Standard flat-bottom rails are usually used for light to medium duty applications, but because of their height-to-width ratio they may be prone to overturning when subjected to large lateral loads. Their relatively thin webs are also subject to a combination of high vertical stress and bending stress from the lateral loads causing what could be a failure of the web.
For heavy and extra-heavy-duty applications, special heavy-duty crane rails should be used.
Crane rails may be either Fixed or Floating
A fixed rail implies that the rail is fastened directly to the top-flange of the crane-girder by means of welding or bolting. For very light-duty, short-span applications a simple square bar welded to the girder flange may be used, but this allows no room for adjustment or alignment, and requires a high level of accuracy during fabrication. If correct welding procedures are not applied the beam section may buckle under heat which will be very difficult to correct.
The second fixed rail option is to bolt the rail through its base directly to the top flange of the supporting girder, as with the previous option there is no room for alignment or lateral adjustment.
Such fixings, apart from not allowing lateral adjustment, will not permit the rail to ‘float’ longitudinally, which for medium to heavy-duty applications is desirable.
A floating crane-rail is achieved by fastening the rail to the supporting girder by means of proprietary rail-clips which will hold the rail down thus preventing the formation of a ‘bow-wave’ ahead of the moving wheels.
As the crane-girder deflects under vertical load, the top flange shortens under the compressive stress and relative movement takes place between the top flange and the rail. After repeated cycles, the clips can work loose, and in severe cases, the flange under the rail can wear due to the ‘fretting’ action. Steel wearing-plates may be placed under the rail to alleviate this, or the application of proprietary resilient mountings which have the additional advantage of redistributing the concentrated wheel-load as well as reducing impact, vibration, and noise, together with a smoother travelling of the crane
There are a number of types of proprietary rail-clips that fall into the ‘floating-rail’ category and are sometimes used with resilient rail-pads that break the metal-to-metal contact between the rail and the girder flange and consequently assure a smoother travelling of the crane. A special feature is the provision for lateral adjustment of the rail on the girder, which facilitates alignment.
Where rails are not continuous, their joints should coincide with the girder ends. A practice sometimes applied is to run the end of one rail some distance beyond the girder in order to ‘smooth-out’ the discontinuity of the girder. However, the benefits are doubtful as the rail now tends to connect the top flanges of the abutting girders together, whereas they should be free to move under end-rotation caused by the vertical loading. As a consequence, the holding-down clips at the ends tend to work loose.
Shear-Plates welded to the top flange at the girder-ends would help to assure rail alignment in the horizontal plane
The best form of rail joint is the welded option as used in a continuous ‘floating-rail’. This arrangement, in conjunction with continuous gantry girders, resilient rail-pads, and stiff columns represents the ideal combination of features incorporated into the design of a gantry.
For large heavy-duty crane-rails the rails may be made fully continuous by means of end-bolting or welding and then be allowed to ‘float’ longitudinally in relation to the girder. The rail is fixed to the girder by means of clips that will hold it in position laterally and also prevent it from ‘lifting’ but will not ‘clamp’ it to the girder. As the rail is free to move longitudinally it’s necessary for it to be site-bolted or welded into a continuous length from end-to-end, the rail terminates by abutting against the rail-stop. The welding procedure used with heavy-duty crane-rails is somewhat specialized and requires skill, but the high additional cost is compensated by savings in maintenance and rail replacement. In the case of flat-bottom rails, the rails joints are close-butted and connected with bolted Fish-Plates (without slotted holes). The main advantage of ‘floating’ rail connections is that the discontinuities at the girder joints, which invariably cause wear on wheel flanges and rail-ends and a loosening of fixings are avoided.
Crane Rail Stops
The most severe longitudinal load occurs at the crane-stop which has to absorb the impact of the crane. It’s necessary not only for the stops themselves to be properly designed, but the force has to be transmitted back along the girders to the vertical column bracing. The normal day-to-day loading will seldom be applied, however the impact load of a full-speed stop will be severe, so they must be designed for worst-case. As the load is in one direction only and not reversible, fatigue should not become an issue and high-strength bearing-bolts should be used to resist the shear forces.
Two examples of end-stops are illustrated below: The first is bolted to the top flange of the girder, A sufficient number of bolts must be used to resist the shear and the forward bolts will have to resist tension as a result of the overturning moment on the stop.
The second is bolted to the end of the girder by means of an end-plate welded to the girder end. The stop itself is usually formed from a standard -H- section, which is bolted to the end-plate. A shear-bracket is welded forward on the stop which in-turn is bolted to the flange of the girder.
Both of these options should be fitted with energy absorbing buffers.