Overview of Limit Switches
A Limit Switch is enclosed in a case to
protect a built-in basic switch from external force, water, oil, gas, and
dust. Limit Switches are made to be particularly suited for applications
that require mechanical strength or environmental resistance.
The shapes of Limit Switches are broadly classified into Horizontal,
Vertical, and Multiple Limit Switches. The structure of a typical vertical
Limit Switch is shown in the following figure as an example. Limit
Switches are generally composed of five components.
Drive Mechanism of Limit Switch
The drive mechanism of the Limit switch is
an important part of the Limit Switch and is directly linked to seal
performance and operating characteristics. Drive mechanisms are classified
into three types, as shown in the following figure.
(1) Plunger
There are two types of plunger (types A and B in the figure) depending on
the sealing method. With type A, an O-ring or a rubber diaphragm is used
for sealing. The rubber seal is not externally exposed, and so resistance
is provided against cutting debris from machine tools, but sand and fine
shavings may become stuck on the sliding surface of the plunger. With type
B, sand and fine shavings will not become stuck, and the sealing
performance is superior to type A, but hot cutting debris striking the
Switch may damage the rubber cap.
Whether type A or type B is required depends on the location in which the
Switch is to be used. With the plunger drive, the movement of the plunger
piston enables
air to be compressed and taken in.
Therefore, if the plunger is left pushed in
for a long time, the air in the Limit Switch will escape and the internal
pressure will become equivalent to atmospheric pressure. This will cause
the plunger to tend to reset slowly even if an attempt is made to quickly
reset it. To prevent this problem from occurring, design the system to
limit the
amount of air compressed by pushing in the plunger to 20% or less of the
total air pressure in the Limit Switch. To extend the service life of the
Limit switch, the plunger drive includes an OT absorption mechanism that
absorbs the remaining plunger movement using an OT absorption spring and
stops the movement of an auxiliary plunger that pushes the Built-in switch
according to the movement of the plunger.
(2) Hinge Lever
The amount of plunger movement is increased at the end of the lever (i.e.,
roller) by the lever ratio, and so an absorption mechanism is generally
not used.
(3) Roller Lever
The structure of the WL is shown as a typical example. Other drives
include those in which the plunger performs the function of the reset
plunger and those in which a coil spring is used for the reset force and a
cam is used to move the auxiliary plunger.
|
|
(1) Plunger |
(2)
Hinge Lever |
(3) Roller
Lever |
|
Drive
mechanism of limit switch |
 |
 |
 |
|
Movement of auxiliary plunger |
 |
 |
 |
|
Force vs. stroke characteristic |
 |
 |
 |
|
Precision |
High |
Normal |
Low to normal |
Switch Component Materials
The main parts of the Switch are
formed from the following materials.
|
Part |
Material |
Symbol |
Characteristics |
|
Contacts |
Gold |
Au |
Gold is
extremely resistant to corrosion and is used for microloads. It is
soft (Vickers strength: HV25 to HV65), which easily results in
adhesion (e.g., contacts sticking together) and the contacts are
easily dented if the contact force is large. |
|
Gold-silver alloy |
AuAg |
This
alloy of 90% Au and 10% Ag is extremely resistant to corrosion, and
its hardness (HV30 to HV90) is higher than that of gold, and so it is
often used in switches for microloads. |
|
Platinum-gold-silver alloy |
PGS |
This
alloy of 69% Au, 25% Ag, and 6% Pt is extremely resistant to
corrosion, its hardness (HV60) is similar to AuAg, and it is often
used in switches for microloads. |
|
Silver-palladium alloy |
AgPd |
This
alloy has good resistance against corrosion but it easily generates
polymers if it adsorbs organic gases.
With 50% Ag and 50% Pd, it has a hardness of HV100 to HV200. |
|
Silver |
Ag |
Silver
has the highest rate of electrical conductivity and heat transfer
among metals. It exhibits low contact resistance, but has the
disadvantage of easily generating a sulfide film in environments with
sulfide gas, and so contact faults easily occur in microload ranges.
The hardness is HV25 to HV45. Silver is used in almost all switches
for standard loads. |
|
Silver-nickel alloy |
AgNi |
With
90% Ag and 10% Ni, this alloy has electrical conductivity about equal
to Ag, and it has excellent resistance to arcing and welding. The
hardness is HV65 to HV115. |
|
Silver-indium-tin alloy |
AgInSn |
This alloy is
very hard, has a high melting point, and exhibits excellent resistance
to arcing, welding, and contact transfer. |
|
Movable
springs
and armatures |
Phosphor bronze for springs |
C5210 |
Phosphor bronze is very ductile and has resistance against fatigue and
corrosion. It is annealed at low temperatures. The spring limit
(Kb0.075) is somewhat low at 390 N/mm² minimum for C5210-H and 460
N/mm² minimum for C5210-EH, but it is often used for armatures of
miniature basic switches. |
|
Age-hardened copper beryllium for
springs |
C1700
C1720 |
Copper
beryllium is pressed and then age-hardened. It has a high rate of
electrical conductivity, and the spring limit (Kb0.075) after age
hardening is extremely high at 885 N/
mm² minimum. for C1700-H and 930 N/mm² minimum for C1720-H. It is used
for basic switches that require a high spring limit. |
|
Mill-hardened copper beryllium for
springs |
C1700-[]M
C1720-[]M |
This
copper beryllium is age hardened by the materials manufacturer before
shipment (i.e., mill hardened). Pressing after age hardening is not
required. At 635 N/mm² minimum (reference value) for C1700-HM and 635
N/mm² for C1720-HM, the spring limit (Kb0.075) is higher than with
bronze phosphor for springs. Mill-hardened copper beryllium is often
used for the movable springs in basic switches. |
|
Stainless steel for springs
(austenite) |
SUS301-CSP
SUS304-CSP |
Austenite
stainless steel has excellent resistance against corrosion.
The spring limit (Kb0.075) is 490 N/mm² minimum for SUS301-CSP-H and
390 N/mm² for SUS304-CSP-H. |
|
Cases
and covers |
Phenol
resin |
PF |
Phenol resin is
heat hardened. It is often used as the material for the casings of
basic switches.
Phenol resin has a UL heat index of 150°C, a UL fire-retardant grade
of at least 94V-1, and a water absorption coefficient of 0.1% to 0.3%.
Material without ammonia is used for basic switches. |
|
Polybutylene terephthalate resin |
PBTP |
This
resin is thermoplastic. A glass-reinforced epoxy type of this resin is
often used as the material for the casings of basic switches.
The resin has a UL heat index of 130°C, a UL fire-retardant grade of
at least 94V-1, and a water absorption coefficient of 0.07 to 0.1. |
|
Polyethylene terephthalate resin |
PETP |
This
resin is thermoplastic. A glass-reinforced epoxy type of this resin is
used as the material for the casings of basic switches.
The resin has a UL heat index of 130°C, a UL fire-retardant grade of
at least 94V-1, and a water absorption coefficient of 0.07 to 0.1. |
|
Polyamide (nylon) resin |
PA |
This
resin is thermoplastic. A glass-reinforced epoxy type of this resin
has heat resistance that is superior to PBT and PET. The absorption
coefficient is large. Select a grade for use with a the lowest
possible absorption rate.
The resin has a UL heat index of 180°C, a UL fire-retardant grade of
at least 94V-1, and a water absorption coefficient of 0.2 to 1.2. |
|
Polyphenylene sulfide |
PPS |
This resin is
thermoplastic. It has heat resistance that is superior even to PA.
The resin has a UL heat index of 200°C, a UL fire-retardant grade of
at least 94V-1, and a water absorption coefficient of 0.1. |
|
Switch
boxes |
Aluminum (die-cast) |
ADC |
Aluminum is often used as the material for the switch box (case) of
Limit Switches. Standards are specified in JIS H5302. |
|
Zinc
(die-cast) |
ZDC |
Die-cast zinc is
more suitable than ADC for thin-walled objects, and its resistance to
corrosion is also superior to ADC. Standards are specified in JIS
H5301. |
|
Rubber
seals |
Nitrile-butadiene
rubber |
NBR |
This
rubber has excellent resistance to oil, and it is often used for Limit
Switches. It is classified into five nitrile levels according to the
amount of combined nitrile: Very high (43% or higher), high (36% to
42%), mid-high (31% to 35%), medium (25% to 30%), and low (24% or
lower). Resistance to oil, heat, and cold somewhat vary with each
level. The ambient operating temperature ranges from –40 to 130°C. |
|
Silicon
rubber |
SIR |
Silicon
rubber has excellent resistance to heat and cold, and the ambient
operating temperature ranges from –70 to 280°C. Its resistance to oil,
however, is inferior. |
|
Fluorine rubber |
FRM |
Fluorine rubber has resistance to heat, cold, and oil that is superior
even to NBR and SIR.
Depending on the constituents of the oil, however, the oil resistance
may be inferior to NBR. |
|
Chloroprene rubber |
CR |
Chloroprene
rubber has good resistance against ozone and climatic conditions. It
is often used as the material for basic switches that require
resistance against climatic conditions. |
Switch Terminology
General Terms
Limit Switch
A Built-in switch enclosed in a metal
or resin case to protect it from external forces,
water, oil, dust, dirt, etc. Also abbreviated to merely “Switch”.
Ratings
Generally, the ratings of the Switch
refer to values that ensures the characteristics and performance of the
Switch, such as rated current and rated voltage under specific conditions.
Contacts
Contacts are mechanically opened and
closed for current switching.
Contact Configuration
The electrical input/output circuit
configuration of contacts which depends on the application.
Resin Molding (Molded Terminals)
Terminals that are hardened by
applying resin after lead wires have been connected
in order to eliminate any exposed current-carrying parts and to improve
sealing performance.
Terms Related to
Configuration and Structure
Terms Related to Switch
Durability
Mechanical Durability
The mechanical durability refers to the
number of available switching operations on condition that the Switch is
actuated to the OT position per operation.
Electrical Durability
The electrical durability is the switching
durability at the rated load (i.e., a resistive load) with overtravel set
as the reference value.
Terms Related to
Characteristics
FP (Free Position)
The initial position of the actuator when no
external force is applied.
OP (Operating Position)
The position where the movable contact
reverses from the free position when an
external force is applied to the actuator.
TTP (Total Travel Position)
The position of the actuator when it reaches
the stopper.
RP (Releasing Position)
The actuator position where the movable
contacts reverse from the operating position to the free position when the
external force on the actuator is reduced.
OF (Operating Force)
The force applied to the actuator required
to operate the switch contacts.
RF (Releasing Force)
The value to which the force on the actuator
must be reduced to allow the contacts to return to the normal position.
PT (Pretravel)
The distance or angle through which the
actuator moves from the free position to the
operating position.
OT (Overtravel)
The distance or angle through which the
actuator moves from the operating position
to the total travel position.
MD (Movement Differential)
The distance or angle from the operating
position to the releasing position.
TT (Total Travel)
The distance or angle through which the
actuator moves from the free position to the
total travel position.
Terms Used in EN 60947-5-1
Standards
The following provides information on the
following terms used in this catalog.
EN 60947-5-1
EN standards applicable to electronic
machine control circuitry, the contents of which are the same as those of
IEC 60947-5-1.
Application Category
Switch application categories. Refer to the
following examples.
|
Type of
current |
Category |
Typical
application |
|
AC |
AC-15 |
Control
of electromagnetic loads exceeding 72 VA |
|
AC-14 |
Control of
electromagnetic loads not exceeding 72 VA |
|
DC |
DC-12 |
Control of
resistive loads and semiconductor loads |
Closed-circuit Counter
Electromotive Voltage
Instantaneous overvoltage generated from the
closed circuit, which must not exceed the Uimp value.
Space Distance
The minimum space distance between two
charged parts.
Creepage Distance
The minimum distance on the surface of the
insulator between two charged parts.
Distance through Insulation
The minimum direct distance between the
charged part and the nonmetal switch housing through air or any other
insulator.
Rated Operating Current (Ie)
Rated current for the Switch to operate.
Rated Operating Voltage (Ue)
The rated switch operating voltage, which
must not exceed the rated insulation voltage (Ui).
Rated Insulation Voltage (Ui)
The maximum rated voltage at which the
insulation voltage of the Switch is maintained. This value is used as the
parameter of the dielectric strength and creepage distance of the Switch.
Conventional Enclosed Thermal Current (I the)
The normal carry current that does not
increase the permissible upper-limit temperature of the Switch if it is a
model with its charged part sealed. The rated permissible upper-limit
temperature is 65°C if the terminals are made of brass.
Rated Impulse Dielectric Strength (Uimp)
The peak impulse voltage that the Switch can
withstand with no insulation breakage.
Conditional Short-circuit Current
The current that the Switch can withstand
until the circuit breaker operates.
Short-circuit Protective Device (SCPD)
The device, such as a breaker or fuse, which
breaks the current to protect the Switch from short-circuiting.
Pollution Degree
The environment in which the Switch is used.
The pollution degree is divided into four levels as shown below. The
Switch falls under pollution degree 3.
|
Level |
Description |
|
Pollution degree 1 |
No
pollution or only dry, non-conductive pollutants exist. |
|
Pollution degree 2 |
Normally only non-conductive pollutants exist, which are expected to
be temporarily conductive due to condensation. |
|
Pollution degree 3 |
Conductive pollutants exist or existing nonconductive pollutants will
be temporarily conductive due to expected condensation. |
|
Pollution degree 4 |
Conductive
pollutants exist or existing nonconductive pollutants will be
conductive continuously due to rain or snow. |
Protection Against
Electric Shock
Electric Shock Preventive Levels.
|
Level |
Description |
|
Class 0 |
Electric shocks are prevented by basic insulation only. |
|
Class I |
Electric shocks are prevented by basic insulation and grounding. |
|
Class II |
Electric shocks are prevented by double insulation or reinforced
insulation with no grounding required. |
|
Class III |
No
countermeasures against electric shocks are required because the
electric circuits in use operate in a low-enough voltage range. |
Recommended Products
|
Two-circuit Limit
Switch
WL / WLM
Wide range of two-circuit Switches;
select one for the operating environment / application. |
|
General-Purpose Limit
Switch
HL-5000
Economical, miniature Limit Switch
boasting rigid construction. |
| 
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