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Post by Cro on Mar 22, 2021 17:31:46 GMT
Chris Why are you changing over to mechanical lubrication, I do not recall you having issues? Also do not bin your redundant hydrostatic system it may be suitable for the Aspinall (obviously beer vouchers would be available) and would save me making one! Dave Hi Dave, only moving back to mechanical to free some space below the footplate, no other reason and sorry but the hydrostatic will be used on the Pansy or Grange  Chris You can fit a mechanical easily under the tender....  Untitled by Adam Cro, on Flickr |
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kipford
Statesman
Building a Don Young 5" Gauge Aspinall Class 27
Posts: 575 
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Post by kipford on Mar 22, 2021 18:08:26 GMT
Chris
Adding weight to the loco using the nose weight of the tender is a very common trick in the smaller scales where models have a very forward cg.
Dave
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Post by ejparrott on Mar 22, 2021 19:24:13 GMT
Lead and carbide are your friend. Last thing I would do is make a cab from 1/4" plate. With our Netta we added some detail to the cab in the form of the false wooden floor and the two side seat boxes. In these we hid 6Kg of scrap carbide glued together with lead poured in. Thanks Ed, you've just talked yourself out of a job there, would have been laser cut I will look into a false footplate and seat boxes but as you see below there's a lot of pipework that needs to move to make room for some simple pieces. Sight feeds are going as changing to mechanical lubricators as is the condensing coil. Then I need to reposition the axle box lubricators and finally (if I can do it easily) move the injector steam pipes each side. This will make the whole area much clearer to get a thick floor and some seat boxes in there.  Below the footplate is the hydrostatic oil tank which is going but it still leaves the steam brake cylinder to go back in if only to stop the brakes moving unintentionally. Also in there is the injector feed pipes to the check valves low down on the back head. So all pretty congested and would be so much easier if adding this to a new build.  Chris D I think you've got more room there than you realise. Nothing stopping you adding some weight up the front of that stretcher, and either side of the steam brake trunnions, and inside the drag beam. you can also add it under the cab side footplating. My 2.5" Gauge Annie Body has a lead filled ashpan, that was necessary to keep he back end down. Conversly my 3.5" Hunslet has several pounds under the front footplate |
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dscott
Elder Statesman
Posts: 2,440
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Post by dscott on Mar 23, 2021 0:59:29 GMT
 In fact the inner set of hornblocks grew to become a 10 mm 3/8" slab of metal on Asia Europa 54. This is right in front of the middle stretcher standing upright. This needs a curve cutting out for the boiler which is slightly smaller so the eccentrics will go round!!! The original had full length frames and inner horns so I copied. Weight just where you need it. There is a cut out each side for a water pump with a small ram. I spent the day making and cutting outer spacers. Yes looking longingly at the cut off saw in bits. These add weight and locate the outer frames with Five 2 BA allen screws up from below. The rear buffer beam has two which I was also doing on Monday. David and Lily. |
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Gary L
Elder Statesman
Posts: 1,208
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Post by Gary L on Mar 23, 2021 2:01:29 GMT
Of course in full-size, axle loadings were critical on the heavier locos, so it was essential to make the effort. In miniature the argument seems less -er- pressing, though some people take it very seriously. Personally I'm happy if I can get the engine level port and starboard and both buffer beams to the same height! Gary Hi Gary, I wouldn't underestimate the effect of uneven loading in each wheel on miniature locomotives. It's been clearly demonstrated that soft springing makes a big difference to the amount of grip available. Regardless of the springing, the wheel with the lightest loading will give up first and cause a cascade of slipping. The closer they are to each other, the higher the overall adhesion will be. In these times where more and more tracks are sadly going over to Aluminium rails, getting as much adhesion as possible seems to me to be more important than ever. In the end, either you measure it or you don't. If you do, then you may as well go for the best possible setup. If you don't measure it, you have absolutely no idea what's going on, and from my measurements, it's not uncommon for some wheels to be carrying twice the load that others do. That has to have a negative impact on the locomotive's pulling power. Maybe on very large locomotives that's not an issue, but for small ones I suspect that it is. Hi Roger and Chris I agree about the soft springing, but WRT excessive slipping, maybe I've just been lucky. I don't mean to deter or denigrate anybody trying to equalise the loadings on their wheels; I'm sure there are benefits. Balance is worth striving for for its own sake; but I do struggle with the adhesion logic, because when the wheels are locked together by coupling rods so they move as one, why would one wheel let go before another? And if the lighter-loaded wheels have less adhesion as they must, don't the heavier-loaded wheels have greater adhesion to compensate?? Gary |
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Post by Roger on Mar 23, 2021 7:54:18 GMT
Hi Gary, I wouldn't underestimate the effect of uneven loading in each wheel on miniature locomotives. It's been clearly demonstrated that soft springing makes a big difference to the amount of grip available. Regardless of the springing, the wheel with the lightest loading will give up first and cause a cascade of slipping. The closer they are to each other, the higher the overall adhesion will be. In these times where more and more tracks are sadly going over to Aluminium rails, getting as much adhesion as possible seems to me to be more important than ever. In the end, either you measure it or you don't. If you do, then you may as well go for the best possible setup. If you don't measure it, you have absolutely no idea what's going on, and from my measurements, it's not uncommon for some wheels to be carrying twice the load that others do. That has to have a negative impact on the locomotive's pulling power. Maybe on very large locomotives that's not an issue, but for small ones I suspect that it is. Hi Roger and Chris I agree about the soft springing, but WRT excessive slipping, maybe I've just been lucky. I don't mean to deter or denigrate anybody trying to equalise the loadings on their wheels; I'm sure there are benefits. Balance is worth striving for for its own sake; but I do struggle with the adhesion logic, because when the wheels are locked together by coupling rods so they move as one, why would one wheel let go before another? And if the lighter-loaded wheels have less adhesion as they must, don't the heavier-loaded wheels have greater adhesion to compensate?? Gary Hi Gary, The key point is that you can't assume that the friction is linear and proportional to the Normal load applied to the wheel. If this were the case, light springing would have no benefits yet we know that it certainly does. I won't pretend to understand this complex topic, but I believe that the shape of the contact patch varies, much as a pneumatic tyre has a flat patch where it contacts the road. As the load increases, you get less and less increase in contact area for each unit of load. In other words it's an ever diminishing gain in grip for each unit of applied load. Once you accept that the force needs to be the same on each wheel to maximise the tractive effort, I think it helps to understand the logic of the wheel slipping if you take it to extremes. Picture a six wheel coupled locomotive which is balancing on the centre axle with no weight on the outer four wheels. Clearly there are only two wheels contributing to the grip. It doesn't matter that there are four wheels coupled to the middle wheels, they contribute nothing to the adhesion. The amount of grip is entirely supplied by the two wheels, which we already know can't provide the same grip as six wheels. It would be interesing to set up a locomotive like this and see what force it takes to drag it along the track, and then compare it to one set up correctly. If you imagine coming slowly from that extreme, gradually adding weight to the outer axles, you can picture the situation improving until all of the wheels carry equal weight. The dynamics of slipping are also complex, where the maximum grip appears to be just as the wheel begins to slip. However, that figure is going to be different for wheels with different loading, so the optimim point won't be reached by all wheels at the same time. Clearly, the lighter the springs, the more evenly the weight can be distributed over each of them. Again, picturing the axles being bolted solidly at the correct riding height makes it easy to see why that's so. Hopefully that makes sense. |
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Gary L
Elder Statesman
Posts: 1,208
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Post by Gary L on Mar 23, 2021 15:11:00 GMT
Hi Roger and Chris I agree about the soft springing, but WRT excessive slipping, maybe I've just been lucky. I don't mean to deter or denigrate anybody trying to equalise the loadings on their wheels; I'm sure there are benefits. Balance is worth striving for for its own sake; but I do struggle with the adhesion logic, because when the wheels are locked together by coupling rods so they move as one, why would one wheel let go before another? And if the lighter-loaded wheels have less adhesion as they must, don't the heavier-loaded wheels have greater adhesion to compensate?? Gary Hi Gary, The key point is that you can't assume that the friction is linear and proportional to the Normal load applied to the wheel. If this were the case, light springing would have no benefits yet we know that it certainly does. I won't pretend to understand this complex topic, but I believe that the shape of the contact patch varies, much as a pneumatic tyre has a flat patch where it contacts the road. As the load increases, you get less and less increase in contact area for each unit of load. In other words it's an ever diminishing gain in grip for each unit of applied load. Once you accept that the force needs to be the same on each wheel to maximise the tractive effort, I think it helps to understand the logic of the wheel slipping if you take it to extremes. Picture a six wheel coupled locomotive which is balancing on the centre axle with no weight on the outer four wheels. Clearly there are only two wheels contributing to the grip. It doesn't matter that there are four wheels coupled to the middle wheels, they contribute nothing to the adhesion. The amount of grip is entirely supplied by the two wheels, which we already know can't provide the same grip as six wheels. It would be interesing to set up a locomotive like this and see what force it takes to drag it along the track, and then compare it to one set up correctly. If you imagine coming slowly from that extreme, gradually adding weight to the outer axles, you can picture the situation improving until all of the wheels carry equal weight. The dynamics of slipping are also complex, where the maximum grip appears to be just as the wheel begins to slip. However, that figure is going to be different for wheels with different loading, so the optimim point won't be reached by all wheels at the same time. Clearly, the lighter the springs, the more evenly the weight can be distributed over each of them. Again, picturing the axles being bolted solidly at the correct riding height makes it easy to see why that's so. Hopefully that makes sense. Yes it does thanks. It's to do with creep and hydrostatic displacement, which I'd never heard of till a few years ago; you have explained that very well, especially the diminishing returns in adhesion as you increase weight for a given wheel diameter. There comes a point when you have run out of ballast-shifting options and only have spring rates left to alter. That in turn is likely to mean making some springs harder, and chances are it would reduce spring travel in the process. How far would you go in that direction? Is there a scientific way of finding the sweet spot? I recall a loco that had very soft springs; it would bounce along the club track rather like a kangaroo, because coil springs have little inherent damping, unlike the leaf springs on full-size locos. So perhaps we generally have stiffer springs than are ideal from an equalisation point of view. Gary |
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Post by Roger on Mar 23, 2021 15:58:00 GMT
Yes it does thanks. It's to do with creep and hydrostatic displacement, which I'd never heard of till a few years ago; you have explained that very well, especially the diminishing returns in adhesion as you increase weight for a given wheel diameter. There comes a point when you have run out of ballast-shifting options and only have spring rates left to alter. That in turn is likely to mean making some springs harder, and chances are it would reduce spring travel in the process. How far would you go in that direction? Is there a scientific way of finding the sweet spot? I recall a loco that had very soft springs; it would bounce along the club track rather like a kangaroo, because coil springs have little inherent damping, unlike the leaf springs on full-size locos. So perhaps we generally have stiffer springs than are ideal from an equalisation point of view. Gary Hi Gary, One further thought about weight distribution is that you probably want more on the front to get the maximum tractive effort. The locomotive will squat and transfer some of the weight to the rear when starting. I guess one way to figure out how much would be to see what the wheel loadings are when pulling against a fixed point. With the locomotive on the scale, you could turn the wheels and see what the new loading is. I guess you would ideally end up with the same weight on each axle at the point where it just slips as you turn the wheels. I guess you could use a slightly higher spring rate at the front in an attempt to get the weights all the same. That might give a better solution than using higher spring rates all round. That way the softer sprung wheels have a better chance of gripping. I've included attachment points for dampers which I might experiment with. Maybe I've missed something, but as far as I'm away, dampers haven't been fitted to Model Steam Locomotives. The damping you get with leaf springs is far from idea, but I suppose it's better than nothing. I'll be interested to see how far out mine is. It's a complete guess that led me to add the massive stretcher under the Cab. At least if it's too much, I can machine some of it away. |
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Post by RGR 60130 on Mar 23, 2021 17:36:59 GMT
I can't help wondering if more consistent results might be obtained if the coupling and connecting rods were removed before weighing. I'm sure a thousand theories will abound!
Reg
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Post by coniston on Mar 23, 2021 22:00:55 GMT
That's a thought worth considering Reg, I guess the rods shortening due to radius of movement could impart more friction in the axle box then. When I get back to weighing mode I'll try and remember to give this a go and see if there is any noticeable difference in the readings or at least any improvement in consistency.
Chris D
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robmort
Hi-poster
3.5" Duchess, finishing 2.5" gauge A3 and building 3.5" King
Posts: 174
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Post by robmort on May 16, 2021 9:15:07 GMT
Once you accept that the force needs to be the same on each wheel to maximise the tractive effort, I think it helps to understand the logic of the wheel slipping if you take it to extremes. Picture a six wheel coupled locomotive which is balancing on the centre axle with no weight on the outer four wheels. Clearly there are only two wheels contributing to the grip. It doesn't matter that there are four wheels coupled to the middle wheels, they contribute nothing to the adhesion. The amount of grip is entirely supplied by the two wheels, which we already know can't provide the same grip as six wheels...... Do we accept that?
Do we know that 2 wheels can't provide the same grip as 6 with the same total load?
Friction is linearly proprtional to the weight applied, and is independent of the area of contact. So 2 wheels should be the same as 6 with the same total weight.
Full-size locos had more wheels to limit each axles load not to provide better adhesion.
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oldnorton
Statesman
5" gauge LMS enthusiast
Posts: 721
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Post by oldnorton on May 16, 2021 9:29:53 GMT
Do we know that 2 wheels can't provide the same grip as 6 with the same total load?
Friction is linearly proportional to the weight applied, and is independent of the area of contact.
Yes, our schoolboy physics experiments with the blocks sliding down a ramp told us that. I could never reconcile that with the real world of friction. For example, the bigger the friction surface on drum brakes the better the braking, up to a limit; and then disc brakes come along with smaller surfaces but greater pressure (force/weight). And were not the railways convinced that an eight coupled wheel set gripped better than a six, and single axle drivers didn't do too well. Would be nice to hear the full physics story. |
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robmort
Hi-poster
3.5" Duchess, finishing 2.5" gauge A3 and building 3.5" King
Posts: 174
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Post by robmort on May 16, 2021 9:55:02 GMT
Yes, our schoolboy physics experiments with the blocks sliding down a ramp told us that. I could never reconcile that with the real world of friction. For example, the bigger the friction surface on drum brakes the better the braking, up to a limit; and then disc brakes come along with smaller surfaces but greater pressure (force/weight). And were not the railways convinced that an eight coupled wheel set gripped better than a six, and single axle drivers didn't do too well. Would be nice to hear the full physics story. The full physics story is the same as your shoolboy experiments, the physics hasn't changed! Static friction (rather than dynamic friction for sliding surfaces) is what we're talking about here for adhesion, and is independent of area. See many explanations on-line as usual, but if you want: zebu.uoregon.edu/1999/ph161/friction.html |
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stevep
Elder Statesman
Posts: 1,073
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Post by stevep on May 16, 2021 10:00:34 GMT
Like oldnorton, I remember the physics experiments at school, and also like him I have never been able to reconcile that with the real world. Why, for instance, do F1 cars have such wide tyres?
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robmort
Hi-poster
3.5" Duchess, finishing 2.5" gauge A3 and building 3.5" King
Posts: 174
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Post by robmort on May 16, 2021 10:55:19 GMT
Like oldnorton, I remember the physics experiments at school, and also like him I have never been able to reconcile that with the real world. Why, for instance, do F1 cars have such wide tyres? Unlike steel wheels, for elastic parts (like tyres, or billiard balls on felt) the contact pressure surface has various shapes. The parts with the highest pressure (sometimes in the middle, and often at the edges) will "stick" more than the unloaded parts. But in the end the total traction to achieve full slipping is still ππ to a good approximation. Also F1 tyres need to dissipate a lot of heat hence need to be wide, and their temperature is also optimised to maximise the π coefficient. |
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uuu
Elder Statesman
your message here...
Posts: 2,858
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Post by uuu on May 16, 2021 10:58:59 GMT
As with all school physics, only the perfect conditions are explored. There's no accounting for one soft surface deforming to accommodate peaks and troughs in the other. Nor making one surface hot and sticky (hence the F1 obsession with keeping the tyres hot).
Wilf
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Post by andyhigham on May 16, 2021 11:13:31 GMT
The rear slick on my race bike feels like any other rubber tyre but softer, you can dig a thumb nail in it. A couple of seconds spinning the wheal to warm the tyre and clean the surface and the rubber feels like freshly chewed chewing gum  20210516_121139 by Sigma Projects, on Flickr |
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oldnorton
Statesman
5" gauge LMS enthusiast
Posts: 721
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Post by oldnorton on May 16, 2021 19:41:32 GMT
Unlike steel wheels, for elastic parts (like tyres, or billiard balls on felt) the contact pressure surface has various shapes. Surely steel wheels are elastic, it is just the scale that is different. Also the rail is elastic, it deforms to make a well, and the steel wheel deforms to meet the rail. Now in that scenario all our 'ideal' friction physics has gone to pot. I remember watching locos at Gilling coming past the steaming yard as the oily aluminium track got wetter in the rain. The engines with uneven wheel loadings lost grip first. Then, a renowned 2-8-2 loco with a mass of added tungsten+lead hidden away was slipping just as badly as all the others. That taught me that adding massive weights was not the golden answer. Norm. |
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dscott
Elder Statesman
Posts: 2,440
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Post by dscott on May 17, 2021 0:32:31 GMT
The Ultimate test is down at the Andover Track where you leave the Station on a curve, you are going uphill.
You are under trees. You are on aluminum track and you are driving a Rob Roy.
Years ago I began a collection of BITS and roughly bunged together became THE BEAST.
Powered by the spare out of scale Speedy Boiler. Superb boiler of course. Plus some spare cast iron cylinders.
(Machined)
Mostly made the frames of blocks for weight just lacking the time to bolt everything together.
Got the chance of some free wheels slightly slim on the turning so have some EN3B steel being cut today to make
some tires from. We give Scott a cutting list at Berkshire Metals and go and collect later.
David and Lily.
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Post by Roger on May 17, 2021 8:22:18 GMT
Unlike steel wheels, for elastic parts (like tyres, or billiard balls on felt) the contact pressure surface has various shapes. Surely steel wheels are elastic, it is just the scale that is different. Also the rail is elastic, it deforms to make a well, and the steel wheel deforms to meet the rail. Now in that scenario all our 'ideal' friction physics has gone to pot. I remember watching locos at Gilling coming past the steaming yard as the oily aluminium track got wetter in the rain. The engines with uneven wheel loadings lost grip first. Then, a renowned 2-8-2 loco with a mass of added tungsten+lead hidden away was slipping just as badly as all the others. That taught me that adding massive weights was not the golden answer. Norm. Absolutely correct. Everything is elastic, and you can't use simple coefficients of friction to work out the adhesion of wheels on rails. Equal loadings on the wheels is the way to achieve the maximum grip in any situation. Modern locomotives allow for a creep/slip situation where the maximum grip is just as it begins to slip. |
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