Figure 1: Percentage breakdown of certain service defect categories.

Bearing Retention

Bearing retention is the ability of the bearing components to remain properly clamped together on the axle journal. This is accomplished through both radial and lateral clamp on the journal. Radial clamp is achieved by an interference fit between the bearing’s inner races and the axle journal. This type of clamp depends upon the components’ dimensions and is established during the bearing mounting process. Lateral clamp is achieved through torquing the bearing’s cap screws. Cap screw torque is translated into a lateral force (as seen in Figure 2) which keeps the bearing component stack secured tightly on the axle journal. Sufficient radial and lateral clamp essentially eliminates the possibility of loose bearing components and their associated problems such as coneface and journal fretting and overheated bearings.

Figure 2: Illustration of bearing radial and lateral clamp.

Some changes in AAR rules that have improved bearing clamp, initiated by Brenco’s research efforts, are:

AAR Circular Date Description of Change

C-7269 3/88 Prohibited the use of cap screw seal rings in new And reconditioned bearings.
C-7286 6/88 Mandatory use of cap screw thread gage in Reconditioning shops.
C-7384 5/89 Mandatory lubrication of second-hand cap screws.
C-7404 7/89 Mandatory lubrication of all new cap screws.
Pending Increase in allowable torque value.

While previous Technical Forums and bearing clamp studies have focused on bearing components and journal sizes, our most recent study focused on the relationship between axle bolt holes and lateral clamp.

Brenco’s Investigation

Rule 1K4 of the AAR Wheel and Axle Manual, Section G – Part II reads as follows,

"…….Axle ends must be clean and free of dirt and contaminants. Visually inspect tapped holes in the axle ends for thread damage, dirt and corrosion."

And Rule 1C5,

"For new axles bearing machined, for axles being converted to a smaller size, or for reclaiming axles with defective cap screw holes, see 2A4 (e).
Suitable jigs, fixtures or machines must be used to correctly locate and
Guide the drill or drills when drilling cap screw holes in the ends of axle.
Taps should be sharp to insure good threads. Clean holes after tapping.
Check hole location, using gage shown in Figure 5.8 and check threads
With standard thread gage."

This study/survey was conducted to evaluate axle bolt hole conditions in service today. A secondary goal was to quantitatively show how these conditions could affect bearing lateral clamp.

Visual Inspections

A total of 503 axle journals (30% new) were surveyed. Conditions identified as possible deterrents to sufficient lateral clamp are as follows:

1. Metal chips/dust in the axle bolt holes.
2. The absence of protective lubrication (rust preventative) on the internal threads (i.e. oil/grease film).
3. Defective internal threads (i.e. nicks, high spots, etc.).
4. Rust on the threads
   Note: Although applying a rust preventative to axle bolt holes is not an AAR mandatory practice, Brenco sees this as bearing essential to preventing axle bolt hole corrosion.
   
   Dimensional Inspections
5. Rejection of a Go Gauge (undersized thread diameter or obstruction).
6. Acceptance of a no-Go Gauge (oversized thread diameter).

Photographs of bolt hole conditions appear in Figures 3 – 6.

Figure 3: Metal Dust	Figure 4: Rust on Internal Threads
Figure 5: Metal Chip	Figure 6: Thread Obstruction

Survey data of the axle bolt hole conditions was obtained using a digital borescope, thread No-Go and Go Gauges, and a high intensity light source. (Fig. 7) In order to quantitatively show the effects these conditions could have on lateral clamp, a clamp-load analysis was conducted. Lateral clamp on an axle with contaminated axle bolt holes was compared to the clamp measured after these bolt holes had been retapped and cleaned. The majority of the conditions identified can cause unwanted cap screw friction and associated reduction in bearing clamp. Approximately 90% of the torque applied to the cap screw is consumed overcoming the cap screw frictional resistance. Any increase in the frictional resistance caused by the above conditions will reduce the bearing lateral clamp force.

Figure 7: Digital Borescope on location

Digital images of bolt conditions recorded by a borescope appear in Figs. 8 – 12.

Figure 8: New threads

Figure 9: Damaged internal threads

Figure 10: Thread Highspot

Figure 11: Metal Chip

Figure 12: Internal thread rust

Results

The following results have been broken down into survey conditions and are represented in percentages for ease of interpretation. Figures 13 and 14 represent axle bolt hole visual and dimensional inspection data for the population and 503 axle journals. The rejection of a Go Gauge and the absence of a protective lubricant were found to be the two most frequent undesirable survey conditions.

Figure 13: Axle Journal survey results - Visual Inspection Percentages

Rejection of a thread Go Gauge indicated that the axle bolt hole was undersized contained defective threads or was contaminated in some manner. Protective lubricant on the axle bolt hole threads is needed to ensure that rust does not form and to assist in the application of the cap screws. As Figure 14 shows for used axles, rejection of the Go Gauge by at least one bolt hole occurred in 52% of the axle journals. Although less frequent, the other bolt hole conditions can still impact bearing performance. Of particular significance is the 20% of used axles containing metal dust and chips, as seen in Figure 13. If any one of the survey conditions are present within the axle bolt hole, the potential exists for a reduction of lateral clamp.

Figure 14: Axle Journal survey results - Dimensional Inspection Percentages

This research has shown that cleaning or retapping defective axle bolt holes increases bearing lateral clamp. Figure 15 displays a chart of clamp load changes from axle bolt holes with contaminants or tight holes, to bolt holes that have been retapped and cleaned thoroughly.

Figure 14: Axle Journal survey results - Clamp Load Analysis

The average increase in lateral clamp was 7,000 lbs., with the largest increase being 17,000 lbs. As measured at the bearing spacer. As previously mentioned, loose bearings caused at least 42% of the bearing setouts in Brenco data since 1988.

A recent procedure implemented to ensure that bearings retain proper retention during service is a bearing retorquing program. This program, initiated by CSX Transportation and adopted by other railroad and private car owners, removes from service bearings that have not retained at least 80% of their original applied torque. Since increases of up to 20% were measured during our clamp load analysis, bearings could be placed in service with an applied clamp 20% less than desired, potentially allowing the bearing to loosen in service.

Summary

Inspection for these axle bolt hole conditions were conducted at a variety of wheelshops in the railroad industry. Two railroad wheelshops, two large independent wheelshops and one small independent wheelshop were visited during the survey. Each shop stored, handled and processed their used and new axles in very different ways. Some shops showed higher quantitites of a certain bolt hole condition than others, but all were identified for potential improvements relating to axle bolt hole conditions.

Recognizing that sufficiently clamped bearings can greatly reduce the possibility of bearing distress due to loose components, greater attention by the railroad industry to axle bolt hole conditions can have economic, performance and safety benefits. Simple steps to reduce the unwanted friction in the axle bolt hole alone can have significant effects upon bearing lateral clamp and be a key factor in improving bearing performance.

Recommendations

Some steps that are recommended at the wheel shop level to ensure optimum axle bolt hole conditions are:

1. Wash and clean with a wire brush all used axle journal bolt holes followed by flushing the holes with a rust inhibiting solvent.
2. When storing axle (outside or inside), coat journals as usual but also spray an oil or rust inhibitor inside the bolt holes to inhibit rusting.
3. Inspect the bolt holes of all used and new axles with Go and No-Go thread gauges. Retap tight or undersized bolt holes, then clean threads after tapping as described above.
4. Retap rusty bolt threads the clean threads after tapping as described above.
5. AAR actions should include issuing specific instructions to be followed for certain axle bolt hole conditions.

Following these recommended practices can eliminate another variable that detracts from proper bearing clamp and will help improve the quality of axle journals returning to service.

The Technical Forum is an information resource for the rail industry and is provided as a courtesy of Amsted Rail Group. Suggestions, inquiries or comments are welcomed and should be directed to:

Editor, Technical Forum
BRENCO, Incorporated
P.O. Box 389
Petersburg, Virginia 23804
804-863-1713

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