How to Select a Pressure Switch for Your Application
FLUID POWER - Design Data Sheet 20
Part of the information on diaphragm and bourdon tube pressure
switches was adapted from information published by the Barksdale
Division of Delaval.
Several types of pressure switches are available. The designer
should choose a type which can be expected to give the most
satisfactory results on his type of application.
At low pressures (compressed air and very low pressure
hydraulics), diaphragm and bellows movements, sometimes Bourdon
tube, are most often used. At high pressures, piston and bourdon
tube movements are most common. Snap action contacts seem to be
universally used except in switches with tilting mercury contacts
actuated with a Bourdon tube.
Expected Service Life
Expected life is usually limited by the type of pressure sensing
mechanism - Bourdon tube, diaphragm, piston, Belleville spring,
etc. The snap action contacts are presumed to have greater life
than the sensing mechanism.
If the service life (number of cycles the switch is expected to
operate) is less than one million, a Bourdon tube or diaphragm type
is indicated. If more than a million cycles, a piston type should
be used. An exception to this rule is when the pressure change in a
system is small (20% or less, of the adjustable range). Under such
conditions a Bourdon tube or diaphragm switch can be used up to 2½
million cycles before metal fatigue or contact failure.
Speed of Cycling
In addition to service life, the speed of cycling must be
considered. If a switch is expected to cycle more than once every 3
seconds, a piston switch should be specified. The metal of any
bourdon tube or diaphragm switch acts as a spring which will heat
and fatigue in extremely fast cycling operations, thus shortening
the life of the switch.
Diaphragm and Bourdon tube pressure switches generally have
greater accuracy than piston switches, and would be preferred where
accuracy is important, provided they meet requirements for service
life and speed of cycling.
Switches with Belleville spring (snap action) seem to provide
the greatest repetitive accuracy, but the manufacturer should be
contacted on life expectancy of the spring.
Figure 1. On
switches with diaphragm and Bourdon tube movements, greatest
accuracy is in the upper 65%,
best life factor in the lower 65%, and the best combination is
usually in the middle 30% of its working range (Zone A).
The term "working range" defines the pressure range a switch may
see under normal working conditions. This is normally the
For greatest accuracy the set point should fall in the upper 65%
of the adjustable range. But for the most favorable life factor the
set point should be in the lower 65% of the adjustable range.
Therefore, the most favorable combination of accuracy and life
factor lies in the middle 30% of the adjustable range, and this is
illustrated in the diagram. This general rule applies to diaphragm
and Bourdon tube pressure switches. Piston switches have a more
nearly uniform accuracy and life factor over their adjustable
Types of Switch Action
- Standard pressure switches sense a single pressure source and
open or close a single set of contacts.
- Differential pressure switches have two connections and sense
the pressure difference across a circuit.
- Dual switches sense a high and low limit on the same pressure
source, and actuate two sets of electrical contacts. Wide range
dual pressure sensing can be accomplished with two standard
pressure switches. See diagram below.
The fluid compatibility with materials of construction must be
considered. Consult switch manufacturers catalog.
Proof pressure is the highest pressure that the switch will stand
without permanent deformation, and usually defined as 1½ times
maximum working range. Although a pressure gauge in a system may
show a constant operating pressure, there may be surges which are
dampened by the orifice in the gauge which would damage diaphragm
and Bourdon tube elements in a pressure switch. For this reason,
the working range should extend well above the actual operating
Figure 2. Unlimited
differential between cut-out and cut-in pressures.
WIDE DIFFERENTIAL PRESSURE SENSING
Although some pressure switches have an adjustment for setting the
differential (pressure difference needed to open and close the
switch contacts), this differential may not be wide enough for some
Figure 2 circuit can be adjusted for a
virtually unlimited switching differential. It uses two standard
pressure switches and one holding relay.
One pressure switch is adjusted for the high pressure cut-out
point, the other for the low pressure cut-in point. The relay, CR,
has one set of N.O. holding contacts, CRl, and one set of load
switching contacts, CR2, which may be either N.O. or N.C. according
to circuit requirements. Circuit action is as follows: Starting
from zero system pressure, as the pressure, as the pressure rises,
the ''low" pressure switch will close, but this will have no effect
on the switching circuit. As pressure rises further, the "high"
pressure switch will close, energizing the coil of Relay CR. The
relay locks in electrically through Contacts CR1 and the "low"
pressure switch. Relay Contacts CR2 will break the switching
circuit. As pressure falls, the "high" switch will break but this
has no effect on the switching circuit. As pressure falls further,
the "low" switch will open, unlocking the holding relay. Contacts
CR2 close the switching circuit.
CLAMP AND WORK CIRCUITS
USING SEQUENCE VALVES
(Continued from Data Sheet #19)
Design Data Sheet 19 showed a sequence circuit
for a large double-acting work cylinder. The present circuit is a
modification using a double acting clamp to replace the
single-acting clamp of the previous circuit.
Sequencing follows the standard clamp and work pattern, with the
clamp retaining full holding force until the work cylinder has
completed its retraction.
Valve 4 is responsible for retaining full air pressure in the
clamp until the end of the cycle. It is a 4-way, closed center,
double piloted valve, 1/4" size. Only one pilot is used. The pilot
on the left end is left in a vented condition, with an air breather
to prevent ingestion of dirt into the end cap. Only the center and
one side position are used.
Circuit action is as follows: The operator shifts and holds
Valve 1 to start the cycle. Air flows to the clamp cylinder through
check Valve 5 and Valve 4 (which is being held in its side position
at this time). When the clamp bottoms out and pressure rises behind
its piston, sequence Valve 2 opens, causing pilot air to shift
Valve 3. The work cylinder makes its forward stroke. Also, the
pilot of Valve 4 vents, allowing the spool of that valve to move to
center position, locking air behind the clamp piston.
To retract the cylinders, the operator releases Valve 1. This
vents the pilot of Valve 3 and allows that valve to return to
normal position causing the work cylinder to retract. Pressure on
the rod end of the work cylinder remains low until that cylinder
has completely retracted. Pressure then builds up and shifts Valve
4 to its side position. Thls powers the double-acting clamp into
its release mode.
As stated in Design Data Sheet 19, if an air
sequence valve is not available, a low range hydraulic relief valve
can be used if it can be adjusted to about 50 PSI, and if the air
line is lubricated. Refer to that sheet for more details.
Download a PDF of
Fluid Power Design Data Sheet 20 - How to Select a Pressure Switch
for Your Application.
© 1990 by Womack Machine Supply Co. This
company assumes no liability for errors in data nor in safe and/or
satisfactory operation of equipment designed from this