Saturday, April 30, 2011

Asymmetric wheel flange wear: A new twist

Written by  Harry Tournay, TTCI, for Railway Age

TTCI seeks a remedy for uneven flange profiles.

As part of a research project funded by the Association of American Railroads, Transportation Technology Center, Inc. (TTCI) researchers determined that coal gondola and hopper cars are reporting an increased incidence of wheel removals due to thin flanges. This appears to be a recent phenomenon associated with cars built after 2004. Cars of this vintage are generally of lighter tare (aluminum construction), have been fitted with an improved-steering (M-976) truck, and have a slightly increased brake ratio.

CRB (Car Repair Billing) data (Figure 1) confirms this information, suggesting that the incidence of WM 60 (thin flange) removals on gondola coal cars has increased from approximately 12% in 2010 to approximately 52% of all wheel removals post-2004. Removals on coal hoppers have, for the same time periods, increased from approximately 12% to approximately 27%.

CRB data, car inspections, and analysis of wheel profile monitoring data reveal that wheelsets replaced for thin flanges invariably have one wheel of the wheelset with a flange on minimum thickness and the opposing flange on the same wheelset on maximum thickness, a condition termed AWFW (asymmetric wheel flange wear).

Teardowns indicate, as Figure 2 depicts, that flange wear is associated with lateral migration of the brake beams in opposite directions in each truck and the orientation of the truck brake pull rod and reaction link of rod-through truck brake rigging.

Further analysis of CRB and owner repair data shows AWFW was present in 809 cars built in 2000 and fitted with three-piece trucks, prior to the increase in brake ratio. It is, though, at a lower rate of approximately 12% of all wheelset removals.

The data also shows AWFW does not occur on the A-trucks of a fleet of 639 cars built in 1997 and fitted with truck-mounted brakes, but does occur on the B-truck. This is presumed to be due to eccentric forces associated with the handbrake pull rod.

Investigations completed by TTCI suggest:

• The asymmetry of the forces on the truck brake rigging results in opposing lateral forces on the brake beams.

• These forces, together with the clearances between the rigging and the truck frame and the “gap” between the brake block and the flange, result in lateral migration of the block on the wheel tread (Figure 3).

• The abrasiveness of the shoe on the wheel tread wears the tread asymmetrically, so the truck loses its tracking ability (Figure 4).

This hypothesis goes a long way to explain the observations in service. However, it does not explain why AWFW has increased since approximately 2004, why it is associated with coal cars, and why it is more prevalent on gondola coal cars than on coal hopper cars.

It is possible that the asymmetric brake forces influence the rotation of the truck under the carbody under light car conditions. This possibility and others are part of a continuing investigation through further data analysis and test by TTCI into the root causes of AWFW.