MECHANICAL LIMIT SWITCHES FOR MACHINE SAFETY CONSIDERATIONS

With these devices the guard door is linked mechanically to the contacts of the switch using positive mode operation in compliance with standards and as applicable to E-stop switches. There are three main types of mechanical actuation. These are:

1. Tongue-operated

2. Hinge-operated

3. Cam/plunger-operated.

Tongue-operated limit switches

Features: These devices comprise two separate elements: a switch body and actuator tongue. Tongues are metal probes specially shaped to fit into the switch rather like a key.

These are to be fitted to edge of a sliding door or on to a removable guard. When the tongue enters the switch body it engages a mechanism that closes or opens internal electrical contacts.

The mechanism is designed to prevent easy bypassing or ‘cheating’ of the switch since the tongue is coded like a key. Usually these types have self-ejecting spring-loaded mechanisms so that the tongue will only remain in place if it is attached to the door of the guard. If the tongue were to be removed from the door and just pushed into the switch it would not stay in place.

• Requires reasonably accurate alignment

• Should not be subjected to constant high-amplitude vibration

• Can be used on sliding, hinged and lift-off guards.

Advantages: Low cost, versatile, certified for safety. Almost tamper-proof if the tongue design is good.

Disadvantages: Not suited to pharmaceutical applications and some food applications where good cleaning is essential.

Hinge-actuated limit switches

The device is mounted over the hinge-pin of a hinged guard. The opening of the guard is transmitted via a positive mode operating mechanism to the control circuit contacts.

Advantages: When properly installed these types of switches are ideal for most hinged guard doors where there is access to the hinge centerline. They can isolate the control circuit within 3° of guard movement and they are extremely difficult to defeat without dismantling the guard.

Disadvantages: Care must be taken on large wide guard doors as an opening movement of only 3° can still result in a significant gap at the opening edge on very wide guard doors. It is also important to ensure that a heavy guard does not put undue strain on the switch actuator shaft.

Cam-operated limit switches

This type of arrangement usually takes the form of a positive mode acting limit (or position) switch and a linear or rotary cam. It is usually used on sliding guards and when the guard is opened the cam forces the plunger down to open the control circuit contacts.

Features: The simplicity of the system enables the switch to be both small and reliable. It is extremely important that the switch plunger can only extend when the guard is fully closed. This means that it may be necessary to fit stops to limit the guard movement in both directions.

Advantages: Wide range of low-cost switches are available. Available in very wide range of sizes. Can be made extremely durable and rugged. Easy for maintenance crews to inspect and repair.

Disadvantages: Relatively easy to defeat. It cannot be used on lift-off guards. Requires careful installation and design of strikers: For example: it is necessary to fabricate a suitably profiled cam which must operate within defined tolerances. This system can be prone to failures due to wear especially in the presence of abrasive materials or with badly profiled cams.

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