How to Size an Axial Piston Motor

When I sit down to design a hydraulic circuit, I don't start with the pump. I start with the work. Sizing an axial piston motor is essentially working backward: you define the "muscle" needed to move the load, and then you figure out what kind of "heart" (the pump) is required to feed it.

If you get this wrong, your machine either won't budge under a heavy load or it will scream along at a speed that shakes the frame apart. Here is my professional roadmap for getting the sizing right the first time.

Step 1: It’s All About the Torque (Especially the Start)

The biggest mistake I see? Designing for running speed and forgetting about the Breakaway Torque.

Think of a loaded winch or a heavy excavator track. It takes much more "grunt" to get that mass moving than it does to keep it spinning. In the hydraulic world, Torque is a function of Pressure and Displacement.

To calculate the theoretical torque (\( T_{theo} \)) needed, use this formula:

Metric: $$ T_{theo} \text{ (Nm)} = \frac{V_g \text{ (cc/rev)} \times \Delta p \text{ (bar)}}{62.8} $$ Imperial: $$ T_{theo} \text{ (lb-in)} = \frac{V_g \text{ (in}^3\text{/rev)} \times \Delta p \text{ (psi)}}{2\pi} $$

Step 2: Calculating Displacement (\( V_g \))

Once you know your required torque (\( T_{req} \)) and your system's working pressure (\( \Delta p \)), you can find the required displacement.

Motor Architecture Selection Guide

If you need...	Consider...	Why?
High Starting Torque	Bent Axis Motor	Better mechanical leverage at zero RPM.
Compact Packaging	Swash Plate Motor	Slimmer profile, easier to "stack" pumps behind it.
Two Speed Ranges	Variable Displacement	Allows you to "shift gears" hydraulically for high torque or high speed.

Step 3: Match the Flow to Your Speed

Now that you have the "size" (displacement), you need to make sure you can feed it enough oil to hit your target RPM.

$$ \text{Flow (L/min)} = \frac{V_g \text{ (cc/rev)} \times \text{RPM}}{1000 \times \eta_v} $$

Here, \( \eta_v \) is the Volumetric Efficiency (usually around 0.95 for a good piston motor).

Watch out for the "Speed Limit"

Every motor has a maximum RPM. If your required flow forces the motor to spin faster than the catalog allows, you have two choices:

1. Increase the motor size (displacement) and decrease the pressure.
2. Add a mechanical gear reducer (like a planetary gearbox) so the motor can spin fast while the load turns slow.

Step 4: The "Silent Killers" (Case Drain and Side Loads)

You can have the perfect torque and speed, but if you ignore the plumbing, you'll blow the motor in a week.

• Case Drain Pressure: Piston motors leak internally by design—that oil lubricates the moving parts. This oil must get back to the tank via a dedicated case drain line. If this line is restricted, pressure builds up inside the motor housing and blows the shaft seal. Never "Tee" a case drain into a high-pressure return line.
• Radial Loads: Are you putting a pulley or a sprocket directly on the motor shaft? That creates a "side load." Check the manufacturer’s data for Allowable Radial Load. If you exceed it, the internal bearings will "flake" and fail prematurely.

Final Checklist Before You Buy