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Post by Roger on Oct 1, 2021 13:44:44 GMT
I disagree with your view Roger...it's not about how many springs you have. It is about contact point and it is about with two springs spread apart they can be softer and in some respect will can be acting against each other further softhenibg the rebound. Both springs don't always work in unison, we are not talking about a static situation where yes you are correct, we are talking about a moving object on an undulating surface. The difference may be small but a difference none the less. The two springs spread the load just as a leaf spring does and why they are used on HGV's to carry heavier loads. Modern railway stock with coil springs use rows of mutiple springs, this not only spreads the load but also means the springs can be softer and thus not bounce along the track leading to derailment as Ed is experiencing with his model. Edit: I fogot to include that a single coil spring does one thing, it supports the load above it. It has no bearing on braking, accelerating which leaf springs do, two coil springs on the fore/aft axis also have some affect here, not the same as they have no dampening but in my mind it does make a difference. Two springs at half the stiffness of one gives an identical point load on the axlebox. I don't see how that isn't true. If the two springs them connect to two points set apart on the chassis, there's an equal force on each of them, so that's directly equivalent to a point load on the middLe. You can't make a stool more stable by clever arrangements of legs that attach the seat to the legs. This is in effect what you are suggesting. If you draw out any arrangement of springs you like along a line, then put arrows, spring forces and dimensions between them, you can work out the moments. It will always balance out. Like you've said elsewhere, you can't beat physics. |
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Post by Deleted on Oct 1, 2021 14:14:11 GMT
Yes, equal on a static item, not when moving, the force is the same, the action of it's movement/operation is not and why if you look into leaf springs you'll note their superior load bearing due in part the the spread of the contact points, two vs one Your example of a stool is not a good one or perhaps it is? try supporting it with one leg. With a static and superb balance yes you can for a short time, add movement...  |
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Post by Jock McFarlane on Oct 1, 2021 16:01:18 GMT
Two springs at half the stiffness of one gives an identical point load on the axlebox. I don't see how that isn't true. If the two springs them connect to two points set apart on the chassis, there's an equal force on each of them, so that's directly equivalent to a point load on the middLe. You can't make a stool more stable by clever arrangements of legs that attach the seat to the legs. This is in effect what you are suggesting. If you draw out any arrangement of springs you like along a line, then put arrows, spring forces and dimensions between them, you can work out the moments. It will always balance out. Like you've said elsewhere, you can't beat physics. I am interested in this subject and its conclusion as I am figuring out the springing on some 4 wheel coaches. I would agree with Roger that whether its leaf spring or a single coil spring it makes no difference as long as the springs will bear the same weight. My thought is that as the axlebox can only go up or down and there is one load bearing point in the middle at the top it cannot possibly make a difference whether you use a leaf arrangement or a single coil. I am favouring a single coil (with perhaps a weaker coil in the middle) with dummy leafs for appearance. The other factor in favour of coils in my case is that these 4 wheel coaches are of mid Victorian vintage and in 7 & 1/4" the top leaf would be around 11 inches long which makes them expensive and probably difficult to get right. Regards JM |
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Post by Deleted on Oct 1, 2021 16:28:22 GMT
fair enough Jock, each to their own but do the two act the same? A spring and leaf spring with equal load capacity does not act the same in operation. The leaf spring is softer or should I say it responds to loads better, less harsh. This is not only due to its naturual dampening effect, but it's also due to the spring hangers which flex/pivot and thus less sharp when responding to undulations. Add shock absorbers to the equation too which is normal practice, certainly on LNER stock and you have a far better suspension system. I would say superior to old car setups which have the leaf secured solid to the chassis one end, usually the front and then the rear has a shackle giving the effect as stated above, the shackle is the loco's spring hanger. If you do use coils on your coaches then yes I would agree with you to add another within to get a better ride. Sorry Ed we seem to be wandering off your loco, I have had my say and given my reasons, perhaps not in a very good way, I'm no teacher but I have tried to explain my thoughts on the subject as best as I can. There are reasons why railways used leaf springs and why in some cases they changed from coil to leaf and leave it at that... Regards Pete |
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Post by terrier060 on Oct 1, 2021 17:16:39 GMT
Hi Pete
I have just come in from a long walk in the sunshine and have not had a chance to read all through yet. I am amazed that now we are on page 4! I am going to cook a rump steak for my brother who will be leaving on Sunday, then I will try and absorb the discussion. Some of it is getting very technical but I am sure we will all learn by it.
Ed
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Post by Roger on Oct 1, 2021 17:36:50 GMT
fair enough Jock, each to their own but do the two act the same? A spring and leaf spring with equal load capacity does not act the same in operation. The leaf spring is softer or should I say it responds to loads better, less harsh. This is not only due to its naturual dampening effect, but it's also due to the spring hangers which flex/pivot and thus less sharp when responding to undulations. Add shock absorbers to the equation too which is normal practice, certainly on LNER stock and you have a far better suspension system. I would say superior to old car setups which have the leaf secured solid to the chassis one end, usually the front and then the rear has a shackle giving the effect as stated above, the shackle is the loco's spring hanger. If you do use coils on your coaches then yes I would agree with you to add another within to get a better ride. Sorry Ed we seem to be wandering off your loco, I have had my say and given my reasons, perhaps not in a very good way, I'm no teacher but I have tried to explain my thoughts on the subject as best as I can. There are reasons why railways used leaf springs and why in some cases they changed from coil to leaf and leave it at that... Regards Pete With two springs, the two contact points press equally against the frame at exactly the same distance either side of the axle box centre line. It doesn't matter whether it's a dynamic or static situation, the forces are precisely shared between the two springs. Therefore the effective force applied to the frame is the sum of the two spring loads exactly above the axle. You can't argue that one end of the spring hinges against the other in one direction without applying the same argument to the other end. The result is that the force is effectively applied exactly between the springs. Picture the two springs being attached to a seesaw with the pivot connected to the frame directly above the axle. You're suggesting that the force will somehow tip the seesaw, but that's clearly not going to happen. You can't magically get the force to favour the outboard spring. If what you say was true, you would have a different response if you then doubled the distance between the two springs. In reality, all that happens is that the effective spring force is still exactly over the axlebox. There's nothing to be gained by having two springs of half the weight. The Axlebox just sees the same force, it's unaware of what you've done above it. Another way to see that this is the case is to invert the whole arrangement. Draw a line at the top of the page to represent what was the track, and one at the bottom to represent the chassis. Then try to argue that the arrangement is more stable. It clearly isn't. The only way to make it more stable with the same spring arrangement is to increase the distance between the axles. |
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Post by chris vine on Oct 1, 2021 18:06:12 GMT
Yes, that is exactly correct, Roger.
Pete, I think that a reason for using leaf springs in the old days, is that they were perhaps easier to make for the old loco works. It must be quite difficult to make a coil spring if you haven't got a coil spring making machine!!
Another advantage of the leaf spring is that there is quite a lot of redundancy if a leaf breaks due to fatigue etc. A coil spring would tend to fail catastrophically, where a leaf spring would sag or show up its defect at an inspection.
Another advantage for the leaf spring is that its load - into the chassis/frame - is spread. I am not meaning the acting load on axle box, but if the frame is not very strong (think Model T Ford) then taking the load into the chassis at two locations could be quite a big advantage; compared to having to have a very strong bit of chassis at the top of a coil spring; and then the chassis flexing either side of it!
However, if you ignore friction, then the action of the leaf spring on the axle box and the frame (as a whole) is identical. (as long as they are acting in the same direction.) It doesn't matter if the situation is static or moving all over the place.
That brings one to the big advantage of the coil spring - its un-sprung mass. if you look at the wheel accelerating up and down over uneven track or road, then only around half of the spring is regarded as un-spring mass. One end is moving fully, the other end is not moving (with the wheel) at all. The ability of the wheel to follow track/road with a fairly constant force depends on having the lowest possible un-sprung mass. EG Alloy wheels in cars and spoked/boxpok wheels etc on railways.
It was quite a while before engineers understood the difference between un-sprung and sprung weight and its damaging effect on track and foundations.
There is the true story of the Bridge Stress Committee who instructed the driver of a small-wheeled goods loco to drive over a bridge as fast as he could while they measured the deflections. He took them at their word and flew across with regulator wide open and sparks from the chimney. The committee were appalled and shaken by the performance and tried to give him a row. But he was only doing what they had told him!!
Chris
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Post by Deleted on Oct 1, 2021 18:07:55 GMT
I'm afraid you and I are never going to agree and we are getting away from Ed's problem, perhaps my fault for not explaining myself well enough, basically road dynamics are what I'm trying to explain in loco terms, the needs are similar.
Anyway, to get back to springs, springs alone are not suitable for this job. Quote: Unless a dampening structure is present, a spring will extend and release the energy it absorbs from a bump at an uncontrolled rate. The spring will continue to bounce at its natural frequency until all of the energy originally put into it is used up. A suspension built on springs alone would make for an extremely bouncy ride and, depending on the terrain, uncontrollable.
Substitute terrain for track and you get the idea, I was trying to explain that IMHO the two spring setup will deal with this better than one, evidently, I failed miserably to get the message over..lol
The fact that the loco with the two spring setup works would back up my view. It's totally irrelevant that it would work better with dampers, yes it probably would but we are dealing with the design of two locos presented to us as they stand, one works, the other doesn't, and the most obvious reason why is staring us in the face.
Pete
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Post by chris vine on Oct 1, 2021 18:12:14 GMT
I agree, Pete, but it isn't the position of the ends and forces from the springs into the chassis which is making the difference.
It must either be their rates (force divided by deflection) or damping from friction - maybe in the springs themselves, or hangers/rods or even in the axleboxes themselves.
Maybe there is a different distribution of mass along the length of the loco too??
Chris.
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Post by Deleted on Oct 1, 2021 18:16:21 GMT
I agree, Pete, but it isn't the position of the ends and forces from the springs into the chassis which is making the difference. It must either be their rates (force divided by deflection) or damping from friction - maybe in the springs themselves, or hangers/rods or even in the axleboxes themselves. Maybe there is a different distribution of mass along the length of the loco too?? Chris. All possible Chris as is the fact that the weight of the chassis is better supported, or should I say, more evening supported by the 8 points rather than 4. |
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Post by chris vine on Oct 1, 2021 21:19:26 GMT
Hi Pete,
I think that would only have any effect if the chassis was super flexible and was whipping about.
As it is, on our scale model locomotives, the chassis will many orders of magnitude more rigid than the spring system.
So it is back to the spring rates and damping (or lack of!), and not the distribution of the spring loads into the chassis.
Chris.
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Post by Deleted on Oct 1, 2021 21:33:37 GMT
Hi Pete, I think that would only have any effect if the chassis was super flexible and was whipping about. As it is, on our scale model locomotives, the chassis will many orders of magnitude more rigid than the spring system. So it is back to the spring rates and damping (or lack of!), and not the distribution of the spring loads into the chassis. Chris. Hi Chris I think I know why we disagree and yes it's down to me not explaining myself properly or more to the point how I see what is presented in front of us. You and Roger are seeing two fixed points with springs, I am seeing a compensating beam although not fully. For me there will be some flex in the dummy leaf spring and the buckle spring rod which holds it. With the weights involved and the size of the components there will be movement of the buckle rod and the spring hangers even if the dummy spring itself doesn't flex. To summarise, I am not seeing fixed points and tried to say this with the springs moving in relation to each other. Hope that makes my side a little clearer. |
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jma1009
Elder Statesman
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Post by jma1009 on Oct 1, 2021 21:41:49 GMT
Hi Ed C,
As we discussed on the phone when you bought the loco, I have an intense dislike of the crappy Milner design of these lovely Hunslet 'Quarry' locos.
Springing is very important for safety on these big lumps with a very short wheelbase and considerable overhangs fore and aft. I've previously mentioned balancing of the wheels. You have an inclined set of cylinders imparting a battering ram effect on the axleboxes, and not inconsequential masses rotating.
Factor in - what is it called? - is it amplitude or harmonic stuff? - and track joints and the speed of the loco, there are so many variables. You presumably want a loco that will not cause risk assessment issues and will be safe to use in public on a club track presumably hauling punters.
A 'bouncy' loco is one that to my mind has inadequate soft springing.
You also need to take into account the compromised valve gear of the Milner design. Others have alluded to this. Run it on the Hall simulator and you will understand. Your assertion that it runs ok notched up cannot be correct. It is incapable of being notched up. Indeed if you run it notched up the irregular valve events will have quite a dramatic effect on the springing - perhaps this is another thing to be factored in? The chassis starts to vibrate within itself, and throw up all sorts of other forces that ought to be avoided, and many of us will recognise this phenomenon. The 'bouncing' could easily be accentuated by notching up when the valve gear events will be going haywire and creating a 'kick back' effect and oscillations. This will be exaggerated on a short wheelbase 0-4-0 with large overhangs.
Cheers,
Julian
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Post by terrier060 on Oct 1, 2021 23:51:27 GMT
Sorry Julian but the loco as I understand it was built with modified valve gear and larger rectangular steam chests and runs very well knotched back. But in any case this discussion is rather irrelevant as we have two very similar locomotives and the only major differences that I can see are the positions of the springs, the travel and spring rates ( we cannot see Nick's springs and I am sure he will not wish to take his loco apart for me to assess them!!), and mine probably has a slightly different weight distribution due to the cab, which is quite heavy and has a high roof which Nick's does not, and mine has a taller and probably heavier chimney. That must affect the centre of gravity somewhat.
I will try and weigh the axles so that we have some extra data.
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jma1009
Elder Statesman
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Post by jma1009 on Oct 2, 2021 10:42:53 GMT
the loco as I understand it was built with modified valve gear... and runs very well knotched back. I am looking, Ed, at your second pic of 25th September 2021 on page 1 of this thread, and I can see no evidence of a modified valve gear, at least not incorporating the changes required so that it can be notched up without the valve events going haywire. Yours is exactly as I remember the Milner design with launch type links and no suspension offset. For direct drive outside admission and launch links, a very generous suspension offset is required. |
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Post by Roger on Oct 2, 2021 11:24:43 GMT
the loco as I understand it was built with modified valve gear... and runs very well knotched back. I am looking, Ed, at your second pic of 25th September 2021 on page 1 of this thread, and I can see no evidence of a modified valve gear, at least not incorporating the changes required so that it can be notched up without the valve events going haywire. Yours is exactly as I remember the Milner design with launch type links and no suspension offset. For direct drive outside admission and launch links, a very generous suspension offset is required. Hi Julian, I think you'd have to have a very keen eye to see any subtle changes in dimensions that might be involved in any modifications. Since the locomotive seems to run well when notched up, I'd suggest that it's not a wholesale modification that you can see. In the end of the day, all that matters is that it runs well, and that seems to be the case. From what I've heard, it steams very freely and is a delight to drive. Maybe you'll have the opportunity to give it a try and see for yourself. |
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Post by coniston on Oct 2, 2021 19:40:54 GMT
I'll just throw this in for some thought, my dad built a 3" scale Hunslet 7 1/4" gauge to the Reeves Elidir design. This has working leaf springs and having been running now for 21 years without any issues of instability even on his own garden railway of rather dubious quality, I can only assume the leaf springs are a good option for this type of short coupled long overhang loco.
For my money the difference in stability of the two locos being discussed is not to do with the location of the springs but the fact that one has twice as many springs of half the rating. Is there someone who knows if there is a difference in the response time for different rated springs? that just may be the argument for increased number of lighter rated springs?
Chris
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Post by Deleted on Oct 2, 2021 19:58:19 GMT
I'll just throw this in for some thought, my dad built a 3" scale Hunslet 7 1/4" gauge to the Reeves Elidir design. This has working leaf springs and having been running now for 21 years without any issues of instability even on his own garden railway of rather dubious quality, I can only assume the leaf springs are a good option for this type of short coupled long overhang loco. For my money the difference in stability of the two locos being discussed is not to do with the location of the springs but the fact that one has twice as many springs of half the rating. Is there someone who knows if there is a difference in the response time for different rated springs? that just may be the argument for increased number of lighter rated springs? Chris Agree Chris |
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Post by andyhigham on Oct 2, 2021 20:48:27 GMT
I have said this before and I will repeat it again.
More important than the suspension movement from rest position on the compression side is the movement on the extension side. Or as it is referred to in car and bike suspension "static sag" this needs to be around 25% - 30% of total movement.
When a wheel hits a depression, it needs to be able to drop into the depression. Also when a wheel hits a bump, the suspension compresses but also lifts the chassis, if there is no static sag it will lift another wheel
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jma1009
Elder Statesman
Posts: 5,922
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Post by jma1009 on Oct 2, 2021 21:14:38 GMT
Hi Roger,,
If as you suggest I don't know what to look for in a suspension offset on Stephensons valve gear, and might have not seen something that to me isn't there, then so be it.
I really don't know what to believe about this loco of Ed's anymore.
I suggested one specific reason relating to the valve gear that would exacerbate the springing problem based on what Ed himself has stated.
If anyone else would like to have a go at me on Stephensons valve gear, please do.
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