Cylinder Selection

Cylinder Selection

• Determined by the force required to move the load and the

speed required.

• Hydraulic cylinders are capable of greater forces; pneumatic

cylinders are capable of greater speeds.

Piston Force

• A small percentage of the applied force is used to overcome

friction, with the remainder applied to the load.

Piston Speed

• The speed v of a hydraulic cylinder is equal to the flow rate

of liquid Q through the cylinder divided by the cross-

sectional area A of the cylinder.  

F = PA-R

 

v =Q

A

Hydraulic Cylinder Example

Consider a hydraulic cylinder used to move a work piece in a

manufacturing operation through a distance of 250 mm in 15

seconds. If a force of 5 kN is required to move the work piece,

what is the required working pressure and hydraulic liquid flow

rate if a cylinder with a piston diameter of 125 mm is available?

𝑄 = 𝑣𝐴 =0.250𝑚

15𝑠× 𝜋

0.125𝑚

2

2

= 0.0002𝑚3 𝑠

Pressure vs. Force for Pneumatic Cylinders

Pressure vs. Force for Pneumatic Cylinders

Cylinder Buckling Strength

• When an excess thrust is applied to

a cylinder, the buckling strength

must be taken into consideration.

• Conditions leading to excess thrust

include:

1. Compression stress.

2. The stressed part (i.e. piston

rod) is long and slender.

• The buckling strength depends

greatly on the mounting method.

• Euler’s column formula (for round

sections):

Pivoting on

both ends. (n = 1)

Rigidly fixed on one

side and loose at the

opposite end. (n = ¼)

Rigidly fixed on one

side and pivoting at

the other. (n = 2)

Rigidly fixed at both

ends. (n = 4)

22 )( gcr RlEAnP

Pcr = critical (maximum) load

n = end condition coefficient

E = modulus of elastisity

A = cross-sectional area

l = length of column

Rg = least radius of gyration

Radius of Gyration

where I is second moment of area of piston rod

where l is the effective (revealed or unsupported)

piston rod length

ratiosslendernesRl g

A

IRg

Cylinder Buckling Strength

• For most cylinders, maximum allowable loads (Pcr) are

determined by the manufacturer:

Pressure and Flow Control

Pressure Control Valves

There are three main types of pressure control valves:

1. Pressure regulating valves

• Used to control the operating pressure in a circuit and maintain it at

a constant value.

2. Pressure limiting valves

• Used as safety devices to maintain the pressure in a circuit below

some safe value.

• If pressure rises above the set safe value, the valve opens and vents

to the atmosphere or returns to the sump.

3. Pressure sequence valves

• Used to sense the pressure of an external line and give a signal when

it reaches a preset value.

Pressure Limiting Valve

• Example to right has one orifice

that is normally closed.

• When the inlet pressure (force

on ball) exceeds the force

applied by the spring, the valve

opens and vents to the

atmosphere, or back to the

sump.

• Spring force (set limit value) is

adjustable.

Pressure Sequence Valve

• Adaptation of pressure limiting

valve.

• Allows flow to occur when the

inlet pressure exceeds a set

value.

• Applications include starting an

automatic machine operation

once the clamping pressure

applied to the workpiece has

reached a certain level.

Flow Rating

• Flow rating is a measure of a component’s ability to pass air at

an acceptable pressure drop.

• Usually pressure drop should not exceed 10% of supply pressure.

• One method of expressing the flow rating is by assigning a

coefficient called the CV factor.

• The higher the CV factor, the greater the flow rating of the

component.

• Components can also be rated in standard cubic feet per minute

(scfm) where flow is measured at 60°F (16°C) and 15 psi (1

bar).

• When defining the flow rating of a component, the temperature

and pressure must be specified.

 

CV » flow in cfm´ 35 ´10-6

»flow in litres/min

1000

 

CV »cylinder bore area ´ stroke ´ compression

pressure drop ´ single stroke time ´ 29

For a cylinder:

Pneumatic Economics—Work from Air

How much work can be performed by 1 m3

of air?

Consider a cylinder 35 mm in diameter

raising packages weighing 20 kg. A second

cylinder of the same diameter pushes the

packages onto a conveyor belt. The stroke

of the first cylinder is 400 mm; the stroke of

the second is 200 mm.

• The compressive force at 6 bar (600 kPa or 90 psi)

is 520 N.

• For both cylinders, 0.001 m3 of air is used per

double stroke (in/out).

Thus, with 1 m3 of air, 1000 packages can be

lifted and pushed onto the conveyor belt.

Effects of Leakage

• The cost of compressed

air can rise

considerably unless

careful watch is kept

for leaks in the piping.

• Even small leaks can

lead to increased costs.

• Graph to right

demonstrates the

relationship between

escape rate and area of

aperture at various

pressures.

Additional Reference Material

• The following slides are were not visited in class

lectures.

• They are provided here for your future reference.

Process Control Valves

• Used to control the rate of fluid

flow.

• May be used to control the rate of

flow of a liquid into a tank.

• The basic principle involves an

actuator that is used to move a

plug into the flow pipe, thereby

altering the pipe cross-section.

• Process control valves consist of

three main components:

1. Actuator.

2. Valve body.

3. Plug.

Diaphragm Actuator

• Common form of pneumatic actuator used with process control

valves.

• Consists of a diaphragm with an input pressure signal from the

controller on one side and atmospheric pressure on the other.

• Pressure difference is gauge pressure.

• Diaphragm is made of a rubber centre sandwiched between

two steel discs.

Diaphragm Actuator

• Movement of the diaphragm is communicated to the final

control element by an attached shaft.

• For a linear-response restoring spring (i.e., F = kx, where k is

constant), the displacement of the shaft is proportional to the

gauge pressure (i.e., kx = PA).

Valve Bodies

• There are many forms of valve bodies.

• Primary forms are single seated and double seated.

• Single seated refers to a valve having a single path for the

fluid through the valve.

• Only one plug is required to control the flow.

• Can be closed more tightly than a double-seated valve.

• Force on plug due to flow is much higher, requiring the

diaphragm to exert considerably higher forces on the

stem.

Valve Bodies

• Double seated refers to a valve where the fluid entering

the valve body splits into two streams.

• Each stream passes an orifice controlled by a plug.

• Requires two plugs.

• Lower stem forces are required to close the valve than a

single-seated valve.

Plugs

• The shape of the plug determines the relationship between the

stem movement and the effect on the flow rate.

Current to Pressure Converter

• Conversion of electrical current into a gauge pressure for

control purposes (specifying diaphragm displacement of a

process control valve) may be accomplished using an apparatus

such as illustrated below:

• Current through coils causes

lever to be attracted to magnet.

• Corresponding movement of

flapper over nozzle is produced.

• Flapper position controls rate of

air escape and hence air pressure

in the system.

• Springs on the flapper are used

to adjust the sensitivity of the

converter.

• Currents of 4 to 20 mA produce

gauge pressures of 20 to 100 kPa.

Hydraulics

Actuator Power to Weight Ratios

Weight [kg]

Hydraulic

actuators

Pneumatic

actuators

DC motors

Shape

memory

metals

104

103

102

10

10-2 10-1

1

102 10 1

Pow

er/

Weig

ht

rati

o [

W/kg]

Hydraulic Systems

Advantages

• Very powerful, suitable for

tasks requiring large

forces.

• Capable of higher

maximum acceleration

than DC motors.

• Strokes from a few

millimetres to metres in

length.

• Speeds and forces are

infinitely adjustable.

• Small time constants result

in smooth motion.

Disadvantages

• Use of fluids can be messy

and noisy when improperly

applied.

• Generally not suitable for

processes requiring a clean

operating environment.

• Hydraulic oil can be

volatile.

• Installation of hydraulic

components is relatively

expensive.

• Components (and fluid) are

heavy.

Time constant

• The time it takes to observe a significant change in a

given process.

Hydrostatic Force Multiplier

 

F = P1A1 = P2A2

Hydrostatic Force Multiplier

1

2

1

2

2

1

A

A

F

F

d

doutWorkinWork

dFdF

2211

2

1

22

1

21 d

A

Ad

F

Fd

2

22

A

FP

1

11

A

FP

1

1

22 F

A

AF

Hydraulic Power Supply

• Pressurized oil is provided by a pump driven by an electric

motor.

• Accumulator is just a container in which the oil is held under

pressure against an external force.

Accumulator