Model vs. prototype adhesion

[Doughless]

Couple more thoughts:

Adding weight to the extreme front and rear of the loco will improve the tractive effort of the loco....as long as the weight is equidistant from the midpoint of the drivers....or in the case of our models, equidistant from the driver(s) that actually does the work. 

If the nickel silver drivers of our models are so slippery, why don't they make the drivers out of the sintered metal that Athearn made for their old BB diesel trucks?  Electrical pickup problems?  Certainly those BB trucks picked up track power decent enough. 

[Overmod]

ROBERT PETRICK

You'd be shocked (and/or amazed and/or amused) at the empirical data they rely on or the reasons they give for changing bats.

I would be even more shocked (and I suspect you would be even more amused) if I could take some of that empirical data or reasons and use them for a career in the majors.

That this is somewhat unlikely is a major point of what I've been trying to say here.

[Overmod]

SeeYou190

Certainly not what you would consider an objective reason, but in my world, tools and equipment that do not meet my needs are unceremoniously disposed of and something that meets my needs is obtained.

This is precisely the difference between objective and subjective that I was describing ... and equally precisely, the importance of subjectivity in the discussion.  In practice people don't "have" to put up with shortcomings in things like feature implementation or user interface -- they vote with their feet.  Don't like iDrive?  Don't buy anything that even threatens to have something like it, as BMW and a couple of copycat companies discovered.  We've had many discussions here about whole families and manufacturers who are 'not patronized' because of the likelihood their products will cause consternation or expense.  Did you expect me to advocate this is 'mistaken' because it's at least theoretically possible to fix what was gotten wrong?

The discussion here is more about mistaken theories that lead to throwing engines that might be relatively easily salvaged out because the problem with them was misunderstood or mischaracterized.  That might not be important to many people, but some (particularly those who had to scrimp to afford the engine in question) may not have the luxury of 'letting it go' -- and for those, if you have a slipping engine it may be valuable to understand something about what you can do and where to put the effort.

[gregc]

Lastspikemike

Observe a locomotive with blind drivers that don't touch the rails. It will still move and pull a train.

if a driver isn't making contact then the friction on the remaining wheels will be greater and the cylinder force will be distributed over fewer drivers

Lastspikemike

I doubt there is a published table comparing the coefficient of friction of those two materials to that of steel on steel, for example.

do you look at the links posted in comments?

here's a partial table, previously posted, Friction and Friction Coefficients (click the link?)

[ROBERT PETRICK]

Overmod

Part of the issue, though, is that you have nothing but black magic and trial-and-error to account for why the stuff you do works -- you cannot, for example, account for why some locomotives appear 'slippery' while others don't.

There are professional baseball players who are paid tens of millions of dollars a year for their skills, knowledge, and judgement. You'd be shocked (and/or amazed and/or amused) at the empirical data they rely on or the reasons they give for changing bats.

Robert

[SeeYou190]

Overmod

There is very little objective reason to junk a model as a 'lemon' when you can't even tell me why it doesn't do what its wheels, and basic physics, say it should

Certainly not what you would consider an objective reason, but in my world, tools and equipment that do not meet my needs are unceremoniously disposed of and something that meets my needs is obtained.

I will not waste my time and energy fixing what a manufacturer should have done right in the first place. Especially if there is something else available that will get the job done.

Why it does not work does not interest me. The designers should have a keen interest in that, not kick the can to the consumer to deal with.

My needs for pulling power are so low, that this should be a non-issue for me.

-Kevin

[Overmod]

SeeYou190

Physical experimentation and real world experience mean more to me than theory.

As they should -- in experienced practice; it's no different than all the empirical factors and constants that plague steam-locomotive design to this day.

Part of the issue, though, is that you have nothing but black magic and trial-and-error to account for why the stuff you do works -- you cannot, for example, account for why some locomotives appear 'slippery' while others don't.  One of the great trends in engineering becoming an actual profession is developing the models that can actually serve as good predictors for performance ... or to explain anomalous or seemingly-mysterious results (or achievements) in reproduceable terms.

There is very little objective reason to junk a model as a 'lemon' when you can't even tell me why it doesn't do what its wheels, and basic physics, say it should.  Not that there aren't plenty of people who do, and not just in models -- and not necessarily for expedience as in the likely case of the PRR T1s... in a world where practice matters, and time is valuable, we have come to specialize design engineering and proof testing as a profession, as nobody else has the time to waste learning to re-invent the watch before learning to build it when the object is to know the time.  (Fortunately or unfortunately, someone will need to know a great deal about how watches are designed and constructed if they have a timepiece that is observed 'not to be keeping time' and it is then important to have watchmakers instead of 'parts changers' finding correct solutions to guide and inform practice.

[SeeYou190]

I don't know much, but I do know some things from experience. I run short trains (6 to 12 freight cars), so pulling power really does not mean much to me.

This I know: My Oriental Powerhouse Light Mikados would pull a 12 car freight train around my layout, but the Athearn Genesis Light Mikado would not even pull a six car train around the 24 inch radius curves.

Physical experimentation and real world experience mean more to me than theory. Any model locomotive that does not live up to my needs must be a total lemon in the drawbar pull department.

The Athearn Light Mikado was junked and its nicer-looking tender is now coupled to one of the Powerhouse locomotives.

-Kevin

[Overmod]

Any actual physics involving friction will note it as a force, with units of a force, and in vector analysis corresponding to a force even though the mechanics that produce it are acting at a substantial angle.  The reason we use coefficients is that they are dimensionless numbers; see Cd in streamlining for a comparable example.

There certainly were tables carefully comparing the coefficients of different materials at different surface finishes (which is an important characteristic so far missing from the present discussion).  As I recall ASTM has standards for how to conduct meaningful research in 'undocumented' combinations and perhaps geometries, and it would be relatively simple for someone with, say, access to a college lab to do careful analysis of various materials on nickel-silver (perhaps even against nickel-silver rail of various states of 'gleam') as part of the investigation I keep hoping to see.

There are some other concerns here.  In the prototype, there is clear deformation and surface-welding in the contact patch, less deformation and wear in the wheeltread than in the railhead by design.  I very much doubt anything of the kind occurs in nickel silver rail, particularly that which is extruded with a sharp shoulder in the contact patch/gauge corner area as so much commercial product is.  On the other hand I have never read an analytic discussion of model wheel wear, which is an obvious effect and often noted as an obvious problem -- I'm sure there are experienced people on this list who can tell us the operative mechanisms that produce it on both driven and non-driven wheels.  In my slim experience with steam-locomotive models that have severely worn wheels, the drivers appear to be little if any more deeply worn than the 'carrying' wheels in the trucks, which I expect to be significant in appraising the various wear mechanisms. 

Note that wheels ought to be both precisely machined and tread-hardened (as all good modern wheels have been since the days of chilled-iron tread) so that sacrificial distortion and wear is as much as possible seen in the railhead (where it can be dealt with effectively with magic-wear-rate techniques).  It occurs to me that a case could be made for hard coatings and even superfinishing of model treads, both for rolling and for traction.  This would particularly involve steam-locomotive driver tires, where hard coating is compromised by soft substrate and perhaps improper plating techniques.  Remember this is rolling friction, not laterally engaging friction (so for example we can expect vertical engagement of asperities, but not sliding surface welding or upsetting), and a little review of why creep control is important in achieving 'best' full-scale adhesion numbers ought to be done even if 'it turns out' there is no comparable effect practically to be achieved at model scale.

As a personal prejudice: I have never understood why people think that power transfer through typical primitive model side rods, with ridiculous bearing clearances and no particular attention to tribology, is going to produce particularly equalized or steady torque at the wheelrims.  I suspect that very careful precision quartering (in models, for sameness and not precise 90-degree, but you almost need the same quartering-machine pin-grinding precision for either...) followed by some sort of acetal bearing accurately pressed to dimension in the rods, would give the proper combination of running freedom and consistent force transfer when loaded to 'share the torque' with the motored driver pair(s).  

I won't get into the theories of slip in steam locomotives as expressed so far, except to note they don't particularly jibe with either observations in practice or actual theory for locomotive design.

[Trainman440]

Lastspikemike

Friction is a coefficient. Unless steel on steel develops the same coefficient as whatever model drivers are made of on nickel silver the model cannot match the prototype.

I agree with greg. friction is not a coefficient, but rather a force. In modeling, we generally are less worried about that, but rather the coefficent of friction, which determines how much the weight affects friction, and is different for all materials/situations. 

Lastspikemike

The nickel silver part is a constant. Bronze rails likely would be stickier, as used in G gauge, but that's just a guess. What is the ideal metal for drivers though?

I believe friction between metals is likely more due to how soft each metal is. Even so, I'd argue that the finish on the metal is more important than the metal itself. In theory if you were to put diamond tread on a steel engine wheel, and placed it on a track that has a similar finish, that will run the sh!t out of any nickel silver plated wheel. 

Lastspikemike

As for weight distribution on the drivers that is not relevant to total tractive force because friction is a coefficient. Uneven weight produces proportionally differing traction force for each driver but the total is the same (not the case for pneumatic tired vehicles just btw, but air filled rubber tires are really weird structures) . For prototypes weight distribution matters only because of rail carrying capacity, not a concern for our models.  

weight distribution is most certainly relevant. I may not know the math behind it all, but I know from experience that if the center of balance for the weight of an engine is off, that it will pull less than one with proper weight distribution. 

In fact, I presume that's why Sheldon placed additional weights near the front of his Bachmann 2-8-4. I dont have one of these engines, but I will soon. I recall from various reviews that the center of balance is between the third and fourth driving wheel, which allowed it to pull a less-than-satisfatory amount of cars(<20). The common solution is to place more weights near the front.

----------------------------------------

In fact, that is why so many MTH steamers perform so pitifully without their stock traction tires. Because they cram a huge smoke unit into the front of the boiler, the center of balance of the engine is so off that it ends up requiring traction tires. 

An MTH USRA 2-8-2 pulls only 9 bluebox 40' cars(theyre under weight) before it slips.

An MTH K4 pulls literally 3 bachmann P70 coaches with walthers oiled trucks, and still slips occasionally. 

With traction tires, performance is satisfactory. Still, that's a compromise I would rather not take. 

Charles

[doctorwayne]

Lastspikemike

As long as the weight is within the driver wheelbase it shouldn't matter where it goes. Adding weight outside that wheelbase gets you into possible leverage effects which should not matter in most situations but it might in some.

The added weight doesn't all need to be within the drivers' wheelbase.  As long as the additional weight is balanced at the mid-point of the driver wheelbase, it will be effective in increasing traction.

Wayne

[ATLANTIC CENTRAL]

Well, the list of variables here is pretty long, and lots of people have touched on many of them.

And, with steam loco models, what is a major factor for one model may be of no concern for another.

I have a pretty good sized steam fleet, I pull long trains, I have done some testing. And I have added weight and made other modifications with considerable success in most cases.

I am not going to try and offer all my thoughts on this topic in one post.

In this converstation some will try to rely completely on known prototype engineering theory, some of that has already come up. I wish it was that simple....

A few basic observations based on my 53 years at this:

Balance of weight on drivers does matter.

Springs on leading or trailing trucks will often reduce tractive force.

Sprung drivers sometimes help, sometimes not.

Vertical curves in track are a problem........

Sharp radius curves are a problem.......

More axles mean more potential losses and problems.

Yes, drawbars can complicate the issues, in several ways.

 

Now, example one:

Bachmann Spectrum USRA Heavy 4-8-2 vs Bachmann Spectrum USRA Light 2-10-2

These two locos share the same boiler, just like their prototypes.

The 4-8-2 is driven by the second driver, and has a sprung third driver, all others fixed with minimal vertical play. This puts most of the weight on the second and forth drivers, with the others more "floating". These locos pull well, mine will pull 35 of my 5 oz piggyback flats on level track.

The 2-10-2, same basic weight, same cast boiler, is driven from the third driver, and has no driver springing. Driver three is blind, like the prototype. The first and second driver have minimal vertical play. Drivers 3 thru 5 have noticable vertical play. This has the effect of putting most of the weight on drivers 2 and 5 on level track, but potentially shifting that weight to drivers 2-4 on some vertical curves.

This loco also pulls well, but not quite as well as the 4-8-2. It handles about 30-32 my piggies comfortably.

So in both cases, due to rigid wheelbase length, the engineers at Bachmann decided to purposely take the weight off some drivers and focus it on others to allow the loco to track better in vertical curves with less shifting of weight from driver to driver.

I have observed this design principle in other brands and models.

That is enough for tonight. Next we can consider my kit bashed Mikados, built from Bachmann 2-8-4's and with added weight.

Sheldon 

  

[gregc]

Lastspikemike

The drive wheels are all locked together. They can't slip just because one wheel develops less tractive force. They all slip as if they were one big wheel.

the force applied by the cylinders is evenly distributed on all wheels because they are locked together, but the friction on each wheel depends on the weight.   

when the force exceeds the friction on that one wheel, the force then shifts to the remaining wheels which usually exceeds the friction on those wheel and they immediately slip.

Lastspikemike

When we speak of friction we are actually speaking about the coefficient and not the force.

huh?     the coefficent of friction will be the same on different types and sizes of locomotives but the friction determining the max tractive effort of a specific locomotive depends on the weight of the locomotive and the number of drivers.

[gregc]

Lastspikemike

Friction is a coefficient.

friction is a force (lbf).    the coefficient of friction is a scalar

Lastspikemike

I suspect but don't know that nickel silver alloy (mostly copper actually) has a pretty poor coefficient of friction.

the coefficient of friction depends on both metals.    No need to hypothesize, there are tables: friction and friction coefficients

 

Lastspikemike

As for weight distribution on the drivers that is not relevant to total tractive force because friction is a coefficient.

Lastspikemike

The locomotive will develop the same tractive effort whether the drive wheels are equally loaded or wildly different loading, until the rail gives.

doesn't matter until one wheel starts slipping and all the rest follow.   max tractive effort therefore depends on the max force of the wheel with the least weight

 

[RR_Mel]

I have little mechanical engineering in me, only personal experience with my locomotives.

I totally agree with the blind drivers being less adhesive.  My Bachmann and Bowser 4-8-4s are not good pullers, run and look good but puny drawbar for such a large locomotive.  All have stock weight and motors.

My Rivarossi articulateds have very nice drawbar, 5.8 to 6 oz.  Most have 8 to 10 ounces of added weight and at least one new can motor, many have dual can motors.  They weigh in at 19 to 20 oz. each or around 30% traction, more wheels higher percentage??  Dual motors don’t have any effect on drawbar they just run better.  The original Rivarossi drive locking the two driver assemblies cause them to have a slight wobble, the dual motor floating driver assemblies don’t wobble.

Most of my diesel fleet are Athearn three axle truck, a few Proto 2K three axle truck and a few Model Power E-7s, again three axle trucks most with a lot of added weight.  My Athearn E-7s have Cary metal shells on SD40-2 frames with can motors, weighing almost two pounds each and have 8 oz drawbar or 25% as mentioned above.

If someone wants to up the drawbar on their locomotives they can add weight but make sure the motor will handle the higher current.  A single can motor in a 4-8-8-2 with the added weight is around 560ma at wheel slip, with dual can motors they run about 275ma each.

My remotored E-7s draw a bit over 800ma at wheel slip, 8 oz drawbar, a pair will pull your sox off.




Mel



 
My Model Railroad   
http://melvineperry.blogspot.com/
 
Bakersfield, California
 
I'm beginning to realize that aging is not for wimps.

[Doughless]

I assume it has to do with having enough weight with the correct distribution to have the steam power appropriately transferred to the rails (not totally unlike like getting a car engine's torque/hp transferred to the tires) in order to get the maximum pulling power.

In the prototype the design reality of the innards were pretty consistent from producer to producer compared to our models....IMO, and the bigger the loco the more it weighed proportionally to a smaller loco. 

Adding ballast mattered some, but the weight of the mechanicals and "shell" of the prototype alone was proportionally heavier than the weight of the mechanical innards and shells of our models.

With the models, weight (or added ballast) is a much larger component of the overall weight of the loco...IMO... and with different producers have different innards, there has to be enough space to add the weight in the right spots.  

Probably not consistent from producer to producer, or even small loco to large.

 

[Trainman440]

Lastspikemike

Interestingly, diesel models seem quite predictably uniform in their pulling power and roughly correspond to prototype patterns: bigger and heavier pull proportionately better. This is not the case for steam locomotive models which is frankly a bit weird

That's why I model steam!

[Trainman440]

Other minor things that are technically covered in the list, but could be expanded upon:

more traction can come from:

1. weaker/no springs on the leading/trailing trucks

2. even springs on wheels*

3. less blind drivers

4. a more rigid wheelbase**

5. drawbar on engine should be (ideally) at the height at which the centers of axles on the drivers are***

Many of these have minimal effect, only apply to certain engines, and potentially hurt the operating characteristics in other ways(for ex sprung leading/trailing truck helps tracking) but they do contribute to pulling power. 

The most common cuprits are usually unbalanced drivers, and a general lack of weight. 

This is a very interesting thread, I will be keeping track of it. 

Charles

--------------------------------

*often times on brass engines, the wheel with the gearbox has stiffer suspension than the rest, creating a sort of seesaw effect, where not all wheels are putting equal force on the track. 

**=flanges scraping against sides of track will provide more traction,thats why engines pull more on curves. rigid wheelbase=more horizontal force on flanges against track. 

***=too high and the load will pull the front of the engine up. 

[gregc]

my understanding is that prototypical adhesion is ~25% of the balanced weight on the drivers and this is also roughly true for models: 20-30%.    Of course wetness, leaves, dirt, frost, ... can affect this.

my understanding of friction is that it depends primarily on weight and less to do with surface area.

i don't understand why the value is as small as it is if the coefficeint of steel-on-steel is ~0.74-0.8.

"balanced" refers to the desire to have an equal amount of weight on each driver.   since friction depends on weight, the first driver to start slipping will be the one with the least weight.    Once it slips, all the force is now imposed on the non-slipping drivers which very likely will exceed their friction limit and also slip.

balancing the weight on drivers appears to be easier with diesels than steam.

 

[Paul3]

Roughly, model diesel locomotives have about 25% adhesion (just like the real thing historically, not counting modern traction controls).  That means if a loco weighs in at 16oz., it should have about 4oz. in tractive effort, more or less.

At my club, we test all registered locos using a spring scale that measures force in 1/4oz. increments.  Right now, we have about 2000 engines on the roster, and since I test most of them, I have a lot of experience in measuring tractive effort.

Things that make an engine pull less:
Less weight on drivers
Vibration
Clean wheels
Clean track
Unbalanced on drivers
Nickel-Silver wheels
Nickel-Silver track
Less powerful motor

Things that make an engine pull more:
More weight on drivers (up to a point)
Smoothness
Dirty wheels
Dirty track
Balanced on drivers
Sintered wheels
Steel track
More powerful motor

[Overmod]

This discussion is starting up, off-topic, in the detailed-RTR-operating-model thread.  It deserves its own named thread for discussion.

Some of the discussion thus far is as follows:

ATLANTIC CENTRAL

Lastspikemike

Yes, thanks, I've actually previously book marked that extensively detailed description. [This is the link to big blue trains.com on improving adhesion for light models.]   Thanks. 

The disparity in pulling power for these steam locomotives has me pondering coefficients of friction on nickel silver...

However, I noted an extensive thread on locomotive drawbar force as it may relate to the coefficient of friction of steel on steel as well as steel on sanded steel comparing prototype to HO.

In light  of the significant disagreements evident there I decided not to make my own contribution.  The topic seems surprisingly controversial, given that the physics have been well understood for over 100 years.

Interestingly, diesel models seem quite predictably uniform in their pulling power and roughly correspond to prototype patterns: bigger and heavier pull proportionately better. This is not the case for steam locomotive models which is frankly a bit weird.

It is not weird at all. Model steam locos, even with spring drivers suffer from a lack of even weight distribution on the drivers, as well as other traction losses on our sharp curves, etc. This problem increases with the number of driven axles.

The physics of the prototype does not scale down.

The flexibility of diesel trucks solves this problem.

Sheldon

There have been many discussions over the years on aspects of this subject.  Aside from gleaming (which is usually not done to enhance adhesion per se, although I'd welcome discussion on how it would or wouldn't in its various specialized forms) I see comparatively little on profiling the gauge side and face of the rail to remove any corner or ridge decreasing the actual contact of driver with rail.

I am sure there is wide and rich experience with both the practical and esthetic aspects of improving adhesion, some of it since the last coverage of the subject here or in books...