Hydraulic Cylinders and
Cushioning Devices
Introduction
 Hydraulic cylinders and hydraulic motors perform a function opposite to
that performed by a pump. They extract energy from a fluid, and convert
it to mechanical energy to perform useful work.
Hydraulic System
Fxv
Hydraulic
Cylinder
VxI
Electric
Motor
Txω
Hydraulic
Pump
PxQ
Hydraulic
Motor
Txω
Introduction
 Hydraulic cylinders, also called linear actuators provide a force that drives
an external load along a straight line.
 Hydraulic motors, also called rotary actuators, provide a torque that drives
an external load along a circular path.
Hydraulic System
Fxv
Hydraulic
Cylinder
VxI
Electric
Motor
Txω
Hydraulic
Pump
PxQ
Hydraulic
Motor
Txω
Hand Operated
Hydraulic Jack
Telescopic
Boom Forklift
Airplane Wing Flaps
and Landing Gear
Telescopic
Boom Forklift
Airplane Wing Flaps
and Landing Gear
Single Acting Hydraulic Cylinders
Piston
Piston Seal
Rod
Extension
Retraction
Barrel
Port
Push Action
Oil to extend Return by External Force
(e.g. Gravity)
Graphic Symbol
(P&ID Symbol)
Single Acting Hydraulic Cylinders
Push Action
Oil to extend, Spring for return
Pull Action
Oil to retract, Spring to extend
Double Acting Hydraulic
Cylinders
Piston
Piston Seal
Rod
Extension
Retraction
Rod
Seal
Barrel
Port B
Port A
Oil to extend.
Oil for Return
Graphic Symbol
(P&ID Symbol)
Double Ended Piston Rod
Double Acting Cylinder
Piston
Piston Seal
Rod Seal
Rod
Rod Seal
Barrel
Port A
Port B
Oil to extend.
Oil for Return
Graphic Symbol
(P&ID Symbol)
Cylinder Construction
End Cap
Tie Rod
Threaded Rod
Barrel
Front Cap
Double Acting Hydraulic Cylinders
Cylinder Mounting Methods
Front Flange
Foot Bracket
Rear Flange
Clevis (Rear Pivot)
Side Lug
Intermediate Trunnion
Cylinder Mounting Methods
Front Flange
Foot Bracket
Direct Rear
Screwed Front
Clevis (Rear Pivot)
Intermediate Trunnion
Combining Cylinders with Mechanical Linkages:
Oscillatory motion with thrust amplification or reduction
First Class Lever
Second Class Lever
Third Class Lever
The three combinations are inverted slider crank mechanisms
Combining Cylinders with Mechanical Linkages:
Straight line motion with thrust amplification or reduction
Thrust Reducer
(Six Bar Mechanism)
2:1 Motion Multiplier
(Rack and Pinion)
Two direction
straight line
Combining Cylinders with Mechanical Linkages:
Continuous Rotary Motion
Continuous Rotation
(Double Ratchet)
Fast Rotary Motion
(Screw and Nut)
Combining Cylinders with Mechanical Linkages:
Motion Transfer
Transfer to Distant Point
(Pantograph)
Cylinder Alignment:
Spherical Bushings and Spherical Bearings
 Much effort has been made by manufacturers of hydraulic cylinders to relieve or
eliminate the side loading of cylinders created as a result of misalignment. It is
almost impossible to get perfect alignment and since the alignment of the cylinder
has a direct bearing on its life, the efforts have been well worth while.
 A spherical bushing or a spherical bearing is commonly used to deal with
misalignment. This approach may not be able to take the loads that the cylinder is
capable of producing. It can act as a complete hinge in one direction only, while
being limited to a maximum misalignment of five degrees in the other directions.
Spherical Bushing
Spherical Roller Bearing
Cylinder Alignment: Universal Joints
 A universal joint alignment accessory may be used. It allows fifteen degrees of
angular misalignment on each side of center. It also provides more load
carrying capabilities.
 It is recommended that not more than a thirty degree maximum misalignment
angle be used on the pins
Cylinder Force, Velocity and Power
Piston
Rod
 Extension Stroke
F ext  p  Ab
v ext  Q in Ab
P  F ext  v ext  p  Q in
 Retraction Stroke
Port
Port
F ret  p   Ab  Ar 
v ret  Q in
 Ab
 Ar 
P  v ret  F ret  p  Q in
Ab
Ar
Cylinder Loading Through 1st class lever
 As the lever rotates an angle ϴ from its initial orientation, the cylinder
rotates an angle фcyl and the load rotates with an angle фload
Neglecting friction and dynamic loading (small values compared to
forces from the cylinder thrust and load), then taking the moments
around the pivot, O, we have
M
O
 0
F cyl L1 cos    cyl   Fload L 2 cos    load
F cyl L1 cos  cos  cyl  sin  sin  cyl

0

 Fload L 2 cos  cos  load  sin  sin  load
L1

ϴ
 For small values of ϴ and фcyl , and фload sin ϴ sin фcyl ≈ 0,
and sin ϴ sin фcyl ≈ 0
O
Fcyl
L 2 cos  load
L1 cos  cyl
Fload
ϕcyl
 Assuming no change on the load orientation, фload =0
F cyl 
L2
L1 cos  cyl
Fload
Fload
ϕload
F cyl L1 cos  cos  cyl  Fload L 2 cos  cos  load
F cyl 
L2
Load Displacement Through 1st class lever
 Assume no change on the orientation of
the load, and using the conservation of
energy (FcylΔcyl = Fload Δload), we have
from the previous equation for small
values of ϴ and ф
L1
 load
 cyl

Fcyl
Fload

L2
L1 cos 
L2
ϴ
O
Fcyl
ϕ
Fload
Cylinder Loading Through 2nd class lever
L2
L1
 Using the previous assumptions, with no change
ϴ
O
on the load orientation, we have we have
Fcyl
M
O
Fload
 0
F cyl  L1  L 2  cos      Fload L 2 cos   0
F cyl  L1  L 2  cos  cos   sin  sin    Fload L 2 cos 
 For small values of ϴ and ф, sin ϴ sin ф ≈ 0, and
Fcyl  L1  L 2  cos  cos   Fload L 2 cos 
Fcyl
Fload

 lo a d
 cyl

 L1 
L2
L 2  cos 
ϕ
Cylinder Loading Through 3rd class lever
L1
 In this case, we have
M
O
ϴ
Fcyl
F cyl L 2  cos  cos   sin  sin    Fload  L1  L 2  cos 
 For small values of ϴ and ф, sin
ϴ sin ф ≈ 0, and
Fcyl L 2 cos  cos   Fload  L1  L 2  cos 
Fload

O
 0
F cyl L 2 cos      Fload  L1  L 2  cos   0
Fcyl
L2
 lo a d
 cyl

 L1 
L2 
L 2 cos 
Fload
ϕ
Buckling and Telescopic Cylinders
 Buckling occurs when the rod of the
cylinder bend or bows sideways under
the action of compressive load. The
longer and lighter the cylinder rod, the
more likely it is for it to buckle. When
selecting a cylinder from catalog, it is
important to calculate the buckling
loads.
 Telescopic cylinders allow a longer
cylinder stroke without buckling. These
cylinders have from 2 to five telescopic
sections with each section sliding inside
a larger section. They are used for lifting
platforms, tipping platforms and other
commercial vehicle applications.
Hydraulic Cylinders Cushions
 Double acting cylinders sometimes
contain cylinder cushions at the
end of the cylinder to slow down
the piston near the ends of the
stroke. This prevents excessive
impact when the piston is sopped
by the end caps.
 Deceleration starts when the
tapered plunger enters the
opening in the cap. This restricts
the exhaust flow from the barrel
to the ports. During the last
portion of the stroke, the oil must
exhaust through an adjustable
opening
Hydraulic Cylinders Cushions
 The cushion also incorporates a
check valve to allow free flow to
the barrel during the piston’s
reversed stroke.
 The maximum pressure developed
by cushions at the end of the
cylinder must be considered, since
excessive pressure buildup would
rupture the cylinder.
 Refer to example 6-6 in the book,
which illustrates how to calculate
this pressure.
Hydraulic Shock Absorbers
 A shock absorber is a
multiple orifice hydraulic
device. When a moving load
strikes the bumper of the
shock absorber, it sets the
rod piston in motion, which
pushes the oil through the a
series of holes from the
inner, high pressure
chamber, to the outer, low
pressure chamber.
 The resistance of the oil flow
caused by the holes creates a
pressure that acts against
the piston to oppose the
moving load.