Hydraulic Pump and Motor: How They Work and How to Size Them

Hydraulic pumps and motors are the core components that generate motion, power, and torque in modern machinery. Whether you are working on industrial equipment, agricultural machinery, construction systems, or power-driven applications, understanding how hydraulic pumps and motors work is essential for performance and safety.

This guide also explains how to size a hydraulic pump and motor correctly in 2025. Our clearance sales are available; check our complete catalog of genuine hydro parts at the best prices.

What Is a Hydraulic Pump?

A hydraulic pump converts mechanical power into hydraulic energy by pushing oil into the system under pressure. Once pressurized, hydraulic flow powers cylinders, actuators, and hydraulic motors.

How It Works

• Mechanical energy (from an electric motor or a diesel/gas engine) rotates the pump shaft.
• The pump creates a vacuum at the inlet
• Oil moves from the reservoir into the pump chambers
• The chamber compresses the oil
• Oil is delivered at high pressure into hydraulic lines

The hydraulic pump generates flow, not pressure. Pressure builds due to load resistance (motor, cylinder, or restriction).

Types of Hydraulic Pumps

• Gear Pumps (simple, rugged, cost-efficient)
• Piston Pumps (high-pressure, high-efficiency)
• Vane Pumps (smooth operation, low noise)

Most mid-to-heavy-duty modern systems rely heavily on piston pump designs due to durability and pressure output.

What Is a Hydraulic Motor?

A hydraulic motor converts hydraulic energy (flow + pressure) back into mechanical power. This rotating movement drives:

• Wheel hubs
• Augers
• Winches
• Rotary attachments
• Track drives

How a Hydraulic Motor Works

• Pressurized oil enters the motor inlet
• Oil forces internal chambers to rotate
• The output shaft delivers torque
• Low-pressure oil exits back to the tank

Torque output is determined by pressure × displacement.

Hydraulic Pump and Motor Flow Relationship

At a basic level:

Pump creates flow → Motor converts flow into rotation

If pump flow increases, → Motor RPM increases. If system pressure rises, → Motor torque increases

How to Size a Hydraulic Pump and Motor (2025 Method)

You must define the required speed and torque for the application. Below is the relationship used in modern hydraulic sizing.

Step-by-Step Sizing Table

Basic Hydraulic Sizing Formulas 

Motor Speed

RPM = (GPM × 231) ÷ displacement

Motor Torque

Torque (in-lb) = (PSI × displacement) ÷ 6.28

Correct Matching Rule

Pump GPM must deliver the required motor RPM

System pressure must deliver the required torque loads

Common Sizing Examples

Hydraulic Pump & Motor FAQs

Final Thoughts

Hydraulic pumps generate flow, and hydraulic motors convert that flow into motion and torque. The key to proper operation is sizing each component based on speed and load requirements, pressure capability, and volume demand. When properly matched, hydraulic systems operate cooler, stronger, safer, and longer.

If your application requires precise hydraulic pump or motor genuine replacements, sizing verification, or system upgrades, review your model specifications before purchasing components to maintain proper performance.