There are many moving components onboard a ship that require stopping at certain points. Examples include closing watertight doors, opening hatch covers, rudders swinging to one side, opening fully or closing fully. It is important to know when these items are at their end point so someone on the bridge or control room knows and to stop the mechanism providing power. This is where limit switches come in. They aren’t glamorous but are small devices that can tell you that ‘this has reached this point’. They are very important from a safety perspective.
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Limit switches are very simple devices. They are fitted within a control circuit and contain electrical contacts. A moving component such as the door, valve stem or cam on a shaft will come into contact with the actuator of the limit switch. This could be a lever, button or roller. The contacts will then change state. If the contacts were closed they will now be open and vice versa. This change in the contacts will cause something to happen within the control circuit. This could be stopping a motor, turning on an indicator, sounding an alarm or stopping something else from happening.
Most switches will have two sets of contacts. One set will be normally open (NO) meaning they will be open until the switch is actuated. The other set will be normally closed (NC). Depending on what you want to happen within the circuit you will wire the correct set of contacts. Often safety circuits will use NC contacts to ensure that if there is a break in the wiring or failure of the switch, the machine will stop. This is because it will look like it has reached its limit. Having a fail safe circuit is much better than having one that fails silently.
Many mechanical switches use a snap action design. This means that the contacts don’t slowly move with the actuator but stay in place until they suddenly snap into place when the actuator reaches a certain point. This creates a clear signal and reduces any arcing that may occur.
This is the most common type of limit switch. The actuator is physically moved and causes the contacts inside to change state. There are a few common types of actuators:-
Sometimes you don’t want anything touching the switch. Proximity switches can detect a target without any contact. There are a few common types:-
Proximity switches will last longer in harsh environments as there is no contact to cause wear. They also change state quickly. They do however need a power source and usually only carry a small current (so they are used to send a signal to a relay or PLC input rather than directly to a motor). They are also more affected by environmental factors such as temperature and electrical interference than a simple mechanical switch.
If you need to know how far something has turned then a rotary cam switch would be a good option. The shaft is driven by the moving part of the machinery. This could be via a gear. As it rotates a number of adjustable cams press onto switches at certain angles. Each cam can be set to trip at different points.
These are commonly found on winches, cranes, windlasses and steering gears. They allow you to have multiple set points from one device. For example a crane hoist might have one switch to slow the hoist as it reaches the top and another to stop altogether. They can be adjusted onboard depending on the machinery.
Once you start looking they are everywhere! Some important places to find them include:
Despite being simple devices they can suffer from many issues due to harsh environments:-
Limit switches are used to tell the system that something has reached a certain point. Depending on the situation there are different types available. Mechanical switches are simple, straightforward and easy to test. Proximity switches have no moving parts to cause wear. Rotary cam switches offer multiple set points from one shaft. It depends on what is moving, where it is and how harsh the environment is. Choosing the correct switch, installing it correctly and ensuring the seals and wiring are in good condition will mean it will do its job for years without anyone noticing.
A limit switch detects when a moving part reaches a certain point and causes the electrical contacts to change state. This will stop a motor, provide an indication or cause an alarm.
Normally open contacts will be open until the switch is actuated and then will close. Normally closed contacts will be closed until the switch is actuated and then will open. Often safety circuits will use normally closed contacts as a break in the wiring will be seen as a fault.
If you don’t want anything touching, the environment is harsh and would cause wear on a mechanical actuator or you require fast switching. Inductive switches are good for detecting metal. Capacitive switches can detect non-metallic objects.
They can provide you with multiple set points from one shaft. This can slow the machinery near the end of travel.
Most commonly due to the bracket or actuator being out of alignment due to vibrations or knocks. Ensure the switch is installed correctly.
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