Steam Engine Traction

[Anonymous]

[?] Why is it that two engines of equal weight one steam one diesel, the diesel will have lots more pulling power than the steam? I suspect it has some thing to do with wheel diameter as small wheeled engines, ie, heislers, climax, shays, all have great power to weight ratios but bigger wheeled engines cannot compare, any insight into this would be apprieciated, Thanks [:)]

[steveblackledge]

how does this sound,
with a Geared locomotive like a shay for every revolution of the drive wheel you get lots of reciprocations of the piston rod on the crank, there is less tendancey for the wheels to slip out of control, with a conventional steamer you get four big pushes of the piston rod for one revolution of the wheel, if you get a slip the high presure steam allready in the steam chest will cause the wheels to race away uncontrolled unless steam's cut off.

Diesel locomotives do slip a little but the powers much more controllable, new diesels have ground speed detecting radar that stops wheelslip electronically before the engineer even notices a problem (in most situations)
hope this helps,

[Anonymous]

I'm sorry , I should have clarified I'm talking about HO models, since Model Railroader started listing drawbar pulling power on models they test they have confirmed my beliefs that there is no comparison in pulling power between the two, although I am a steamer fan the smaller wheels on the diesels and geared steamers have lots more pulling power, even though the power is transferred pretty much the same in all models, the only significant difference I can see is wheel diameter. Any thoughts?

[Anonymous]

I suspect gear ratios, heavier frames as well as the spacing of trucks, then either two or three powered axles facilitates better traction/pulling power.

[jrbernier]

In the model world, weight on drivers is the key factor. Other factors are traction tires and are all the wheels really level on the track? Most HO motors are more than powerful enough no matter what the gear ratio is.

Jim Bernier

[Anonymous]

QUOTE: Originally posted by beamerd

I'm sorry , I should have clarified I'm talking about HO models, since Model Railroader started listing drawbar pulling power on models they test they have confirmed my beliefs that there is no comparison in pulling power between the two, although I am a steamer fan the smaller wheels on the diesels and geared steamers have lots more pulling power, even though the power is transferred pretty much the same in all models, the only significant difference I can see is wheel diameter. Any thoughts?

The smaller wheels probably have some advantage in leverage and the steam models have a tender that can be heavry also. The steam models use a gear box on one driver set and drive the rest of the drivers with side rods and all of the valve gear and piston rods. This extra mechanism requires power to move it.

The Lionel Challenger tender was extremely heavy, being cast in metal and had a weight in it in addition to the fact the unit was cast metal. Some of th tenders use a friction rub for electical pickup on the axles, and that causes drag also.
Most of the weight of an HO engine is on the drivers and not the lead and trailing trucks as on a real engine. In some cases, the lead truck has a firm spring to keep it on the track, and this also removes weight from the drivers.

Add it all up, and the diesel HO model has all its weight on the driving wheels and that allows it to pull more. In the case of the E series, the HO model drives all six axles instead of four like the prototype units, and like the real ones, you can add units and increase the effective trafic force greatly.

[Virginian]

Okay, what outpulled the BLI Class A?
Seriously, there are more factors here than can be adequately covered. Weight on drivers is a biggie, and many model steam engines do not have all the drivers sprung, so you don't have the best wheel to track contact.
If everything else was equal, larger wheels would have better pulling force because there will be a longer contact patch between each wheel and the rail.
I think part of the answer is in your question post. On a loco where all or most of the driving wheels are gear driven, diesel or shay, there is a better even application of force.

[Anonymous]

Although I don't study traction i noticed my 4-8-4 had problems pulling long trains,
whereas my big boy & F-7 had no trouble pulling an heavyweight 30 car train up a
scale 4% grade. I think steam has less power because it loses some energy from
the amount of output into several multi-large wheels linked by rods whereas diesels
can have balanced output making operations more powerful.

[jsoderq]

One thing you are all missing is the rigid wheelbase of diesel (short) compared to any steamer(longer). Any deflection in trackwork is amplified across the length resulting in less wheel contact with the track. The same is also true with any curvature - the longer wheelbase puts more force sideways against the rail causing higher friction or resistance.

[cefinkjr]

QUOTE: Originally posted by jsoderq

One thing you are all missing is the rigid wheelbase of diesel (short) compared to any steamer(longer).

I think that, after weight on drivers, this is the second most likely reason for the unfortunate discrepancy between model steam and diseasel tractive effort. Then the fact that most steam models do not have sprung drivers just makes everything worse.

[2c]

Chuck

[Virginian]

Since almost all model motors have the ability to slip the wheels, any power consumed by friction or moving the rods, etc., has nothing to do with the lesser tractive effort. Unless it stalls rather than slips, you have more than enough power, the issue is you cannot translate that power to tractive effort.
Because the models have a cetain amount of designed in play in the side rod linkages, the power transmission to all drivers is not as constant/even as with all geared driver wheels.
Note that on the better brass steamers the drivers are usually all sprung, and the rod tolerances are closer, and they weigh more, and they often pull like nobody's business. I have never had diesels so never did any comparisons, but I used to have more brass and it all pulled well, although in some cases not until I changed the driver springing. My brass Class A's were ALMOST as strong as my BLI Class A's, and had no traction tires. My brass 'J' and 'K' are both also quite strong pullers, but I could still stand to play around with the springs on the 'K' some more. The springs are usually too stiff. I had a Y6b that was a lousy puller (quite unacceptable in that model, especially), and only after I changed all the springs and added a spring loading to the boiler pad on the front engine did she pull pretty well. Never quite matched the 'A's though, they weighed more.

[Anonymous]

Several possibilities why some locomotives diesel or steamers, vary in pulling power.

Sintered vs. polished nickel-silver wheels. Sintered wheels have better traction but get dirtier faster. Most better steamers have polished wheels, Athearn diesels have sintered wheels.

Diesels typically have more drive wheels than steamers. While on paper this should not make a difference, it often does. Sprung drive wheels helps to even out the tractive force, and increases traction.

Gearing and such should make NO difference, as long as the motor is capable of spinning the wheels.

Total weight, or more specifically weight on drivers can greatly affect the available tractive effort. Diesels are often easier to add lead. I once added 12 ounces each to a couple of AC4400's, now they're unstoppable.

When in doubt, add more lead!

Mark in Utah

[Anonymous]

I don't know what kind of steamers and diesels you guys have, but my HO Rivarossi UP FEF-3 (with traction tires equipped) can out-pull Katos and even Athearn BBs on a 3% grade! And the FEF-3 has 80" drivers (usually the larger the drivers, the less traction up hill), so I don't know if I just have a freak loco or if those Athearns and Katos were bad runners. But, the FEF-3 does weight a hefty 25 oz............

[steveblackledge]

AH HAA, on my BLI 4-8-2 the first and last driven axles are flanged but the two inner drivers are not flanged so on a bend say under 25" diameter the unflanged drivers slip sideways off the rail top, so a loss in traction is inevitable.
Diesels have trucks that swivel on bends so traction on bends is greater

[Anonymous]

I think it's usually due to the reduced weight - I have a OO scale passenger train (9 cars) that most steamers can't shift due to their being too lightweight, while a Bachmann 4-axle diesel happily ambled away with the same load. Interestingly the old tender-drive Hornby steamers may be better heavy haulers - they have some hefty ballast in the tender and traction tyres. I must get round to DCC-chipping my A3 "Royal Lancer" so I can check this!

[Anonymous]

QUOTE: Originally posted by steveblackledge

AH HAA, on my BLI 4-8-2 the first and last driven axles are flanged but the two inner drivers are not flanged so on a bend say under 25" diameter the unflanged drivers slip sideways off the rail top, so a loss in traction is inevitable.
Diesels have trucks that swivel on bends so traction on bends is greater

You might be surprised to find out the model is accurate to the real M1a. PRR had blind middle drivers for their short curves on the branch line and Y's. The I1 had the same feature.
On the real railroad with long mainline curves, this lack of flanges did not make any less traffic effort, but it might lessen the traffic effort on the HO model.


Larry