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Balancing Steamers to Maximize Driver Traction

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  • Member since
    January, 2010
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Posted by peahrens on Saturday, October 06, 2018 3:34 PM


peahrens: when the force on the light wheel gets to it's tractive effort limit, by it will start to slip. At that point it's tractive effort will go down (below max) as I understand it because it's slipping coefficient of friction is lower(?).

greg: i believe once the wheel/axle slips, the force of the piston is redistributed on the remaining wheels. In your 0-4-0 case, it doubles. the force of the piston has to be balanced by frictional forces.

Thanks, Greg, 

I get it now.  I was very much overlooking the incoming (piston) force aspect as well as the dynamic cycle aspects of the rods to the wheels.  I can see how exceeding the first wheel adhesion force starts it spinning and that the other (0-4-0 case) wheel's max adhesion ability cannot handle the whole incoming force, so it all starts spinning.

And that the balancing therefore is important for a steam loco.  

I enjoyed the learning process, thanks to you and others.


Modeling HO with a transition era UP bent

  • Member since
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  • From: somerset, nj
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Posted by gregc on Saturday, October 06, 2018 3:55 PM

yes.   threads like these are interesting and educational.

greg - Philadelphia & Reading / Reading

  • Member since
    December, 2010
  • From: Portland, Oregon
  • 446 posts
Posted by Attuvian on Saturday, October 06, 2018 6:23 PM


Since I won't use traction tires, I solve the problem of insufficient tractive effort by adding more well, even using DC control.




At the risk of generating a potentially major side issue, may I ask why you avoid traction tires?  Is it simply one of on-going loco maintence that is a pain in the neck, or something else?  And as long as I've just threatened this thread, why not juice it a bit more by asking what would happen if I chucked the tires on my MT-4s?  I'll be glad to post the matter as a separate subject if it gets in the way of this one.



  • Member since
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  • From: Canada, eh?
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Posted by doctorwayne on Sunday, October 07, 2018 12:30 AM

No major side issue, John, it just seems that traction tires are too artificial for my tastes. 
I had a steam locomotive some time ago that had traction tires and it was simply a lousy-running locomotive.  I realise that such things are much improved, but they simply don't interest me as much as trying to improve the pulling power in other ways.
In normal operations, my longest train would likely be 20 cars or less, and many might be much shorter.  Most operations, when I get around to that stage of the game, will involve a loco going from town to town, dropping-off or picking-up cars, and perhaps simply re-spotting one or two as needed.  Then it's off to the next town.
The through trains will run mostly between staging yards, stopping only for water, and they're the ones more likely to be longer and to use more than one locomotive.

As for removing traction tires from your loco, do you have replacement drivers without the grooves?  It would be interesting to compare the tractive effort with or without them, but, regardless of the results, I'm unlikely to become a convert.


  • Member since
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  • From: A Comfy Cave, New Zealand
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Posted by "JaBear" on Sunday, October 07, 2018 7:10 AM
This has been an informative thread as I had previously believed that knowing wheel/axle weights were important because there have been examples where, in the early days, the New Zealand Railways had purchased locomotives from overseas companies that were heavier than the specifications, and therefore too heavy for the existing bridges, culverts, and in some areas, the rail.
However, this link gives the actual reasons for weighing.
And while this is from a New Zealand Standards Manual, I believe that the standards described would be pertinent the world over.
National Rail System Standard / 6 Engineering Interoperability Standards
Issue 4 Page 11 of 44
Effective Date: 19 April 2013 All printed copies are uncontrolled
6.1 Axle Weight Ratio
The ratio of axle load (kg) divided by wheel diameter (mm) shall not exceed 30.
6.2 Weight Imbalance
Rail vehicles must not exceed a 10% weight imbalance over a wheelset.
Example: If wheel weights in a wheel set are 7 tonnes and 11 tonnes respectively, the average weight is 9 tonnes and the weight imbalance is (11 – 9)/9 x 100% = 22%, which exceeds 10% and therefore does not comply with this requirement.
Out of interest , I quickly checked my copy of NZR Steam Locomotive Drawings and out of the 26 locomotives, ranging from an 0-40T to a 4-6-2-2-6-4 Garret, bar three, all meet the less than 10% weight imbalance criteria, in working trim, with some being the same weight on each driving wheel, though it should be pointed out that as the working trim weight would vary during operation, the accuracy was not required to be measured in ounces.
While there are modern weighing systems that appear to use load cells, it would appear that 7j43k Eds description of using a hydraulic jack and pressure gauge is correct, though there appears to have been variations of how the jack was attached to the wheel to lift it. One account I’ve found mentions that the wheel height off the rail was 0.040”, measured with a special feeler gauge, but I haven’t been able to find a full photo of a hydraulic jack system in use.
There was a controversial issue between labor, management and the ICC regarding older, non-stoker, locomotives having to be retrofitted with stokers based on their weight on drivers.
Interesting comment regarding mechanical stokers Ed. Here it was determined on firebox grate area.  I believe that the “modern” K class steam locomotives were close to the limit at 47.7 sq. ft.
Cheers, the Bear.Smile

"One difference between pessimists and optimists is that while pessimists are more often right, optimists have far more fun."

  • Member since
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  • From: somerset, nj
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Posted by gregc on Sunday, October 07, 2018 8:33 AM  

pg 5, 1(a) notes one purpose for ensuring "weight is correctly distributed" is to "reduce wheelslip".

greg - Philadelphia & Reading / Reading

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