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Technical Guide
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 Technical Guide > Switches

Limit Switches

 

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.

 

Structural Diagram of Typical Vertical Limit Switch: Seals, Actuator (Head), Connectors, Enclosure Case, and Built-in Basic Switch

 


 

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

Drive mechanism: Plunger

Drive mechanism: Hinge lever

Drive mechanism: Roller lever

Movement of auxiliary plunger

Movement of auxiliary plunger: Plunger

Movement of auxiliary plunger: Hinge lever

Movement of auxiliary plunger: Roller Lever

Force vs. stroke characteristic

Force vs. stroke characteristic: Plunger

Force vs. stroke characteristic: Hinge Lever

Force vs. stroke characteristic: Roller Lever

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.

 

Dog (used for operating the actuator of the switch), movable contact (part of a mechanism to touch or separate from the fixed contact), contacts (the metal parts that touch when the armature reverses), contact gap (distance between the fixed contact and movable contact when they are separated from each other.)

 

 

Terms Related to Configuration and Structure

 

Head, switch casing, built-in switch, conduit opening, terminals, cover and actuator

 

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.

 

Diagram related to characteristics

 

 

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.

 

 

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