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 second. 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
