User:Depwaldrontoo/sandbox

A vibration switch is type of fail-safe device used to monitor industrial rotor applications, such as heat-exchangers, industrial ventilation and pumping apparatus. Vibration switches are installed for a variety of reasons, such as damage limitation from a failure in the machine, as a means of ensuring the safety of employees and to reduce machine maintenance costs. Often the operation of the switch will stop a minor machine failure escalating into a cascading failure.

Typical machine suitable for vibration switch protection.

Principle of operation

On a simplified level, a spinning rotor will vibrate at a set frequency (the easiest model for this is simple harmonic motion). This vibration frequency will be maintained for as long as the system continues to operate in that way. In general the slower the speed of the machine, the greater the amplitude of vibration. Should an error occur in the system (a bearing begins to wear or a fan blade becomes damaged) then the system will no longer vibrate at its set frequency - with the ensuing oscillations increasing in power until a catastrophic failure occurs. A vibration switch is sensitive to the increase in oscillation force and, once it passes a certain threshold cuts power to the machine.

Design

There are several different designs of vibration switch, that whilst sharing a common principle of operation function quite differently to one another.

A typical mechanical vibration switch.

Mechanical Switches

These switches are comprised of a metal sphere that is held in place by a permanent magnet. The magnetic attraction between the magnet and the metal sphere is set to be only slightly stronger than the oscillation force of the monitored machine. As a result, during normal operation the metal sphere will be held securely by the permanent magnet. However, if there is an increase in the degree of oscillation force experienced by the unit then the force of the oscillation will be larger than the attraction between sphere and magnet, leading to the sphere falling out and activating a switch that cuts power to the unit.

The degree of attraction between the sphere and magnet can be controlled by varying the distance between the two components. In most versions, the sphere is held in place by a conical fitting called a seat, which if properly designed allows the unit equalised omni-directional response to changes in the vibrational oscillation of a rotor.

This variety of switch has either a manual reset (where a plunger physically pushes the sphere back into contact with the seat) or a solenoid resetting mechanism which allows the unit to be reset remotely. Some mechanical switches will offer both re-set modes.

Electronic Switches

An electronic vibration switch.

Electronic switches function by using a dedicated electronic sensor to monitor the changes in the frequency of vibration for a machine. Typically using a piezoelectric accelerometer, unlike the mechanical version this type of vibration switch requires constant power. However, there are benefits in terms of measurement sensitivity when compared to mechanical devices, which are inherently binary in their measurements (either the system is operating correctly or the system is turned off).

Limitations

Vibration switches, whilst very effective at slower rotation rates are not sufficiently sensitive to detect changes in oscillation for higher rpm rotors. This is because the amplitude of oscillations is significantly reduced in higher frequency systems - this results in only exceptionally large changes in oscillation registering on the switch. In such applications a more expensive vibration monitor is more suitable.

See also

• Centrifugal switch
• Inertial switch
• Isolator switch
• Latching switch

External links

• http://www.ffeuk.com/product-vib-sw-intro.html