Re: Why are railway wheels so big?

On 2010-10-03 18:28:10 +0100, Michael Bell said:

In message <201010031626052159-coppercapped@gmailcom>
Robert Cox <coppercapped@xxxxxxxxx> wrote:

On 2010-10-03 05:59:03 +0100, Michael Bell said:

In message <6b950252-5de4-4417-8edb-8bd1e9cceb6b@xxxxxxxxxxxxxxxxxxxxx
ups.com>
The Real Doctor <ian.groups@xxxxxxxxxxxxxx> wrote:

On 29 Sep, 15:12, Robert Cox <coppercap...@xxxxxxxxx> wrote:

The arguments that other posters have made also hold true, certainly
the comment about the Hertzian stresses being set up in the materials
at the contact point is very valid. These stresses have to be
controlled so that the deformation at the contact point is elastic (so
the surfaces regain their original shape when the load is removed) and
not plastic (which results in permanent deformation).

Fretting fatigue is the killer at the wheel/rail interface, and
smaller wheels make it worse.

Ian

Fretting fatigue! I've never heard of it. Can you suggest any reading?
If it is a matter of small diameter steel onbjects rolling on a
surface you would expect it to be significant in the rollers of roller
bearings, or is it a matter of lubrication?

Michael Bell

It's described in any good mechanical engineering text book. Basically
it's wear that occurs at the contact area between two materials under
load which move relative to each other, the amplitude of the movement
being small, from nanometres to micrometres. The sort of thing that can
happen to cylinder head gaskets and cylinder heads.

In this context on the railways it can be caused by the microslipping
of the wheel on the rail caused by adhesion control systems

So better traction and braking control is at least a partial answer?

The traction control of things like the Class 59 and later intentionally use microslip to improve adhesion. So it depends what you mean by 'better'? They can haul heavier trains at the cost of marginally increased wheel and railhead wear.

or the two wheels on an axle vibrating relative to each other because
of torsional oscillations in the axle between them; the wheels then
rub backwards and forwards rapidly on the rails. This motion is
superimposed on the normal rolling motion.

So would you approve of having a road-type differential in the middle
of axles in the fashion of Lewis Lesley's "Trampower"?

Why add unsprung weight? The coning of railway wheels enables them to go round the gentle curves found on railways without the extra complication. Tram systems are different. The torsional vibrations are an inherent feature of a system consisting of two masses connected by an elastic strut. In a railway wheelset they can be excited by variations in the co-efficients of friction at each wheel or the micro-creep necessary to generate the cornering forces to steer the wheels in a curve.

On the rail it can cause fatigue cracking, which if not checked can
result in accidents such as Hatfield.

This was described as "gauge corner fracture". Is this different?

Not really, that is a specific case (at the 'gauge corner' of the rails) but it can happen anywhere where two surfaces are in contact and pressed against each other and relative motion occurs between them.

Chunks can also fall out of the surface of the wheels. It is not to be
trifled with.

Is it GREATLY influenced by wheel diameter, such as to stop wheels
from being made smaller to bring floors level with platforms?

Don't know. The contact patch will be smaller so the loads will be greater, but I have no idea if the effect is linear. I suspect it depends on the individual design and materials used.

Is this a reliable start? http://en.wikipedia.org/wiki/Fretting

Looks as if has been written by someone in the USA. It's a very brief outline with not many links. I would suggest finding a good university text book. I've come across fretting from playing with old car engines, it's not uncommon on the mating surfaces of crankshaft bearing caps where they meet the cylinder block and the bolts are not long enough to act as springs. There are also some electro-chemical aspects as fretting corrosion can occur due to different crystal structures being in contact even if the two parts are nominally the same material.



Or do I recognise some of the words?

How else do you describe two surfaces which are in intimate contact, where the contact pressure is high and which move relative to each other?

Michael Bell

Michael - engineering is both a science and an art. The details are the science and the technology; the system is the art. There are many ways to achieve a goal and each one requires the designer to make lots of compromises. The most successful designs are those which meet the customers' requirements most closely. By trying to achieve a level entrance from platform to coach in a system that started out nearly 200 years ago expecting people to climb into a train as they did a stagecoach, i.e., from ground level, by changing some of the design parameters you will create a new set of problems you never thought existed. By all means design a /new/ system with level access, like BART, but don't try to do it with an existing system. A small step from platform to train is not the end of the world, and if it is, ask somebody to help you.
--
Robert

.

Relevant Pages

  • Re: Why are railway wheels so big?
    ... the comment about the Hertzian stresses being set up in the materials ... They can haul heavier trains at the cost of marginally increased wheel and railhead wear. ... that is a specific case but it can happen anywhere where two surfaces are in contact and pressed against each other and relative motion occurs between them. ... By trying to achieve a level entrance from platform to coach in a system that started out nearly 200 years ago expecting people to climb into a train as they did a stagecoach, i.e., from ground level, by changing some of the design parameters you will create a new set of problems you never thought existed. ...
    (uk.railway)
  • Re: The page I forgot
    ... steel before designing a steel structure? ... Typically, to determine the elastic modulus of a metal, one performs a tensile test with both load and strain being measured. ... Sorry, I was taught that was the proper way to determine elastic modulus by a MATERIALS SCIENTIST, not an engineer. ... My wheels on my bicycles do not flex enough to rub the brakes, so how would greater lateral stiffness benefit me? ...
    (rec.bicycles.tech)
  • Re: The page I forgot
    ... steel before designing a steel structure? ... Typically, to determine the elastic modulus of a metal, one performs a tensile test with both load and strain being measured. ... Sorry, I was taught that was the proper way to determine elastic modulus by a MATERIALS SCIENTIST, not an engineer. ... My wheels on my bicycles do not flex enough to rub the brakes, so how would greater lateral stiffness benefit me? ...
    (rec.bicycles.tech)
  • Re: The page I forgot
    ... steel before designing a steel structure? ... Typically, to determine the elastic modulus of a metal, one performs a tensile test with both load and strain being measured. ... Sorry, I was taught that was the proper way to determine elastic modulus by a MATERIALS SCIENTIST, not an engineer. ... My wheels on my bicycles do not flex enough to rub the brakes, so how would greater lateral stiffness benefit me? ...
    (rec.bicycles.tech)
  • Re: Rebuilding derailleurs
    ... you are looking at a faulty design. ... auto lug nut on left side wheels do not. ... so the faces of the wheels are moving down as you face ... improvement and a proper design. ...
    (rec.bicycles.tech)