The eliptical wheel scandal

[towerman]

In around '71 or '72 when the first 67TS units started going in for programme lift,we started getting lots of reports of excessive wheelspin on starting up causing the wheelspin relays to operate and a bad juddering sensation on braking.It took ages and lots of unsucessful fixes,like recalibrating the wheelspin relays,till somebody had the bright idea to gauge up a wheelset for roundness and it was found that the wheels coming back from Acton Works were being turned slightly out of centre.

[domh245]

I'm guessing that this would be caused by not setting the wheel lathe correctly, probably turning them at what would be the centre for 'normal' mainline wheelsets? Although, not being versed in wheel lathe operation, i'm just making a guess. Is this how the wheelsets came to be elliptical?

[class411]

To be pedantic, if you turn a wheel using a different centre to that which will be used to mount it, it will not become elliptic. It will (assuming no other errors) be perfectly circular.

Of course, it will not work very well as a wheel once mounted off centre.

[Chris M]

The rim of the wheel though would travel through an elliptical path.

[whistlekiller2000]

Nov 21, 2014 21:52:53 GMT Chris M said:

The rim of the wheel though would travel through an elliptical path.

You mean like a clown's car Chris?

[norbitonflyer]

Nov 21, 2014 17:19:55 GMT class411 said:

To be pedantic, if you turn a wheel using a different centre to that which will be used to mount it, it will not become elliptic. It will (assuming no other errors) be perfectly circular.

Of course, it will not work very well as a wheel once mounted off centre.

Correct, it won't be elliptic. It will be eccentric. The axle (and thus anything attached to the axle), will follow a sinusoidal path in the vertical plane. The wheel will move backwards and forwards relative to the axle: how that affects the motion of the train depends on whether the axle is motored (in which case it will turn at a constant rate) or non-powered (in which case the speed at the rail interface will be constant) Both will cause the vehicle to lurch to and fro as the axle speeds up and slows down relative to the track.

[norbitonflyer]

To add some mathematical rigour to my previous post, the path travelled by the axle (and thus the vehicle attached to it) will be a cycloid
www.daviddarling.info/encyclopedia/C/cycloid.html

Specifically a "curtate" cycloid unless the axle is actually on the rim. Note that as well as the up and down motion, the horizontal motion will not be constant, but will vary with the distance from the surface on which the wheel is rolling - at a minimum when the axle is at its lowest.

A driven wheel, with constant angular velocity, will behave differently from a freewheeling wheel, which will try to keep the velocity of the centre of gravity of the wheel constant, and also differently from a wheel which is on a vehicle subject to external propulsion (i.e towed or pushed) where the forward velocity of the axle (attached to the vehicle) will be constant, and the rim velocity and velocity of the wheel's centre of gravity will vary. When the wheel is part of a train, it will be subject to towing forces from the other vehicles in the train, as well as the driven/freewheel forces that would affect that wheel on its own. This would lead to slippage at the wheel/rail interface, as well as strain on the couplings as the vehicle lurches forwards and back relative to its neighbours. Problems would multiply if there were more than one eccentric wheel in the train, as the different degrees of eccentricity (how far the axle is off centre) and phases (the axles of different wheels being at the bottom of their travel at different times) result in more complex motion.

[towerman]

My bad memory it was indeed described at the time as eccentric wheels.