Best practices for turnout, crossing diamond grinding

Railway Track and Structures, July, 2000 by David Davis, Don Guillen, Satya Singh, V. Terrill, D. Mesnick

Heavy-haul presents challenges for turnouts and crossing diamonds. Several railroads are tailoring a grinding program to head off problems.

Most railroads grind switches and frogs for the same reasons they grind rail in curves and tangents. They do this to establish a band of contact in the center of the rail or frog to keep contact stresses at acceptable levels, thereby minimizing metal cracking and flow.

This information was gleaned from a recent survey conducted on turnout and crossing diamond grinding best practices for heavy-haul operations in revenue service. Terrill Track Consultants conducted the survey on behalf of the Transportation Technology Center, Inc., under a research project sponsored by the Association of American Railroads.

The survey found that for special track work the reported grinding process becomes more challenging as traffic steers through running surface discontinuities, such as switch entry angles and frog open flangeways.

In addition, there are many abrupt transitions in materials, support conditions and stiffness. These lead to impacts and subsequent rapid deformation of the running surface in special track work.

Special trackwork conditions

Hollow-tread wheels cause considerable degradation to switches and frogs (Figure 1). This is because wheels that do not have a cone-shaped tread profile will generate higher impacts or high-stress contacts at the switch and frog.

The typical AREMA switch does, however, accommodate hollow-tread wheels safely by use of a riser. The switch point running surface is raised above the stock rail from just beyond the point of switch to past the point where the switch and stock railheads separate.

This riser, typically 1/4 or 3/8 inches high, lifts the low hanging field side of the wheel tread over the stock rail as the vehicle travels through the switch. This prevents unwanted deformation of the running surface of the stock rail in facing point moves.

More importantly, it also prevents the possibility of the low-hanging edge of the wheel from pushing or rolling out the stock rail on trailing point moves (Figure 2).

However, the riser causes increased wear and flow to the switch point by forcing the vertical load to the point where it is still thin in sections. The riser is also less than completely successful in preventing stock rail flow, due to the range of possible worn-wheel profiles.

The typical frog is interesting to a track dynamicist, but challenging to a track maintenance worker. All sorts of things are going on under the train.

The frog is designed to accommodate cone-shaped wheel treads. While going from the wing of the frog to its point (i.e., facing point movements), the cone-shaped wheel makes a smooth transition.

In most freight-railroad frogs, the point elevation is lowered at the tip. In the past, frog wing risers were also popular.

On a No. 20 frog, the point slopes up to nominal rail elevation within 24 inches. Thus, the point engages a cone-shaped wheel prior to its leaving the wing.

With a hollow-tread wheel, the wheel does not transition from wing to point at the optimum elevation. This results in a drop-off impact (on the point) in facing point moves and a wheel-climb impact (on the wing) in trailing point moves.

Most frogs have running surfaces that are non-conformal to both design and hollow-worn wheel profiles. The frog running surfaces are flat with a 3/8- to 5/8- inch radius at the gauge corner.

The rationale behind this running surface shape is twofold:

(1) austenitic manganese steel flows rapidly and easily when new. The wheels will deform the frog to a conformal shape,

(2) agreement has not been achieved on the design running surface--and maintaining an inventory of unique casting patterns or grinding each new frog is expensive.

The initial deformation leads to significant initial flow and deformation, which must be ground off. Additionally, there are transitions that must be ground as the wheel goes from rail and its profile to frog and its profile.

Less dramatic transitions occur at the switch point/stock rail transitions. The wing and heel ends of the frog casting are wider than rail running surfaces.

On the point heel, allowance is made for hollow-tread wheels by cutting a channel in the casting.

On the wing, no allowance is made for the hollow tread, except on some spring frogs.

Grinding practices survey

AAR member railroads were surveyed to determine the best practices for special-trackwork running-surface grinding. The results of the survey were similar to those found for rail grinding in open track.

There are railroads that are doing preventive grinding and others that are doing repair and restoration grinding. The best practices are described in subsequent sections. In addition, a list of potential follow-up projects is included.

Methodology

Working with TTCI, Terrill Track Consultants developed a set of questions to help determine the methodology and materials used in switch and frog grinding today.

Additional questions attempted to elicit the philosophy and economics of switch and frog grinding. Areas for additional research and improvements were identified.