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HYDRAULIC SYSTEMS
Introduction to Hydraulic Systems
What are hydraulics?
Answer – The study of the mechanical properties of fluids
What is Fluid Power?
Answer – The use of fluid motion under pressure to transfer power & energy from a
source to a sink (receptor).
Commercial Definitions:
Hydraulics – The transmission of power from a power generation source to a sink using
an engineered incompressible hydraulic fluid for the sake of creating leverage or
motion.
Basic Hydraulic Fluid Principles
Elements of Fluid Mechanics
Fluid Flow = Q
Volumetric rate gal/hour, L/min
Fluid Pressure = P
Force per square area Lbs/sq in, Kg/sq m
Fluid Velocity = V
Distance over time ft/sec, m/sec
Fluid Temperature = T
°F or °C
Fluid Viscosity = ν
Basic Hydraulic Fluid Principles
Fluid Mechanics Relationships
Flow – Velocity
Q= A * v
Where Q= Volumetric Flow Rate, A= Cross sectional Area & v= Fluid Velocity
Fluid flow in a system is additive
Bernoulli’s Law for incompressible fluids
H = z + p/ρg + v
2/2g
(fluid is flowing with a significant difference in height between
source & sink)
Where H=total head pressure, v= fluid velocity, g= force of gravity, z= the height of the fluid source, p=fluid pressure & ρ=fluid density
p
0= p + v
2/2
(fluid height is insignificant)
Basic Hydraulic Fluid Principles
Hydraulic Fluid Power
Fluid power depends on a viscous fluid flowing under pressure from a sink to a
source. The systems efficiency is dependent on fluid density, temperature and
pressure loss due to decreased fluid velocity.
Fluid Flow
Turns fluid pressure / energy into leverage Gravity
Basic Hydraulic Systems Overview
Typical Lift / Ram Circuit (mobile or industrial – open center system)
Pump
Tank Relief
Valve
Control Valve
Cylinder
Filter
Cooler
Basic Hydraulic Systems Overview
Typical Motor Power Circuit (mobile – closed center system)
Tank
Hydraulic
Motor Blower Fan
Filter
Cooler Variable
Pump
EH Servo Control Valve
Hydraulic Motors
Hydraulic Motors Overview
Purpose
A hydraulic motor converts hydraulic energy from pressure into rotary motion and
torque to drive an implement or system.
Types
Fixed positive displacement – gear, piston, geroter / geroler & vane types
Variable positive displacement – piston
Typical Applications
Wheel Motors – drive mobile equipment wheels (skid steers, tractors, lifts)
Fan Drives – hydraulic fan drives (engine cooling, industrial equipment, drive train cooling,
gen sets, grain driers)
Hydraulic Motors
Fixed Positive Displacement Motors
Motor displacement is fixed
Torque is proportional to inlet pressure
Speed is proportional to flow rate
Regulate torque and speed with either valves, variable displacement pump or pump speed.
Gear Motors
Inlet flow / pressure rotates a gear set causing the output shaft to rotate and create torque
Advantages
Low cost – initial and rebuild Good availability / many suppliers
Cast iron motors have high pressure capability Tolerant to contamination
Compact - desirable packaging
Disadvantages
Lower efficiency compared to other types
Hydraulic Motors
Fixed Positive Displacement Motors
Fixed Displacement Piston Motors
Axial Piston, Radial Piston & Bent Axis Types
Swash plate is fixed on an angle to achieve a specified displacement
Number & size of pistons in rotating group determine flow, torque and speed capabilities
Advantages
High efficiency / Performance
Higher torque capability per unit displacement
Radial type packages well for wheel
motor applications
Bent axis type available for improved
packaging
Good serviceability
Disadvantages
Higher cost
Not as tolerant to contamination
Fixed Displacement Bent Axis Piston Motor
Fixed Displacement Axial Piston Motor
Fixed Displacement Radial Piston Motor
Fixed Angle Swash Plate
Hydraulic Motors
Fixed Positive Displacement Motors
Fixed Displacement Vane Motors
Fluid flow over vanes produce rotational speed and torque
High speed and pressure capability
Number & area of vanes determine flow, torque and speed capabilities
Advantages
High efficiency / Performance
Higher speed capacity
Reliability & durability
Forward or reverse rotation
Superior cold start performance
Good power output per motor size
Disadvantages
Lower torque capability
Hydraulic Motors
Fixed Positive Displacement Motors
Fixed Displacement Geroter / Geroler Motors
Spool valve, disc valve & valve in star types Low speed and high torque capability
Works on the “Orbit Principle” – star, drive and output shaft
Gerotor & Geroler have similar performance characteristics for equal frame sizes. In the Geroler type,
the drive gear rides on roller bearings in the star for reduced friction, improved mechanical efficiency and useful life.
Advantages
High efficiency
Higher torque capacity
Reliability & durability – only three main components Compact with high power density
Can be connected in series with same pump source High systems pressure capability
Low speed constant with change in load Disadvantages
Hydraulic Motors
Variable Positive Displacement Motors
Variable Displacement Piston Motors
Axial Piston, Radial Piston & Bent Axis Types
Swash plate angle is variable – manual, hydraulic, EH or electric control
Number & size of pistons in rotating group determine flow, torque and speed capabilities
Advantages
High efficiency / Performance
Higher torque capability per unit displacement
Radial type packages well for wheel
motor applications
Bent axis type available for improved
packaging
Good serviceability
Disadvantages
Higher cost
Not as tolerant to contamination
Variable Displacement Axial Piston Motor Variable Displacement
Hydraulic Motors
How to Choose & Size a Hydraulic Motor
Step 1 – Document Motor Requirements
What is the application (wheel drive, fan, auger, winch, machine tool, turf care, etc.)
Space requirements - packaging
What torque is required for driving the application component? What hydraulic system pressure is available to the motor?
What hydraulic system flow is available to the motor?
What speed range is required for the motor?
Does the motor have to stall or reverse direction?
What is the hydraulic oil cleanliness levels? What are the cost factors?
How many motors will be run in series off of the same source?
What is the ambient temperature range of operation?
What hydraulic fluid will be used?
Step 2 – Choose the Motor Type
Fixed or variable displacement?
If fixed displacement – use the motor type selection chart to determine which type of fixed displacement
Hydraulic Motors
How to Choose & Size a Hydraulic Motor
Fixed Motor Type Selection Chart
Selection Criteria Gear Motor Piston Motor Gerotor / Geroler Motor Vane Motor
Low Cost X X
High Pressure X (cast iron) X X
High Speed / Low Torque X X X
Low Speed / High Torque X
High Efficiency X X X
High Reliability / Durability X X
Superior Cold Start Performance X
Availability X X
Compact Size / Displacement X X X
Large Displacements X
Wide Range of Displacements X X
Tolerant to Contamination X X X
Serviceability X X X
Hydraulic Motors
How to Choose & Size a Hydraulic Motor
Step 3 – Determine Motor Displacement
How much max horsepower or torque is required to drive the devise?
Torque (in-lbs) = 63024 Horsepower / Speed (rpm)
What max displacement is required?
Displacement (cubic in/rev) = 2π* Torque (in-lbs) / Δ Pressure (psi)* Mechanical Efficiency (%)
Mechanical efficiency varies from 80-90% depending on the type of motor.
What flow is required at the motor?
Flow (gpm) = Motor Displacement (cubic in / rev)* Speed (rpm) / 231* Volumetric Efficiency (%)
Volumetric efficiency varies from 85-95% depending on the type of motor.
Step 4 – Determine the motor that meets the requirements
Find a supplier that makes a motor of the type and size determined
Determine the best model motor to meet all or as many of the requirements for the
application that is at least equal to or larger than the displacement calculated.
Compare the selected motor specifications to the motor requirements and qualify it for the
application.
Recalculate the motor torque and flow with the selected motor’s specs to ensure the torque
Hydraulic Motors
How to Choose & Size a Hydraulic Motor
Motor Sizing Example
A hydraulic motor is needed to power a blower fan for a combine separation system. The fan
speed will vary from 0 to 1500 rpm and the fan requires 15 hp at max speed and load
conditions. The system pump supplying flow is a variable displacement axial piston pump with
a max flow of 30 gpm. What type of motor and displacement will satisfy these requirements?
Requirements:
• Pressure available at the motor inlet = 2000 psi • Max pressure for motor return to tank = 100 psi • Clockwise rotation only
• Only one motor in the system • System is unfiltered
• Low cost is important
• Ambient temp range 0 °F to 110 °F.
• Hydraulic fluid – Hydraulic Oil w/viscosity at 15 cST
normal operation, 10 cST min
Motor Type – See selection chart
Hydraulic Motors
How to Choose & Size a Hydraulic Motor
Fixed Motor Type Selection Chart
Selection Criteria Gear Motor Piston Motor Gerotor / Geroler Motor Vane Motor
Low Cost X X
High Pressure X (cast iron) X X
High Speed / Low Torque X X X
Low Speed / High Torque X
High Efficiency X X X
High Reliability / Durability X X
Superior Cold Start Performance X
Availability X X
Compact Size / Displacement X X X
Large Displacements X
Wide Range of Displacements X X
Tolerant to Contamination X X X
Serviceability X X X
Bidirectional X X
Hydraulic Motors
How to Choose & Size a Hydraulic Motor
Motor Sizing Example Continued
Theoretical Motor Displacement Calculation
• Torque (in-lbs) = 63024 Horsepower / Speed (rpm)
Torque = 63024 (15 hp) / 1500 rpm = 630 in-lbs
• Δ Pressure (psi) = Max systems pressure @ inlet – Max motor return to tank pressure
Δ Pressure = 2000 psi – 100 psi = 1900 psi
• Displacement (cubic inch / rev) = 2π* Torque (in-lbs) / Δ Pressure (psi)* Mechanical Efficiency (%)
Gear pump mechanical efficiency = 85%
Displacement = (2π * 630 in-lbs) / (1900 psi * 0.85) = 2.45 cubic in/rev or 40.1 cc/rev
Gear pump supplier chosen is Sauer Danfoss Group 3 frame size 44
• Specs vs. Requirements
Requirement / Spec Requirement Specification
Displacement (cubic in / rev) 2.45 2.69
Max speed (rpm) 3000 1500
Min speed (rpm) 800 800
Rated pressure (psi) 3625 2000
Hydraulic Motors
How to Choose & Size a Hydraulic Motor
Motor Sizing Example
Continued
Verify actual motor torque
Torque (in-lbs) = Δ Pressure (psi)* Mechanical Efficiency (%) * Displacement (cubic in/rev) / 2π
Torque = (1900 psi * 0.85 * 2.69) / 2π = 691 in-lbs > 630 in-lbs
Verify actual motor flow
Flow (gpm) = Motor Displacement (cubic in / rev)* Speed (rpm) / 231* Volumetric Efficiency (%)
Volumetric Efficiency = 88%
Hydraulic Cylinders
Hydraulic Cylinders Overview
Purpose
A hydraulic cylinder converts hydraulic energy from pressure into linear motion and
force to actuate, move or lift an implement or object.
Types
Dual Acting / Single Acting
Multi-stage Telescoping
Pressurized struts – Mobile Applications
Head & Cap Arrangements
• Welded – Medium duty applications / size
• Threaded – Light duty applications / size
• Bolted – Heavy duty applications / size
Hydraulic Cylinders
Hydraulic Cylinders Overview
Typical Applications
Construction Equipment – (implements, dump trucks, suspension struts, stabilizers, steering systems)
Lifts – (scissors lifts, aerial lifts, cranes, fork lifts, lift gates)
Industrial Machinery – (presses, rams, loading docks, injection molding machines)
Agricultural Equipment – (tractor implements, bailers, combine heads, sprayers)
Hydraulic Cylinders
Hydraulic Cylinders Overview
Single vs. Dual Acting Cylinders
Single acting cylinder only actuates the rod
• The rod extends under pressure and contracts under force or weight
• Typically used in applications where load is lifted hydraulically and gravity returned
• A spring in the system can be used to achieve contraction
Dual acting cylinder actuates the rod and the head ends
• Both extension and contraction occur under hydraulic pressure
Hydraulic Cylinders
Typical Cylinder Construction
Barrel or body
Rod
Hydraulic Cylinders
How to Choose & Size a Hydraulic Cylinder
Step 1 – Document cylinder requirements
What is the application (lift, press, steering, hoist, implement, ram, crane etc.)
Space requirements – packaging & end attachments
What is the max collapsed length What is the max extended length
What max force or weight is necessary to actuate the attached object?
What hydraulic system pressure is available to the cylinder?
What hydraulic system flow is available to the cylinder?
How many cylinders will be used to move the load
What max time is required to go from min length to max extended length?
What are the cost factors?
What is the ambient temperature range of operation?
What hydraulic fluid will be used?
Step 2 – Choose the cylinder type
Dual or Single Acting?
Hydraulic Cylinders
How to Choose & Size a Hydraulic Cylinder
Step 3 – Determine cylinder bore size
Force = Required Force / # Cylinders
Cylinder Bore (in) = [.7854 * Force (lbs) / Pressure (psi)]
½
This is the minimum bore size required. To decrease the time to fully extend the cylinder, the
bore size can be increased.
Find a Cylinder of the type chosen with the next larger bore size available
Step 4 – Determine if the flow rate required for max extension .
Flow Rate (gpm) = Fluid Velocity (ipm) * Cylinder Piston Area (in) * 0.00433
Cylinder Piston Area = π * [Cylinder Bore (in) / 2]
2
Fluid Velocity (ipm) = [Extended Cylinder Stroke (in)] / [Max Extension Time (sec) / 60]
Is Flow Rate equal to or less than the required flow rate? If not, the cylinder bore size has to
be increased to ensure the max time to full extension is satisfied within the flow rate
available.
Step 5 – Determine the piston rod diameter & column size
Determine the column strength factor from Table 1.1
Corrected Length = Actual Stroke * Column Strength Factor
Hydraulic Cylinders
Determine the appropriate piston rod diameter
using Table 1.2
Determine the stop tube length if necessary
Internal stops are sometimes required to limit rod
stroke to prevent rod buckling
Stop Tube Length (in) =
[Corrected Length – 40 in] / 10
How to Choose & Size a Hydraulic Cylinder
Hydraulic Cylinders
How to Choose & Size a Hydraulic Cylinder
Step 6 – Choose the type of cylinder ends for attachment
Step 7 – Determine a cylinder that meets the requirements
Find a supplier that makes a cylinder of the type and size determined
Determine the best model cylinder to meet all or as many of the requirements for the
application that is at least equal to or larger than the bore and rod diameter calculated.
Compare the selected cylinder specifications to the cylinder requirements and qualify it for
the application.
Recalculate the cylinder load capability and time for full extension with the selected cylinder’s
Hydraulic Cylinders
How to Choose & Size a Hydraulic Cylinder
Cylinder Sizing Example
A cylinder is needed to lift and lower a dump truck bed. The design calls for two cylinders. The
maximum load that the cylinders have to lift is 58,350 Lbf. The maximum stroke is 70 inches.
The maximum time to fully extend the cylinders into the full dump position is 12 seconds. The
system relief pressure is set to 2400 psi and the max available flow rate is 25 gal/min. The
empty dump bed weight is not enough to fully retract the cylinder.
Requirements:
• Max fully extended length – 125 inches • Max fully retracted length – 42 inches • Clevis Pivot Mount
• System is filtered
• Ambient temp range 0 °F to 110 °F.
• Hydraulic fluid – Hydraulic Oil w/viscosity at
15 cST normal operation, 10 cST min
Cylinder Type
Telescoping dual acting cylinder is chosen
Reason – the fully extended length is more than ½ the
Hydraulic Cylinders
How to Choose & Size a Hydraulic Cylinder
Cylinder Sizing Example Continued
Cylinder Bore Size
Force = Required Force / # Cylinders = 58,350 lbs / 2 Cylinders = 29,175 Lbs Cylinder Bore (in) = [.7854 * Force (lbs) / Pressure (psi)] ½
Cylinder Bore = [.7854 * 29175 / 2400] ½ = 3.09 inches
Standard multistage cylinder has bores – 4.5” 1rst stage, 3.5” 2nd stage & 2.5” 3rd stage and is capable of
supporting up to 30,000 Lbs static load.
Determine if the flow rate required for max extension.
Cylinder Piston Area = π * [Cylinder Bore (in) / 2]
2Stage 1 piston Area = π * [4.5 (in) / 2] 2 = 15.9 sq in (largest section)
Fluid Velocity = [Extended Cylinder Stroke (in)] / [Max Extension Time (sec) / 60] Fluid Velocity = [ 70 in ] / [12 / 60] = 350 in / min
Flow Rate (gpm) = Fluid Velocity (ipm) * Cylinder Piston Area (sq in) * 0.00433
Flow Rate = 350 in/min * 15.9 sq in * 0.00433= 24.1 gal / min < 25 gal / min
Determine the piston rod diameter & column size
Hydraulic Cylinders
How to Choose & Size a Hydraulic Cylinder
Cylinder Sizing Example Continued
Determine the piston rod diameter & column size
From Table 1.1 the Column Strength Factor =
2.0
Corrected Length = Actual Stroke * Column Strength Factor = 70 in * 2.0 =
140 in
Cylinder Thrust (lbs) = Max System Relief Pressure (psi) * Cylinder Piston Area
Cylinder Thrust = 2400 psi * 15.9 sq in =
38,160 lbs
Use Table 1.2 to determine the minimum rod diameter
Stop Tube Length (in) = [Corrected Length – 40 in] / 10 = [140 -40] / 10 = 10 in
Cylinder ends – Clevis
Pivot Mount
Corrected Length =140 in
Thrust Load =38,160 Lbs
Hydraulic Cylinders
How to Choose & Size a Hydraulic Cylinder
Cylinder Sizing Example Continued
Determine a cylinder that meets the requirements
The supplier chosen is Prince – PMC/SAE 62 a 3-stage telescoping cylinder with 5”x4”x3” rod sizes and
5.5”x4.5”x3.5” bore sizes.
Requirement / Spec Requirement Specification
Minimum Bore Size (in) 3.09 5.5 / 4.5 / 3.5
Minimum Rod Size (in) 4.5 6 / 5 / 4
Max extended Load (lbs) 38,160 50,000 lbs
Max Closed Length (in) 42 38.58
Hydraulic Cylinders
How to Choose & Size a Hydraulic Cylinder
Cylinder Sizing Example Continued
Determine a cylinder that meets the requirements
Cylinder time to full extension
Recalculate the cylinder load capability and time for full extension with the selected cylinder’s specs to
