This document provides an introduction to hydraulic components and systems. It describes common applications of hydraulics in machinery and discusses the advantages of hydraulic systems over other types of power transmission. The key components of hydraulic systems are identified as pumps, valves, actuators, hoses, and hydraulic oil. Examples of different types of pumps, valves, cylinders, and motors are illustrated and their basic functions and designs are explained. An educational electro-hydraulic positioning system is presented as an example application.

1. University of Illinois at Urbana-Champaign
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Hydraulic Components:
An Introduction
Don Carter
Team Partner: Rong Zhang
Advisor: Prof. Andrew Alleyne
Project Sponsor: Caterpillar

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Introduction
Applications
Why hydraulics?
Pumps
Valves
Actuators
Hoses and Hydraulic Oil
EVPS
Conclusion

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Applications
Hydraulic presses
Automobile applications
Motion control of airplanes
Civil engineering applications
Agricultural machinery
Industrial robotics
Earthmoving machinery!

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Advantages of Hydraulics
Versatility in design
Not constrained by mechanical linkages
Simply route the hoses
Actuator speed can be controlled via valves
or pump displacement
Reversibility of actuators without damage
Overload protection
High power density; High power/small size
Actuators can be stalled

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Advantages of Hydraulics

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Disadvantages
Can be ‘messy’; LEAKAGE
Not as much of an issue as it used to be
Hydraulic oil is not exactly
environmentally friendly; must be
replaced; can also be flammable
Control of these inherently nonlinear
systems can be challenging
Noisy systems
Others?

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Hydraulic Components
Pumps
Fixed Displacement
Variable Displacement
Valves
Flow control
Pressure relief
Actuators
Cylinders
Motors

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Hydraulic Components
Hoses
Other
Reservoirs
Filters
Heat Exchangers
Accumulators
• Stores hydraulic pressure
• Absorb shock, maintain pressure, develop
system flow

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Hydraulic Symbols

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Pumps
Fixed Displacement
Vane
Lobed
Advantages: cheap and simple!
No control over amount of flow output for given
rotational speed
Variable Displacement
Piston-type
Advantages include ability to adjust flow output by
varying swash plate angle
Expensive and mechanically complex

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Vane Pump

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Lobed Pump

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Variable Displacement Axial
Piston Pump

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Hydraulic Valves
Responsible for controlling and
directing flow
Directional flow control valves
• Spool and internal passages dictate flow paths
Proportional flow control valves
• Variable orifice
• Gate, globe, and needle valves
Check valves (similar to a diode in an
electrical circuit)

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Directional Control Valves

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Proportional Flow Control
Valves

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Pressure Relief Valves
Found in all hydraulic circuits
Normally closed
Can be pilot operated (i.e. Spool is
biased by both spring and fluid
pressure…less override)
Provides protection against exceeding
system pressure rating which can
damage components

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Pressure Relief Valves

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Check Valves

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Hydraulic Cylinders
Provides linear actuation
Single vs. Double acting
Single vs. Double ended
Complexity varies, but basic operation
and principles the same
Basic design terminology
Stroke length
Piston diameter
Piston rod diameter

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Hydraulic Cylinders
Can generate very large forces
Contamination of hydraulic oil and
proper maintenance directly affect the
life of a hydraulic cylinder

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Hydraulic Cylinders

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Hydraulic Motors
Provides rotary actuation/torque
Can be thought of as a pump working
in reverse
Motor ratings
Displacement
Torque capacity
Speed
Max pressure

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Hydraulic Motors
Constant displacement
Vane (can be variable)
Gear
Piston
Variable displacement
Axial piston
High Speed, Low Torque (HSLT)
Low Speed, High Torque (LSHT)

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Hydraulic Motors
Advantages over electric motors
Instant reversal of rotation of motor’s shaft
Can be stalled without damage
Torque control through operating speed
Dynamic braking is easily accomplished
Weight to horsepower ratio of .5 lb/hp
compared to 10lb/hp for electric motors

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Gear Motors

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Radial Piston Motor

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Hoses
Used to connect the various
components
Analogous to wires in an electrical
circuit
Compliance of the hose can affect the
effective bulk modulus
Modeled as a fixed volume

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Hydraulic Oil
Responsible for transmitting hydraulic
power from source to actuator
Dissipates heat!
Lubricates parts
Seals clearances
Bulk modulus, viscosity, and density
are the most important properties
Petroleum based most common

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EVPS Hydraulics
DriveDrive
Hydr.
Pump
Hydr.
Pump
SteeringSteering
Implement
Implement
EngineEngine5
4
3
2 1
ControlControl

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EVPS Hydraulics
MOTOR
LEGEND
high pressure oil
low pressure oil
control signal
sensor signal
LEGEND
high pressure oil
low pressure oil
control signal
sensor signal
LOAD UNIT #1
) (
LOAD UNIT #2
) (
LOAD UNIT #3
) (
Reservoir 1 Reservoir 2 Reservoir 3
3000psi
Main Reservoir

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References
Merritt H.E. Hydraulic Control
Systems. John Wiley & Sons, NY.
1967.
Vickers. Industrial Hydraulics Manual.
Copy in ARG!
Watton. Fluid Power Systems.
Prentice Hall. 1989

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Conclusions
Hydraulic systems offer versatility and
high power density as compared to
other types of power transmission
Relatively small number of
components; pumps, valves,
actuators…that’s basically it!
EVPS has relatively simple hydraulic
setup similar to that of an actual piece
of earthmoving machinery

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Bent Axis Piston Pump