Limit Switches
http://www.saleplc.comOverview 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.
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(1) Plunger |
(2) Hinge Lever |
(3) Roller Lever |
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Drive mechanism of limit switch |
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Movement of auxiliary plunger |
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Force vs. stroke characteristic |
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Precision |
High |
Normal |
Low to normal |
Switch Component Materials
The main parts of the Switch are formed from the following materials.
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Part |
Material |
Symbol |
Characteristics |
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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. |
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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. | |
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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. | |
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Silver-palladium alloy |
AgPd |
This alloy has good resistance against corrosion but it easily generates polymers if it adsorbs organic gases. | |
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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. | |
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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. | |
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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. | |
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Movable springs |
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. |
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Age-hardened copper beryllium for |
C1700 |
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/ | |
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Mill-hardened copper beryllium for |
C1700-[]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. | |
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Stainless steel for springs |
SUS301-CSP |
Austenite stainless steel has excellent resistance against corrosion. | |
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Cases and covers |
Phenol resin |
PF |
Phenol resin is heat hardened. It is often used as the material for the casings of basic switches. |
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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. | |
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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. | |
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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. | |
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Polyphenylene sulfide |
PPS |
This resin is thermoplastic. It has heat resistance that is superior even to PA. | |
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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. |
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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. | |
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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 C40 to 130C. |
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Silicon rubber |
SIR |
Silicon rubber has excellent resistance to heat and cold, and the ambient operating temperature ranges from C70 to 280C. Its resistance to oil, however, is inferior. | |
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Fluorine rubber |
FRM |
Fluorine rubber has resistance to heat, cold, and oil that is superior even to NBR and SIR. | |
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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.
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Type of current |
Category |
Typical application |
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AC |
AC-15 |
Control of electromagnetic loads exceeding 72 VA |
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AC-14 |
Control of electromagnetic loads not exceeding 72 VA | |
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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 65C 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.
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Level |
Description |
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Pollution degree 1 |
No pollution or only dry, non-conductive pollutants exist. |
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Pollution degree 2 |
Normally only non-conductive pollutants exist, which are expected to be temporarily conductive due to condensation. |
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Pollution degree 3 |
Conductive pollutants exist or existing nonconductive pollutants will be temporarily conductive due to expected condensation. |
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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.
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Level |
Description |
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Class 0 |
Electric shocks are prevented by basic insulation only. |
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Class I |
Electric shocks are prevented by basic insulation and grounding. |
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Class II |
Electric shocks are prevented by double insulation or reinforced insulation with no grounding required. |
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Class III |
No countermeasures against electric shocks are required because the electric circuits in use operate in a low-enough voltage range. |