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The New Zealand Railways Magazine, Volume 3, Issue 6 (October 1, 1928)

Derailment of Trucks on Curves

Derailment of Trucks on Curves.

Arnold Stucki throws interesting light on this class of derailment, which apparently has no specific cause. The condition of a truck passing round a curve is divided into three distinct periods—entering, passing and leaving the curve.

On curves the outer rail is elevated so that, at a certain critical speed, the centrifugal force is balanced and the load on all the wheels is equalised. The speed under which this condition takes place will be called normal speed. When the truck is passing over the curve at normal speed the load is carried on the centre bearing and the bogies are free to swivel. If the speed is greater than normal the truck will tilt outward, and if it is less than the normal the car will tilt inward. In both of these cases the side bearers toward which the truck tilts assist the centre bearing to carry the weight of the truck body. This pressure on the side bearings grips the truck and prevents it from swivelling freely.

When the truck enters a curve, the front wheels being on a partially elevated track and the rear wheels on a level track, it is subjected to a severe twist, and if the framing is rigid the load is carried by two side bearings diagonally opposite each other. (Where vehicles such as passenger coaches have good springing and long bodies and are therefore more or less flexible, this does not result, but on short trucks and tenders where the distance between the wheel centres is comparatively short, it holds true.)

There is a tendency for wheels to climb the rail of curves, and the only reason that wheels do not climb in their effort to swivel the truck around the curve is because of the excessive vertical load which comes on them.

Stucki examines at great length the action of trucks passing through curves (a) entering, (b) passing, (c) leaving, and in conclusion arrives at:—

page 38

(1) That curves should be maintained true curves, and that there should be sweet transition from curve to curve and curve to straight, and that great attention should be paid to super-elevation.

(2) Under normal conditions there would be no pressure between wheel flanges and rails.

(3) That where track or truck imperfections exist there are, in every curve, at least six points where there is a danger of wheels climbing the rail.

(4) Therefore, to minimise these tendencies, it is advisable to make the side bearings as frictionless as possible.

(It will be noticed that Stucki depends more upon lubrication of side bearers than does Shannon.)

As a distinguished engineer has said, the great majority of derailments can be traced back to imperfections in the track surface, and that variations in cross level do produce lateral oscillations of the vehicles which, if not absorbed in the springs, must result in derailment.

The best protection against derailments is good track. Modern progress demands greater load's and higher speeds, and I feel that if engineers could only have the time to carefully investigate the true cause of accidents and remove them, the remarkable advance of recent years in speed and load could be even further improved upon. Beaton in his paper on Speed certainly had no doubt about it.

Early Morning at the Locomotive Running Shed, Auckland. (Photo, W. W. Stewart.)

Early Morning at the Locomotive Running Shed, Auckland. (Photo, W. W. Stewart.)

I trust that there will be something that will be useful in the above to our young railway men, and especially to those whose duty it may be to serve on Enquiry Boards.

If we have anything that will help our brother worker, let us pass it on, even if it is only a smile. If we can only induce our “modest wise ones” to come into print, there is a huge mass of workers only too thankful to receive the crumbs that fall from the rich man's table.