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Chapter 3 Hydraulic Cylinders
Objectives:
The purpose of this chapter is to describe:
1. the types of cylinders, the different situations of application,
the configurations of different cylinder components.
2. the calculation of cylinders.
3. the construction and seal of cylinders
4. the basic design steps of cylinders.
Upon completing this chapter, you should be able to:
Distinguish the different types of cylinders, the different
working conditions, the configurations of different cylinder
components.
Master the basic calculation of cylinders, especially the
velocities and the output forces of cylinders.
Understand the characteristics of two kinds of seal.
Be familiar with the basic design steps of cylinders.
Chapter 3 Hydraulic Cylinders
Classification:

 piston cylinders

rams


reciprocating cylinders booster cylinders

Telescopic cylinders



rack and pinion cylinder

oscilating cylinders / motors
3.1 Types and Calculation of cylinders
3.1.1 Piston type cylinders
3.1.1.1 Double-end rod cylinders
A double-end rod cylinder has two piston rods in both ends.
(a)
(b)
Fig. 3.1 Double-end rod cylinders
a. The cylinder body is stationary. b. The piston rod is stationary.
4qv
q
 2
2
F  ( D  d )( p1  p 2 ) m v  
A  (D 2  d 2 )
4
A double-end rod cylinder has identical drive force and speed
in both directions.
3.1.1.2 Single-end rod cylinders
A single-end rod cylinder has only one rod.
F1
A1
d
d
D
A1
A2
D
A2
1
p1
p2
q
(a)
p1
p2
q
(b)
q 4qv

F1  [ D p1  ( D  d ) p2 ]m v1 
2
A

D
1
4
4
4qv
q
 2
 2
2
F2  [ ( D  d ) p1  D p2 ]m v2  
2
2
A

(
D

d
)
4
4
Compare the equations above, we can draw a conclusion that
v2>v1, F1>F2. The velocity ratio is   v 2 / v1  D 2 /( D 2  d 2 )

2

2
2
3.1 Types and Calculation of cylinders
F3
A1
d
d
D
A1
A2
D
A2
u3
q+q '
q'
p1
q
(c) Differential connection
F3 

d p1 m
2
4
F4  0
(d) floating connection
4q v
v3 
d 2
v4  uncertain
The differential connection are usually used to increase the
velocity of an end use device.
The floating connection can make an end use device move
freely.
3.1 Types and Calculation of cylinders
3.1.2 Rams
The bore of a ram need not accurately machining , so the
initial cost is substantially low. However, a ram is controlled in
single direction.
A
柱塞
缸筒
p
（a）
v
v
F
F
p
q
A
q
( b)
If double-acting is needed, a couple of rams should be used.
3.1 Types and Calculation of cylinders
3.1.3 Oscillating motors
1
2
2
3
3
4
1
D
d
1
4
4
Vane motors can be used to provide oscillatory motion of less
than one complete rotation.
Single vane motors can rotate less than 280 degrees and
double vane motors can be limited to about 150 degrees.
3.1 Types and Calculation of cylinders
3.1.4 combination cylinders
3.1.4.1 booster cylinders
Booster cylinders can convert the low input pressure into the
high output pressure.
A1
A2
p1
p2
Fx  p1 A1  p2 A2  0
A1
p2 
p1
A2
3.1.4.2 Telescopic cylinders
A telescope cylinder consists of multilevel piston rods. The
piston of previous level is the sleeve bore of rear level.
The sequence of extension motion is:
the first extension is stage 1, then stage 2, etc.
The output force is decreassive and the velocity is increasive.
on the contrary, the retraction sequence is reverse.
The telescopic cylinder is used when a long stroke length and a
short retracted length are required.
3.1 Types and Calculation of cylinders
3.1.5 Rack and pinion cylinder
The rack and pinion cylinder incorporate a piston rod with a
rack and a pinion. The rack and pinion drive is adaptable for
rotational motions. It is usually used for angle displacement and
graduators in automatic product line or combination machine,
etc.
3.1 Types and Calculation of cylinders
3.2 Cylinder Construction
Fig 3.9 Construction of double acting single-end rod cylinder
l-cap 2-stirrup 3、5、9、11-seals 4-piston 6-barrel
7-rod end 8-bush 10-cover
3.2.1 cylinder body component
Chambers sealed by cylinder body component and piston
component withstand oil pressure. Therefore, cylinder body
component should be enough strength, high surface accuracy
and reliable tightness.
3.2.1.1 connection of cover and barrel
( a)
( d)
( b)
( e)
( c)
( f)
(1) The flange connection is simple, reliable and convenient to
machine. However, the thickness of barrel end is required more
to mount screw.
(2) The whitney key type connection is convenient, reliable
and compacted.
( a)
( d)
( b)
( e)
( c)
( f)
(3) The advantage of thread connection is little space, little
weight and reliable. But the construction of barrel end is
complicated.
(4) The draw-bar type connection is simple, convenient. But
volume and weight of cover is great.
(5) The welding type connection is high strength and convenient
to machine. But it is easy for the barrel to deform.
3.1 Types and Calculation of cylinders
.2.1.2 basic requirement of barrel ,cover and guide sleeve
The surface roughness of the cylinder bore is from 0.1 to 0.4m
The barrel is endured large force of oil pressure, therefore it
should be enough strength and rigidity.
Covers are set up at both ends of barrel. It is also endured large
force of oil pressure, therefore covers and its connection should
be enough strength.
The guide sleeve guides and sustains the piston rod or ram.
Therefore, the cover should be replacement after it is worn out.
The material selection of barrel, cover and oriented sleeve can
refer hydraulic design handbook and specifications .
3.1 Types and Calculation of cylinders
3.2.2 Piston component
3.2.2.1 Connection forms of piston and rod
The threaded connection is simple and convenient to
disassemble and assemble,
The semi-cyclic linkage has a advantage of high strength, it’s
disadvantage is complicated and hard to disassemble and
assemble.
3.1 Types and Calculation of cylinders
3.2.2.2 Piston component seal
1 Gap seal
The gap seal depends on the slight clearance between two
surfaces with relative motion to prevent leakage.
It is simple in structure and has long life, but it does not ensure
not to leak at all. In addition, it needs high machining accuracy.
It is rational for the gap seal to be used in the case of rapid
movement.
3 Seal by using seal packing ring
The seal by using seal packing ring has good tightness , does
not need very high machining accuracy, but has not very long
expected life,
It is rational for the seal by using seal packing ring to be used
in the case of slow motion.
（1）O-ring
(2) V-ring
（a )
（b )
a) pressure ring
(3) Y-ring
（c）
b)V-ring c) supporting ring
( a)
( b)
d
3.2.3 Cushioning devices
Mounting cushion devices is usually needed when cylinder
demand rapid movement and heavy load.
The principle of cushion is increasing the back pressure of
outlet port to reduce the cylinder velocity and protect the
compact of piston and cover when piston or barrel is close to
end of stroke.
u
u
( a)
( b)
u
（c）
u
( d)