Overview

Vibration Overview

VIBRATION SWITCH SENSORS

How Vibration Switches Work and Protect You Equipment from Destructive Vibration

The Robertshaw vibration switch offers many features not usually available in competitive vibration instruments. Some of these features are:

1.   Models are available with ELECTRIC or PNEUMATIC switching action.
2.   Units respond to the DESTRUCTIVE forces, not displacement.
3.   No MAINTENANCE required. no moving parts to wear insures long life.
4.   CONTINUOUS PROTECTION for operating machines.

There are more than 125,000 Robertshaw vibration switches in use all over the world making these instruments by far the most widely used vibration detection system available.

HOW THE VIBRATION SWITCH OFFERS POSITIVE PROTECTION

Figure 2 is a line diagram of the basic operating principle. The detector mechanism consists of an armature (pendulous mass) suspended on a flexure pivoted beam. It is restrained from motion in its normal (reset) condition by means of a permanent hold-down magnet acting through a small air gap. A compression spring provides an adjustable force opposing the magnetic force and this serves as the adjustable SET-POINT which is calibrated directly in G's (acceleration units). The armature acts as a beam: it is forced in one direction by the adjustable spring, and in the other direction it is balanced by the magnetic force as indicated in Figure 2.

The armature is constrained to a single direction of movement (sensitive axis) by using a relatively wide flexure pivot assembly composed of two overlapping blocks and a leaf spring loaded in one direction so as to hold the blocks together as shown in Figure 3.

When the entire assembly is subjected to vibration perpendicular to the base. the peak acceleration (A) times the effective mass (M) of the armature produces an inertial force (F = MA). This force is aided by the spring, which tends to pull the armature away from the hold-down magnet and the stop pin, When the peak acceleration exceeds a pre-selected set-point level, the armature leaves the stop pin. and moves up to the latch magnet. which holds it in the "alarm" (excess vibration) position until it is reset.

Motion of the armature actuates a snap-action switch, either an electrical or pneumatic type, which signals that the set-point has been exceeded. The vibration switch is an accurate and dependable but simple mechanical accelerometer with "on-off" control output.

To the basic design the following refinements are added:

1. Temperature Compensation. The effect of temperature in the mechanism is negligible as the elastic modulus of the adjustment spring and the magnetic flux through the air gaps both decrease slightly with increasing temperature; these two effects are, therefore. compensating.

2. Set-Point Adjustment. This screw allows setting the force on the spring-loaded end of the armature to compensate for "normal" machine vibration plus the desired "safety" factor of the particular machine involved. The dials are calibrated directly in G units through a range of 0 to 4.5 G's.

3. Electric or Pneumatic Reset. An electromagnet attracts the armature when energized with a voltage, or a pneumatically actuated plunger forces the armature back to the "reset" position. It is used for remote resetting after actuation, and for "locking out" the vibration switch during abnormal machine vibrations that may occur during start-up or transient shock conditions.

4. Manual Reset Button. This permits manual resetting of the vibration switch after actuation (tripping).
Figure 4 shows the basic operating parts of the Model 368 vibration switch for operation on pneumatic power. It is basically the same unit as the electric type except for the pneumatic valve assembly.

SIMPLICITY OF INSTALLATION AND ADJUSTMENT

The vibration sensitive axis of the vibration switch is in a direction perpendicular to its mounting base. Therefore it must be mounted on the machine in a plane that will detect the vibratory motion for which protection is desired. Normally bent shafts, unbalances on the rotating mass of the shaft, worn bearings, and other mechanical abnormalities are best detected near the bearing housings and at right angles to the rotating shaft. Do not mount the vibration switch perpendicular to the ends of rotating shafts unless an end-play or end-thrust measurement of the shaft is desired.

The base must be rigidly mounted on a flat surface so that all four corners of the base are contacting the surface. If a mounting plate is attached to the machine, it must be very rugged in order to prevent introduction of mechanical (plate) resonances into the vibration switch.

Adjustment

The set-point is initially adjusted to its maximum G value by turning it fully counterclockwise. If the machine is operating normally (with-out excessive vibration), slowly turn the set-point clockwise until the switch actuates. Then advance the set-point counterclockwise five divisions on the graduated dial (0.5 G). Adjustment is then complete.