I've just read this in a book:
"Though there was no normal hammer blow in the wheels due to excess mass of revolving weights introduced to balance reciprocating parts, there was a true 'hammer blow' from the vertical component of the 71,000 lb thrust of the inside piston, which generally would be greatest at low speed."
I'm familiar with the strict definition of HB or dynamic augment (ie how to calculate it) and where it comes from (as is stated in the first part above).
My question specifically is how does the piston thrust manifest itself as an increase in the vertical load on the rail? eg if I'm sitting on some scales and push down with my hand I don't increase the reading. I can only do that intermittently by doing some body part accelerations as with HB.
Thank you
Pete
If you're squatting on the scales, then stand up, the scale reading will go up while you're rising, then drop as you're coming to a stop, then return to its original reading.
I agree with that because I'm an accelerating mass so there's a changing force to go with it. If on the other hand I stand on the scales and bend over and push down on my foot then, due to inertia of my body parts moving, the reading will change, but not because I'm pushing on my foot.
Take the 71,000 lb piston thrust with locomotive at rest, nothing is moving. Is it a force which adds to the weight on the rail? If not, then why would it do so if things were moving?
pete1950 "My question specifically is how does the piston thrust manifest itself as an increase in the vertical load on the rail? eg if I'm sitting on some scales and push down with my hand I don't increase the reading. I can only do that intermittently by doing some body part accelerations as with HB.
"My question specifically is how does the piston thrust manifest itself as an increase in the vertical load on the rail? eg if I'm sitting on some scales and push down with my hand I don't increase the reading. I can only do that intermittently by doing some body part accelerations as with HB.
The way I understand it is that the piston thrust, which is largely fore-and-aft, is compensated by adding extra wheel weights beyond what is needed for balance in the absence of any piston thrust. That extra balancing weight is what goes lump-lump-lump applying varying apparent weight to the rails as the wheels role.
One way to eliminate this "hammer blow" (besides going to 3 or 4 cylinders requiring the complication of between-the-frame cylinders and cranked axles) is to simply not compensate for the piston thrust, letting the locomotive surge back-and-forth "rur-rur-rur", jostling the crew and maybe the passengers in the first coach or two. On the 5AT project, they propose something they call a "Franklin buffer" between the locomotive and the tender to take up those back-and-forth surges. They have in mind a rather lightweight locomotive coupled to a big tender and they were planning on using the inertia of the tender to help.
If GM "killed the electric car", what am I doing standing next to an EV-1, a half a block from the WSOR tracks?
pete1950there was a true 'hammer blow' from the vertical component of the 71,000 lb thrust of the inside piston, which generally would be greatest at low speed."
Strangest because "true" hammer blow is an offset effect due to leverage; this would be absent from a center piston and rod.
What has to be meant is exactly what they say, the vertical component of rod thrust. This is due to rod angularity, just as in an internal-combustion engine, and is greater the shorter the main in question. Do not confuse this with the vertical component of rod momentum (which is, for example, what overbalance is intended to help address).
It is greater ... somewhat greater ... at starting because you have lower cyclic rpm and longer cutoff (starting cutoff, whatever that is) so you have higher effective pressure in the piston during the portions of the stroke where the vertical component of actual thrust is greatest. But at low speed what you have is a 'jacking up' and then an 'unloading' of the axle load on the main due to this vertical component (in the center rod, for the example given). Johnson essentially dismisses this (for American power) as being a comparatively slight percentage of the adhesive weight on the axle; Withuhn mentioned (in 1983) that the reason Voyce Glaze retained 75lb overbalance in the N&W class J mains was to balance this vertical component of active thrust (the remaining limited overbalance being carried in the other three driver pairs).
If you are on the scale and push down (fast enough), I assure you that you will see a momentary deflection of the scale plate relative to the base, and it will indicate a larger number; likewise, if you glue a handle to the scale and pull hard on it, you will see a momentary reduction. Did you forget that your body has inertia?
[The effect would be minimized further than I've indicated, on an American locomotive, because of the action of the equalization; I presume you're reading this out of a British reference where the individual driver axles are sprung but not equalized...]
Paul MilenkovicOn the 5AT project, they propose something they call a "Franklin buffer" between the locomotive and the tender to take up those back-and-forth surges.
No, that's not really what the Franklin radial buffer does -- see here for an instructive description. The buffer couples the surging in the locomotive much more effectively to the 'coaches' following, rather than allowing the surge to build up momentum, as it would if there were a coupling with play (or drawbar with wear or slack) between engine and tender, and then display a kind of longitudinal equivalent of hammer-blow to the first car's drawbar (or buffers, etc. in British practice).
Large American passenger power used a combination of lighter rodwork and much greater engine AND tender mass to help overcome the effect of surge. Note that without a tender, a Q2 under test on the PRR locomotive test plant exhibited considerable resonant longitudinal surge, enough to be alarming to some of the test personnel, the notable thing being that it was only observed at low speed (so not so much a consequence of inertial rod forces). But put a coast-to-coast tender on the locomotive, with a spring-controlled radial buffer controlling surge, and you had less difficulty...
Doesn't any change in rail load have to be caused by the acceleration of some part of the locomotive (as in overbalance hammer blow)? So in this case the vertical rod force at the crankpin is reacted at the slidebar so bridging the springing which opens up. The locomotive sprung mass is accelerated up with the required force to do that appearing as increased load on the rail.
If I am still on the wrong track perhaps another approach would be: how would we go about calculating the increased rail load from the 8,000 lb, say, vert rod force. ie what factors have to be considered?
I do apreciate your comments and thoughts on this.
(I cannot find the quote now although I was reading Read's Pennsylvania Duplexii).
pete1950Doesn't any change in rail load have to be caused by the acceleration of some part of the locomotive
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