CG position from 4 corner load cells
CG position from 4 corner load cells
(OP)
All,
We are attempting to use 4 load cells at corners of a railcar to determine its CG.
Using 3 points instead (which I'd rather do because it's statically determinate) is not feasible.
A discussion on thread404-117973: Weight Distribution on Four Wheels touched on this. Does anyone have a repeatable method of establishing the CG position (in plan) using the 4 load cell readings? We even have the capability of raising and lowering 1 of the corners slightly and taking new readings. These additional ones could provide the missing equation!
Any ideas?
tg
We are attempting to use 4 load cells at corners of a railcar to determine its CG.
Using 3 points instead (which I'd rather do because it's statically determinate) is not feasible.
A discussion on thread404-117973: Weight Distribution on Four Wheels touched on this. Does anyone have a repeatable method of establishing the CG position (in plan) using the 4 load cell readings? We even have the capability of raising and lowering 1 of the corners slightly and taking new readings. These additional ones could provide the missing equation!
Any ideas?
tg





RE: CG position from 4 corner load cells
RE: CG position from 4 corner load cells
Statically determinant only means that the loads internal to the car can be calculated.
One item that is critical is to eliminate cross-coupling between horizontal loads and vertical loads. Load cells should be interally balanced to eliminate moment inputs and off-axis components from being resolved to on-axis values. Retesting with the load cells turned 90 and 180 should verify that if there is any question.
RE: CG position from 4 corner load cells
Quando Omni Flunkus Moritati
RE: CG position from 4 corner load cells
Quando Omni Flunkus Moritati
RE: CG position from 4 corner load cells
We are attempting to narrow this down to within 1/4 inch.
Railcar is approx 10 ft wide by 89 ft long by approx 14ft high, and we're trying to nail down lateral CG pos'n. Load cells essentially at corners.
It may be that load cells are not precise enough for what we're looking for.
tg
RE: CG position from 4 corner load cells
Quando Omni Flunkus Moritati
RE: CG position from 4 corner load cells
Fundamentally, though, in response space, the CG sits on a flattish hump, i.e., minor perturbations around the CG are not necessarily that detectable. Your requirement of 1/4 inch requires better than 1/8 inch accuracy, and out of 10 ft, that's a 0.1% accuracy, which doesn't sound like something that's doable on a rail car. Are the wheels even that well balanced?
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RE: CG position from 4 corner load cells
Quando Omni Flunkus Moritati
RE: CG position from 4 corner load cells
If you've done this more than once, and get a shift from one station to the next, is it somewhat repeatable? or random? If just one load cell were off, it'd throw the calculated CG off, but would be similar each time you measured identical items.
RE: CG position from 4 corner load cells
If, however, the car teeters on those two wheels and, because the CG has a vertical component, the CG shifts, then the load cells will get the new distribution of loads, and the calculation will show the side-to-side location of the CG has changed, because it has.
There is also the potential for the car to change shape over time due to uneven heating from solar load. You may wish to account for this as well.
RE: CG position from 4 corner load cells
RE: CG position from 4 corner load cells
RE: CG position from 4 corner load cells
RE: CG position from 4 corner load cells
if the car is rigid then the height of the CG doesn't change the location of the CG.
if the car is flexible then the height of the CG will work it's way into the results and add an unknown into the calcs ... the scales will tell you the point the CG is acting through, the line that it is on.
i guess that's a quesion to tg ... do you want all three CG co-ords or only two ?
Quando Omni Flunkus Moritati
RE: CG position from 4 corner load cells
Why bogies in UK - trucks in USA?
RE: CG position from 4 corner load cells
why lorries in UK and trucks in the US? ... this could go on forever!
Quando Omni Flunkus Moritati
RE: CG position from 4 corner load cells
Are the trucks still on the car, how have you gagged them up to the body, or are you just working with the car body? What kind of a car is it, along with a few more dimensions and the lt.wt.? A flat car won’t have many vertical component issues, but a high cube box or passenger car will have some vertical CG issues Where are you jacking or weighing it, at the jacking pads, at the side bearings, where and how? Could you put the car on a knife edges (round bars) on or near the center of each center plate, and move it until it is balanced. Then use a load cell under each center plate to find the longitudinal CG, or balance it on the side sills or center sill near the center. Maybe you should level the car as best you can on four jacks, then shim the load cells under their weighing points, so they all have 100lbs. on their dials; that’s zeroing them out, and should be subtracted out. Then lower the car onto the load cells. I would check a few of the critical dimensions on the car, such as: does the center line match the centers of center pls., of side bearings, of jacking pads, of side sills or sides, any other heavy components, for two reasons; so you have a sense of any obvious off center wts. and secondly so you know where you are weighing w.r.t. the centerline. If I could get within .25",with some repeatability, I think I would be pretty happy. Purchasing just told me that the guys who welded the left side of the underframe consistently used about 28 lbs. more welding consumables one each car, go figure.
RE: CG position from 4 corner load cells
Good god man, why?
You won't be able to manufacture them sufficiently repeatably to keep the CG location of each one that close.
The amount of grease and crud that accumulates on the underside will change the position by more than that.
It will never be loaded sufficiently uniformly for it to matter.
Measure as best you can as you have in the past.
Preferably, repeat on a few cars.
Calculate the average location of multiple readings.
Draw a circle around that average that is large enough to contain all of the individual readings, plus a little more.
For whatever balance calculations you are trying to do, calculate them for the average location and for a point on that circle that is the worst case for the calculation at hand.
Report nominal and "not expected to exceed" values.
RE: CG position from 4 corner load cells
The "real world" railcar is suspended BY SPRINGS in each of four corners between 2 sets of wheels on 4x axles. Those springs will settle and move as the car moves, and as the car is loaded and unloaded, weighed on the load collars and re-loaded on the springs. Not much, true. But what 2-3 percent?
Let's assume that you can reliably and consistently put your load back on the "perfect" car within 1/16 of an inch left/right and fore/aft every time.
Let's assume your load itself is consistently built every time within 1/16 of ITS center of gravity and at the same mass every time and at the same vertical center of gravity every time.
Let's assume your railcar is reliably and accurately and consistently re-built every time with its center of gravity within 1/16 of an inch every time with the same mass everytime.
Even with those three (almost impossible) assumptions, just placing the load "within spec" +1/16 on the car with a load CG change of 1/16 inch + a rail car CG change of 1/16 of an inch would throw the final CG 1/16 + 1/16 + 1/16 = out-of-measurement tolerance.
RE: CG position from 4 corner load cells
Have been away on family related matters - but we succeeded in getting pretty good repeatability when weighing last week, using 4 load cells that have domed tops.
Will provide details soon.
tg
RE: CG position from 4 corner load cells
A nearly similar analysis was requested in an old thread no. 404-279555, however, the request was to find the reactions at four corner points of an assumingly rigid, flat plate subjected to a load at a particular point. If you study my reply, it may provide an insight to a procedure related to your question. As stated above, railroad car spring deflections will tilt the car and that may or may not have to be taken into consideration depending on the accuracy that you would allow.