About chain stetch and tolerance.
I think these are two different things:
Stretch:
Using a caliper, I measured a selected segment of 23 contiguous chain links under maximum top center position tension. (In my case the chain tension is around 40 pounds, with a counterweight mecanism behind the sprocket to offload the motor partially and keep backlash in only one direction). Then another measurement at around 15 pounds chain tension. And constant temperature (don’t touch the chain).
Experimental measurements seem to show chain stretch is almost nil with my chains. (down to 0.02 mm on a digital caliper readout.)
REVIEW 2018-11-17 : As referenced by @Joshua
I could find some more Tsubaki details in RS-25 Technical datasheet where a basic RS25 chain made by Tsubaki has a maximum of 0.64 kN (presumably at the 2.6mm stretch identified by @Joshua). This yields a stretch factor of 2.6 mm /640N / 1000 mm /m: 4.1E-6 elongation per Newton of tension per mm of chain under that tension.
This is not measurable with a caliper but amounts to milimeters at a combination of high tension on the workspace top edge.
Here I prepared a simulation of chain stretch taking into acount a sled weigth of 11.6kg, but weightless chains (on a frame similar to the Standard maslow dimensions).
Note1: that this specific simulation suggests the total sled error is up near 5mm at top center.
Note 2: Reducing the motor distance parameters in this case would somehow cheat the calibration and reduce somewhat the chain elongation error. But that would not be an accurate correction.
Below is the simulated effect of a 2.5 mm reduction on the motor distance parameter, what I call “distBetweenLRMotorsGearBoxShafts”:
END OF REVIEW
Chain tolerance:
That is more a chain link sizing error that would depend on production batch, manufacturer, or something else, but is relatively constant for a chain segment. I tuned up to 0% on my right chain over a 4 feet measurement, then 0.13% on my left chain due to 1/16" longer tape measured value ( possibly due to a different supply as I extended my chains on my 11.5 feet wide shafts distance). It interesting to see how 4 links are dead on a 1 inch length. Makes it rather easy to compare a tape measure big mark to a chain link pin.
Do you have similar readings?