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A comparison of bearing life in new and refurbished railway axle boxes

Toowoomba Foundry / Koyo Australia Pty Ltd

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Toowoomba Foundry


Koyo

Introduction

Australian railway companies commonly employ an axle box and spherical type roller bearing assembly for railway wheel axle journal applications. Two types of axle boxes are shown below.

Axle boxes

Railway axle box and journal assemblies are fitted to rolling stock, sealed and operated on a non-field service maintenance system for a period of approximately five (5) years. During this period the average distance travelled is estimated to be 800,000 kilometres.

After the specified period of operation, the wheel sets are withdrawn from rail traffic for axle box removal, disassembly and inspection.

Both axle boxes and bearing assemblies are subject to strict dimensional and wear tolerance guidelines, and components falling outside of the tolerance envelope are scrapped.

One main area of axle box wear is to the internal bore of the axle box body. In recent years and as an attempt to reduce operating costs, some railway operators have embarked upon a policy of axle box reclamation. The internal bore dimensions have been restored by either the fitting of an internal sleeve, or by the hot spray application of a molten metal to the axle box bore. With the latter method of bore restoration, the metal layer is built up to a suitable oversize dimension and the bore is then machined to tolerance. The axle box is then returned for normal application use.

Background information

Australian railway companies operate rolling-stock to standards and specifications determined by the Association of American Railroads (A. A. R.).

In Australia, the railway track system gauge (width between rail centres) is not standard and varies between individual state railway operators. The three railway track gauge standards accepted in Australia are identified in Table 1 below.

Gauge Track width Operators State
Narrow 3' 6" (1067mm) Queensland Rail Queensland
Standard 4' 8 .5" (1435mm) Maintrain New South Wales
Broad 5' 3" (1600mm) Clyde Engineering Victoria

The gauge system employed will determine several rolling-stock design considerations. Of primary consideration is the permissible maximum operating mass, physical size and the operating speed of railway locomotives, passenger carrying and freight carrying rolling-stock.

Railway axle box housings are cast in Spheroidal Graphite (S. G.) Iron (Australian standard grade 500/7), which has a malleable metallic structure with elastic properties, able to absorb shock loading over a wide range of operating temperatures.

The railway axle box assembly comprise a backing ring and two (2) spherical roller bearing units separated by a distance piece, press mounted onto a wheel axle journal. Bearings are then precisely packed with a grease lubricant before the axle box body is fitted (transition fit) and environmentally sealed with a back cover. The back cover is fitted in two halves and interlocks via a labyrinth seal arrangement with the backing ring. Figure 1 illustrates a typical axle box arrangement.

Axle box
Figure 1: A typical axle box.

The mounting of railway journal bearings is a critical operation, and the correct seating of all components is important to ensure correct alignment between assembly components.

The wheel set and axle box assemblies are mounted into a bogie pedestal and secured into position with a keeper plate. See Figure 2.

Axle box mounting
Figure 2: Axle box mounting.

Axle box operation

Bearing inner rings are effectively stationary (fixed tight) on the wheel axle journal. The bearing outer ring turns relative to the axle. The bearing outer ring is held stationary inside the axle box by the combined mass of the bogie and the mounted vehicle; typically, a force of between 12~18 tonnes is exerted onto each wheel set. The bearing outer ring may turn or creep very slowly inside the axle box under the influence of the rotational forces generated.

Some critical factors affecting bearing & axle box life

  1. Correct seating of components during axle box assembly and internal component tolerances (including axle box bore).
  2. Stability and purity of grease lubricant. (Contamination by water will cause grease to break down and foreign material particles will cause internal wear).
  3. Effectiveness of axle box environmental sealing. (Corrosion/mechanical abrasion caused by moisture/dust ingress)
  4. Maximum operating temperatures. (Normal operating temperature 55°~60°C).
  5. Accurate alignment of wheel set (axle assembly) in bogie pedestal. (Some slight misalignment will be absorbed by the bearing design features).
  6. Shock loading. (The hammering action caused by a flat spot on the wheel tyre area can generate extreme shock loads, leading to the catastrophic mechanical failure of axle boxes and bearing assemblies).

MISG project objectives

To compare the difference in journal bearing operational life in new and refurbished railway axle boxes. To ascertain the effectiveness of axle box refurbishment by the above mentioned methods, and to compare the mechanical properties of new and refurbished axle boxes.