Comparing Steel Core and Glass Core Belt Stretch in UMMD's Z Axis

A recent debacle led me to replace the belt clamps, and while I was at it, the belts, in UMMD's Z axis.  I took the 10 mm wide steel core belts out and installed 9mm wide, glass core, Gates LL2MR09 belts.

In a previous post I had tested the stretch of the steel core belts under a print load up to 4 kg (plus the mass of the bed and it's support structure, another 3.5 kg).  I found that the steel core belts stretched about 42 um/kg of print load, which translates to worst case stretch in any 0.25 mm layer of 1.2 um- completely inconsequential.

Today I clamped my digital gauge to the printer's frame with the bed about mid way down the Z axis and loaded it up the same way- just stacked a few spools of filament on it.  Photos below show the resulting stretch:

Unloaded and zeroed.

Glass core belt stretch when loaded to about 4 kg.

0.58 mm/4kg = 0 .145 mm/kg which is 3.4 x the stretch I measured with the steel core belts.:

Steel core belt stretch.

The steel core belts stretch came to about 1.2 um maximum in any 0.25 mm thick layer (entire bed covered with a layer of PLA).  That means these glass core belts will stretch a maximum of about 4 um in any 0.25 mm layer.  It seems unlikely to cause any issues in a real print situation, but I'll probably put the steel core belts back on the machine with an improved belt clamp design.