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Coefficients of Friction, or is steel better than ali?

George Coles

Joined: 4-08-09

Topics: 19

Replies: 90

Posted: Wed 29th Oct 2014, 3:43pm
Coefficients of Friction, or is steel better than ali?

In a discussion in the club house the other day one member expressed the opinion that aluminium rail gives a better grip than iron rail, which was completely contrary to my perception. Does any-one have any scientific knowledge to back this up?
George C
 

Replies To This Post

Peter Beevers

Joined: 9-10-01

Topics: 3

Replies: 134

Posted: Wed 29th Oct 2014, 4:04pm

George, I have heard this one quoted before. According to the tables I have, steel wheels on steel rail and steel wheels on ali rail are about the same, although steel on steel has a marginal edge. Cast iron wheels are significantly worse than steel ones on either rail.
Peter
 

johnnicholson

Joined: 1-01-77

Topics: 12

Replies: 69

Posted: Wed 29th Oct 2014, 8:58pm

I once read a very interesting article, many moons ago, in model engineer by the late Ted Martin. This was about the molecular change in the steel rail due to a loco wheel running over it. As best as I can remember the jist of it was that the rail minutely dips under the weight of a wheel and therefore increases the contact area of the wheel and this is a rolling change that occurs as the wheel traverses down the rail. If this is practically accurate then the deformation between steel and aluminium would possibly explain the difference.
 

George Coles

Joined: 4-08-09

Topics: 19

Replies: 90

Posted: Thu 30th Oct 2014, 11:18am

John. If I understand you correctly, you are suggesting that the ali rail would deform more, thus increasing the contact area? I would have thought that meant the wheel was pushing a bigger wave in front of it thus using more energy, in a similar way to your car tyres deforming as you roll along the road. Surely this would increase the reluctance of the wheel to proceed, and hence the probability of loss of adhesion as it tried to climb the wave.
I think I am going to have to have a go with a spring balance unless someone can come up with a table of friction coefficients.
George C
 

Mick

Joined: 1-01-89

Topics: 7

Replies: 114

Posted: Thu 30th Oct 2014, 9:17pm

According to my engineers' tables the coefficient of friction for mild steel on mild steel is .74 static and .57 sliding. Mild steel on aluminium is less at .61 and .47 respectively so in theory is poorer. Having driven extensively on both types of rail I can't say I have noticed much difference in practice, although after changing from steel to aluminium the wheels pick up a silvery coating which wears off once the loco returns to running on steel. I agree with the previous post that cast iron wheels are not so good for serious operations - steel can run in cast iron bearings with suitable lubrication, and also a groove will wear in the wheel tread if running on square edged bar stock rail.
 

Richard_Langford

Joined: 1-01-70

Topics: 10

Replies: 37

Posted: Fri 31st Oct 2014, 12:27am

surely most of the time the wheels and the rails are coated with oxides as well as other things such as water, mud, leaf residue, oils etc. These must all have a serious effect on levels of adhesion.
 

Mick

Joined: 1-01-89

Topics: 7

Replies: 114

Posted: Fri 31st Oct 2014, 7:49pm

A well maintained track should be kept clean with a leaf blower and then will self clean through the passage of wheels on the rails if subject to heavy traffic. Lightly used tracks will suffer from the problems mentioned above. Oddly enough, heavy rain has little effect on adhesion (washes the rails) whereas light drizzle or an afternoon dew can have serious effects! The photo shows the effects of usage at the Sheffield Model Engineers' track at Abbeydale, on the left, the elevated 7¼" gauge track gets moderate use but light loads, centre, the ground level 7¼" gauge gets a lot of heavy use, the 5" gauge barely any, on the right the carriage sidings get used as and when needed.
A well maintained track should be kept clean with a leaf blower and then will self clean through the passage of wheels on the rails if subject to heavy traffic. Lightly used tracks will suffer from the problems mentioned above. Oddly enough, heavy rain has little effect on adhesion (washes the rails) whereas light drizzle or an afternoon dew can have serious effects! The photo shows the effects of usage at the Sheffield Model Engineers' track at Abbeydale, on the left, the elevated 7¼" gauge track gets moderate use but light loads, centre, the ground level 7¼" gauge gets a lot of heavy use, the 5" gauge barely any, on the right the carriage sidings get used as and when needed.
 
 
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