Chair keys for bullhead track on a gradient

[Western 52]

Some time ago when doing some PW work on a preserved line, I was told that for bullhead line on a gradient, the keys should be hammered in from the uphill direction. The reason, apparently, was that for a train climbing the gradient, the force exerted on the rail would tend to pull the keys into the chairs rather than loosen them. For a train braking going downhill, the effect would be the same. Is this a real concept though, and would this be normal practice? Would the severity of the gradient make a difference?

 

[John Webb]

The movement of rails on a gradient due to trains climbing up or braking as they come down is known as 'creep'. It tends to alter expansion joint widths and at intervals may require rails to be loosened in the chairs and adjustments made. I have no idea however how this effect varies with steepness of the gradient.

 

[edwin_m]

Physics says that when the train is under power, the frictional force between the wheel and the rail is pushing the rails backwards, and when braking it is pushing them forwards. When coasting the frictional force is essentially zero. It's more likely to be under power when climbing and braking when descending, both of which would push the rails in the downhill direction. So it's correct that the tendency to dislodge the keys would be less likely if they were hammered in from the uphill end, though I've no idea how big the difference might be. Logically the same effect ought to apply in areas such as approaches to stations where more trains are accelerating or braking.

 

[Western 52]

As a section of rail is joined to adjacent rails and there would be considerable friction between the rail and chair when the weight of the train is on it, would a rail really move enough to affect the key?

 

[zwk500]

As a section of rail is joined to adjacent rails and there would be considerable friction between the rail and chair when the weight of the train is on it, would a rail really move enough to affect the key?

Yes, because as the weight moves over the rail in one direction it squeezes the key. You would normally insert keys with the direction of travel, so that they are pushed into the chair rather than pushed out of it.

 

[Ayrshire Roy]

In 2006 there was a derailment in Ayrshire and in the accident report they mention that the area had ongoing problems with downhill track creep due to loaded coal trains decending a 1in70 gradient.
The section of line was bullhead rail.