Hydraulic Motors

Overview of Lesson

• How they work

• Types of Motors

• Performance

• Applications

How a hydraulic motor works

• High pressure fluid is

used to turn a shaft.

• This is done in many

different ways.

• Much like a cylinder

the power comes from

the pressure acting

over a large area and

creating a large force.

Hydraulic Motors

• Hydraulic motors are called rotary actuators

• They convert fluid pressure and flow into

torque and rotational movement

Hydraulic Motors

• Typical hydraulic motor application

Hydraulic Motors

• All basic hydraulic motors consist of three

component groups:

– Housing

– Rotating internal parts

– Power output shaft

Hydraulic Motors

• Parts of a typical

hydraulic motor

Hydraulic Motors

• System fluid enters the housing and applies

pressure to the rotating internal parts

• This, in turn, moves the power output shaft

and applies torque to rotate a load

Hydraulic Motors

• Primary parts that produce the rotating

motion in most hydraulic motors are either:

– Gears

– Vanes

– Pistons

Hydraulic Motors

• Four requirements of a motor

Hydraulic Motors

• Displacement of a hydraulic motor indicates

the volume of fluid needed to turn the output

shaft one revolution

– Fixed displacement

– Variable displacement

Hydraulic Motors

• In a fixed-displacement motor:

– Internal geometry cannot be changed

– Same volume needed per output shaft

revolution

Hydraulic Motors

• In a variable-displacement motor:

– Internal geometry can be changed

– Displacement per shaft revolution can be

adjusted

– Motor can operate at variable speeds with a

constant input flow

Hydraulic Motors

• Hydraulic motors may be classified by the

type of load applied to the bearings of the

output shaft

– Unbalanced indicates the output shaft is loaded

from one side, side loading the shaft bearings

– Balanced indicates the bearing load is balanced

by use of two inlet ports arranged opposite of

each other and two outlet ports similarly

arranged

Types of Hydraulic Motors

• Gear Motors

• Vane Motors

• Piston Motors

• Limited Rotation Actuator

Gear Motors

• The external gear hydraulic motor is the

most common and simplest of the basic

motor types

– Fixed displacement

– Unbalanced load on the bearings

External Gear Motors

• 2 rotating gears, the

area of the gear teeth

is where the pressure

acts to create force.

• Both gears turn

simultaneously.

• One gear is

connected to the

output shaft and the

other is an idler.

Model 21300 “B1 Series” Gear Motor

Eaton® Heavy Duty Series 1 Variable Motor

Gear Motors

• Internal Gear Motors

• Two categories

– Direct drive gerotor, works much like a rotary engine.

– Two gears, an inner and an outer.

– The pressure pushes them around a center point, turning a shaft

Internal Gear Motors

• The most common internal gear motor has a

gerotor design

Courtesy of Eaton Fluid Power Training

Internal Gear Hydraulic Motors

• The specially shaped gear teeth of the

gerotor form variable-volume chambers that

allow system fluid flow and pressure to turn

the motor output shaft

• Gerotor motors are fixed-displacement units

operating with an unbalanced bearing load

Internal Gear Hydraulic Motors

• An orbiting gerotor motor is a variation of

the basic gerotor design

– Uses a fixed outer gerotor gear with internal

teeth and an inner gear with external teeth

– Center point of the inner gear orbits around the

center point of the fixed gear with internal teeth

– Motor operates at a slower speed, but has a

higher torque output

Internal Gear Hydraulic Motors• Orbiting gerotor motor

Courtesy of Eaton Fluid Power Training

Vane Motors

• Spring loaded vanes are connected to a rotor

• The rotor turns inside a cam ring (elliptical hole)

• The vanes slide in and out of the slots in the rotor to make contact with the cam wall.

Vane Motors

• Basic vane motor has a slotted rotor located off center in a circular chamber and fitted with movable vanes

– Space between the vanes creates a number of variable-sized chambers

– Forcing fluid into the small-size chambers causes the volume of the chambers to increase, turning the motor shaft

– Basic vane motor is fixed displacement with an unbalanced bearing load

Vane Motors

• Basic vane motor

Vane Motors

• Balanced vane motors evenly distribute the

load on the bearings

– Achieved by operating the rotor and vanes in a

slightly oblong chamber

– Allows two inlet ports and two outlets ports to be

used in the motor

– Placing ports opposite each other balances bearing

loading

Vane Motors

• A basic, balanced vane motor

Vane Motors

Vane Motors

• Vane motors are available as either fixed or

variable displacement

• The variable-displacement feature allows an

operator to change the speed of a motor

without changing the system flow rate

Vane Motors

• In variable-displacement designs, the

chamber in which the rotor and vanes

operate is contained in a moveable ring

– When the center point of the rotor and ring are

concentric, the displacement is zero

– Moving the ring so the center points are not

concentric increases the motor displacement

and changes motor speed

Piston Motors

• Many different types of piston motors

• All of them use the same basic principles

• Much like a cylinder, only turns a shaft like

the cylinders in your car engine.

• Generally the most efficient

• High power, high speed, high pressure

Piston Motors

• Most efficient

• Often used in

aerospace applications

due to high power to

weight ratio

Piston Motors

• Piston motors are available having either

fixed or variable displacements

• In variable-displacement designs, the length

of the piston stroke is changed to vary the

volume of fluid needed to rotate the motor

one revolution

Piston Motors

• Two basic classifications of piston motors

are axial piston and radial piston

– An axial piston motor has pistons with

centerlines parallel to the axis of the output

shaft

– A radial piston motor has pistons with

centerlines perpendicular to the axis of the

output shaft

Piston Motors

• Axial piston motor

The Oilgear Company

Piston Motors

• Axial piston motors are available in two

configurations:

– Inline

– Bent axis

Piston Motors

• In an inline piston motor:

– Centerline of the barrel is concentric with the

centerline of the power output shaft

– A swash plate transmits force from the pistons

to the shaft

In-line Piston Motors

• Simple construction

• Low cost

• Used in low torque

high speed

applications, such as

machine tools

In-line Piston Motors• Inline piston motor

The Oilgear Company

Radial Piston Motors

• High Torque

• Low speed

• Possible application

could be a roller.

Bent Axis Piston Motor

• Is one example of variable

displacement motor.

• The angle is altered to

change the displacement.

Bent Axis Piston Motors

• In a bent-axis piston motor:

– Centerline of the barrel is at an angle to the

centerline of the output shaft

– A universal joint and other fittings are used to

transmit force between the barrel and the output

shaft

Bent Axis Piston Motors

• Bent-axis piston motor

Courtesy of Eaton Fluid Power Training

Limited Rotation Actuator

• Also sometimes called

a oscillator.

• Has limited movement

• High torque

Performance

• The mechanical torque

desired must be

specified in order to

find required working

pressure.

• Pin= (T*2 )/Disp.

• The rpm desired must

be specified in order to

find required flow

rate.

• Q= (rpm * Disp.)/231

Power

• Hpout= (Tlb-ft * rpm)/5252

• Effoa= (Hpout * 100)/ Hpin

Hydraulic Motors

• A number of alternate motor designs are used

in specialized hydraulic applications

– Screw motor designs for quiet, continuous

operation

– Special piston-motor designs allowing the direct

mounting and drive of wheels for off-road, heavy-

transport vehicles

Hydraulic Motors

• Hydraulic motors may be incorporated into

circuits using series or parallel connections

– Series circuits: total system pressure is

determined by adding the loads placed on each

unit

– Parallel circuits: each motor receives full

system pressure; loads must be matched or

equal flow supplied to each motor if constant

speed is desired from each unit

Hydraulic Motors

• Motors in series

Hydraulic Motors

• Motors in parallel

Hydraulic Motors

• Motors in parallel with flow control

Hydraulic Motors

• Braking circuits are used to slow hydraulic

motors to a stop

– Inertia of a heavy rotating load can continue to

turn the motor shaft

– Braking occurs when fluid discharged from the

motor outlet port is forced to pass through an

adjustable pressure control valve before

returning to the reservoir

Hydraulic Motors

• Braking circuit

Hydraulic Motors

• An open-loop hydraulic motor system uses

a layout typical of a basic hydraulic system

– Pump moves fluid from a reservoir, through a

directional control valve, to the motor

– Fluid is then returned from the motor to the

reservoir through the same control valve

Hydraulic Motors

• Closed-loop hydraulic motor systems

continuously circulate fluid between the

pump and the motor without returning it to a

system reservoir

• These systems use a replenishment circuit

to replace fluid lost through leakage

Hydraulic Motors

• Replenishment circuit

Hydrostatic Drives

• Hydrostatic drive systems consist of the basic

components typically found in other hydraulic

motor circuits

MDMA Equipment—Menomonie

Hydrostatic Drives

• Hydrostatic drives provide effective

transmission of power and allow easy

adjustment and control of:

– Output shaft speed

– Torque

– Horsepower

– Direction of rotation

Hydrostatic Drives

• When compared to conventional

transmissions, hydrostatic drives:

– Have a high power output–to–size ratio

– May be stalled under full load with no internal

damage

– Accurately maintain speed under varying load

conditions

– Provide an almost infinite number of

input/output speed ratios

Hydrostatic Drives

• Hydrostatic drives may be open or closed

circuits

– Open circuit has the layout of a basic hydraulic

motor circuit

– Closed circuit has the outlet of the pump

directly connected to the inlet of the motor and

the outlet of the motor directly connected to the

inlet of the pump

Hydrostatic Drives

• Open circuit design

Hydrostatic Drives

• Closed circuit design

Sauer-Danfoss, Ames, IA

Hydrostatic Drives

• Four combinations of pump/motor

arrangements can be used

– Fixed-displacement pump and motor

– Fixed-displacement pump and variable-

displacement motor

– Variable-displacement pump and fixed-

displacement motor

– Variable-displacement pump and motor

Hydrostatic Drives

• Fixed-displacement pump and motor:

– Maximum horsepower, torque, and output shaft

speed are fixed

– Pump and motor have fixed displacement, so

these characteristics cannot be changed

Hydrostatic Drives

• Fixed-displacement pump and variable-

displacement motor:

– Maximum horsepower is fixed

– Torque and speed are variable

– Due to use of a relief valve, efficiency is lowered

– Output shaft rotation may be reversed if the pump

is reversible

Hydrostatic Drives

• Variable-displacement pump and fixed-

displacement motor:

– Torque output is fixed

– Horsepower and output shaft speed are variable

– Output shaft rotation may be reversed if pump

is reversible

Hydrostatic Drives

• Variable-displacement pump and motor:

– Horsepower, torque, output shaft speed are

variable

– Output shaft direction is reversible

– Most versatile of the four pump/motor

combinations

Hydrostatic Drives

• Hydrostatic drives are typically considered

hydrostatic transmissions when both the

pump and motor have variable displacement

• This combination allows manual or

automatic control of torque, speed, and

power output

Hydrostatic Drives

• Two different general techniques are used

in the construction of hydrostatic

transmissions

– Integral

– Nonintegral

Hydrostatic Drives

• Integral construction combines all of the

transmission parts into a single housing

• Nonintegral construction involves separate

pump, motor, and accessories connected by

hoses or tube assemblies