Drive Axle Problems

Theory and Information

• Information

• Gear Tooth Terminology

Setup Procedure

• Preparing the vehicle

Measurements

• In-Stall Loaded Testing

Specifications

• Backlash and Runout Specifications

Diagnostics

• Improper Pinion Bearing or Side Bearing Preload
• Excessive Gear Tooth Backlash Variation
• Pitch-Line Runout
• Bent Pinion stem
• Cocked pinion bearing
• Mis-bored axle housing
• Two-Speed or double reduction MD/HD truck axles

Theory and Information

Numerous drive axle problems can result in a vibration which is propshaft speed or tire speed related.

If the vehicle has a vibration that is equal to any order propshaft or tire rotation, and it is not present when testing the vehicle in the stall, then it is possible that the vibration is being generated by internal front or rear axle components. This may also be true if the vibration was correctable in the stall, but returned when the vehicle was driven on the road. These vibrations tend to be aggravated by the load of the vehicle working against the ring and pinion gear set.

• Since the pinion gear operates at the exact same speed as the propshaft (they are bolted together through the pinion flange), the vibrations it produces will have the same frequency and symptoms.  

• Since the ring gear operates at the exact same speed as the tires (except while turning corners), the vibrations it produces will have the same frequency and symptoms.

Gear Tooth Terminology

• Heel and Toe identify the ends of the gear tooth.  Imagine you and 12 other people are standing in a circle facing each other, your toes are pointing inside of the circle and your heels are pointing outside of the circle.

  • The Toe end of a ring gear is on the inside diameter of the gear.

  • The Heel end of a ring gear is on the outside diameter of the gear.

• The Pitch Line is an imaginary line that divides the upper and lower halves of the gear tooth contact area.

• The Pitch Point is the theoretical point where the pitch lines of both meshing gear teeth meet while rotating relative to one another.

• The Root Line is an imaginary line that indicated the bottom of the meshing gear tooth contact area.

• Face and Flank identify the area in the contact area of the gear tooth. 

  • The Face of a gear tooth is the upper half of the gear tooth contact area.

  • The Flank of a gear tooth is the lower half of the gear tooth contact area.

• The Drive Side and Coast Side of the gear tooth identify the sides of the gear tooth used under acceleration and deceleration.

  • The Drive Side is the convex side of the gear tooth and is used during acceleration and steady speed driving.

  • The Coast Side is the concave side of the gear tooth and is used during deceleration.

• Backlash is the endplay between the gear teeth as the gear sets rotate relative to one another.

• Clearance is the distance between the top of one gear tooth and the bottom of the area between two gear teeth it is meshing with.

Setup Procedure

• Support the vehicle on a suitable hoist or jack stands. The rear axle should be at curb height; the drums and tire/wheel assemblies should be removed.

Measurements

In-Stall Loaded Testing

This test is designed to load the pinion so it will produce the vibration on the hoist, then allow the technician to identify and isolate the source.

• While holding the EVA’s sensor touching the pinion nose or axle shaft housing, have another technician accelerate and decelerate the vehicle through the speed range at which the vibration was first noted during the road test.
• Load the front pinion bearing and the left differential side bearing by accelerating.  Listen for a noise change.
• Load the rear pinion bearing and the right differential side bearing by decelerating.  Listen for a noise change.
• Click here to perform additional bearing diagnosis.

Example: if the vibration was originally noted at 55 mph, accelerate and decelerate from 45 mph to 65 mph, back to 45 mph and so on. Repeat this while taking note as to whether or not the pinion nose vibrates under load during the acceleration and/or deceleration.

Diagnostics

IMPORTANT- Make sure that both axle shafts are rotating at the same speed. The action of a differential when one tire is spinning faster than the other may mask the vibration. Adjusting the rear brakes equally will usually correct this condition.  

• If the pinion nose vibrates under acceleration/deceleration, and the other propshaft components have been eliminated as a cause, then a problem within the rear or front axle may exist.
• If the vibration is not experienced, reinstall the brake drums and tire/wheel assemblies. This helps to put an additional load to the system. Repeat the test.
• If the vibration still cannot be reproduced, try lightly applying the brakes to further load the system, while maintaining the vibration complaint speed. Use caution not to overheat the brakes.

Improper Pinion Bearing or Side Bearing Preload. 

Improperly loaded, specifically under loaded, pinion bearings and side bearings will allow excessive gear movement resulting in noise and vibration.

• Under loaded acceleration, If the pinion bearing preload is too low:
  • The pinion gear will push away from the ring gear. 
    • This will load the pinion bearing closest to the pinion gear teeth.  If the bearing is failing, it will make more noise under acceleration.
    • This will unload the pinion bearing furthest from the pinion gear teeth.  If the bearing is failing, it will make less noise under acceleration.
    • This will also increase the backlash.  If the backlash was too tight or if backlash variation was excessive, the noise may decrease under acceleration.
    • This will also create an improper gear tooth contact pattern.  Specifically a gear tooth pattern closer to the heel (outside) of the ring gear and upwards towards the face (top) of the tooth resulting in gear noise.
  • If the differential side bearing preload is too low, the ring gear will try to push the differential case towards the side bearing closest to the ring gear.
    • This will load the side bearing closest to the ring gear. If the bearing is failing, it will make more noise under acceleration.
    • This will unload the side bearing furthest from the ring gear. If the bearing is failing, it will make less noise under acceleration.
    • This will also increase the backlash.  If the backlash was too tight or if backlash variation was excessive, the noise may decrease under acceleration.
    • This will also create an improper gear tooth contact pattern.  Specifically a gear tooth pattern closer to the heel (outside) of the ring gear and upwards towards the face (top) of the tooth resulting in gear noise.

• Under loaded deceleration, if the pinion bearing preload is too low:
  • The pinion gear will be pulled towards the ring gear. 
    • This will load the pinion bearing furthest from the pinion gear teeth.  If the bearing is failing, it will make more noise under acceleration.
    • This will unload the pinion bearing closest to the pinion gear teeth.  If the bearing is failing, it will make less noise under acceleration.
    • This will also decrease the backlash.  If the backlash was OK or if backlash variation was excessive, the noise may increase under deceleration. This can allow the gears to bind under a load resulting in gear tooth failure, noise and vibration.
    • This will also create an improper gear tooth contact pattern.  Specifically a gear tooth pattern closer to the toe (inside) of the ring gear and downwards towards the flank (bottom) of the tooth resulting in gear noise
  • If the differential side bearing preload is too low, the ring gear will try to pull the differential case towards the side bearing opposite the ring gear.
    • This will load the side bearing furthest from the ring gear. If the bearing is failing, it will make more noise under acceleration.
    • This will unload the side bearing closest to the ring gear. If the bearing is failing, it will make less noise under acceleration.
    • This will also decrease the backlash.  If the backlash was OK or if backlash variation was excessive, the noise may increase under deceleration. This can allow the gears to bind under a load resulting in gear tooth failure, noise and vibration.
    • This will also create an improper gear tooth contact pattern.  Specifically a gear tooth pattern closer to the toe (inside) of the ring gear and downwards towards the flank (bottom) of the tooth resulting in gear noise

Excessive Gear Tooth Backlash Variation.

Theoretically, every tooth of a gear set should have the same backlash when setup properly.  In the real world this is rarely the case.  Backlash variation is a difference in the backlash from one gear tooth to another gear tooth.  Gear tooth backlash may be within specifications on one tooth and out of specifications on the next tooth.  Backlash variation should be checked on EVERY tooth of a gear set. 

Heat induced gear tooth expansion can decrease backlash and worsen the vibration as the gears warm up.  If any of the following conditions occur, the heat induced gear tooth expansion will occur more quickly.

• The gear lubrication level is low.
• The gear lubrication is contaminated.
• The gear lubrication the incorrect lubrication.
• The initial backlash is set too tight.
• The vehicle is overloaded.
• The vehicle is pulling a trailer which is too heavy.

On a ring and pinion gear set with a 3.73:1 gear ratio (41 ring gear teeth and 11 pinion gear teeth) the following vibrations can occur:

If the ring gear was machined incorrectly:

• If the ring gear has one gear tooth with zero backlash, the gear would bind once per revolution of the ring gear resulting in a first order tire speed related vibration and a 0.278 order propshaft speed related vibration.
• If the ring gear has two gear tooth with zero backlash, the gear would bind twice per revolution of the ring gear resulting in a second order tire speed related vibration and a 0.537 order propshaft speed related vibration.
• If the ring gear has three gear teeth with zero backlash, the gear would bind three times per revolution of the ring gear resulting in a third order tire speed related vibration and a 0.805 order propshaft speed related vibration.
• If the ring gear has 41 gear teeth with zero backlash, the gear would bind 41 times per revolution of the ring gear resulting in a forty-first order tire speed related vibration and an eleventh order propshaft speed related vibration.

If the pinion gear was machined incorrectly:

• If the pinion gear has one gear tooth with zero backlash, the gear would bind once per revolution of the pinion gear resulting in a 3.73 order tire speed related vibration and a first order propshaft speed related vibration.
• If the pinion gear has two gear tooth with zero backlash, the gear would bind twice per revolution of the pinion gear resulting in a 7.46 order tire speed related vibration and a second order propshaft speed related vibration.
• If the pinion gear has three gear teeth with zero backlash, the gear would bind three times per revolution of the ring gear resulting in a 11.19 order tire speed related vibration and a third order propshaft speed related vibration.
• If the pinion gear has 11 gear teeth with zero backlash, the gear would bind 11 times per revolution of the pinion gear resulting in a forty-first order tire speed related vibration and an eleventh order propshaft speed related vibration.

Backlash variation can be detected by measuring the backlash of the ring gear on every tooth around the gear.

• If the backlash is within specifications AND varies by more than 0.002” from the lowest to the highest backlash, then it is excessive and the differential case should be checked for excessive lateral and radial runout as shown below.
• If the differential case runout is less than 0.002” then the ring and pinion gear set should be replaced.
• If the differential case runout is more than 0.002” then the differential case should be replaced.

Specifications

• Typical ring gear backlash = 0.005" to 0.009" (0.127 - 0.2286mm)

• Maximum ring gear backlash variation = 0.002" (0.0508mm)

• Maximum differential case radial runout = 0.002" (0.0508mm)

• Maximum differential case lateral runout = 0.002" (0.0508mm)

Pitch-Line Runout.  

A variation in gear tooth depth and width consistency of the ring gear or pinion gear teeth which can cause gear tooth binding.  Click here to diagnose Pitch-Line Runout.

Bent Pinion stem. 

Possibly resulting from a rear-end collision where the rear axle was forced into the propshaft and transmission with excessive force.

Cocked pinion bearings or bearing cups. 

Possibly resulting from improper installation of the bearings or from the bearings being installed into a dirty or contaminated surface.

Mis-bored axle housing, etc.

Anything that can affect the pinion gear and how it contacts the ring gear as it rotates can contribute to any order, torque sensitive propshaft vibration. The only way to correct these conditions is to replace the affected components. In most cases this means the ring and pinion gear set and related bearings, but in some cases, it may include the axle housing. Presently, there is no way to effectively measure or identify the exact component at fault, except close visual inspection for unusual wear marks that may or may not be present.

Sometimes the installation of a "known good" axle assembly from a stock unit is the best way to quickly isolate an internal axle problem as the cause. Always qualify or evaluate the "known good" stock unit to ensure that it does not have a vibration problem.

Once an internal axle problem has been corrected, perform a road test. Determine if the vibration has been eliminated. It may be necessary to 'line tune" the vehicle by performing a system balance of the propshaft

Two-Speed or double reduction MD/HD truck axles

A two-speed or double reduction MD/HD truck axle has additional components which can also cause a vibration.  These components can rotate at speeds that do not match the speed of the propshaft or the tires.  For example:  A two speed axle might have a gear ratio set of 6.14 in high, and 8.38 in low.  The gear reduction ratio = 8.38 / 6.14 = 1.36:1.  (This reduction is typically accomplished with a planetary gear set). This means that in low range, there is an additional 1.36: gear reduction taking place.  

A vibration caused by any of the reduction components may cause a vibration that is 1.36 times faster than the speed of the propshaft.  The vibration frequency of this type of component will not match any on the lines on the vibration graph.  Shifting the axle from high to low may assist in isolating the source of the vibration.

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

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