Chapter 7 Hydraulic Motors
Topics:
7.1 Introduction.
7.2 Limited Rotation Hydraulic Motors.
7.3 Gear Motors.
7.4 Vane Motors.
7.5 Piston Motors.
7.6 Hydraulic Motor Theoretical Torque, Power, and Flow Rate.
7.7 Hydraulic Motor Performance.
7.8 Hydraulic Transmissions.
7.1 INTRODUCTION
*Hydraulic Motors extract energy from a fluid and convert it into mechanical energy.
* Three basic types:
1). Gear
2) Vane.
3) Piston.
Low Speed High Torque Hydraulic Motors (LSHT) – Special class of hydraulic motor that
produces high torque.
Rotary Actuator or Oscillating Motor – Limited rotation motor either clockwise or counter
clockwise but always less than one complete revolution.
Figure 1 Rotary Actuator
Hydraulic Motor – continuous rotating motor.
Figure 2 Fixed Displacement- Unidirectional Hydraulic Motor
*Hydraulic motors are “pumps” designed to withstand different forces involved in motor
applications.
7.2 LIMITED ROTATION HYDRAULIC MOTORS
Figure 3 Rotary Actuator
*Provide rotary output motion over a finite angle.
*Torque capacity 3 to 1 million lb∙ft.
*Pressures up to 5000 psi.
*Can have cushioning devices.
Symbol :
Torque for single vane rotary actuator:
𝑇𝑜𝑟𝑞𝑢𝑒 𝑜𝑓 𝑅𝑜𝑡𝑎𝑟𝑦 𝐴𝑐𝑡𝑢𝑎𝑡𝑜𝑟 (𝑇) =
𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒 (𝑝) ∙ 𝐷𝑖𝑠𝑝𝑙𝑎𝑐𝑒𝑚𝑒𝑛𝑡(𝑉 )
2∙𝜋
𝐷𝑖𝑠𝑝𝑙𝑎𝑐𝑒𝑚𝑒𝑛𝑡 𝑜𝑓 𝑅𝑜𝑡𝑎𝑟𝑦 𝐴𝑐𝑡𝑢𝑎𝑡𝑜𝑟 (𝑉 ) = 𝜋 ∙ (𝑅𝑎𝑑𝑖𝑢𝑠 𝑉𝑎𝑛𝑒(𝑅 ) − 𝑅𝑎𝑑𝑖𝑢𝑠 𝑅𝑜𝑡𝑜𝑟 (𝑅 ) )
7.3 GEAR MOTORS
Figure 4 External Gear Motor
* Since case of hydraulic motor is pressurized by outside source most hydraulic motors have
case drains to protect shaft seals.
*Types of Gear Motors:
1.
2.
3.
4.
External Gear.
Internal Gear.
G-Rotor.
Screw Motors.
* All Gear Motors are fixed displacement models (no variable displacement models exist).
* Gear Motors are mostly limited to about 2000 psi, 2400 rpm and 150 gpm.
* Simplest design with lowest pressure ratings, and lowest overall efficiency but most dirty oil
tolerant along with lowest manufacturing cost.
* Internal gear type motor operates at higher pressures and speeds and greater displacement
than external gear pumps.
7.4 VANE MOTORS
Figure 5 Balanced Vane Motor
Types of Vane Motors:
1. Unbalanced Vane.
2. Balanced Vane.
* Unbalanced Vane Motor can variable of fixed displacement model. Balanced Vane Motor is
only available as a fixed displacement model.
* Between Gear Motor and Piston motor for pressure ratings and overall efficiency ratings
* Vane motors must have some type of force to extend vanes against case of pump.

Use springs or pressure loaded vanes.
* Pressure up to 2500psi and 4000 rpm for balanced design.
7.5 PISTON MOTORS
Three Basic Types:
1. Axial Piston Motor
2. Bent Axis Piston Motor
3. Radial Piston Motors
*Axial and Bent Axis motor are available a variable or fixed displacement models. Radial Piston
Motors are available as only fixed displacement models.
Figure 6 Axial Piston Motor
In-line or Axial Piston Motors (Swash Plate Design)
*Swash Plate design can be variable or fixed displacement type.
*Swash Plate angle increases the displacement of motor.
Figure 7 Bent Axis Motor
In-Line or Axial Piston Motor (Bents Axis Design)
*speed and torque depend on angle between cylinder block and driveshaft.


Larger the angle the larger the displacement and the slow the speed but the higher the
torque output of motor.
Angle varies between 7.5° and 30°.
*piston motors are most efficient of the three designs and capable of the highest speeds and
pressures.

Up to 12,000 rpm and pressures up to 5000 psi and flow rates up to 450gpm.
*Available as variable or fixed displacement models.
Radial Piston Motors
Figure 8 Pintel Type Radial Piston Motor
*Three Basic Types:
1. Eccentric Ring Radial Piston Motor.
2. External Cam Radial Piston Motor.
3. Rotary Cam Radial Piston Motor.
* Considered to be low steed but high torque motors (LSHT).
7.6 HYDRAULIC MOTOR THEORECTICAL TORQUE, POWER, AND FLOW RATE
Motor Theoretical Torque (TT):
𝑇ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝑇𝑜𝑟𝑞𝑢𝑒 (𝑇 ) =
𝐶ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒 (∆𝑝) ∙ 𝑀𝑜𝑡𝑜𝑟 𝐷𝑖𝑠𝑝𝑙𝑎𝑐𝑒𝑚𝑒𝑛𝑡 (𝑉 )
2∙𝜋
Break Power of Motor (PB):
𝐵𝑟𝑎𝑘𝑒 𝑃𝑜𝑤𝑒𝑟 (𝑃 ) = 𝑇ℎ𝑒𝑜𝑟𝑒𝑐𝑡𝑖𝑐𝑎𝑙 𝑇𝑜𝑟𝑞𝑢𝑒(𝑇 ) ∙ 𝑀𝑜𝑡𝑜𝑟 𝑆𝑝𝑒𝑒𝑑 (𝑁)
Motor Theoretical Flow Rate (QT):
𝑇ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝐹𝑙𝑜𝑤 𝑅𝑎𝑡𝑒 (𝑄 ) = 𝑀𝑜𝑡𝑜𝑟 𝐷𝑖𝑠𝑝𝑙𝑎𝑐𝑒𝑚𝑒𝑛𝑡 (𝑉 ) ∙ 𝑀𝑜𝑡𝑜𝑟 𝑆𝑝𝑒𝑒𝑑 (𝑁)
7.7 HYDRAULIC MOTOR PERFORMANCE
Introduction
* Performance depends on precision of manufacture and service requirements
* Leakage contributes to volumetric efficiency ( 𝜂 )
* Friction between parts and fluid turbulence contribute to mechanical efficiency ( 𝜂 )
* Overall Efficiency ( 𝜂 ) of Common Motors:
1. Gear Motors: 70 to 75%
2. Vane Motors: 75 to 85%
3. Piston Motors: 85 to 95%
*Motors starting under load have a stall torque factor.

Only 80% of max torque can be expected to start under load or operate under speed of
500 rpm
Volumetric Efficiency:
* Inverse of pump volumetric efficiency. Pump does not produce as much it should. And motor
uses more fluid than it should.

Due to leakage.
𝑀𝑜𝑡𝑜𝑟 𝑉𝑜𝑙𝑢𝑚𝑒𝑡𝑟𝑖𝑐 𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 (𝜂 ) =
𝑀𝑜𝑡𝑜𝑟 𝑇ℎ𝑒𝑜𝑟𝑒𝑐𝑡𝑖𝑐𝑎𝑙 𝐹𝑙𝑜𝑤 𝑅𝑎𝑡𝑒 (𝑄 )
𝑀𝑜𝑡𝑜𝑟 𝐴𝑐𝑡𝑢𝑎𝑙 𝐹𝑙𝑜𝑤 𝑅𝑎𝑡𝑒 (𝑄 )
Mechanical Efficiency:
* Inverse of pump because of friction. Pump requires more torque than it should and motor
produces less torque than it should.
𝑀𝑜𝑡𝑜𝑟 𝑀𝑒𝑐ℎ𝑎𝑛𝑖𝑐𝑎𝑙 𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 (𝜂 ) =
𝑀𝑜𝑡𝑜𝑟 𝐴𝑐𝑡𝑢𝑎𝑙 𝑇𝑜𝑟𝑞𝑢𝑒 (𝑇 )
𝑀𝑜𝑡𝑜𝑟 𝑇ℎ𝑒𝑜𝑟𝑒𝑐𝑡𝑖𝑐𝑎𝑙 𝑇𝑜𝑟𝑞𝑢𝑒 (𝑇 )
Overall Efficiency:
𝑀𝑜𝑡𝑜𝑟 𝑂𝑣𝑒𝑟𝑎𝑙𝑙 𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 (𝜂 ) =
𝑃𝑜𝑤𝑒𝑟 𝑂𝑢𝑡 𝑜𝑓 𝑀𝑜𝑡𝑜𝑟 (𝑃 )
=𝜂 ∙𝜂
𝑃𝑜𝑤𝑒𝑟 𝐼𝑛𝑡𝑜 𝑀𝑜𝑡𝑜𝑟 (𝑃 )
7.8 HYDRAULIC TRANSMISSIONS
Hydrostatic Transmission
*Provides adjustable speed drives for practical applications such as Tractors, Rollers, Front-end
Loaders, Lift-trucks, ect.
* Advantages:
1.
2.
3.
4.
Infinitely variable speed and torque in either direction over full speed and torque ranges.
High power to weight ratio.
Ability to be stalled without damage
Low inertia rotating members, permit fast starting and stopping with smoothness and
precision.
5. Flexibility and simplicity of design.
Three types:
1. Variable displacement pump feeding a fixed displacement motor (Constant Torque).
2. Fixed displacement pump feeding a variable displacement motor (Constant Power).
3. Variable displacement pump feeding a variable displacement motor (Variable
Independent).