Failed Bearings or Gears

Theory and Information

Theory and Information
Bearing Vibration Frequencies

Setup Procedure

Preparing for a road test

Measurements

Loading wheel bearings

Diagnostics

Theory and Information

Failed bearings or gears can cause vibrations also. The vibration frequency of such a vibration might not match any line on the vibration worksheet.
If the vibration is Engine Speed Related. divide the vibration frequency (RPM) by engine RPM. Compare this number with all the belt-driven accessory speeds. Look for multiple order vibrations. Example: A six cylinder A/C compressor will make three shakes for every one revolution of it's own crankshaft.
If the vibration is Vehicle Speed Related, try to determine whether it is Tire Speed Related or Propshaft Speed Related. To make this determination, divide the vibration frequency (RPM = Hz x 60) by tire RPM or propshaft RPM and look for multiple order vibrations of tire or propshaft speed. For example:
- A vehicle speed related vibration which produces a loud whine type sound has a vibration frequency of 24,600 RPM (410 Hz). The tire speed = 600 RPM and propshaft speed = 2238 RPM. Nothing on the vehicle rotates at 24600 RPM, so it must be a multiple order of the speed of a rotating vehicle component.
- 24600 RPM / 600 RPM (Tire Speed) = 41.000, a direct multiple. Look for something which could cause 41 shakes per revolution; a 41 tooth ring gear in the rear axle.
- 24600 RPM / 2275 RPM (Propshaft Speed) = 10.813, not a direct multiple.
- Sometimes detecting a direct multiple will not be so obvious, try to pick the multiple that is the closest one and look for bearings or gears which can cause the vibration you are trying to diagnose.

Bearing Vibration Frequencies

Timken bearings has a web site where you can calculate the frequency of the bearing roller, cup, and cone vibration based upon the failure type and the bearing cone speed.

First, calculate the rotational speed of the suspected bearing cone. The bearing cone speed is typically the speed of the component it is connected to. For example, a pinion gear bearing cone will turn the same speed as the pinion gear. The pinion gear rotates the same speed as the propshaft connected to it.
Second, click here to select the part number of the Timken bearing you use for the vehicle you are diagnosing.
Third, click here to go to the Timken bearing vibration frequency calculator. Enter the bearing part number and the cone speed. The frequency calculator will show the vibration frequencies of the bearing roller, cup, and cone at that speed.

The Timken pinion bearing (HM88649) shown above has 17 taper roller bearings attached to the cone (inner race), it takes 2.476 revolutions of the cone (inner race) to have all 17 rollers make their way around the cup (outer race). According to the Timken calculations for the HM88649 bearing:

A roller with one irregularity (Nick or Spall) would have a vibration frequency of 1.0 Hz at 8.738 cone RPM. By converting shakes per second (Hz) to shakes per minute (Hz x 60) the roller irregularity would vibrate 60 times for every 8.738 revolutions of the cone. (60 shakes/minute) / (8.738 revolutions/minute) = 6.687 shakes per revolution. So, a roller with one irregularity on it's bearing contact surface results in a vibration equal to a 6.867 order propshaft speed related vibration.
A cone with one irregularity (Nick or Spall) would have a vibration frequency of 1.0 Hz at 6.212 cone RPM. By converting shakes per second (Hz) to shakes per minute (Hz x 60) the cone irregularity would vibrate 60 times for every 6.212 revolutions of the cone. (60 shakes/minute) / (6.212 revolutions/minute) = 9.659 shakes per revolution. So, a cone with one irregularity on it's bearing contact surface results in a vibration equal to a 9.659 order propshaft speed related vibration.
A cup with one irregularity (Nick or Spall) would have a vibration frequency of 1.0 Hz at 8.176 cone RPM. By converting shakes per second (Hz) to shakes per minute (Hz x 60) the cup irregularity would vibrate 60 times for every 8.176 revolutions of the cone. (60 shakes/minute) / (8.176 revolutions/minute) = 7.339 shakes per revolution. So, a cup with one irregularity on it's bearing contact surface results in a vibration equal to a 7.339 order propshaft speed related vibration.

Setup Procedure

Inspect the vehicle to insure it is safe to road test.
Find a safe location to drive in such as an empty parking lot.

Measurements

Load the left front and left rear wheel bearings by turn sharply to the right while driving. Listen for a noise change.
Load the right front and right rear wheel bearings by turn sharply to the left while driving. Listen for a noise change.
Load the front pinion bearing and the left differential side bearing by accelerating while driving. Listen for a noise change.
Load the rear pinion bearing and the right differential side bearing by decelerating while driving. Listen for a noise change.

Diagnostics

Bearings - If the bearing noise was louder while turning the vehicle right on the road test, inspect the bearings on the left side of the vehicle for roughness. Replace as necessary.
Bearings - If the bearing noise was louder while turning the vehicle left on the road test, inspect the bearings on the right side of the vehicle for roughness. Replace as necessary.
Bearings or Gears -If the bearing noise was louder while accelerating or decelerating vehicle on the road test, you probably have drive axle problems. click here to diagnose. You may also have a center support bearing failing on vehicles with multiple piece rear propshafts.

This page was last modified Thursday, March 05, 2009 09:02:24 PM