc/rev.

How to make a hydraulic motor go faster: increase flow to the motor, reduce motor displacement, or improve volumetric efficiency. Be sure the inlet is not starved and that the case drain is within limits. Never overspeed beyond the manufacturer’s rating.

Pressure, torque, and starting behaviour

Pressure makes torque. Displacement sets torque constant.

• Ideal torque (N·m) = Motor displacement (cc/rev) × Pressure (bar) ÷ 62.8

• Real torque = Ideal torque × mechanical efficiency

For many motors, mechanical efficiency ranges from 85 to 95 percent at operating pressure. Starting torque is the torque available to begin rotation from rest, typically 60 to 90 percent of theoretical depending on motor type and load conditions. Low-speed high-torque (LSHT) gerotor/geroler motors have good starting torque characteristics, while some piston motors need adequate back pressure and correct control setup to start smoothly.

How to calculate the hp of a hydraulic motor:
Hydraulic power (kW) = Flow (L/min) × Pressure (bar) ÷ 600.
Motor output power = Hydraulic power × overall efficiency (volumetric × mechanical).

Horsepower requirements for pumps

How much horsepower do you need to run a hydraulic pump?
Use the same core equation, then divide by pump efficiency:

• Hydraulic power (kW) = Flow (L/min) × Pressure (bar) ÷ 600

• Input power (kW) = Hydraulic power ÷ overall pump efficiency

• Horsepower (hp) = kW × 1.341

Example 2: A circuit needs 40 L/min at 200 bar. Hydraulic power = 40 × 200 ÷ 600 ≈ 13.3 kW. If pump overall efficiency is 85 percent, input power ≈ 13.3 ÷ 0.85 ≈ 15.6 kW ≈ 20.9 hp. Size the prime mover with headroom for cold start, transient spikes, and altitude.

Sizing a hydraulic pump and motor together

Follow a simple chain: load torque and speed define motor displacement and pressure, which define pump flow and prime mover power.

1. Define the load

• Required shaft torque (N·m) and speed (RPM) at the motor shaft.

• Duty cycle and ambient temperature.

• Acceptable overshoot, smoothness, and stall tolerance.

2. Choose a motor technology

• LSHT gerotor/geroler for compact, moderate-pressure, good starting torque.

• Axial piston for higher efficiency and pressure with good dynamic control.

3. Calculate motor displacement and pressure

• From torque: Displacement = 62.8 × Torque ÷ Pressure (bar).

• Check motor maximum pressure rating and verify starting torque with mechanical efficiency.

• From speed: Verify required flow at your chosen displacement and RPM with volumetric efficiency.

4. Select the pump

• Pump flow = Motor flow ÷ transmission ratio (1:1 if direct).

• Choose fixed or variable displacement based on duty and control needs.

• Confirm prime mover RPM and calculate pump displacement.

• Verify NPSH and suction plumbing to avoid cavitation.

5. Check horsepower and heat

• Compute hydraulic power and input power.

• Confirm cooler capacity or reservoir surface area if expected heat load is high.

• Add a margin for cold oil and pressure spikes.

Example 3, step-by-step

• Goal: 450 RPM at 75 N·m continuous, with intermittent peaks to 100 N·m.

• Assume system pressure limit 210 bar.

• Displacement for 75 N·m ideal = 62.8 × 75 ÷ 210 ≈ 22.4 cc/rev. With 90 percent mechanical efficiency, real displacement target ≈ 22.4 ÷ 0.9 ≈ 24.9 cc/rev. Choose a 25 cc/rev motor rated for 210 bar continuous.

• Flow for 450 RPM with 88 percent volumetric efficiency ≈ 450 × 25 ÷ 1000 ÷ 0.88 ≈ 12.8 L/min.

• Hydraulic power at 75 N·m and 450 RPM: P (kW) = Torque × RPM ÷ 9550 ≈ 75 × 450 ÷ 9550 ≈ 3.5 kW.

• Cross check with pressure and flow: 12.8 L/min × 210 bar ÷ 600 ≈ 4.5 kW at pressure. The difference reflects load not always at peak pressure and efficiency assumptions.

• If the prime mover runs the pump at 1800 RPM, pump displacement ≈ 12.8 × 1000 ÷ 1800 ≈ 7.1 cc/rev.

• With 85 percent pump efficiency, input ≈ 4.5 ÷ 0.85 ≈ 5.3 kW ≈ 7.1 hp. Add margin for starts and peaks, so a 10 to 12 hp prime mover is practical.

Fixed vs variable displacement piston pumps

Fixed displacement

• Simple, robust, lower initial cost.

• Best when demand is steady and control is via valves.

• Heat risk rises when flow is mostly throttled or spent across reliefs.

Variable displacement

• Flow adjusts to demand, often with pressure-compensated, load-sensing, or electronic controls.

• Reduces heat in idle and partial load conditions and can cut input power.

• Excellent for multi-function mobile machines and industrial setups with varying duty.

Closed-centre systems make sense when the pump can unload or de-stroke at pressure in neutral, holding actuator positions without constant flow. Pairing a load-sensing directional valve stack with a variable displacement piston pump helps maintain stable motion across varying loads and keeps oil cooler.

If you are evaluating options, explore our variable displacement piston pump selection for compact, efficient builds where duty varies.

Selection checklists

Pump checklist

• Flow at rated RPM and expected duty

• Maximum and continuous pressure ratings

• Efficiency data and control type (fixed, pressure-comp, load-sense)

• Suction conditions, filtration target, case drain limits

• Prime mover horsepower with cold-start margin

Motor checklist

• Displacement and speed range

• Continuous and intermittent torque/pressure

• Starting torque and low-speed smoothness

• Case drain requirements and mounting orientation

• Shaft, porting, and seal compatibility with fluid and temperature

Supporting components

• Directional control, relief, and flow control sizing

• Hoses and fittings rated for peak pressure and temperature

• Cleanliness targets, filtration, and commissioning gauges

To round out your build, you can source hydraulic motors, hydraulic valves including options like a 3-way hydraulic valve, and hydraulic pressure gauges for setup and diagnostics from Island Hydrostatics.

Common pitfalls that cause heat or sluggish performance

• Oversized pump with fixed orifices: wastes energy as heat in neutral.

• Relief valve cracking constantly: set too low or excessive backpressure keeps it bleeding.

• Starved inlets: undersized suction lines or clogged suction strainer causing cavitation and noise.

• Leaky internals: worn pumps or motors reduce volumetric efficiency, demanding more flow for the same speed.

• Restrictive returns: small return plumbing or partially closed ball valves lift backpressure and slow actuators.

• Mismatched valve centre condition: open-centre valves on a closed-centre pump or vice versa lead to heat and instability.

• Inadequate cooling: no allowance for duty cycle or ambient temperature.

Quick FAQ

• How do you size a hydraulic pump and motor?
  Start with required shaft torque and speed. Select motor displacement from torque and pressure. Confirm speed with flow and volumetric efficiency. Choose the pump to supply that flow at pressure, then verify prime mover horsepower with pump efficiency.

• How much horsepower to run a hydraulic pump?
  HP ≈ Flow (L/min) × Pressure (bar) ÷ 600 ÷ efficiency × 1.341. Always add margin for cold starts and transients.

• What is the most commonly used hydraulic pump?
  Gear pumps are most common in mobile for cost and durability, while piston pumps are widely used where higher pressure and efficiency are needed.

• What is the starting torque of a hydraulic motor?
  It varies by design, typically 60 to 90 percent of theoretical. Check the manufacturer’s curve; LSHT motors usually offer strong starting torque.

• How to calculate the hp of a hydraulic motor?
  Hydraulic kW = Flow × Pressure ÷ 600. Motor hp = kW × overall efficiency × 1.341.

• How to make a hydraulic motor go faster?
  Increase flow, reduce displacement, or reduce downstream restrictions. Stay within maximum RPM and inlet conditions.

Complete your system with reliable components

Build reliability starts with clean oil, correct plumbing, and the right controls. For practical upgrades and commissioning:

• Use hydraulic flow control valves to tune motion without excessive heat.

• Add hydraulic test gauges at key ports to set reliefs and diagnose spikes.

• Choose high pressure hose and the right hydraulic fittings to match thread and seat types.

Island Hydrostatics is a Canada-based hydraulic equipment supplier that can help you size, select, and ship the parts you need. Explore components, get support, and keep downtime low.

Summary and next step

Match flow to speed and pressure to torque, then check horsepower with realistic efficiencies. Choose fixed pumps for steady duty and variable displacement for varying loads or closed-centre control. Avoid heat by preventing constant relief flow, keeping inlets and returns free, and confirming valve centre conditions.

Need help running the numbers or choosing the right package of hydraulic pumps and motors, valves, and gauges? Contact Island Hydrostatics for guidance and Canada-wide supply. You can also browse hydraulic motors, find a 3-way hydraulic valve for your circuit logic, add hydraulic pressure gauges for commissioning, and specify a variable displacement piston pump to keep heat in check.