Wheel Loss Still a Problem

While wheel loss is still plaguing the industry, several companies are working to find solutions to a problem that continues to give a bad name to the trucking industry. G Allan Hagelthorn of Rather Engineering and Barry Braun of Stemco Incorporated presented their findings at the Society of Automotive Engineers Truck and Bus Meeting held November 17 to 19 at the Cleveland Convention Center, Cleveland, Ohio. Hagelthorn warned of government action such as the province of Ontario, Canada, requiring certification of all mechanics working on wheels. This is a result of a rash of wheel loss accidents.

Hagelthorn reviewed castellated nuts manufactured with excessive thread tolerance and how thread deterioration can result in excessive wheel endplay. As deterioration increases, lateral forces on the wheel assembly can become great enough to cause the wheel to separate from the axle.

"Another common problem is overheating of roller bearings due to worn oil seals," says Hagelthorn. Seals often are worn prematurely due to excessive lateral movement. Movement results from loose adjustment or an increase of clearance due to wear of wheel end components. The resulting loss of lubricant can lead to potential wheel loss.

Hagelthorn also explained bearing-on-axle rotation when the inner race of a tapered roller bearing rotates on the journal of the axle spindle. Finding Solutions

Rather Engineering conducted a survey of 111 full members of The Maintenance Council in 1984 to determine if development of a new axle nut system would be justified. The 13-year-old survey showed a definite trend in the attitudes of Maintenance Council members. The survey showed that 55% of respondents had been involved personally in a wheel loss accident. Of these incidents, 80% were attributed to overheated bearings, likely resulting from a loss of lubricant.

Hagelthorn surveyed full fleet members of The Maintenance Council again in the spring of 1997. The latest survey asked opinions of members regarding preload installation of tapered roller bearings on trailer axles. Respondents were clear in their message to manufacturers, with 97% stating that manufacturers should advise how much preload their tapered roller bearings will safely withstand. The same percentage stated that if preload is acceptable, mechanics can be taught how to install bearings with preload. The survey also sought to find the opinion of members regarding the most important advantage to be gained by improved wheel bearing adjustment. "Reduction of possible wheel loss" was ranked first 51% of the time by survey respondents followed by "oil seal service life."

Several manufacturers have developed axle nut designs that can control the amount of internal preload on the bearings. According to Hagelthorn, these systems are of little value to the industry in applying controlled preload until bearing manufacturers come fourth with information stating the maximum preload that may be applied to their bearings.

In support of this idea Hagelthorn presented the findings of Rather Engineering on tapered roller bearing axial load pressures with clamping forces introduced by tightening the axle nut. According to Hagelthorn, manufacturers of wheel end components will be able to develop appropriate installation instructions for their products using the information from this testing.

In an effort to correlate internal axial bearing pressure with the physical tightness of the axle nut, Rather Engineering built a test stand and developed testing procedures that allowed rotation of the wheel assembly while measuring internal bearing pressure applied by tightening the nut system. The test stand included the following:

1. A base structure supporting a horizontally mounted axle spindle with sufficient clearance for a wheel or hub (machined to reduce its diameter).

2. A matched set of bearings and cups specified by the axle and wheel manufacturers.

3. The transducer mounting ring with load cell (transducers) and signal instrumentation.

4. A chain drive with sprockets adapted for mounting to the wheel or hub driven by a gear reducer with a flexible coupling to an electric drive motor.

5. An axle nut system to be evaluated.

Using a patent-pending axle nut, Rather Engineering performed calibrated testing of the axle nut torquing.

Testing yielded an average preload of 463.2 pounds of force when axle nuts were tightened to 25 ft-lb. A maximum deviation of 58.8 pounds from the mean was recorded over the course of ten installations. "An average deviation of only 7.8% proves consistency in preload pressure values when using a constant torque setting to tighten axle nuts," says Hagelthorn.

With reasonable control of preload pressure thus demonstrated, Hagelthorn hopes that tapered roller bearing manufacturers will be prompted to issue advisories of acceptable preload limitations, promoting axle nut manufacturers to publish recommendations for addressing the limitations. "Mechanics can then be taught reliable wheel bearing adjustment," said Hagelthorn.

Testing New Technologies Stemco Incorporated, a seal manufacturer, presented a design strategy including the utilization of preloaded bearings and a bearing spacer. Braun stated several benefits of using bearing preload, including maximizing of bearing life by reducing clearances that can create uneven loading. Axial clearances can create radial motion that directly impacts a seal's ability to track properly, according to Braun.

Stemco conducted testing using data analysis equipment to determine the results of seal displacement due to axial clearances around bearings. Tests were conducted on a unitized hub with a bearing spacer and a conventional wheel end system preloaded to 50 ft-lb minus one-half turn at the nut. Initial analysis showed considerably less movement from the unitized hub than from the conventional assembly. Endplay in the conventional assembly resulted in 15 to 20 times greater seal motion. Using a bearing spacer can increase bearing life by more directly controlling the load path around the bearings.

Stemco's research also showed that centrifugal force directs oil out of bearings and into the hub cavity at high speed. To solve this problem, Stemco designed a stationary vane that takes advantage of the hub's rotational energy to redirect oil into bearings. Qualitative road testing showed a 17-degree reduction in temperature that can result in a 48% longer seal life, according to Braun

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