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Model vs. prototype adhesion

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Posted by gregc on Saturday, October 3, 2020 1:17 PM

weight distribution doesn't affect drawbar force.

it affects when slip occurs

slip occurs when the force on the driver exceeds the friction force.   while all drivers have the same force, the friction is lowest on the driver with the least weight.

greg - Philadelphia & Reading / Reading

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Posted by Overmod on Saturday, October 3, 2020 3:35 PM

gregc
weight distribution doesn't affect drawbar force.

Keep in mind there is static weight distribution, and there is "weight transfer" (when the drawbar pull is not in line; think of it as rotation degrees of freedom in addition to translation).

If a model locomotive were effectively equalized like the prototype, most of the pernicious efforts of individual-axle loading would be solved just as on that prototype: the equalizers would move until all axles were balanced-loaded, within the limits of suspension travel.  That would have to be very carefully laid out and made, minimizing any friction or interference causing the equalizing to hang up, but it would require little lubrication other than perhaps at the pedestals to function efficiently if the contact surfaces in the equalizers were sufficiently hard and radius-formed and polished.

Instead, we get drivers that are sprung nominally to increase electrical contact, and while (as with locomotives in England built consciously without equalized drive) the spring pressure can be carefully adjusted to give 'equal' weight apportionment for adhesion to all the driver pairs, this will become wrong if the sprung driver pair is then floated up or down.  Much more likely, I think, is the use of lighter springs pressing these down without much effect on actually helping suspend the locomotive itself in reaction -- I suspect that in some engines the actual vertical suspension is carried (hopefully very unprototypically but you never know) on the lead and/or trailing truck(s).  Now in my opinion if all the drivers were sprung (with the corresponding results on drive design) you would still have to be very careful balancing the locomotive fore-and-aft and then accounting for weight transfer, as all the springs will compress until in force balance with gravity, but this is not dictated by axial tilt but by weight distribution, and stopping the balancing short of equilibrium (e.g. by supporting the ends with solid or sprung trucks) will necessarily result in some axles having less imposed load than others, and hence lower factor of adhesion.  (You would then also need to balance any spring pressure putting tracking weight on the engine and trailing trucks, but that follows logically.)

The issue of lateral accommodation is critical in prototype steam design, although probably less so in models, where dynamic augment is not commonly recognized as an issue (even in the Arbour Models build!) -- my personal suspicion is that when drivers are sprung there's enough play that the cross-level articulation in suspension will 'take care of itself' and the difference in adhesion between drivers in a particular pair would be slight.  ("Springing" over the axle center would likely solve any issue concerning this for even exaggerated superelevation in model curves or cross-level defects)

I'm sure that someone, somewhere, has carefully studied the practical effects of traction tires, and how much extra 'friction' they provide for adhesion.  Certainly the Michelin experiments in the '20s and '30s established that a wider contact patch with a conformal elastomer increases some measures of adhesion; to an extent they also indicate that even small quantities of 'lubricant', including water, destroy much of the advantage when present.  I have not seen data or microphotographs about the conformed contact area of a traction tire with a "scale rail" head profile, but on typical rail with a square gauge corner and near line contact with tapered tread, it could only be considerably larger, quite possibly outweighing the effect of some or all of the other drivers together.

A little complication is the contribution to slip of decreasing adhesion on some wheels that are driven by rodwork.  Since no driver actually breaks into slip conditions (full coefficient of sliding friction) you need to consider what is actually happening when a given driver loses some of its effective adhesion in your theoretical 'model' of friction effects -- there is clearly a loss of adhesion vs. applied torque, but not something measured by a sliding or slipping/skidding test until the actual slip starts and propagates.

The statement that "slip occurs when the force on the driver exceeds the friction force." is more correct in the aggregate, but this is something of a truism.  The greater concern is, as far as practical, ameliorating any particular driver's or pair's adhesion so that all drivers 'depart' only at the highest applied torque -- which may involve suspension, compliance, and weight distribution, but the range of corrective action may be comparatively slight.

It might be interesting to consider the likely methods of action of 'traction increasers' like Bullfrog Snot in these models.  Think of the ways this is like or unlike a fixed elastomer traction tire, or a tire in which the surface activity has been greatly increased (as in 'Gecko Grip')

Do we need to discuss the effects of weight transfer or methods to correct it (a couple of which already appear in the Arbour Models thread)?

 

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Posted by Lastspikemike on Sunday, October 4, 2020 11:32 AM

Two comments: it isn't useful to consider the behaviour of rubber contact patches in a pneumatic tire when assessing how steel on steel develops drawbar force. Anyone who has driven an indoor only forklift intuitively understands why.

My point was about why only one driver is typically equipped with a traction tire.

Locomotive suspension was never about tractive effort as far as I can see. Locomotive springing would be disadvantageous were track to be perfectly level. The springing had to do with point loading at the rails. 

Weight transfer effects may be useful to consider for sprung road vehicles but I doubt springing a locomotive has anything to do with that. Weight transfer is of course a misnomer, it refers to leverage effects on sprung suspension caused by driving or braking forces exerted at the contact patches. Weight is not changed much less transferred anywhere.  

Total drawbar force is not affected by suspension of locomotive driving wheels and weight transfer has to be minimal given the very low internial moments as compared to sprung road vehicles. Increasing the load at any drawbar height above the rails will force the locomotive drive wheels downwards and deliver enhanced traction as a result. It should not matter whether that increased leverage is equally distributed across all drivers and springing certainly could not achieve that in any event.

The effectiveness of the single traction tire should prove this.

Alyth Yard

Canada

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Posted by SeeYou190 on Monday, October 5, 2020 2:34 PM

Lastspikemike
Two comments: it isn't useful to consider the behaviour of rubber contact patches in a pneumatic tire when assessing how steel on steel develops drawbar force. Anyone who has driven an indoor only forklift intuitively understands why.

Why would this be intuitive for a forklift operator, since "indoor only" forklifts are (conventionally, by an overwhelming majority of standard practice) neither equipped with pneumatic tires nor steel tires?

Please do explain.

-Kevin (formerly an OSHA qualified forklift operator instructor)

Wink Happily modeling my STRATTON & GILLETTE RAILROAD. A Class A line located in a personal fantasy world of semi-plausible nonsense on Tuesday, August 3rd, 1954.

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Posted by Lastspikemike on Monday, October 5, 2020 6:47 PM

SeeYou190

 

 
Lastspikemike
Two comments: it isn't useful to consider the behaviour of rubber contact patches in a pneumatic tire when assessing how steel on steel develops drawbar force. Anyone who has driven an indoor only forklift intuitively understands why.

 

Why would this be intuitive for a forklift operator, since "indoor only" forklifts are (conventionally, by an overwhelming majority of standard practice) neither equipped with pneumatic tires nor steel tires?

Please do explain.

-Kevin (formerly an OSHA qualified forklift operator instructor)

 

Because any reasonably experienced forklift operator would likely have driven a forklift equipped with solid tires and maybe even one with pneumatic tires. You obviously have. But do you understand why solid rubber tires behave quite differently to pneumatic tires? That'd be interesting.

In the case of model locomotive traction tires you're dealing with solid rubber and the friction effect of "creep" for solid rubber behaves quite differently to that of solid rubber tread on a pneumatic tire. Model traction tires also no doubt creep around the groove in the metal tire which complicates things somewhat. The object of my remark was to encourage some clear thinking about how a set of connected drivers actually develops drawbar force. I am saying that weight distribution cannot be one of the relevant factors, at least at 1/87 scale. 

Part of this thread refers to "creep" as a part of transition from static to kinetic friction for steel on steel (or perhaps nickel silver on nickel silver) but rubber, now that's interesting stuff. Given the very low contact area pressures of a model locomotive I'd be surprised if the metal to metal creeps appreciably but obviously I've never measured it. Creep is a result of  pressure created by load. 

Jeepers, you need instruction to drive a forklift? They steer just like a boat, mind you lots of people are also confused by a boat tiller.

Alyth Yard

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Posted by Overmod on Tuesday, October 6, 2020 9:39 AM

Lastspikemike
Model traction tires also no doubt creep around the groove in the metal tire which complicates things somewhat.

You know, I never thought about that -- and it would be very easy to test and then measure: just matchmark a point on the driver rim with the corresponding edge of the tire, then operate for a while and periodically measure the 'displacement'.  I'm tempted to put out a 'call' for large numbers of operators to start doing this...

I am saying that weight distribution cannot be one of the relevant factors, at least at 1/87 scale.

I think it is, but perhaps for reasons not in the representational models we've been making.  Certainly several threads have noted that at least some models 'pull better' when properly balanced on the driver wheelbase.

Given the very low contact area pressures of a model locomotive I'd be surprised if the metal to metal creeps appreciably but obviously I've never measured it.

I think we should set up some sort of experimental protocol to conduct reproduceable testing.  Remember that although the pressures are low, so is the effective 'contact patch' between an unworn plated wheel and the square railhead profile at the gauge corner in a great deal of track.

I agree with you that the 'null hypothesis' is that little if any observable creep (in the sense that effect is relevant to prototype traction control) takes place, and I will raise the hypothesis by saying that its 'scale' equivalent would involve hopelessly minuscule modulation of driver rotation even with very fast, somehow effective pulse control of motor rotation.

Jeepers, you need instruction to drive a forklift? They steer just like a boat, mind you lots of people are also confused by a boat tiller.

The first objection that comes to mind is that boats don't periodically have most of their effective mass wobbling near the top of a very long vertical lever arm whose axis may not go through the machine's effective center of mass for best balance.  A great deal of safety training involves how to run the fork when actually lifting or moving items high up.  More of it involves the difference in handling when you have a load outboard on the forks with an inertial 'mind of its own' that drivers may not appreciate without experience -- experience that if they gain it on their own may be both expensive and prospectively written in blood.

There's some other stuff, too, along the same lines that while experience is the best teacher, some lessons are better imparted in a classroom or as advice in training ... 

(Helicopters are easy to fly, too ... until they aren't. Whistling)

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Posted by Lastspikemike on Tuesday, October 6, 2020 8:58 PM

I was of course perversely referring to operating a fork lift as a vehicle. Of course the safety training relates to its primary function.

As for lever arms operating on boats, my default boat is a sailing ship. Now leverage on sailing vessels is extremely interesting and makes operating a forklift look like child's play by comparison. 

Also, I concede I was still mildly annoyed by a persistent and completely unnecessary difference in perspective which I intended to defuse, albeit somewhat aggressively. Hopefully it worked as intended, we shall see.

Alyth Yard

Canada

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