Two key aspects in rail grinding-effectiveness and efficiency; researchers look at the effectiveness and efficiency of rail grinding to prolong rail life and reduce maintenance costs

Railway Track and Structures, Dec, 2004 by Huimin Wu

Effectiveness and efficiency are two key aspects in rail grinding. Effectiveness measures how well the grinding objectives are reached--low-contact stress, good curving performance, high-speed lateral stability and successful surface defect removal. Efficiency is influenced by how the grinding interval and pattern choices affect the cost of grinding.

Rail grinding is commonly used by North American railroads as a maintenance procedure for removing rail corrugations, for surface defect removal and for restoring the shape of the rail to improve wheel and rail interaction.

The total cost of rail life and rail grinding is related to wheel/rail interaction. Reduced surface rolling contact fatigue and wheel/rail wear resulting from favorable wheel/rail contact could extend grinding intervals and reduce metal removal rate, thereby prolonging rail life and reducing maintenance cost.

Establishing an adequate grinding program includes properly designed ground profiles, grinding procedures and wheel/rail interaction monitoring procedures.

Two rail grinding trials conducted by Transportation Technology Center, Inc., on curves of two freight service lines discussed here demonstrate the need to further improve current grinding practices.

Properly-designed ground rail profiles

Rail profiles are generally not ground back to the new rail shapes. The ground rail shapes are important for wheel/rail contact conditions after grinding.

On curves, conformal or close conformal contact is advisable for the high rail. High rails in curves generally become worn to a conformal stage with contacting wheels. This stage is considered as a desirable condition of contact for producing lower contact stress and lower wear. Grinding may only be needed to remove a layer of metal for reducing or eliminating surface defects.

Aggressive removal of metal from the gauge corner of the high rail during grinding is not recommended. Figure 1 shows an example of high-rail profile measured before and after the first pass of grinding on a six-degree curve. The wheel in Figure 1 was measured on a car operating on one of the service lines. The wheel produced desirable conformal contact with the worn high-rail profile. A gap of about 0.8 mm at the wheel flange root/rail gauge corner resulted from the first pass of grinding, which gave a rolling radius difference of eight mm at two-contact points on the same wheel. A considerable amount of metal removal (either by grinding or wear) is required to return the wheel and rail profiles to conformal contact conditions.

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Low rails on curves can wear to undesirable shapes. Rail begins with a crowned head and then generally wears into a flat top, as Figure 2 shows. After the first rail grinding, the low-rail contact condition is highly dependent on the ground rail shape. Figures 3 and 4 are examples of ground low-rail profiles observed on two freight service lines.

The ground rail shape in Figure 3 was the result of intentionally removing metal from the field side of rail to avoid false flange contact from hollow-worn wheels. Over cutting the field side, however, led to rail shoulder contact that produced high contact stress due to a smaller contact radius in that region. The rail shoulder contact was likely repeated by both leading and trailing wheels of each passing truck. Combined with the effects of tangential forces, as Figure 3 shows, considerable head checks and spalls occurred at the gauge shoulder of the low rail (severe at about one-thrid of rail width that agreed with the contact position in the left figure).

The ground rail shape in Figure 4 was the result of intentionally removing metal from the gauge side of rail to move the contact position towards the field side of the rail to increase rolling radius difference during curving. As Figure 4 shows, rolling contact fatigue occurred at both center and field side of the rail, possibly because the passing wheels have different tapers due to different wear levels. Locating the contact position towards the field side of the rail increases the risk of rail rollover derailment, especially under heavy-axle-load traffic. The false flange contact from even slightly hollow-worn wheels would produce very high contact stress under this contact situation.

TTCI has recommended a set of templates to both lines to use when grinding low rail with the intention of locating the contact of both leading and trailing wheels around the rail center area where there is large contact radius of approximately 10 inches.

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Conduct pre-grinding inspection

The economic benefit of rail grinding increases when grinding procedures are well planned. Big rail grinders are generally used for conducting rail grinding in North American heavy-haul service. Costs can reach $20,000 daily. Therefore, the more track miles ground each work day, the lower the cost of grinding. The pass mile/track mile ratio is taken as an indicator of grinding efficiency. One-pass grinding makes this ratio equal to one.