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Very strange, one more... Inertia when starting

#1 User is offline   Jean-Paul 

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Posted 02 March 2018 - 08:54 AM

In the same spirit that trials made with steam engines, (see wheelslip problems posts), I tested my 141 R with a "standard" train constituted of ten sand hoppers loaded by 80 metric tons. I noticed, when friction parameters were expressed with Davis coefficients ( ORTSA= 910.90 ; ORTS B = 13.1624 ; ORTS C = 1.197900), Friction @ 0 km/h was "enormous" (see attached files, around 5767N / 80 t ), absolutely not in correlation with Davis equations which give, ]@ 0 (R = A + BV + CV^2) : 910.90 N. Inertia exists, but with roller boxes, certainly not so high. So, always using FCalc 2.0 and with same parameters, I obtained, in the old MSTS style : "910.9N/m/s -0.10 1.6mph 3.735N/m/s 1.752", which is mathematically almost equivalent for considered speeds.
Then, inertia @ 0 was around 1770 N; which is widely more acceptable for a modern roller-bearing freight wagon.
I think there's a bug in the way friction is calculated from Davis coefficients, but I'm sure that there is some "cristalline" explanation, no ?
By some way, this could explain why it's so difficult to start under sipping conditions in steps, no ?
Best Regards,
Jean-Paul

Attached thumbnail(s)

  • Attached Image: FCalc_Davis Type.jpg
  • Attached Image: FCalc_MSTS Type.jpg


#2 User is offline   copperpen 

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Posted 02 March 2018 - 12:45 PM

I have a question. How have you set up your wag files, do they include a line like ORTSBearingType ( Roller ) ?. If you do not, then OR will base the inertia and initial friction on plain bearings.

My 100 ton test wagons, bogie stock, roller bearings show 1593lbf at rest, if low friction bearings are used that drops to 771lbf.

#3 User is offline   Jean-Paul 

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Posted 02 March 2018 - 01:34 PM

 copperpen, on 02 March 2018 - 12:45 PM, said:

I have a question. How have you set up your wag files, do they include a line like ORTSBearingType ( Roller ) ?. If you do not, then OR will base the inertia and initial friction on plain bearings.

My 100 ton test wagons, bogie stock, roller bearings show 1593lbf at rest, if low friction bearings are used that drops to 771lbf.


Hi !
They do, as you can see (attached files)
Best regards,
Jean-Paul

Attached File(s)



#4 User is offline   copperpen 

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Posted 03 March 2018 - 01:53 AM

Comparing your wag file with mine yours is 20 tons lighter which is reflected in the lower "at rest" friction figure. All stock regardless of bearing type will have a high initial starting resistance including inertia which rapidly drops off as they start to move. The bearing resistance remains low while moving. The resistance figure shown then starts to climb again as speed increases, this is the Davis figures working showing overall resistance figures. If you want lower starting resistance, try changing Roller to Low.

#5 User is offline   Jean-Paul 

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Posted 03 March 2018 - 02:54 AM

Thanks for explanation, but how to explain that with 20 tons less than your 100 t hopper, resistance at start is more than 3 times higher ? Di you also use ORTS Davis parameters, or old configuration "MSTS type" (which gave more correct results).. Did you configure in "Low" if you used Davis parameters.
Must say I don't understand such a difference..
Cheers,
Jean Paul

#6 User is offline   copperpen 

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Posted 04 March 2018 - 01:26 AM

When I used Fcalc to get the Davis figures I used the default bearing type. As far as I know, Davis resistance figures are used when the train is moving, not at rest. The figure you see when not moving is the total bearing resistance for the bearing type used. As the train begins to move this bearing resistance drops quite fast for the first 5mph and then remains low until speed drops below 5mph at which point it climbs towards maximum.

The Davis figures work the other way. The train resistance figures get higher as the speed increases, most of which is wind resistance going up with speed increasing. The bearing resistance here is minimal.

Comparing your 80 ton with my 100 ton you actually have a lower bearing resistance than I do because your figures are in Newtons and mine are in lbf, there are about 4.5 newtons to 1 lbf.

#7 User is offline   Jean-Paul 

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Posted 04 March 2018 - 02:50 AM

OK ! I forgot your values were not metric !
Thanks,
Jean-Paul

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