The following are the two primary components of vibration
diagnosis:
• | The physical properties of objects |
• | The object's properties of conducting mechanical energy |
The repetitive up/down or back/forth movement of a component causes
most customer vibration complaints. The following are the common components
that vibrate:
Vibration diagnosis involves the following steps:
- Measure the repetitive motion and assign a value to the measurement
in cycles per second or cycles per minute.
- Relate the frequency to 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 per 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 (1800 cycles per minute). The total travel, or amplitude, is
less.
Vibration
Vibration is the repetitive motion of an object, back and forth, or
up and down. The following conditions cause most vehicle vibrations:
• | The engine combustion process firing impulses |
Rotating components will vibrate with excessive imbalance or runout.
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.
A vibration concern will occur when the firing impulses of the engine
are not properly isolated from the passenger compartment.
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 doesn't mean the source of the 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 figure, the source of the vibration 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 which is 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.
Cycle