simond
Western Thunderer
Go with Tony’s comment: you have X grammes of weight, which you can have spread around one pair of driving wheels or five pairs, or anything in between. (Are there any six-coupled locos?). You can of course “waste” some of the available weight on bogies and pony trucks too.
The limiting tractive effort is mu * X. The area in contact makes no difference. Mu is the coefficient of friction, which, steel on steel will vary from 0.1 to 0.8. Probably a bit less on NS rail.
(in practice, the area in contact probably does make a difference but this is likely due to local contamination changing the mu.)
some values here
engineeringlibrary.org
And here
roymech.org
notice how the friction coefficient drops significantly from static to sliding.
So imagine the effect of loose/sloppy coupling rods. The driven axle might well slip before the other axles are actually seeing any torque at all, and the shock might well cause all of them to slip. Stop your loco, (reverse to slacken the couplings?) and start again gently.
oh, and ignore Gresley & Bulleid: Churchward had it right. Trailing trucks are bad for adhesion, due to weight transfer.
In response to Martin’s point, in most models, I’d expect the motor to provide more than enough torque to provoke wheelslip unless the model is overweighted. If not, the motor will be stalled and will rapidly overheat, which can destroy it.
The stalled current is the deciding factor for the DCC chip current limit too, unless it can’t stall, of course, in which case the peak current at wheelslip will do. I’ve used standard “HO” decoders since I started playing with DCC sone 28 years ago, and I’ve only fried a few chips, and none of them were due to stall current overload, except one where the gearbox jammed.
You're a lucky modeller if the trains you’re running are big enough to stop your locos pulling them
The limiting tractive effort is mu * X. The area in contact makes no difference. Mu is the coefficient of friction, which, steel on steel will vary from 0.1 to 0.8. Probably a bit less on NS rail.
(in practice, the area in contact probably does make a difference but this is likely due to local contamination changing the mu.)
some values here
Coefficient of Friction | Engineering Library
This page provides an overview of friction force and several tables of friction coefficients from the literature.
engineeringlibrary.org
Coefficients Of Friction - Roy Mech
roymech.org
notice how the friction coefficient drops significantly from static to sliding.
So imagine the effect of loose/sloppy coupling rods. The driven axle might well slip before the other axles are actually seeing any torque at all, and the shock might well cause all of them to slip. Stop your loco, (reverse to slacken the couplings?) and start again gently.
oh, and ignore Gresley & Bulleid: Churchward had it right. Trailing trucks are bad for adhesion, due to weight transfer.
In response to Martin’s point, in most models, I’d expect the motor to provide more than enough torque to provoke wheelslip unless the model is overweighted. If not, the motor will be stalled and will rapidly overheat, which can destroy it.
The stalled current is the deciding factor for the DCC chip current limit too, unless it can’t stall, of course, in which case the peak current at wheelslip will do. I’ve used standard “HO” decoders since I started playing with DCC sone 28 years ago, and I’ve only fried a few chips, and none of them were due to stall current overload, except one where the gearbox jammed.
You're a lucky modeller if the trains you’re running are big enough to stop your locos pulling them
