Metal-To-Metal Maintenance

Joel B. Marsel

Introduction:

This paper is intended to provide general information and describe routine maintenance issues involved in the upkeep of any rapid or mass transit system built on technology derived from the “railroad”, that is a system in which steel flanged wheels ride on steel running rails, regardless of size.  For actual industry standards and specifications, refer to the AAR (Association of American Railroads). 

 http://www.aar.org/

Rail Gauge:

Rail Gauge is the spacing between the two running rails of a railway system. Maintaining proper gauge, or rail spacing, is essential to the safe and reliable operation of any rail system. The relationship between the outer wheel flange-to-wheel flange axle width and the inner rail ball-to-rail ball width must be maintained to ensure that the rail vehicle has adequate clearance while traveling upon the rails.  Typically the width between the rails is just wide enough to permit the vehicles wheels to pass without interference from the rails themselves. At worst case a wide rail gauge would permit the wheels to fall between the rails. When the width of the rails becomes too wide, the vehicle can oscillate or pinball back and forth between the rails. An extreme case of this effect can cause the flanged wheels to jump over the ball of the rail causing a derailment situation.  If rail gauge becomes too narrow, the wheels and the rails have an “interference” fit relationship. In this case the wheels and/or the rails will shear material from each other.  This condition is undesirable not only because of the physical wear to the rail and wheel components, but a worst case scenario of this condition can cause the wheels to climb over the ball of the rail, once again resulting in a derailment situation.  This condition also produces an audible metal-to-metal squeal that can disturb nearby communities and alarm passengers.  Whether fastened to a tie and ballast base or to concrete super-structure via steel or rubber pads with spring clip retainers, rails will shift position slightly during normal rail traffic operation. It is therefore essential that the gauge of the rails be checked and corrected on a routine basis.

Rail Wear:

The very nature of two relatively hard surfaces coming in constant contact with each other ensures considerable wear to one or both of the surfaces.  In a rail based transit system the rail surfaces can wear in several ways. 

First, the top surface of the rail in which the vehicles weight primarily rests, will flatten or “mushroom” over time from bearing the load of the rail vehicles. Rails must be re-surfaced periodically to ensure a smooth ride for passengers. Rails must be also be tested periodically via x-ray or ultra-sonic techniques to check for metal fatigue and stress cracking, which can lead to actual, breaks in the rail structure itself.

Secondly, the sides of the rail ball will wear as a result of contact with the vehicle wheels themselves.  The wheels can act much the same as a milling machine, actually removing material from the rails with every pass. This effect is most prominent on the inside edge of the outside rail on any curve in the track right of way on a high-speed rail system.  As a rail vehicle navigates the curve, the majority of the vehicle’s weight is thrown to the outside of the curve. The inside surface of the rail ball on the outside rail is essentially the only thing holding the vehicle in place, and therefore takes the full brunt of the vehicles weight. The result is the rapid loss of material from this rail edge. This loss of material makes re-gauging the rails necessary as the rails wear effectively increasing rail gauge. Depending on the vehicle weight, speed, frequency of travel, and metal consistency or “hardness” of the wheels and rails,  rails themselves will wear out and need to be replaced.  In a typical high-speed rapid transit system, rails are replaced on regular schedules to maintain safety standards.  Some rail sections may last years, while others last only a few short months between replacements.

Wheel Wear:

When rail conditions become slippery due to weather such as rain or ice, or simply morning dew upon the rails, rail vehicles have a tendency to slide or skid along the rails.  The results of this action are “flat spots” on the wheels. These flat spots cause uneven rotation of the wheels upon the rails.  The most notable side effect of this is the famous “clackety-clack” sound of the wheels on the rails as the vehicle travels down the right of way. Although minor flat spots may not substantially affect vehicle safety, they are a substantial cause of noise. This repetitious sound can be quite undesirable close in proximity to modestly and heavily populated suburban communities.  To correct this condition, the wheels are machined or “turned” down in diameter until they are once again fully round in shape. This process is similar to the re-surfacing of brake rotors on passenger cars, although the equipment necessary to complete this operation is a bit more substantial.  Wheels are manufactured in various diameter sizes. At some point, the wheel diameter falls below the minimum acceptable diameter (as determined by the AAR- Association of American Railroads) after re-surfacing and must be discarded.  For example, a wheel is manufactured to a diameter of 28.00 inches. A slide results in a 1.00- inch “flat spot” on the wheel. The wheel must be “turned” down to approximately 27.940 inches to remove the flat spot and restore the wheel to a true round shape. In the re-surfacing process, the amount of material removal necessary to eliminate repeated 1.00-inch flat spots increases as the wheel diameter decreases.  The frequency and severity of these “flat spots” is dependant on many factors, such as the weight of the vehicle, temperature, and relative wetness of the rails. In some geographical locations these slides and “flat spots” are a daily occurrence, which can reduce the life span of metal-flanged type wheels considerably.  Wheels also have limits on “hours of service” and require replacement when these periods are exceeded. 

About the Author:

Joel Marsel held employment with the Washington Metropolitan Area Transit Authority (MetroRail) mass transit system in Washington D.C. from 1981 until 1997. Mr. Marsel has had extensive firsthand experience with rail based transit system maintenance. He has performed preventive and corrective maintenance on track and structures, rail switching devices, electro-mechanical controls, railcar electronics & propulsion systems, Automatic Train Control Systems, and Automatic Fare Card Systems.  Mr. Marsel is now a Printed Circuit Board Design Engineer and resides in Austin Texas.

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