Monday, December 17, 2018

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.


  1. Would be interested in comparison with proper (Gates) Kevlar belts. As I recall, Kevlar has highest stiffness (and perhaps better adhesion to the rubber?).

    Also wonder if the results you see are more about the quality of fabrication, rather than the material in question. Kind of suspect even fiberglass reinforced belts should be more than adequate.

    1. I wouldn't count on adhesion to the rubber to have much of an effect when it comes to avoiding the problem I had with the steel core belts. The reinforcement cords in the belts are made of hundreds or thousands of tiny fibers, and the rubber adheres to the outermost layer but doesn't penetrate the entire cord. I think a bad clamp design could result in the same sort of failure where the rubber slides over the cords and stretches.


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