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.
DISPLACEMENT:
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.
VELOCITY:
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.”
ACCELERATION:
(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!
FREQUENCY:
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.
SINE WAVE:
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.
COMPLEX WAVE:
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.
800-929-2845
Acceleration sensitive—measures total
