Vibration Theory
The designs and engineering requirements of vehicles have undergone
drastic changes over the last several years.
Vehicles are stiffer and provide more isolation from road input than
they did previously. The structures of today's stiffer vehicles are less susceptible
to many of the vibrations which could be present in vehicles of earlier
designs, however, vibrations can still be detected in a more modern vehicle
if a transfer path is created between a rotating component and the body
of the vehicle.
There are not as many points of isolation from the road in many vehicles
today. If a component produces a strong enough vibration, it may overcome
the existing isolation and the component needs to be repaired or replaced.
The presence/absence of unwanted noise and vibration is linked to the
customer's perception of the overall quality of the vehicle.
Vibration is the repetitive motion of an object, back and forth, or
up and down. The following components cause most vehicle vibrations:
| • | The engine combustion process firing impulses |
Rotating components will cause vibrations when excessive imbalance or
runout is present. During vibration diagnosis, the amount of allowable imbalance
or runout should be considered a TOLERANCE and not a SPECIFICATION. In
other words, the less imbalance or runout the better.
Rotating components will cause a vibration concern when they not properly
isolated from the passenger compartment: Engine firing pulses can be detected
as a vibration if a motor mount is collapsed.
A vibrating component operates at a consistent rate (km/h, mph, or RPM).
Measure the rate of vibration in question. When the rate/speed is determined,
relate the vibration to a component that operates at an equal rate/speed
in order to pinpoint the source. Vibrations also tend to transmit through
the body structure to other components. Therefore, just because the seat
vibrates does not mean the source of vibration is in the seat.
Vibrations consist of the following three elements:
| • | The source - the cause of the vibration |
| • | The transfer path - the path the vibration travels through
the vehicle |
| • | The responder - the component where the vibration is felt |
In the preceding picture, the source is the unbalanced tire. The transfer
path is the route the vibrations travels through the vehicle's suspension
system into the steering column. The responder is the steering wheel, which
the customer reports as vibrating. Eliminating any one of these three elements
will usually correct the condition. Decide, from the gathered information,
which element makes the most sense to repair. Adding a brace to the steering
column may keep the steering wheel from vibrating, but adding a brace is
not a practical solution. The most direct and effective repair would be
to properly balance the tire.
Vibration can also produce noise. As an example, consider a vehicle
that has an exhaust pipe grounded to the frame. The source of the vibration
is the engine firing impulses traveling through the exhaust. The transfer
path is a grounded or bound-up exhaust hanger. The responder is the frame.
The floor panel vibrates, acting as a large speaker, which produces noise.
The best repair would be to eliminate the transfer path. Aligning the exhaust
system and correcting the grounded condition at the frame would eliminate
the transfer path.
Basic Vibration Terminology
The following are the 2 primary components of vibration diagnosis:
| • | The physical properties of objects |
| • | The object's properties of conducting mechanical energy |
The repetitive up and down or back and forth movement of a component
cause most customer vibration complaints. The following are the common components
that vibrate:
Vibration diagnosis involves the following simple outline:
- Measure the repetitive motion and assign a value to the measurement
in cycles per second or cycles per minute.
- Relate the frequency back on terms of the rotational speed of
a component that is operating at the same rate or speed.
- Inspect and test the components for conditions that cause vibration.
For example, performing the following steps will help demonstrate the
vibration theory:
- Clamp a yardstick to the
edge of a table, leaving about 50 cm (20 in) hanging over the
edge of the table.
- Pull down on the edge of the stick and release while observing
the movement of the stick.
The motion of the stick occurs in repetitive cycles. The cycle begins
at midpoint, continues through the lowest extreme of travel, then back past
the midpoint, through the upper extreme of travel, and back to the midpoint
where the cycle begins again.
The cycle occurs over and over again at the same rate, or frequency.
In this case, about 10 cycles in one minute. If we measure the frequency
to reflect the number of complete cycles that the yardstick made in one
minute, the measure would be 10 cycles x 60 seconds = 600 cycles
per minute (cpm).
We have also found a specific amount of motion, or amplitude, in the
total travel of the yardstick from the very top to the very bottom. Redo the
experiment as follows:
- Reclamp the yardstick to the edge of a table, leaving about 25 cm
(10 in) hanging over the edge of the table.
- Pull down on the edge of the stick and release while observing
the movement of the stick.
The stick vibrates at a much faster frequency: 30 cycles per
second (1,800 cycles per minute).
Cycle
