ScaleSeven Wheel and Rail Relationship
Understanding the principles outlined on this page is an essential preliminary to appreciating the reasons for the tolerances and working dimensions specified on the Working Dimensions and Tolerances page. In the drawings on this page, the numbers refer to dimensions on Track and Wheel Standards and Working Dimensions and Tolerances.
Note that the running rail is shown upright, whereas in the prototype it is inclined inwards at 1 in 20 to match the coning of the wheel tread. The main reason in the prototype is to reduce wear on rail and wheel, and although it is not relevant for our purposes, track construction systems from C&L and Exactoscale incorporate this aspect in both point kits and plain track.
At crossing vees the crossing flangeway between running and wing rails is maintained as close as possible to 1.750 inches on the prototype. The check rail gap can vary depending on tolerances on other dimensions and gauge widening.
When a vehicle negotiates a curve, the wheels on the outside rail tend to climb up and over the rail head. If this tendency is not checked it would lead to an eventual derailment. To prevent this happening, a second rail is provided alongside the inner rail of the curve, known as a check rail. The back of the wheel bears on the check rail to prevent the other wheel riding up over the rail head.
Wing rails have a similar function and form part of the crossing vee construction.
The cause of many difficulties and derailments is incorrect relationships between rails and wheels at the crossing vee. These Information Sheets deal with this important subject in detail and the drawings illustrate the dimensions which are critical in successful turnout construction. The drawings should be read in conjunction with the dimensions table on the previous page. ScaleSeven gauges are available to give close control of those fundamental dimensions which cannot be reliably and consistently achieved by direct measurement of the track itself.
Fundamental relationships which must not be adversely affected by manufacturing tolerances are:—
1. Back-to-back plus one effective flange must not exceed check gauge.
2. Back-to-back plus two effective flanges must not exceed track gauge.
3. Over check rail faces must not exceed back-to-back.