If
the free air flow is known, the minimum inside diameter
to keep velocity below 6 m/s, can be found from:
d
in mm =
For
normal installations, where the pressure is about 7 bar
gauge, this can be simplified to:
d
in mm should be greater than
p
=
Pressure
drop (bar)
v
=
Flow
velocity in metres/s
Q
=
Free
air flow (m3/s)=L/s x 10-3
p
=
Initial
pressure (bar)
1
=
Pipe
length (metres)
d
=
Inside
pipe diameter (mm)
Output
force and maximum rod lengths Example: Knowing the output force required (200kN) and
the pressure of the system (160 bar), connect Output force through
pressure to cut cylinder diameter. Answer: 125 millimetres
To find the maximum length of a piston rod. Connect output force
required (200kN) through rod diameter (70mm) to cut the maximum
rod length scale; this gives you the (Lm) dimension. Answer:
2800mm.
To find the actual length stroke (LA) for a specific mounting
use formulae below.
Maximum
stroke lengths for specific mounting cases
Foot mounted, eye rod end
LA = Lm x 0.8
Foot mounted, rigidly supported rod
LA = Lm
Front flange, eye rod end
LA = Lm x 0.8
Front flange, rigidly supported rod
LA = Lm
Rear flange, eye rod end
LA = Lm x 0.4
Rear flange, rigidly supported rod
LA
= Lm x 0.8
Rear eye, eye rod end
LA
= Lm x 0.3
Trunnion head end, eye rod end
LA
= Lm x 0.3
Trunnion gland end, eye rod end
LA
= Lm x 0.6
Trunnion gland end, rigidly supported end
LA
= Lm x 0.8
For intermediate
trunnion positions scaled multiplier factors must be taken.
Clevis and spherical eye mountings have the same factor as eye
mountings. Example: Having found Lm (2800mm) for rear flange mount
with eye rod end
LA = Lm x 0.4 = 2800 x 0.4 = 1120mm.
Nomogram
for determining pipe sizes in relation to flow rates and recommended
velocity ranges.
Based on the formula:
Velocity
of fluid in pipe (m/s)
=
Flowrate(L/min)
x 21.22
d2
where d =
Bore of pipe (mm)
Recommended velocity ranges based on oils having a maximum viscosity
grade of 70cSt at 40°C and operating between 18°C and
70°C.
Storage
Applications Formula to estimate accumulator volume for
storage applications.
Slow
charge
Slow discharge
Fast
charge
Fast discharge
Slow
charge
Fast discharge
The
precharge pressure is chosen to 90% of the min. working
pressure. n varies between 1 and 1.4 depending on whether
the course is slow (isothermal) or fast (adiabatic).
Pump
Pulsation Formula to size accumulator to reduce pump pulsations.
a)
Minimum
effective volume (litres) V1 =
k.
Q
n
Note:
It is good engineering practice to select an accumulator with
port connection equal to the pump port connection.
b)
To
check the level of pulsation obtained.
Volume
of fluid entering accumulator = D.C
For
pulsation damping precharge pressure P1 = 0.7
. P2
and
assuming change from P1 to P2 is
isothermal, then V2 = 0.7 . V1
Hence:
Percentage pulsation above and below mean is
V1
=
effective
gas volume
V2
=
min.
gas volume
V3
=
max.
gas volume
P1
=
precharge
pressure
P2
=
min.
working pressure
P3
=
max.
working pressure
Va
=
V3-V2
= working volume (fluid)
k
=
a
constant *
Q
=
Pump
flow (L/min)
n
=
Pump
speed (rpm) if n>100 use 100
D
=
Pump
displacement (L/rev)
C
=
a
constant *
*
Dependent upon no. of piston. For multi piston pumps >3
pistons. k=0.45 and C=0.013.
COOLING
The
tank cools the oil through radiation and convection.
P=
T
. A . k
so
T
=
P
. 1000
1000
A.k
k
= 12 W/m2,°C
at
normally ventilated space
24
at
forced ventilation
6
at
poor air circulation
Required
volume of water flow through the cooler:
Q
= 860 x Power loss
T
water
HEAT
EQUIVALENT OF HYDRAULIC POWER
in
kj/sec
=
Flow
(L/min) x Pressure (bar)
600
HEATING
Heating
is most necessary if the environmental temperature
is essentially below 0°C.
Requisite heating effect:
P=
V
. T
kW
35
. t
ENERGY
(J)
=
M.C.T
M
=
Mass
(kg)
C
=
SHC
J/kg°C
T
=
temp
change (°C)
t
=
time
(mins)
Note
1MJ
=
0.2777
kW/Hr
CHANGE
OF VOLUME AT VARIATION OF TEMPERATURE
Change
of volume V
= 6.3 x 10-4.V. T
CHANGE
OF PRESSURE AT VARIATION OF TEMPERATURE
Note:
With an infinite stiff cylinder.
Change
of pressure p
= 11.8 . T
(in general - affected by many variables)
Example:
The temperature variation of the cylinder oil from night time
(10°C) to day time/solar radiation (50°C) gives:
P
= 11.8 x 40 = 472 bar
KEY
T
=
Temp
change (°C)
P
=
Power
(Kw)
k
=
heating
coefficient (W/m2°C)
A
=
Area
of tank excluding base (m2)
t
=
time
change (mins)
p
=
change
in pressure
C
=
Specific
heat capacity (J/Kg°C)
V
=
Volume
(L)
TERMINOLOGY
The main source of fluid power terms and definitions is the
International Standard - ISO 5598 - Fluid Power Systems and
Components - Vocabulary, 1985, however, new definitions are
arising from recent work on E.E.C. - CEN standards.
The following are just a few of the fluid power terms in every
day use for hydraulic and pneumatic applications:-
Fluid
Power - The means whereby signals and energy can be transmitted,
controlled and distributed, using a fluid as the medium.
Hydraulics
- Science and technology which deals with the laws governing
liquid flow and pressure.
Pneumatics
- Science and technology which deals with the use of air or
neutral or gases as the fluid power medium.
System
- Arrangement of interconnected components which transmits and
controls fluid power energy.
Machinery
- An assembly of linked parts or components, at least one of
which moves, with the appropriate actuators, control and power
circuits etc., joined together for a specific application.
Component
- An individual unit (e.g. actuator, valve, filter) comprising
one or more parts, designed to be a functional part of a fluid
power component or system.
Actuator
- A device which converts energy into force and movement. The
movement may be linear (e.g. cylinder), semi-rotary (e.g. torque
unit), or rotary (e.g. motor).
Operating
conditions - Operating conditions are indicated by the numerical
values of the various factors relating to any given, specific
application of a unit. These factors may vary during the course
of operations.
Working
Pressure - Pressure at which the apparatus is being operated
in a given application.
System
pressure - Nominal pressure usually measured at the inlet
to the first valve or at pump outlet (normally the relief valve
setting).
Pilot
pressure - Pressure in a pilot line or circuit.
Hydraulic
pumps - Units which transform mechanical energy into hydraulic
energy.
Compressors
- Devices which cause a gas to flow, against a pressure: they
convert mechanical energy into pneumatic fluid power.
Directional
control valve - Device connecting or isolating one or more
flow paths.
Control
mechanisms - The means whereby components change their state.
Control mechanisms may be manual, mechanical, pressure or electrical
in operation.
Pressure
relief valve - Valve which limits maximum pressure by exhausting
fluid when the required pressure is reached.
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p
= Pressure change (bar)
(m)
T