Sizing an axial piston motor requires calculating the required mechanical torque, displacement, hydraulic flow rate, and operating pressure to match specific load requirements. Selecting the correct motor size prevents low-speed stalling, fluid overheating, and shaft seal failures in industrial hydraulic circuits.
For complete motor dimensions and technical datasheets, consult our Huade Axial Piston Motor Selection Guide.
Step 1: Calculating Required Torque and Mechanical Efficiency ($T_{req}$)
The primary parameter in axial piston motor sizing is breakaway torque. Breakaway torque is the mechanical torque required to overcome static friction and initiate load movement from zero RPM.
Formula for Theoretical Torque ($T_{theo}$)
To calculate theoretical torque based on working pressure and displacement, use the following formulas:
- Metric System:
$$T_{theo} \text{ (Nm)} = \frac{V_g \text{ (cc/rev)} \times \Delta p \text{ (bar)}}{62.8}$$
Factoring in Starting Mechanical Efficiency ($\eta_{hm}$)
Axial piston motors exhibit a starting mechanical efficiency ($\eta_{hm}$) of 70% to 80% at zero RPM. To calculate actual required torque ($T_{act}$), divide theoretical torque by starting efficiency:
$$T_{act} = \frac{T_{theo}}{\eta_{hm}}$$
*Sizing Rule: If a mechanical load requires 100 Nm of torque, size the motor to produce at least 130 Nm. Insufficient torque causes motor stall and bypasses fluid through the relief valve.*
Step 2: Determining Motor Displacement ($V_g$) & Operating Pressure
Hydraulic motor displacement ($V_g$) is the volume of hydraulic fluid required to turn the motor shaft through one complete revolution.
To extend bearing operational life, select an operating pressure ($\Delta p$) that equals 70% to 80% of the motor's maximum continuous pressure rating. Operating continuously at relief valve pressure causes fluid overheating and bearing fatigue.
Structural Comparison: Bent-Axis vs. Swashplate Motors
Step 3: Aligning Hydraulic Flow Rate ($Q$) with Target RPM
The hydraulic flow rate ($Q$) determines the rotational speed (RPM) of the motor shaft.
To calculate required flow rate based on motor displacement and volumetric efficiency ($\eta_v$), use the following formula:
$$Q \text{ (L/min)} = \frac{V_g \text{ (cc/rev)} \times \text{RPM}}{1000 \times \eta_v}$$
Step 4: Critical Installation Rules (Case Drain & Radial Load Limits)
Improper installation of external lines causes premature shaft seal blowout and bearing failure in axial piston motors.
1. External Case Drain Line Pressure
Axial piston motors leak hydraulic fluid internally to lubricate internal pistons, shoes, and swashplate bearings. This fluid drains back to the reservoir via a dedicated case drain line.
- Rule: Keep case drain pressure below 2 bar.
- Warning: Never connect a motor case drain line directly into a high-pressure return line. High backpressure blows out the shaft seal.
2. Allowable Radial and Axial Shaft Loads
Direct drive connections using pulleys or sprockets exert radial forces on the output shaft. Always check the manufacturer's data sheet for maximum allowable radial load ($F_q$). Exceeding radial load limits causes bearing flaking and shaft deflection.
Sourcing Equivalent Axial Piston Motors
Replacing worn OEM piston motors with ISO-compliant alternatives reduces equipment downtime and procurement costs.
For direct drop-in replacements, explore our Huade A2FM Fixed Piston Motor and Huade A6VM Variable Piston Motor series, designed to match Rexroth mounting dimensions and displacement specifications.