Views: 0 Author: Site Editor Publish Time: 2026-10-01 Origin: Site
Sizing a hydraulic pump and motor starts with the machine load, not with a preferred pump or motor model. The designer first needs to know how much torque the driven equipment requires and how fast it must rotate. These two values determine the mechanical power needed at the hydraulic motor shaft.
The hydraulic motor is then sized according to displacement, pressure difference, speed, and efficiency. Once the required motor flow is known, the hydraulic pump can be sized to provide that flow at the available pump drive speed.
A correctly matched hydraulic pump motor system should deliver enough torque and speed without continuously operating near maximum pressure, maximum rpm, or thermal limits. Oversizing can increase cost, heat generation, and energy consumption, while undersizing can cause low speed, insufficient torque, overheating, cavitation, and shortened component life.
Xiamen Senhengyuan Machinery Equipment Co., Ltd. supplies hydraulic pumps and hydraulic motors in piston, vane, gear, and other configurations for mobile and industrial hydraulic systems. The following sizing process provides a practical starting point for matching a hydraulic pump and hydraulic motor.
The first step is defining what the machine actually needs from the motor shaft.
For a conveyor, this may be the torque required to start a loaded belt and the rpm needed to maintain production speed. For a wheel drive, the calculation may involve vehicle weight, wheel radius, rolling resistance, grade, and acceleration. For a mixer, required torque depends on material viscosity, impeller size, and operating speed.
At minimum, determine:
Required Data | Why It Matters |
|---|---|
Continuous Torque | Determines normal motor load |
Starting Torque | Determines startup capability |
Peak Torque | Covers temporary shock or overload |
Normal Speed | Determines required flow |
Maximum Speed | Checks motor rpm limit |
Duty Cycle | Affects thermal sizing |
Rotation Direction | Determines circuit and motor requirements |
Radial/Axial Load | Affects shaft and bearing selection |
Do not size the system only from motor horsepower or existing pipe size. Torque and speed should come directly from the driven machine.
Motor displacement is normally expressed in cubic centimeters per revolution, or cc/rev. It indicates approximately how much hydraulic fluid the motor requires for one shaft revolution.
A larger displacement motor produces more theoretical torque for the same pressure difference, but it also requires more flow to achieve the same rotational speed.
A smaller displacement motor needs less fluid per revolution, so it can reach higher rpm at the same flow. However, it produces less torque for a given pressure.
This creates one of the most important hydraulic design relationships:
Larger displacement = higher torque potential + lower speed
Smaller displacement = higher speed potential + lower torque
Motor displacement should therefore be selected from torque and speed together.
A practical preliminary formula is:
Motor Torque (N·m) ≈ Pressure Difference (bar) × Displacement (cc/rev) × Mechanical Efficiency ÷ 62.8
If the required torque is known, the same formula can be rearranged to calculate the approximate pressure requirement:
Pressure Difference (bar) ≈ Torque × 62.8 ÷ (Displacement × Mechanical Efficiency)
For example, suppose a machine requires 220 N·m and an 80 cc/rev motor is being considered. If mechanical efficiency is estimated at 90%:
Pressure ≈ 220 × 62.8 ÷ (80 × 0.90)
≈ 192 bar
This means the motor would need approximately 192 bar of effective pressure difference to produce the required torque under the assumed conditions.
The actual product must still be checked against its continuous pressure, peak pressure, starting torque, and manufacturer performance data.
Motor speed depends primarily on flow rate and motor displacement.
A practical formula is:
Motor Speed (rpm) ≈ Flow (L/min) × 1000 × Volumetric Efficiency ÷ Displacement (cc/rev)
If the required speed is already known, calculate the flow instead:
Required Flow (L/min) ≈ Motor Speed × Displacement ÷ (1000 × Volumetric Efficiency)
Suppose the same 80 cc/rev hydraulic motor must operate at 800 rpm and estimated volumetric efficiency is 92%.
The required flow is:
800 × 80 ÷ (1000 × 0.92)
≈ 69.6 L/min
The hydraulic pump therefore needs to provide roughly 70 L/min to the motor at this operating point, before additional circuit losses, auxiliary flows, and design margins are considered.
Using the previous example:
Parameter | Design Requirement |
|---|---|
Required Motor Speed | 800 rpm |
Required Motor Torque | 220 N·m |
Selected Motor Displacement | 80 cc/rev |
Estimated Mechanical Efficiency | 90% |
Estimated Volumetric Efficiency | 92% |
Required Pressure Difference | ≈192 bar |
Required Motor Flow | ≈69.6 L/min |
This gives the designer a starting point for comparing actual hydraulic motor models.
A variable piston motor such as the A6VM axial piston variable motor can provide additional flexibility because displacement can be changed to alter the speed-to-torque relationship. HUA—rather, SHY's current A6VM product data includes multiple displacement sizes and operating-speed ranges, making it suitable for hydraulic drives that need changing operating conditions.
For applications that do not require variable displacement, a fixed motor such as the AA2FM axial piston fixed motor can provide a simpler relationship between flow, displacement, and shaft speed.
Once the motor flow requirement is known, the next step is sizing the hydraulic pump.
Pump flow depends on pump displacement, pump shaft speed, and volumetric efficiency.
The basic relationship is:
Pump Flow (L/min) ≈ Pump Displacement (cc/rev) × Pump Speed (rpm) × Volumetric Efficiency ÷ 1000
If required flow is already known:
Pump Displacement (cc/rev) ≈ Required Flow × 1000 ÷ (Pump Speed × Volumetric Efficiency)
Using the previous motor requirement of approximately 69.6 L/min, assume the pump will operate at 1,500 rpm and has an estimated volumetric efficiency of 93%.
The required pump displacement becomes:
69.6 × 1000 ÷ (1500 × 0.93)
≈ 49.9 cc/rev
A pump around 50 cc/rev would therefore be a reasonable preliminary sizing point.
The final selection should use an actual pump displacement and control configuration that can provide the required flow without exceeding its continuous speed, pressure, and power ratings.
Parameter | Example |
|---|---|
Required Pump Flow | 69.6 L/min |
Pump Drive Speed | 1,500 rpm |
Estimated Pump Volumetric Efficiency | 93% |
Calculated Pump Displacement | ≈49.9 cc/rev |
Preliminary Pump Size | About 50 cc/rev |
A variable piston pump can be useful when the system does not require full flow continuously.
For example, the A7VO axial piston variable hydraulic pump is available across multiple displacement sizes and can be used in applications requiring variable hydraulic output.
Another option is the A10VO axial piston variable pump, which is available with different control configurations for hydraulic systems requiring pressure or flow adjustment.
Pump flow creates motor speed, but system pressure must be high enough for the motor to produce the required torque.
The pump should therefore be selected for both flow and pressure.
If the motor requires approximately 192 bar pressure difference to generate the target torque, the pump and hydraulic circuit need to operate above that level once pressure losses are included.
Additional pressure losses can come from:
Directional valves
Flow-control valves
Hydraulic hoses
Pipes and fittings
Filters
Heat exchangers
Check valves
Manifolds
If the motor requires 192 bar and the hydraulic circuit loses another 10–15 bar at the design flow, the pump may need to produce more than 200 bar during full-load operation.
The exact design pressure should include appropriate engineering margin without unnecessarily setting the system much higher than required.
Pressure and flow together determine hydraulic power.
The common formula is:
Hydraulic Power (kW) ≈ Pressure (bar) × Flow (L/min) ÷ 600
Using the example:
192 × 69.6 ÷ 600 ≈ 22.3 kW
This is the approximate hydraulic power delivered to the motor inlet before motor losses.
The prime mover driving the hydraulic pump must provide more power because the pump itself is not perfectly efficient.
If the pump's total efficiency is assumed to be 88%:
Required Pump Shaft Power ≈ 22.3 ÷ 0.88
≈ 25.3 kW
The electric motor or engine driving the hydraulic pump should therefore be selected above this theoretical input requirement, considering duty cycle, startup conditions, system margins, and manufacturer recommendations.
Parameter | Result |
|---|---|
Motor Torque Requirement | 220 N·m |
Motor Speed Requirement | 800 rpm |
Motor Displacement | 80 cc/rev |
Required Pressure Difference | ≈192 bar |
Required Flow | ≈69.6 L/min |
Pump Speed | 1,500 rpm |
Calculated Pump Displacement | ≈50 cc/rev |
Hydraulic Power | ≈22.3 kW |
Estimated Pump Shaft Power | ≈25.3 kW |
This is a preliminary engineering calculation, not a substitute for model-specific performance curves.
No. The hydraulic pump and motor do not need to have the same displacement.
Pump displacement determines how much fluid is delivered for each pump-shaft revolution. Motor displacement determines how much fluid the motor requires for each output-shaft revolution.
Their relationship depends on pump speed and desired motor speed.
For example, a 50 cc/rev pump operating at 1,500 rpm can theoretically supply approximately 75 L/min before leakage losses.
An 80 cc/rev motor receiving that flow would rotate much slower than the pump shaft.
This difference is completely normal and is often the reason hydraulic transmission is used.
The pump must deliver enough flow to achieve the required motor rpm.
If pump flow is too low, the motor will rotate below the desired speed even if system pressure is high.
If pump flow is much higher than required, excessive flow may need to pass through control or relief valves, generating unnecessary heat and wasting energy.
The preferred design is usually to produce only the flow required by the actuator or to use a variable-displacement pump that can adjust delivery according to system demand.
Pump Flow Condition | Motor/System Effect |
|---|---|
Too Low | Motor speed too low |
Correct | Target speed achievable |
Excessive | Overspeed or excess throttling |
Unstable | Motor speed fluctuates |
Insufficient at Load | Speed drops under operation |
Flow sizing should also consider leakage that increases as hydraulic components wear.
Pressure should be high enough to overcome the load, but higher system pressure is not automatically better.
If the motor only requires 180 bar to operate the machine, designing the entire system around unnecessarily high operating pressure can increase leakage, heat, stress, seal loading, and energy loss.
The pump must have a pressure rating above the maximum expected operating pressure, and the motor must also tolerate the pressure difference across its ports.
Peak pressure during acceleration, sudden stopping, load changes, or valve shifting should be considered separately from continuous pressure.
Pressure relief valves are used to protect the system from excessive pressure, but they should not operate continuously as a normal method of controlling power.
A variable hydraulic motor requires additional calculations because displacement changes during operation.
At maximum displacement, the motor generally produces greater torque per unit pressure but rotates more slowly for a given flow.
At reduced displacement, the same pump flow can produce a higher shaft speed, but torque per bar decreases.
This is particularly useful for mobile machinery that requires:
High torque during working
High speed during travel
Variable power demand
Hydrostatic transmission control
For an A6VM-type motor, the system should therefore be checked at both maximum and minimum operating displacement.
Do not size only at one nominal operating point. Maximum speed at reduced displacement and required pressure at maximum torque should both remain within the motor's published limits.
A high speed hydraulic motor usually has lower displacement relative to its flow capacity.
Sizing begins with the required rpm and shaft power. From those values, calculate the approximate torque requirement and then determine a displacement that can reach the target speed using the available pump flow.
High-speed selection must also check:
Maximum rated rpm
Continuous and peak pressure
Inlet pressure
Case pressure
Case-drain requirements
Oil viscosity
Bearing loads
Fluid cleanliness
Cooling
Cavitation risk
A motor may mathematically reach a certain rpm from the flow calculation while still exceeding its mechanical speed limit.
This is why theoretical sizing must always be compared with the manufacturer's performance data.
A fixed-displacement hydraulic pump produces flow mainly according to its displacement and shaft speed.
If the engine or electric motor runs at a constant speed, pump flow remains relatively predictable. This can be suitable for simple hydraulic systems with stable actuator demand.
A variable-displacement pump can change its output displacement according to load or control requirements.
Feature | Fixed Pump | Variable Pump |
|---|---|---|
Pump Displacement | Constant | Adjustable |
Flow Control | Mainly pump speed/valves | Pump displacement + speed |
System Complexity | Lower | Higher |
Energy Efficiency | Good for stable demand | Better potential for variable demand |
Control Flexibility | Moderate | High |
Typical Use | Simple hydraulic circuits | Mobile and advanced industrial systems |
Variable pumps are often useful where a hydraulic motor operates over a wide range of speeds and loads.
The hydraulic circuit architecture also affects pump and motor selection.
In an open circuit, the pump draws oil from the reservoir and sends it to the hydraulic motor. Oil leaving the motor returns to the reservoir.
Open circuits are common in general industrial machinery.
The reservoir, return filtration, cooling, and suction design all need to support the full operating flow.
In a closed hydrostatic circuit, the main flow travels directly between the pump and motor.
Oil leaving one side of the motor returns directly to the opposite side of the pump. A charge pump normally supplies make-up oil and supports cooling, flushing, and positive inlet pressure.
Closed circuits are common in mobile travel drives and hydrostatic transmissions.
Variable pumps and motors can provide wide speed-control ranges in these systems.
Sizing must include the main loop, charge flow, flushing requirements, pressure protection, and case-drain conditions.
Hydraulic components should not normally be selected to operate continuously at their absolute maximum ratings.
However, simply oversizing every component by a large percentage is also poor design.
The required margin should reflect:
Duty cycle
Peak pressure
Starting torque
Temperature
Component efficiency
Expected wear
Contamination conditions
Shock loads
Desired service life
Continuous operating pressure should remain within the manufacturer's continuous rating. Temporary peaks should remain within specified peak limits.
Motor flow should also leave suitable margin below maximum rated speed.
Pump input power must be compatible with the available electric motor or engine throughout the complete operating range.
Ideal formulas assume that all fluid displaced by the pump reaches the motor and all pressure-generated force becomes shaft torque.
Real hydraulic systems have losses.
Pump volumetric efficiency reduces actual delivered flow. Pump mechanical losses increase the shaft power required from the prime mover.
Motor volumetric losses reduce actual motor speed for a given flow. Mechanical losses reduce actual output torque.
Pressure losses also occur through valves, hoses, filters, and fittings.
Ignoring these losses can produce a system that works on paper but fails to achieve the required speed or torque in real operation.
Pump and motor calculations should not stop at the components themselves.
Hydraulic lines must carry the required flow without excessive pressure loss or fluid velocity.
Undersized suction lines can create excessive restriction at the pump inlet and increase cavitation risk.
Undersized pressure and return lines create additional pressure loss and heat.
Line sizing depends on:
Flow rate
Fluid viscosity
Line length
Hose or tube diameter
Number of bends
Fittings
Pressure level
Suction or return conditions
A correctly sized pump can still perform poorly if the surrounding hydraulic circuit is too restrictive.
Hydraulic inefficiency becomes heat.
If the pump uses 30 kW of shaft power but the actuator receives less mechanical power, part of the difference appears as heat in the hydraulic oil.
Continuous-duty systems therefore require adequate oil volume and cooling.
Reservoir design should support:
Heat dissipation
Air release
Contamination settling
Stable pump suction
Oil expansion
System maintenance
A heat exchanger may be required when natural tank cooling cannot maintain the recommended oil temperature.
Variable-displacement pumps can reduce unnecessary heat in systems with fluctuating demand because they can reduce flow when full output is not needed.
One common mistake is sizing the hydraulic pump first and then trying to find a motor that matches it. The better approach is usually to start from the load.
Another mistake is assuming system pressure determines motor speed. Pressure primarily relates to torque, while flow primarily determines speed.
Ignoring efficiency creates another common error. Theoretical flow and torque values will not exactly match actual operating performance.
Other common mistakes include:
Mistake | Possible Result |
|---|---|
Pump too small | Motor cannot reach required speed |
Pump too large | Heat and energy loss |
Motor displacement too small | Insufficient torque |
Motor displacement too large | Speed too low |
Pressure rating too low | Premature component failure |
Ignoring peak torque | Motor stalls or overloads |
Ignoring maximum rpm | Motor damage |
Ignoring line losses | Low actual performance |
Poor suction design | Pump cavitation |
No cooling calculation | Hydraulic overheating |
The complete system should therefore be checked rather than sizing each component independently.
If the hydraulic motor runs too slowly, first measure actual pump flow before assuming that the motor is too large.
If the motor cannot move the load, check available pressure and pressure difference across the motor.
A motor that reaches normal speed without load but slows dramatically under load may indicate insufficient pressure, pump wear, internal leakage, or an undersized power source.
A system that continuously operates against the relief valve may have an incorrectly sized actuator, excessive load, poor control strategy, or an insufficient motor displacement.
Excessive heat often indicates unnecessary throttling, high leakage, undersized components, or operation near maximum ratings for too long.
Actual pressure, flow, temperature, case-drain flow, and shaft speed measurements are more useful than replacing components based only on symptoms.
Before ordering the components, confirm the following:
Item | Required Information |
|---|---|
Motor Torque | Continuous, starting, and peak |
Motor Speed | Normal and maximum |
Motor Displacement | cc/rev |
System Flow | L/min |
Operating Pressure | bar |
Peak Pressure | bar |
Pump Drive Speed | rpm |
Pump Displacement | cc/rev |
Hydraulic Power | kW |
Prime Mover Power | kW or HP |
Circuit Type | Open or closed |
Rotation | One-way or reversible |
Duty Cycle | Continuous/intermittent |
Fluid | Type and viscosity |
Temperature | Normal and maximum |
Shaft Load | Radial/axial |
Control Method | Fixed or variable displacement |
This information provides a much stronger basis for selecting the hydraulic pump and motor than simply matching model numbers.
Xiamen Senhengyuan Machinery Equipment Co., Ltd. supplies hydraulic pumps, hydraulic motors, valves, and related hydraulic components for industrial and mobile machinery.
Its hydraulic pump range includes piston, vane, and gear pump options, while the motor range covers piston, vane, gear, and orbit motor designs.
This allows buyers to consider both sides of the hydraulic pump motor system when selecting replacement components or designing a hydraulic drive.
Variable piston pumps can support changing flow requirements. Variable piston motors can provide different speed and torque operating ranges. Fixed-displacement motors offer simpler behavior where operating conditions remain predictable.
For accurate product matching, buyers should provide pump drive speed, required motor speed, motor torque, operating pressure, peak pressure, required flow, displacement, circuit type, shaft and flange dimensions, port configuration, hydraulic oil, and equipment application.
Matching these parameters is more reliable than selecting a hydraulic pump or hydraulic motor from displacement alone.
Sizing a hydraulic pump and motor starts with the mechanical load.
First determine the torque and speed required at the hydraulic motor shaft. Then select a suitable motor displacement and calculate the approximate pressure and flow needed to achieve those targets.
Pressure difference primarily determines motor torque. Flow primarily determines motor speed. Displacement connects the two.
Once required motor flow is known, pump displacement can be calculated from pump drive speed and expected volumetric efficiency.
Hydraulic power should then be checked to ensure the electric motor or engine driving the pump has enough input power. Efficiency losses, valves, lines, filtration, temperature, peak loads, and circuit configuration must also be included.
A high speed hydraulic motor requires additional attention to maximum rpm, inlet conditions, case pressure, lubrication, cooling, and cavitation.
Xiamen Senhengyuan Machinery Equipment Co., Ltd. supplies different hydraulic pump and motor configurations that can be matched to industrial and mobile hydraulic applications.
The most reliable sizing approach is therefore:
Load → Torque and Speed → Motor Displacement → Pressure and Flow → Pump Displacement → Input Power → Circuit Verification
Start with the required shaft torque and rpm. Select a motor displacement that can produce the required torque at an acceptable system pressure. Then calculate the flow needed to reach the target rpm and verify continuous pressure, peak pressure, maximum speed, efficiency, and shaft-load ratings.
First calculate how much flow the hydraulic motor needs. Then use pump drive speed and pump volumetric efficiency to determine the required pump displacement. The pump must also have enough pressure capacity to allow the motor to generate the required torque.
Yes, flow is the main hydraulic parameter controlling motor speed. Higher flow generally produces higher rpm, while lower flow reduces rpm. Motor displacement and volumetric efficiency also affect the final speed.
Pressure difference across the hydraulic motor is a major factor determining torque. Greater pressure difference generally provides more torque, while motor displacement also has a direct influence.
No. Pump and motor displacement do not need to be equal. Pump displacement is selected according to required flow and pump drive speed. Motor displacement is selected according to required torque, speed, and pressure.
A practical formula is:
Pump Displacement (cc/rev) ≈ Flow (L/min) × 1000 ÷ [Pump Speed (rpm) × Volumetric Efficiency]
Always compare the calculated result with actual available pump sizes and manufacturer ratings.
A practical formula is:
Flow (L/min) ≈ Motor Speed × Motor Displacement ÷ (1000 × Volumetric Efficiency)
The actual system may require additional flow to account for leakage and auxiliary circuit demands.
The required pressure depends on motor displacement, required torque, and mechanical efficiency. Higher load torque requires a greater pressure difference or a larger motor displacement.
A larger pump can provide more flow and may increase motor speed, but the motor must remain below its maximum rated rpm. Excess flow can also create heat, high pressure losses, and control problems.
Increasing available pressure can increase motor torque if the system and motor are rated for the higher pressure. However, exceeding continuous or peak pressure ratings can damage the motor, pump, hoses, valves, and other components.
Determine the required shaft rpm and power first. Select a displacement that can reach that speed with the available pump flow while still producing enough torque. Then verify maximum rpm, continuous pressure, peak pressure, case pressure, bearing loads, oil viscosity, cooling, and cavitation limits.
Provide required motor speed, continuous and peak torque, operating pressure, maximum pressure, pump drive rpm, available prime-mover power, required flow, motor and pump displacement if known, circuit type, duty cycle, shaft and flange dimensions, port configuration, fluid type, and machine application.
