Switch Gap Design

by Ken Scheer, from Modeltec, July 1993, reprinted by permission of author

Thankfully, the International Brotherhood of Live Steamers has set Wheel and Track Gauge Standards which are generally adhered to by most people building in The Hobby. This permits one person's equipment to be operated on someone else's trackage of the same gauge, be it a club or otherwise (after noting minimum curve radius compatibility), and to do so without fear of damage to either the equipment or the trackage due to constructional differences. A point of some concern, however, is that no track turnout (switch) guardrail flangeway gap standards have been set by the IBLS. After some study, it appears this task would be difficult to achieve at best, for reasons that will become apparent shortly. This article will attempt to try to clarify some of the design problems encountered in switch guardrail and frog construction in order to show how you go about finding the minimum and maximum allowable flangeway gap tolerances. We will also look at some other limiting design factors for track switches used on any track gauge from 3-1/2 inch to 7-1/4 or 7-1/2 inch. Though the IBLS does offer suggestions for recommended flangeway gap dimensions, these may not always be applicable for all cases due to subtle differences occurring in switch construction.

For the smoothest, most reliable, and safest operation, it is only natural that precision-built, scale-sized locomotives and cars demand a certain amount of attention to precision in trackage components. When studied with this in mind, switch flangeway gap dimensions are seen to depend closely on, and vary proportionately with, the actual track gauge tolerances used in constructing a switch. And these, as with other aspects of scale-size railroad construction, may be snug or built to wider tolerances, to suit the engineering strategies of the build--yet still comply with IBLS Wheel and Track Gauge standards. Also, since some extra track gauge tolerance should be included in the curved section of a switch, there may be some difference in gap sizes from those found on the tangent section of the same switch. Likewise, since the flangeway gap dimensions should reflect the characteristics of a particular track switch, any two switches in the same trackage may be built with slight differences, even if both are the same degree of turnout.

The correct relationship between the dimensions of the various parts is of the most importance, especially in regard to facing point wheel movements. When properly balanced, these relationships minimize wheel pounding over the frog and the resultant frog point wear and also prevent wheel flanges from picking the frog point, prevent back-to-back wheel flange binding, and minimize both wheel flange wear and rolling friction through the switch. The two most critical dimensions--aside from the need to have a uniformity of track gauge throughout the switch (including the added tolerance in the curved section)--are those of the Guardrail Span and the Rail Check Gauge.

Guardrail Span

'Guardrail Span is the actual track gauge, less the two flangeway gaps formed by the frog wingrail and the stock guardrail, and should always be a lesser dimension than the IBLS Wheel Back-to-Back Gauge (minimum tolerance). If the Guardrail Span is greater than this, the wheel flanges will bind along the inside of--and may tend to climb onto--the railheads of the stock guardrail and the frog wingrail. The maximum Guardrail Span is equal to the IBLS Wheel Back-to-Back Gauge. It should be apparent that the minimum design width for each flangeway gap is found by subtracting the IBLS Wheel Back-to-Back Gauge from the actual track gauge, then diving the result by two (since there are two flangeway gaps).

Rail Check Gauge

Rail Check Gauge is the actual track gauge less one flangeway gap, and is not to be confused with the Wheel Check Gauge. The Rail Check Gauge should always be a greater dimension than the IBLS Wheel Check Gauge. If the Rail Check Gauge is less than this, wheel assemblies will be allowed too much lateral movement, and the wheel flanges may have a tendency to pick the frog point in a facing movement. The minimum Rail Check Gauge measurement is equal to the IBLS Wheel Check Gauge dimension. It might not be apparent, but the maximum design width of each flangeway gap is found by subtracting the IBLS Wheel Check Gauge from the actual track gauge. Now, being armed with both the minimum and maximum design flangeway gap allowances, it looks like it should be easy to pick a gap dimension anywhere between the two figures and proceed to build both the from wingrail gaps and the stock guardrail gaps accordingly, right?

Well, unfortunately this is not necessarily true. There are other limiting design factors to be considered for typical construction in the 3-1/2 inch to 7-1/4 and 7-1/2 inch gauges. In reality, the frog flangeway gap widths may not always end up being the same width as the stock guardrail gaps. So much for theory!

The best place to begin is with the actual track gauge. Too snug a track gauge sometimes yields a very small Rail Check Gauge and results in questionably minimal wheel-flange-to-frog-point clearances, as well as what probably is too narrow a flangeway gap width for smooth operation. The track gauge tolerance difference doesn't seem like much, but every little bit helps. Some people feel that construction of the actual track gauge to the widest recommended tolerances on both the tangent and curved sections of a switch is a better strategy. This strategy allows maximum wheel-flange-to-frog-point clearances to be achieved. It also allows for more overall variation in track and wheel assemblies--due to normal wear and tear or constructional differences--and results in less overall rolling friction in the switch. There is no difference in the control of lateral wheel movements. It must be realized that typically, two different sets of Rail Check Gauge and Guard Rail Span measurements will be found on the same switch, and they are relative to whether they are being measured on the tangent or the curved section.