Understanding how hydraulic motor displacement translates into torque and flow helps you select the right motor for a project. By combining bore, stroke, and rotational speed, you can estimate the chamber volume a motor displaces per revolution and, thus, the overall hydraulic throughput. This page provides a straightforward calculator and clear explanations to help engineers, technicians, and buyers design more efficient hydraulic systems.
Short calculator title
Introduction
In hydraulic systems, displacement tells you how much fluid a motor moves in one turn. This volume, usually measured in cubic centimeters per revolution (cc/rev), affects torque, speed, and overall flow. A clear grasp of bore size, stroke, and rpm helps you predict performance without needing full prototype testing. The calculator on this page translates those physical dimensions into actionable numbers you can compare when selecting motors or sizing a hydraulic circuit.
How to use the calculator above
Start with three basic inputs: bore diameter, stroke length, and rotation speed. The bore determines the cross-sectional area the piston moves through, the stroke sets the piston travel per revolution, and rpm tells you how many revolutions occur each minute. The tool then outputs the displacement per revolution and the displacement per minute, letting you estimate both mechanical output and hydraulic flow for your design.
- Enter bore diameter in millimeters (mm).
- Enter stroke length in millimeters (mm).
- Enter rotation speed in RPM.
Results are shown as:
- Displacement per revolution: how much hydraulic fluid is moved in a single turn
- Displacement per minute: the total fluid moved each minute at the given RPM
Tip: to compare motors, normalize data to cc/rev and cc/min, then relate those figures to system pressure and efficiency. Remember that real-world losses from seals, valving, and viscosity can reduce effective flow, so treat the numbers as baseline estimates.
Worked example with specific numbers
Let’s use a common small hydraulic motor configuration: bore diameter 50 mm, stroke 40 mm, and rotation speed 1800 RPM. First, compute the displacement per revolution:
Displacement per revolution = π × (50/2)^2 × 40 / 1000 = π × 625 × 40 / 1000 = 78.54 cubic centimeters (cc) per rev.
At 1800 RPM, displacement per minute = 78.54 cc/rev × 1800 rev/min = 141,372 cc/min, which is about 141.37 liters per minute (L/min).
These figures help you estimate the hydraulic power and flow requirements for the system. For instance, if your operating pressure is 150 bar, you can gauge theoretical hydraulic power using standard relationships, keeping in mind efficiency losses that occur in real machinery.
Other helpful information
Displacement is just one piece of the performance puzzle. Several additional factors influence how a hydraulic motor behaves in practice:
- Efficiency: Volumetric and mechanical efficiency reduce actual flow and torque. Older or worn components typically perform worse.
- Pressure and flow relationship: Higher pressure demands more robust components, while flow rate affects speed and torque at a given pressure.
- Unit conversions: cc/rev is a convenient motor metric, but engineers often translate to cubic inches per revolution (in³/rev) or liters per minute (L/min) for broader readability.
- System dynamics: Piping losses, valve restrictions, and fitting friction can alter the actual displacement delivered to the load.
- Motor selection: A motor with a larger displacement might provide more torque at low speeds, while a smaller displacement can offer higher speeds at the same pressure.
When engineering a hydraulic drive, you typically start with displacement and rpm to estimate flow, then cross-check with available pump capacity, available pressure, and target torque. The calculator helps you quickly iterate different bore and stroke combinations to find a motor that balances speed, torque, and efficiency for your application.
Frequently Asked Questions
What is hydraulic motor displacement?
Hydraulic motor displacement refers to the volume of fluid moved by the motor per revolution, usually expressed in cubic centimeters per turn (cc/rev). It is determined by the bore size and stroke and directly influences the motor’s torque and flow at a given pressure.
Why does bore size matter for displacement?
The bore sets the cross-sectional area through which fluid acts. A larger bore increases the piston area, which increases the volume moved per revolution. In turn, this raises the potential torque and flow the motor can deliver.
How does stroke affect displacement?
Stroke is the distance the piston travels in one cycle. A longer stroke increases the displaced fluid per revolution, boosting both torque and flow potential, assuming other factors remain constant.
How do you calculate displacement per rev?
Displacement per rev is computed as π × (bore/2)² × stroke, typically converted to cubic centimeters by dividing by 1000 when using millimeters for bore and stroke. The formula is PI * (bore_diameter_mm/2)^2 * stroke_mm / 1000.
What units are used for displacement and flow?
Displacement per revolution is commonly given in cubic centimeters per revolution (cc/rev) or cubic inches per revolution (in³/rev). Flow rate is often expressed as cc/min, L/min, or gpm depending on regional preferences and system requirements.
How is rpm involved in flow rate?
Flow rate equals displacement per revolution multiplied by RPM. Higher RPM with a fixed displacement per rev yields greater hydraulic flow, which can increase motor speed and load capability, but may also raise system pressure and energy consumption.
Can this calculator help with motor selection?
Yes. By adjusting bore, stroke, and rpm inputs, you can compare potential motors and see how changes affect displacement and flow. This supports decisions about torque, speed targets, and compatibility with pump capacity and operating pressure.
How accurate is the calculator?
The calculator provides baseline estimates based on geometric displacement, assuming ideal conditions. Real-world performance is affected by efficiency losses, fluid properties, temperature, and mechanical wear, so use the results as guides for initial sizing and selection.
How do I convert cc to liters?
There are 1000 cubic centimeters in a liter, so simply divide the value in cc by 1000. For example, 141,372 cc/min equals 141.372 L/min.
What other factors influence motor performance?
Important factors include system pressure, valve configuration, hose and fitting losses, fluid viscosity, temperature, and the mechanical condition of the motor. A motor with a larger displacement may deliver more torque at low speeds, but could require higher flow to achieve the same speed at a given pressure.