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!
Start with Track Gauge
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.
When applying a wider choice of actual track gauge tolerances, and designing what appears to be ideal flangeway gap widths with optimum Rail Check Gauge measurements, too great a Guardrail Span often results (along with attendant back-to-back wheel binding problems). This happens when equal gaps are used on the frog wingrails and the stock guardrails. The most critical gap is the stock guardrail gap. To allow for a variety of ideal dimensional choices for stock guardrail gap width, and yet be able to reduce the Guardrail Span to an acceptable measurement, it's necessary to design the frog wingrail gaps to be wider than the stock guardrail gaps.
Frogs
Most switch Frogs can easily allow a much wider wingrail gap with no harm done. In fact, they can be (and are) constructed with universally applicable flangeway gap widths, no matter what the other switch construction tolerances may be. This uniformity makes basic frog construction and production methods easier and more efficient. It also permits, for instance, 4-3/4 inch gauge and 7-1/2 inch gauge equipment to use the same frogs. The wingrail gaps must be wide enough to provide a narrower Guardrail Span, but not so wide as to abnormally enlarge the open space between the frog's point and the bend of the toerails. If that space is too large, wheels will pound heavily over the frog point. In cases where commercially available frog castings are used, attention must be given to the wingrail gap widths (which may vary between manufacturers).
Also, since dimensional relationships of the switch may be affected, the various parts (including, perhaps, the actual track gauge tolerance) would need to be designed accordingly. It must also be noted here that since frog wingrail gaps are wider than stock guardrail gaps, the Rail Check Gauge must always be measured from the frog point rail to the outside edge of the stock guardrail. As the stock guardrail gap is narrowed by increasing the Rail Check Gauge, a larger wheel-flange-to-frog-point clearance may be obtained (up to the point of being limited by the Guardrail Span).
After realizing the limiting design factors above, the idea stock guardrail flangeway gap widths can finally be determined for both sections of the switch, in accordance with the Guardrail Span, Rail Check Gauge, the actual track gauge tolerance and other design strategies of the builder. On both the tangent and curved switch sections, the stock guardrails normally provide a relatively light guidance--called flange draw--to the wheel assemblies. The flanges should be forced laterally only enough to ensure that the designed wheel-flange-to-frog-point clearance is maintained. Too harsh a flange draw should be avoided as it causes rough riding, at best.
Guard Rails
On the tangent switch section, the stock guardrail is basically there to gently slap the occasional hunting (lateral yaw cycling) or laterally-loaded wheel assembly into truer alignment. Track and wheel design physics permit wheel assemblies to center themselves naturally along the track centerline, so wheel flanges usually contact the trailheads infrequently. On the curved switch section, the rails through the frog are also tangent, the actual curve occurring in the moveable switch points and in the part of the closure rail. Here, a centrifugally-created lateral load thrusts the outside wheel flanges against the outside rail of the curve, but by the time the wheels are on the frog tangent and entering the guardrail gap, this lateral load is reduced or eliminated, and the guardrail provides the same (or only slightly heavier) guidance or flange draw as on the tangent section of the switch.
All things being generally well-built and consistent otherwise, it has been noteworthy to see some very wide switch flangeway gaps--in some cases so wide as to no longer qualify as a gap--yet no unusual lateral wheel movements, which would create a tendency toward derailment or frog point picking were shown. Nor was there any abnormal frog point wear. In most cases, it would take quite a large lateral load placed at the level of the axle at the exact instant a wheel crosses from the toerail bend to the frog point for any undesirable incident to occur. (Allowing this condition to exist intentionally would be pushing our luck, but it seems to prove that guardrail gap dimension don't have to be overly snug for a switch to work without a lot of nasty surprises.)
Guard Rail Spacer Blocks
The guardrails can be installed and the gap widths held to accurate dimensions by using spacer blocks of the correct width. These are through-bolted between the webs of the stock rails and guardrails (three short blocks per guardrail should do it). The correct spacer block dimensions depend not only upon the chosen gap dimensions, but also on the cross-section of the rails used. The railhead width, web thickness, and height will define the spacer block's shape. Spacer blocks can be easily shimmed or trimmed if finer gap tuning is necessary. A little material usually must also be trimmed off the outside foot of the guardrail for toe-to-toe fit against the stock rail, and to keep the guard rail standing flat on the crossties.
Summary
To construct a smooth-running track switch, a builder can consider all of the above information while working out the mathematics of the design and comparing all of the dimensional relations. It is necessary for a builder to balance these relationships, not only to suit the characteristics of a particular switch and his own engineering strategies, but also to be assured of conformance to IBLS Wheel Standards. The procedures are really quite simple when the figures are played with a while, and it will be gratifying to know that more reliable, smoother and safer train operation over track switches will be the result.