what kinds of loco modification resulted in changes in weight on drivers resulting in less than ideal balance?
presumably, the ideal weight distribution is the same on all drivers, but i see in the chart below that modifications (e.g. I5 abc) sometimes exacerbate weight distributions. the weight distribution of the I5b & c are worse than the I5a. The I7b, k and i improved weight distribution. The I8b improved over the I8a but the I8sd which presumanly added a superheater resulted in an acceptable (?) degradation. There were no improvements to the I10sa which had a near ideal distribution
when did loco designers recognize the value of balanced weight on drivers?
Since total tractive effort is not affected by weight distribution on the drivers the consideration would be the strength of the rails. Put another way, at the weight limits of the rails it is beneficial to spread the load equally among the drivers.
Otherwise, since the coefficient of friction remains the same regardless of weight on any particular driver the total tractive effort remains constant regardless of weight on any one driver.
The connecting rods tying all the drivers into one unit appear to make a difference but do not. They are tied together only because there are not enough pistons.
I do not understand? If you were to lift one set of drivers 1/16 inch off the rails the rest of the weight would be on the remainder of the drivers. The one set off the rails would not have any traction, would it?
Keep in mind that most of these kinds of change in modern practice involve equalized locomotives, where tinkering with the lever geometry is what determines the effective load on drivers and weight transfer to associated carrying wheels. This also fully addresses any issues with drivers supposedly ‘raised’ above or below their neighbors. (this was a timeless topic on steam_tech where John Knowles would advocate the English practice of individually springing each wheel ‘just right’ without the added weight of all those levers)
Weight distribution is also affected by where various auxiliaries and components are placed. This is sometimes discussed in detail (as with some of Larry Brashear’s comments about ATSF design ‘engineering’ in the early '20s, for example about what was necessary for 325psi boiler pressure). Since freight motive power design in that era was more a matter of low-speed expediency (with necessary frequent track repair for other reasons) changes that gave unequal driver load might be tolerated if fixing them would cost more than the perceived inconveniences.
There is such a thing as ‘tapered loading’ Some references call it a misguided design principle, but I’ve always liked the concept in principle (one perhaps naive idea being that both axle load and peak augment would be rolled progressively onto less-than-perfect track, another being that less load on the lead driver produced lower shock going into curves).
If wheel arrangements were modified with different trucking, I would assume that the equalization would be one of the first concerns.
Reading did very famously get things awful wrong from time to time, but not (to my knowledge) with weight distribution. They very famously tried a pin-guided Adams truck under the back of an otherwise-good Atlantic (it guided and teetered more or less exactly as you’d expect it would looking at its pictures) and tried spring late
You must have a reading-comprehension ‘issue’ [:)] - equalization is perhaps the most important consideration in spring rigging on American locomotives, particularly on lead and trailing trucks in modern practice.
I could go into the finer points of spring rigging and snubbing but there are people here who would pay to keep me from it…
There is a limited number of places to put, and then plumb, devices like air compressors or feedwater heaters – and if you have just one it will affect what and where something else has to go on the other side. Sand dome needs to be reasonably above driver wheelbase for both forward and reverse.
More likely, priorities changed. Weight distribution on drivers was discussed extensively in Baldwin literature by I think 1893 – perhaps well earlier.
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when did “weight distribution (?)” get figured out? was weight distribution just a matter of adjusting driver or truck suspension or did it require moving components?
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All the above, as required.
In the old days this was a very tedious operation. Essentially the locomotive was divided into longitudinal ‘slices’, the weights of which were then figured; side-to-side lever arms and torque moments were likewise doped out, then everything was correctly added up and any longitudinal moments calculated. Even small nominal changes might result in a need for substantial recalculation – all of it charged to motive power, whereas impact (no pun intended) on track expense would not be.
Not too often, as there was usually ‘something’ that could be put on if you needed more adhesive weight. I think I recall N&W experimenting with lead weighting in the forward engine of one of the articulateds, but I’ll leave that for feltonhill or Big Jim to address more knowledgeably.
I would not be surprised to see some weights applied to one side or the other to counterbalance an auxiliary moved to a different location, e.g, compressors moved from hanging off the side of the boiler to the pilot deck. But that would s
There is a limited number of places to put, and then plumb, devices like air compressors or feedwater heaters – and if you have just one it will affect what and where something else has to go on the other side. Sand dome needs to be reasonably above driver wheelbase for both forward and reverse. You see some awful results from some of the decisions… NYC K6b’s, anyone?
More likely, priorities changed. Weight distribution on drivers was discussed extensively in Baldwin literature by I think 1893 – perhaps well earlier.
All the above, as required.
In the old days this was a very tedious operation. Essentially the locomotive was divided into transverse ‘slices’, the weights of which were then figured; side-to-side lever arms and torque moments were likewise doped out, then everything was correctly added up and any longitudinal moments calculated. Even small nominal changes might result in a need for substantial recalculation – all of it charged to motive power, whereas impact (no pun intended) on track expense would not be.
Not too often, as there was usually ‘something’ that could be put on if you needed more adhesive weight. I think I recall N&W experimenting with lead weighting in the forward engine of one of the articulateds, but I’ll leave that for feltonhill or Big Jim to address more knowledgeably. I’m sure there were others.
I would not be surprised to see some weights applied to one side or the other to counterbalance an auxiliary moved to a diff
It does not contribute to traction if actually off the rail but the total friction force remains unaffected provide the drivers don’t transition from static friction to kinetic friction by slipping. Slip point remains the same, disregarding complex effects from pressure affecting the steel and so on.
Counterintuitively but understood empirically even by toddlers less contact area between materials does not cause traction problems. Reason is friction is proportional to weight, well force actually. Reduce the total contact area of the drivers and you increase the pressure (weight per unit area )of the contact areas remaining on the rail which then generates the same total tractive force as you had before you lifted a driver off the rail.
This is why big heavy people don’t need special shoes to avoid falling down, or, if you prefer small light people don’t need different shoes to avoid falling down.
Often you hear that heavy trucks take longer to stop than lighter vehicles because of the weight. Although that is all complicated by the behaviour of rubber in pneumatic tires it is the case that heavy vehicles can be made to stop as well as lighter vehicles, same reason. A big 6,000 lb Bentley can stop in as short a distance as a 3,000 lb Toyota.
agreed if the driver is actually off the rail (not likely) the weight distribution changes
if the driver is actually slipping while on the rail, the weight distribution is unchanged but the force applied by the cylinder is redistributed to the remaining wheels which is not very likely to cause them to slip.
There’s actually more to it than that (equally understandable by toddlers). Think about a vehicle the size of a Prius that has to be driven on an icy road. Take tires all made of the same rubber compound, with the same tread depth. If you have road-bicycle-size tires you will slip and slide; normal car tire tread patch, better; wide ‘performance’ (or 22" showoff rim) treadwidth, you’ll just sit there with the wheels going around. It’s the contact patch characteristics that determine the adhesion – the reason we say ‘tires steer the car; tires brake the car’ – and there is some combination of patch size and contact under particular conditions that will be most effective.
Interestingly enough, in railroad adhesion the contact patch actually does deform elastically under axle load, and it is ‘enough’ larger for larger drivers that references mention the difference as significant.
It is possible to make a heavy track car perform as well as an ‘optimally light’ one … if for example you put skirts, fans, and a 40hp snowmobile engine on it for downforce. But you will immediately and instinctively recognize your tire life will be measured in seconds if you do… which is also the issue with decelerating heavy road vehicles with the right weight on the right area of contact patch…
I’m not going there. Pneumatic tires with rubber contact patches are pretty complex devices. I do know my stuff in that area.
My contributions here are intended to illustrate a very common misconception about friction and load.
I’m not sure about the context for the slipping driver situation. The usual engineering of steam locomotives means all drivers slip at the same speed or none of them do.
Up to the limit of slip of the lightest loaded driver, no driver will slip. Once that limit is reached for the lightest loaded driver then weight distribution does matter because of the difference between kinetic and static friction coefficients. The phenomenon of stiction is also related to this transition zone. Driving on ice gives an ordinarily skilled driver a good lesson on stiction effects.
The classic weight distribution modification was on the M-1sa class where the Reading aded a box of lead on the pilot deck to weight the nose of the engine for better tracking and to reduce derailments.
I know you do. That’s partly why I used that example.
The actual situation is different, because what determines the adhesion at the contact patch can, and often does, vary wildly by driver, and the propensity of a conjugated ‘set’ of drivers to break is related to the sum of the individual adhesions.
Assume for a moment that torque deflection in the axles is minimal and there is no tolerance in the rod bearings, and the rods do not deflect. If the locomotive encounters oil or a patch of that peculiar plastic material that leaves become when trains run over them, some of the drivers will experience diminished – sometimes radically diminished – “static” coefficient of friction, while others do not. At some point the balance between resultant of thrust and adhesion friction can ‘go negative’ at which point the drivers will begin to slip. Very (very) shortly after this the conditions between the rotating contact patches and whatever is under them on the railhead assume the characteristic of “sliding” friction, and if nothing changes at that point, the breakaway torque would now be sufficient to accelerate the spin (this doesn’t happen proportionally in reciprocating locomotives unless you have enough admission, precise enough valve gear, etc. to ‘make it’ around to the following admission, but it happens dramatically with electric motors or hydraulic drives) and you will have to reduce torque until the speed differential between patches is reduced enough for the asperities
Added weight, whether dead weight presumably in search of additional tractive effort or accessories needed or beneficial to add in some fashion. How much weight?
Where relative to the driver axles to put the weight?
Leverage effects from adding weight within the driver wheelbase or outside it, outside the total wheelbase including either the pilot or trailing truck or both?
Springing of each driver axle and the separate issue of springing the trucks.
Equalization connections between sprung axles.
I think those are the topics for discussion raised by the OP?
Brings to mind the equalizing spring pivots between the twin and triple leaf sprung axles under the mobile homes I built in the early 70’s (not single handed, but I did start on axles and hitches.) The lead and trailing ends of the pair of leaf springs on twin axles were connected as usual by shackles to spring hangers welded to the frame. But the center hanger was connected to the trailing end of the forward leaf spring and to the leading end of the rear leaf spring by a swinging casting that pivoted in the hanger and on through bolts on each leaf spring. Triple axles had the centre axle fully suspended on these swinging castings. The result going down the road would be to couple the spring rates for the two or three axles that would give a much softer ride but still fully support the weight. Also, impact forces from the lead tire riding over the bump first would be eased by rearward movement of the axle as the first spring compressed and the pivot, well, pivoted and also while at the same time extending the lead end of the next leaf spring downwards toward the road surface and so on. Net result would be less vertical motion imparted to the frame over a given bump and less bump impact force transfers too. Clever, cheap and effective.
