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A2
If your load is well under the young modulus you don't have "meaningful" stretch.
Well, as many times as it is below the young modulus, that many times the stretch is less meaningfull for unit length of belt. Anyway this is meaningless claim without some concrete numbers/examples in comparison to our delta bots. At least rmat claims dynamic stretch is a problem. Maybe he bought low quality belts or pulleys. Or maybe it is a real problem.
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A2
PowerGrip GT2
There is minimal stretching during the wear-in period.
After the burn in period there is
no stretching due to wear.
The effects of belt elongation and tooth deflection
do not have any influence on the registration accuracy of this type of system.
[
www.cad.sun.ac.za]
That is about static stretch during wear-in. That is not what I'm concerned about. I believe that is not what rmat was talking about. I was not able to find modulus (or the linear stifness) of the belt itself in the linked PDF. That is what I'm interested in. How does it compare between GT2 (fiber glass reinforcement) and T2.5 (steel reinforcement).
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A2
Timing Belt Theory
pg13
the total elongation (deformation) of the belt operating under load is equal to the total belt elongation resulting from the belt pre-tension.
For most practical cases the difference between the deformations of the belt in contact with both pulleys during pretension and during operation
is negligible.
Tensile tests show that in the tension range timing belts are used, stress is proportional to strain.
Theoretically, the
tooth stiffness increases with increasing belt tension over the tooth, which has also been
confirmed empirically.
This results in the practical recommendation for linear actuators to operate under high pre-tension in order to achieve higher stiffness, and hence, better positioning accuracy.
[
www.gatesmectrol.com]
Note that on page 16 of the paper they explicity take belt stiffness
Kr into account when computing the total drive stiffness. So clearly dynamic belt stretching (I do not mean the initial wear-in stretching) needs to be taken into account for precise positioning.
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A2
The amount of force to produce a specified stretch is known as the belt modulus.
[
www.epi-eng.com]
Yes, belt modulus for both GT2 and T2.5 is the number I would like to see. Then I would be able to guess whether rmat might have been right with his claim that GT2 is not good since it is too stretchy.