To grind, or not to grind? Among other benefits, rail grinding can extend the service life of one of the biggest-ticket items in a railroad's m/w budget - Brief Article - Industry Overview

Railway Age, Nov, 2002 by Tom Judge

Rail can account for as much as 60% of a railroad's total asset value, and replacing worn or defective rail can cost anywhere from $250,000 to $400,000 per mile. "Obviously, it's to a railroad's advantage to defer rail replacement as much as possible without incurring risk," says Fred Prahl of the National Research Council of Canada.

Rail life is determined by wear, contact fatigue damage, and internal defects. Most wear occurs at the gauge face in gentle as well as sharp curves. Some vertical head wear is caused by wheel contact, but often most is due to corrective grinding.

"Ironically, rail that does not wear will eventually succumb to rolling contact fatigue, where microcracks initiate on the surface, and then propagate laterally and vertically," Prahl says. "Some cracks may penetrate the railhead and contribute to a transverse defect and broken rail. Most will grow to a characteristic depth (5mm to 15mm, 0.04 to 0.06 inches) and then travel horizontally, linking up with others and causing material to spall on the rail surface. Maximum rail life is achieved when there is just sufficient wear from wheel/rail contacts and/or a properly designed rail grinding program to control contact fatigue.

"Effective rail grinding has two main goals," Prahl says. "The first is shaping the rail to the desired profile, one which will mate with wheel profiles to improve steering through curves, minimize contact stress, control lateral forces, reduce hunting, and inhibit corrugation formation. The second is achieving the so-called 'magic wear rate,' which means removing just enough metal to prevent the initiation of rolling contact fatigue defects on the rail surface without wasting serviceable rail steel. Combined with a proper lubrication program, a carefully planned grinding program can extend rail life from 50% to 300%."

Whether a system is freight, passenger, or transit, rail profiles must be designed to match the distribution of new and worn wheels that use it. "The optimum rail profiles will usually be different in curves and in tangent track," says Prahl. "Properly mated rail and wheel profiles can tremendously reduce the stress state, thereby minimizing damage and contributing to significant increases in rail and wheel life. During curving, the stresses at the gauge corner are often twice as high as those between the rail crown and the wheel tread. To reduce contact stresses at the gauge corner and the gauge shoulder in curves, the wheel/rail profiles should be conformal, meaning the gap between the profiles is less than .020 inches at the center of the rail (in one-point contact) or at the gauge corner (in two-point contact)."

Another important benefit of conformal contact in curves is that a relatively large rolling radius difference can be achieved between the wheel on the high rail and that on the low rail, without unduly increasing wear, "This improves wheelset steering, which in turn reduces the lateral load and wear at both the gauge corner and top of rail," says Prahl. "In tangent track, however, conformality between wheels and rails is not a good thing as it leads to hunting and short-pitch corrugations. This can be avoided by controlling the crown radius on the head of tangent rail to between 200mm and 250mm (eight to ten inches). In some systems it may be necessary to actively vary the lateral position of the contact band on the rail to spread wear evenly across the wheel tread and retard the development of hollow worn wheels."

"Ideally, rail should only be replaced for wear, which is a safe and predictable process," Prahl says. "But a wear rate that is too lowcan actually decrease rail life. This seeming paradox is because the high contact stresses that exist between rail and wheel cause surface fatigue cracks to initiate that, if not removed by natural wear or grinding, lead to subsequent flaking, spalls, deep-seated shells, and other contact-fatigue defects. When this happens, the rail may fail prematurely due to rolling contact fatigue, never reaching its available wear limits. Preventive grinding (frequent, light, single-pass grinding) simulates a natural wear rate that removes surface microcracks and, at the same time, maintains the rail profile, dresses welds, and removes incipient corrugation without wasting precious rail metal. The 'magic wear rate' is an optimal value in which fatigue and wear are in balance. Its value changes with many variables including track quality, traffic mix, and truck and wheel maintenance. In mos t systems where rail is lubricated, vertical wear is insufficient and grinding is a necessity if rail life is to be maximized."

Machines to do the job

Today's grinding machines have advanced to the point where computers operate the grinding stones to provide precise rail profiles. Environmental impacts have been minimized.

The C-Series rail grinder for switches and crossings from Harsco Track Technologies is a 20-stone unit equipped with Harsco's Jupiter computer control system. Features include dust collection, noise suppression, fire suppression, and improved head controls with the Jupiter system to give increased performance and productivity through increased speed and better control, according to Tony Origer, director-rail grinding operations.