MAINTENANCE MATTERS


Dont get uptight about bearing torque


Torque is rotational force or the ability to overcome resistance to rotation. It is the cross product of force and radius. Torque is the amount of force applied tangentially to a circle.

Ask any mechanic who works around a wheel hub, and he will likely tell you that "Happiness is a well-adjusted wheel bearing." In recent years, we have all seen a change in bearing technology, which has changed certain aspects of how you service and maintain your coach wheel ends. For your fleet to stay productive, your service team needs to be aware of all of the new bearing designs; their features and benefits; and required service procedures.

Likewise, methods of locking the wheel-bearing pack into adjustment have evolved. In addition to the conventional double-nut locking designs, we now have the PRO-TORQ™ locking device. The Pro-Torque™ nut is a six- or eight-sided nut that is torqued into place; a mechanical locking spring is then installed into the body of the nut, locking it into position on the spindle.

This month's Maintenance Matters will look at the three bearing and two locking-system designs currently in use in MCI coaches, along with the inspection, adjustment, and service procedures for each.

LOCKING MECHANISMS

Once the bearings and hub assembly are installed and adjusted, a locking nut system is used to ensure that the adjustment does not change during operation. MCI currently uses both the conventional double-nut locking system and the Pro-Torq™ nut system. Both are very secure locking systems but employ different locking methods. The two locking systems are different and are not interchangeable.

Conventional

The conventional double-nut system uses two spindle nuts separated by one or more locking washers. One of these washers generally has a bendable tab that is used to secure the outer nut from loosening, while an optional second washer may have a series of alignment holes that index onto a pin on the inner nut. When the outer nut is tightened, it acts as a jam nut holding the inner nut in position.

Pro-Torq™

The Pro-Torq™ nut system uses a single nut and a spring steel keeper ring to lock the nut into adjusted position in place of the multi-piece double-nut system.

Once the bearing package is installed and adjusted, the keeper engages into both the index flat or slot on the axle and into the keeper groove machined into the face of the Pro-Torq™ nut.

The design of the Pro-Torq™ nut uses both six- and eight-sided axle nut sockets currently available from most tool suppliers. The spring steel keeper should be carefully inspected before each use and replaced if necessary. Caution must also be taken to ensure that the ring is fully seated into the nut locking groove.

WHEEL END ASSEMBLIES

MCI currently uses three wheel bearing configurations in our modern wheel-end assemblies, as supplied by the axle manufacturer:

  • Conventional Bearing
  • Pre-Set Bearing
  • Unitized or U.W.E. (Unitized Wheel End)

Conventional

This is the standard inner/outer tapered roller bearing configuration that we all grew up with and have grown to love for its simple design and ease of service. Positive camber helps the larger inner bearing (cone) accept most of the vehicle load with its larger surface area. The smaller outer bearing accepts the remainder of the rolling load and keeps the hub stable and in place on the spindle.

During hub installation and bearing adjustment, a small amount of "free play" is normally adjusted into the bearing pack. This allows for expansion of the bearing assembly during operation as it heats up, and the expansion then removes the free play, placing a minor pre-load on the bearings. For tapered roller type bearings, this is an optimum operating condition. The bearings may be either oil or grease lubricated.

Inspection

At routine intervals as normally prescribed by the axle manufacturer, inspect the bearing assembly for operating condition. This generally involves the service technician rotating the hub assembly while checking for smooth and quiet rotation of the hub, and measuring the free play or end play of the bearing and hub. If the bearing end play measures beyond the prescribed limits, it should be adjusted. Any roughness or noise in the bearings will require that the bearing and hub components be serviced as needed.

Measuring End Play

With the coach securely supported and wheel(s) removed:

  • Remove the hubcap or dust cover.
  • Mount a magnetic base dial indicator to the spindle, and zero the indicator against the hubcap mounting surface.
  • Push and pull the hub assembly straight in and out while reading the indicator needle.
Readings beyond specifications require bearing service.

Adjustment (always consult the appropriate maintenance manual for your coach for exact specifications and procedures). As mentioned, adjusting a conventional tapered roller bearing arrangement is performed to allow for expansion of the bearing pack during operation.

  • Once the lubricated bearings are installed and the hub assembled onto the axle, torque the bearing nut to approximately 100 foot pounds while rotating the hub assembly. This will force the seating of the bearing pack and ensure that all components are centered correctly.
  • Back off on the bearing nut at one turn and then lightly seat again, at 50 foot pounds while rotating the hub assembly.
  • Back off a specified amount (at ⅛ to ½ turn) and install nut locking mechanism.
  • Check end play (as above) which should be .001" to .005". Re-adjust if necessary.

Ensure that the oil-filled hub is filled with the proper lubricant.

Service

At routine intervals as normally prescribed by the axle manufacturer, disassemble, clean, inspect, and then reassemble and adjust the entire bearing package. Any worn or damaged parts should be replaced with new ones during reassembly.

Pre-Set

The Pre-Set bearing design is a low-maintenance assembly. Like conventional bearings, Pre-Set bearings use the tapered roller design. Unlike the conventional bearing, it also uses a precision spacer between the inner and outer bearings, and it is used in conjunction with a precision-machined hub assembly. When installed, the spacer correctly positions the bearings' seating depth into the bearing cups, automatically setting the correct bearing preload onto the assembly. No adjustment is necessary.

Inspection

At routine intervals as prescribed by the axle manufacturer, inspect the bearing assembly for operating condition. This generally has the service technician rotating the hub assembly while checking for smooth and quiet rotation of the hub, and measuring the free play or end play of the bearing and hub. If the bearing end play measures beyond the prescribed limits, it should be adjusted. Any roughness or noise in the bearings will require that the bearing and hub components be serviced as needed.


Adjustment

 
(always consult the appropriate maintenance manual for your coach for exact specifications and procedures)

As mentioned earlier; the Pre-Set bearing requires no adjustment since the bearing preload is set by the spacer.

  • Torque the inner (or Pro-Torq™) bearing nut to 300 foot pounds while rotating the hub.
  • Check end play, which should be .001" to .005". Readjust if necessary.
  • If end play is not within specifications, the bearing assembly and/or spacer should be disassembled, cleaned, inspected and replaced as needed.
  • Ensure that the oil-filled hub is filled with the proper lubricant.

Service

At routine intervals as normally prescribed by the axle manufacturer, disassemble, clean, inspect, and then reassemble and adjust the entire bearing package. Any worn or damaged parts should be replaced with new ones during reassembly. Note that the spacer may be replaced alone if needed. Replacing the bearings requires replacement of all components, bearings, cups and spacer.

Unitized

The Unitized Wheel End (UWE) is a maintenance-free complete hub assembly that uses a pre-lubricated and sealed bearing assembly. The hub assembly is composed of a wheel-end adaptor, rotor and unitized hub bearing mounted to the spindle. The bearing assembly requires no maintenance and is not adjustable. Periodic inspection is required to maintain performance.

Inspection

As with all other bearing assemblies, rotate the hub assembly while checking for noise or rough operation. Check end play of the bearing assembly as described above. Any roughness, noise, or end play will require that the bearing assembly be serviced.

Adjustment

(always consult the appropriate maintenance manual for your coach for exact specifications and procedures)

The UWE wheel end is a nonadjustable bearing assembly. Any end play of the bearing requires that the bearing be serviced.

Once the assembled UWE bearing is mounted to the spindle, the spindle nut is torqued to 600 foot pounds.

Service

The UWE wheel bearing assembly is a non-serviceable unit. Any roughness, noise or end play requires that the bearing assembly be replaced.

Always consult your MCI Maintenance Manual or Arvin-Meritor manual MM-0409 for specific information, procedures and specifications. The Arvin-Meritor manual is available by calling MCI Customer Service at 800-323-1290.

The FYI from MCI editorial staff values your feedback. Please e-mail any suggestions, comments, or ideas for future articles to fyi@mcicoach.com.

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