Why is Autumn Rubbish for Train Adhesion?

[peri]

Could they create greater friction on rails near stations to reduce chances of slippage?

 

[Flying Phil]

Could they create greater friction on rails near stations to reduce chances of slippage?

Not quite sure how...and I think it is safe to assume all the realistic options have been investigated several times over the past 40 odd years - it has been a long standing problem.
I believe the original specification for the APT included plasma jet treatment of the rails to maximise adhesion......

 

[Ken H]

Not quite sure how...and I think it is safe to assume all the realistic options have been investigated several times over the past 40 odd years - it has been a long standing problem.
I believe the original specification for the APT included plasma jet treatment of the rails to maximise adhesion......

dont some french rapid transport use rubber tyres as well as traditional steel wheels? or there is VAL which is almost a guided bus.

 

[MichaelAMW]

There is no pressure term in F=μN.

No, but your μ there is implied as being a constant, whereas friction can be both pressure and speed dependent, so you definitely cannot assume pressure won't affect adhesion.

 

[TractiveEffort]

I don't think it's that modern units are lighter, so much as that the bogies are better designed in that there is less friction between wheel and railhead. I could be wrong though.

I think you could be.... reducing friction between and railhead is undesirable and friction doesn’t affect TC current.

 

[MichaelAMW]

I think you could be.... reducing friction between and railhead is undesirable and friction doesn’t affect TC current.

Except that it's the low friction that is rail's USP - rubber tyres would work a lot better for grip but also put fuel consumption through the roof. I appreciate that it's rolling friction for the movement of the train but static friction for braking, but they tend to go hand in hand.

 

[MarkyT]

dont some french rapid transport use rubber tyres as well as traditional steel wheels? or there is VAL which is almost a guided bus.

Paris Metro used rubber tyres on modernised lines starting from the 1950s. Although they later abandoned this policy generally, they continued to specify rubber for some completely new lines in more recent years. VAL is a development of the technology that got rid of the conventional rail bogies, wheel-sets and rails which are retained in the original Paris system in-board of the tyres and their running surfaces for emergency support following deflation and for guidance through junctions. The latest incarnation of VAL technology, NeoVal, is now sold by Siemens and uses the Translohr centre guidance rail instead of the previous metal side guidance elements copied from Paris, which also carry traction power. Translohr centre guidance is also used for some modern 'rubber tyred tram' systems in Paris and other European cities. In it's first major city metro application, NeoVal is being used for the new Rennes Line B in France, currently under construction.

 

[Pigeon]

μ there is implied as being a constant

Lubricated contact, so it pretty much is, and if anything extreme pressures will raise it.

 

[dcbwhaley]

There is no pressure term in F=μN.

Could you elucidate please. I don't recognise that equation

 

[Pigeon]

Frictional force (F) = Coefficient of friction (μ) x Force normal to the plane of contact (N). μ depends on the materials, and takes values between 0 and 1.

Works well for situations where the conditions at the point of contact don't cause any local change in the nature of the contacting surfaces - eg. lubricated surfaces, where the film of lubricant keeps them apart so there is no actual contact, or hard metals, where the stiffness of the material is such that there is no significant deformation in response to the contact pressure. Steel wheels on steel rails are pretty close to this "ideal" condition.

Works really badly for surfaces which deform under pressure, which heat up and significantly change their properties at the contact point, which chemically bond with each other instead of merely undergoing proximity-based Van der Waals type interactions - like rubber tyres on the road, which do all these things, hence motorcycles being able to achieve lean angles greater than 45 degrees which should be impossible if μ<1, and cars getting more grip in the dry (but not when lubricated, in the wet) from having wider tyres with a larger contact area, when it shouldn't make any difference because there is no pressure term, only force.

The notion that the small contact patch makes trains more liable to skid arises from applying expectations based on situations commonly experienced directly via the senses - in which all wheels have rubber tyres - to a situation which looks similar at first glance but in fact is not because of the very different materials involved. There is further confusion from everyday experience being a conglomeration of factors many of which are not relevant at all, such as humans finding it more difficult to balance on a small area than a large one, so it is a natural reaction to regard a small contact patch as "insecure", even when the human sense of balance is not involved.

If anything trains are more liable to skid because the contact patch is too large. A much smaller contact patch giving a much higher contact pressure that caused significant deformation at the contact point would move things out of the "ideal" region and make it behave as if μ was larger than it really is. It would also be better able to disrupt the lubricant film. But it would also cause a whole lot of problems, rolling contact fatigue up the wazoo for a start, which is among the reasons that rails are not made from a harder form of steel than they are.

 

[superkev]

Difficult for drivers with so many variations in train brakes i.e. disc; disc with scrubbers; tread; with or without Sanders or wsp. I believe Northern even has supposidly identical 158s with and without wheel scrubbers depending on where theve been handed down from.
Personally i've never understood why suburban trains don't have tram like magnetic track brakes as a last resort. I believe some foreign railways do have these.
K

 

[Meerkat]

Isn’t because NR soil themselves at the electrical interference issues that a whacking great magnet might cause?

 

[Shaw S Hunter]

Isn’t because NR soil themselves at the electrical interference issues that a whacking great magnet might cause?

In particular it would cause interference with track circuits so a complete no-go on most of the network.

 

[Kneedown]

Difficult for drivers with so many variations in train brakes i.e. disc; disc with scrubbers; tread; with or without Sanders. I believe Northern even has supposidly jdentical 158s with and without wheel scrubbers depending on where theve been handed down from.
Personally i've never understood why suburban trains don't have tram like magnetic track brakes as a last resort. I believe some foreign railways do have these.
K

Our Porterbrook 156's have all been fitted with WSP, however the Angel owned ones are still without as an agreement to fit it hasn't been agreed at this time.

 

[superkev]

In particular it would cause interference with track circuits so a complete no-go on most of the network.

Trams seem to use them (track brakes) without problems although only used in emergencies.
If they could be made to work think of the benefits in terms of cost, wheel life, no sanders needed and train availability as well as the ability to "stop on a sixpence".
I seem to remember reading somewhere here that such devices are used at least in Germany.
K

 

[philthetube]

Trams work by line of sight, not track circuit signalling.

 

[trebor79]

Tyne and Wear Metro has track brakes and is conventionally signalled.

 

[Pigeon]

A magnet moving parallel to the rails will not induce any voltage along the rails.

What track brakes do do is cause massively accelerated wear on the rails at all the braking spots, which is why where they do exist they are usually kept for emergency use only.

 

[MichaelAMW]

Lubricated contact, so it pretty much is, and if anything extreme pressures will raise it.

I'm not sure if you are agreeing or disagreeing with me there.

 

[MichaelAMW]

Frictional force (F) = Coefficient of friction (μ) x Force normal to the plane of contact (N). μ depends on the materials, and takes values between 0 and 1.

Not true. There is no reason, in general, why it can't be more than 1.

 

[superkev]

A magnet moving parallel to the rails will not induce any voltage along the rails.

What track brakes do do is cause massively accelerated wear on the rails at all the braking spots, which is why where they do exist they are usually kept for emergency use only.

So it's network rail who prefer the train operators to take the hit with damaged wheels rather than wear out there rails with the odd emergency stop. Sounds like an example of fragmented railway thinking to me
K

 

[Eccles1983]

So it's network rail who prefer the train operators to take the hit with damaged wheels rather than wear out there rails with the odd emergency stop. Sounds like an example of fragmented railway thinking to me
K

I'd rather the TOC's take the hit.

A damaged rail unnoticed has the potential to kill a thousand people, worn down rails can go for a while before being inspected or noticed. A emergency brake application on a damaged rail could be unexpected, sudden and deadly.

A damaged wheel will not go unnoticed, the driver will whine at maintenance and it will get inspected at depot that day, or if it's really bad immediately by a technican.

 

[Ken H]

Trams seem to use them (track brakes) without problems although only used in emergencies.
If they could be made to work think of the benefits in terms of cost, wheel life, no sanders needed and train availability as well as the ability to "stop on a sixpence".
I seem to remember reading somewhere here that such devices are used at least in Germany.
K

I was on a brussels tram then it did an emergency stop. people ended up on the floor. tram ended up inches from the passenger door of a car with a baby in a pram top strapped in. Jeez

 

[Meerkat]

How brutal is a train emergency stop?
Obviously I don’t want to find out the hard way, but presumably it isn’t as bad as on a bus.
If you were sat down would you even really notice (ie having to brace against seat in front)?

 

[trebor79]

I was on an HST once that performed an emergency stop. I was walking to the buffet at the time. It was a stronger deceleration than usual but not enough to make people fall over.

 

[6Gman]

I was on an HST once that performed an emergency stop. I was walking to the buffet at the time. It was a stronger deceleration than usual but not enough to make people fall over.

That was my experience on a Voyager too - immediately obvious that it wasn't a normal application, but kept my footing.

 

[Shaw S Hunter]

I was on an HST once that performed an emergency stop. I was walking to the buffet at the time. It was a stronger deceleration than usual but not enough to make people fall over.

The experience will clearly depend on the traction/brake type. Emergency stop on a 142 is decidedly abrupt and more than capable of throwing standees to the floor.

 

[380101]

Manuals are great eh! I can promise you you, no such assistance is available on all 150/2..’s

I assume the gearbox fault light illuminates when you get wheel spin and the power gets cut? If it does, this is the "wheel spin protection" that is talked about above. Certainly what happens in our 156 and 158 units.

 

[TheEdge]

There was quite an interesting video on YouTube somewhere about what makes leaves so bad for adhesion. I can't find the link right now but its more complex than oils from leaves on the rails. Its actually down to a chemical bonding of pectins within the cell walls to iron within the steel rails which forms a plaque. It then gets wet and effectively becomes a lubricated gel that is chemically bonded to the railhead.