New bearing design improves reliability, extends lifeWhen railroad operators promote their ability to get customer shipments to their destinations on time, it’s more than just good marketing. For America’s railroad companies, the ability to keep locomotives operating smoothly represents a key competitive advantage. An unscheduled interruption of service involving just a single locomotive can cost a railroad upwards of $25,000 – including labor cost, lost revenue, and subsequent repair or replacement costs.
   Among its many, many components, the diesel-electric freight locomotive depends heavily on reliable traction motor bearings to stay on track. Locomotives and these large, extremely powerful DC motors place significantly higher stress levels on bearings than their counterparts of 10 years ago. For instance, older generation bearings were designed to accommodate 3,000 horsepower locomotives. Today, many models are rated as high as 4,400 horsepower. As the horsepower increases, so do the demands on the bearings – which has a direct negative impact on reliability and service life.

   Given its need for high reliability, the severe financial consequences of an unscheduled breakdown, and the pressures exacted by ever increasing horsepower, the railroad industry clearly needed a new generation bearing – one that could meet the challenge of increasing performance standards. Working closely with one of the largest locomotive builders in the world, the Electro-Motive Division (EMD) of General Motors, SKF created such a bearing: the redesigned Traction Motor Bearing. This bearing is specifically designed for use in North American diesel-electric freight locomotives with DC motor drives.
Responsive approach
Realizing the need to improve bearing performance, a team from SKF and EMD engineers tackled the issue of how to create a bearing that could withstand the loads and harsh conditions of the locomotive application. Over a two-year period, SKF researched the various options and tested alternatives, until a newly designed traction motor bearing was developed.
   Introduced in 1998, the new design incorporates features that are of particular interest to railroad companies and the motor repair shops that service their equipment. At the heart of the design is a redesigned roller with a logarithmic profile, used on the pinion end bearing. Why did SKF hone in on the roller design?

   “The roller is typically under tremendous stress,” explains David Lease, director of North American Railway Business. “This is especially true for the pinion end bearing, which is subject to high speeds, high temperatures, and particularly, combinations of heavy load and misalignment.” Standard, cylindrical-shaped rollers create high pressure at the point where they come into contact with the bearing ring. The use of a logarithmic design helps to relieve some of that pressure, Lease explains.
   “Unlike conventional crown rollers, the contour of the new roller simulates a mathematical logarithmic curve,” he says. “The roller surface is flat in the center, then drops away at an increasing rate as it approaches the ends.”
   The benefit of this roller design is threefold. First, it provides a wider, more evenly distributed area of contact between the roller and the pathway. Second, as the crown drops at the roller ends, it minimizes stresses at the edges of the roller. And third, it provides a continuous surface from end-to-end – with no intersections or discontinuities that could be subject to higher stress.
Longer life, lower cost
As a result of these roller design improvements, the bearing is better able to withstand the heavy load and misalignment conditions characteristic of pinion-end service. And that translates to improved reliability and significantly longer service life. In fact, Lease says the new pinion-end bearings have been designed for service lives as long as 2 million miles under “heavy haul” conditions, or double the life of their predecessors.
   “The ability to nearly double traction motor bearing service life can mean significant savings for railroad companies and motor repair firms,” he notes.
   A tangential, but equally important benefit resulting from the new roller design is lower operating cost. The logarithmic design eliminates the need for premium, expensive ring materials or supplementary pathway crowns – options previously used to help minimize bearing stress and to improve reliability. To the railroad, the new roller means that SKF can supply a bearing that provides twice the reliability with no increase in price.
Customer advantages
Beyond the advantages of the roller design, the new traction motor bearings offer other advantages for customers. For instance, the bearings yield less operational noise thanks to new pathway “waviness” limits and they experience less vibration as a result of their reduced radial runout. The new design also means the bearing will operate at lower temperatures – with reductions of up to 14 degrees noted in testing against the previous design.
   While much of the innovation behind the new traction motor bearings centers around the pinion-end bearing – where a great deal of the stress is focused – the design also incorporates improvements in the commutator-end bearing. The fall of 1999 will mark the launch of a new polyamide cage for commutator end bearings – a design optimized for cooler operation, higher reliability, and lower cost.
   “By replacing the brass cage with a polyamide alternative, we can provide customers with greater performance at a better price,” Lease explains. Polyamide 6.6 was deemed the material of choice for the new cage, based on the results of a four-year study in which various non-metallic materials were evaluated against conventional brass cages. The study revealed the potential for significant advantages through the use of polyamide.
   For instance, operating temperatures with a polyamide cage average 25 percent less than with a brass cage. Wear, running noise, drag and friction are also reduced, while corrosion is virtually eliminated. And the polyamide cages are capable of withstanding rapid acceleration and shock loads more readily than their brass counterparts. The result is a commutator-end bearing cage with a much longer service life than a typical brass cage.
   “In testing, we found that the new cages performed without a problem in over 250,000 miles of service,” Lease says. “Compared to conventional brass cages, wear was virtually eliminated.”
   While the development of the new traction motor bearings was spurred by a specific need within the Electro-Motive Division of General Motors, it has since been quickly adopted by most railroads and major motor repair shops. “Given the very tangible and significant benefits,” Lease says, “it’s not difficult to see why.”
   “Reliability and continued operation are important for any industry – particularly one where the costs associated with “road failures” are so high,” he adds. “The new traction motor bearings give railroad operators and motor repair firms the ability to greatly extend service life and keep diesel locomotives on track much longer, without incurring a higher cost.”