Thursday, May 29, 2014

Robertshaw Vibration Detection

Robertshaw Industrial Division Vibration Malfunction Detection Switches
The Robertshaw VIBRASWITCH from Sords Electric

366A8  366D8  365A-A8  365A-D8

The 366 switch is Nema 4 and the 365 switch is explosion proof.

Robertshaw Vibration Switches and Monitors
What is Vibration?

Vibration is the mechanical motion of a machine or part back and forth from its point of equilibrium.  The term “vibration” may mean different things to different people.  Some may feel it only denotes movement and to others it may suggest both movement and its relative speed per unit time.  To most people, it describes, on an operating machine, a condition indication impending danger when the vibration appears to be exceeding a level previously considered normal. Note the statement, “a level previously considered normal.” All running machines have some amount of normal or inherent vibration. Example: an automobile engine, in perfect condition and tune, does have some level of inherent vibration.  But, when it starts to “run rough”, the vibration level increases and this is the vibration we desire to detect before damage can occur to the engine.

The our most frequently used terms in any discussion n vibration are: Displacement, Velocity, Acceleration and Frequency. Therfore, defining the meanings of these terms is mandatory if an understanding of vibration and it effects is to be had.

Also sometimes referred to as amplitude.  This is defined as the magnitude of the movement or motion of the machine or part under study.  The total distance traveled by the vibrating part- from one extreme limit of travel to the opposite extreme limit of travel - is noted as the “Peak to Peak Displacement.” The value of displacement is normally expressed in mils – one thousandth of an inch. Displacement measurement refers then to the “bending” of the moving part.  The greater amount of bend (displacement) the more likely the part is to fail.

Speed of movement.  Since the part or machine is moving, it is therefore understood that it is moving at some speed.  Noting a sine wave of the motion, the spped is greatest at the neutral point since at each extreme limit of motion, the part must come to a complete stop before it can change its direction of movement.  It will then increase its speed until it reaches the neutral point when it commences to slowing down prior to reaching the other extreme point of movement. The neutral point – reference to velocity – is referred to as the point of PEAK VELOCITY.  Velocity measurement is expressed in “inches per second peak.”

(What Robertshaw measures)
Acceleration is the first derivative of velocity with respect to time.  On the Sine wave of vibration it is shown that the point of maximum acceleration occur at the extreme limits of travel when the part must stop its movement entirely and restart in the opposite direction.  Therefore, acceleration can be stated as the vibratory “force” being applied to the machine or part.  Large forces of acceleration can occur at hig frequencies even though displacement and velocity of the vibration are small. This can be understood by the fact that acceleration is a function of the displacement and the square of the frequency. This force can result in the breakdown of bearings, lubricant, etc.  Acceleration is sometimes referred to as the measurement or detection of shock. Think of a pre-ignition in one cylinder of an automobile engine.  At the time of pre-ignition, displacement would be very small, velocity is also small but the shock (acceleration) is very high!

The frequency is the number of complete Sine waves the motion of movement a machine of par will make in one unit of time.  Normally, this unit of time is: per minute or per second. Referring to the definition of “displacement,” it was stated that “the greater the amount of bend, the more likely it is to fail.” It can be added that the increasing the rate of bending (the more times per unit of time that the metal is bent), the quicker it will fail.  The “rate” is the frequency. This can also be stated as “the time rate of change of displacement” – or – the first derivative of displacement with respect to time.

The type of vibration produced by a single frequency. This type of vibration rarely, if ever, occurs on an operating machine. Note the Sinusoidal Motion Waveform Figure which is the resultant shape of displacement with respect to time pure harmonic motion-sinusoidal single frequency.

Most operating machines produce vibration waveforms which are non-sinusoidal.  These are called complex periodic waveforms. Recognizing that many frequency components and harmonics are added to produce a given condition of vibration on any particular machine, the resulting motion will assume a complex wave form. The distortion from a pure sine wave is most apparent.  Displacement measurements give little or no information with respect to the harmonic content of the resulting vibratory motion. The complex wave which has a second harmonic with an amplitude 40% as large as the fundamental and a fourth harmonic 20% as large as the fundamental.

Why Monitor Machine Vibration:

The purpose of monitoring vibration is to detect a mechanical malfunction (malfunction detector) as it first begins to develop and wither save an expensive machine or greatly reduce the cost of repairs and down time.  On many machines, dangerous conditions of operation can be corrected IF the machine is stopped before damage is done. Saving from ONE operation of a Vibraswitch can frequently amount to more money than it would cost to equip an entire plant, refinery or pipeline. Thousands of large, electrically driven centrifugal fans or blowers are now protected with circuit breakers on the driving of motors which will respond only to pver current condition of the motor. Others use temperature monitors on anti-friction bearings.  The latter does not respond to unbalance caused by a damaged rotor or accumulated deposits and frequently they respond too late to a failing bearing.  The controls normally installed on large prime movers, for example, respond to all important variable with, in many case, one exception – ACCELERATION due to vibration.  Large machines can and do tear themselves apart without any of the conventional protective devices responding.  When this happens, it is obvious that something is missing in the protective controls.  If the need for vibration protection is so obvious, why isn’t it standard practice for all large machines to have protective devices specified?

Vibration is a difficult quality to measure, comprehend or compare since it is composed of a fundamental, higher harmonics and aperiodic components (transients), all of which vary greatly with different machines and different locations on the same machine. The typical prospective user assumes that the only significant quantity to measure is mils (displacement) and has no idea as to what level would be safe.  The Vibraswitch offers a solution to the machine vibration problem in nearly all cases buy many users do not know of the Robertshaw Vibraswitch.

Many potential users relied on the ability of the operating people to hear any changes in the sound of machines which might reflect mechanical trouble. The dependence on sound was better than nothing but had important disadvantages because sound at the fundamental (low) frequency of rotation is usually inaudible; the human ear is very poor in remembering and comparing sound levels and the sound of a machine Is frequently masked by extraneous noise.  A great disadvantage is that constant operator attendance is required. This is not possible or economical on many machines with automation and remote operation required many places.

How the Robertshaw VIBRASWITCH measure Acceleration:

The Robertshaw detector mechanism consists of an armature suspended on a flexure type pivot and restrained from motion by a permanent magnet acting through a small air gap. A compression spring provides an adjustable force opposing the magnet, and serves as the adjustable set-point calibrated in”G’s.”  The armature acts a s lever; it is forced in one direction by the adjusting spring and balanced by the magnetic orce; as shown in Figure 1.

The Armature is constrained to only one freedom of movement by the flexure pivot composed of two overlapping blocks and a leaf spring loaded in one direction to hold the blocks together as shown in Figure 2.  When the entire assembly is subjected to vibration perpendicular to the base, the peak acceleration times the effective mass of the armature produces an inertial force aided by the spring, which tends to pull the armature away from 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 in it the “alarm” (excess vibration) position.
The motion of the armature actuates a snap-action switch which signals that the set-point has been exceeded. The Robertshaw Vibraswitch is a simple mechanical accelerometer with on-off control output!

To get a quote, please use our Robertshaw Parts Form HERE.