Advantages of GT2 over a rack Many 3D printers employ a GT2 band for the y-axis plate and the printhead in the x-axis carriage.
However, there seems to be the obvious disadvantages of:

stretching (and need for adjustment) and;
possibly slight inaccuracy, or &quot;wobble&quot; (for want of a better word (as wobble is often used in describing z-axis deviations)), due to the elasticity of the band.

Is there any particular reason why GT2 is used over a straight forward rack and pinion system?

In particular, for the y-axis plate, as a rack would appear to me, be:

easy to install;
more accurate, and;
require a lot less adjustment1.

If the rack is made from aluminium, surely weight can not be a major factor...  or can it?
Is cost a factor? A pinion seems to cost around \$10-\$15 (650 B\$, here in BKK, Thailand), which is obviously more than a reel of GT2 and a couple of GT2 pulleys.
Would the rigidity of the rack be less forgiving of an imperfectly aligned axis, something which the GT2 band and pulleys combination would not be so affected by?

TL;DR
Which of these factors cause designers to use GT2 en lieu of a rack?

Accuracy
Weight
Cost
Maintenance
Tolerence of non-square axes, manufacturing errors.


1 Maybe constant adjustment is not required on an everyday basis, but the tension would still need to be checked now and again, whereas a pinion would not have this requirement.
With a belt system, the belt engages roughly half the pulley. This, and the tension in the belt, ensures the belt always engages the pulley tightly. A belt and pulley system is thus relatively forgiving.

With a rack and pinion system, only a few teeth engage at any given time. To avoid backlash and get the same kind of "tight" engagement, both the gear and the rack need to be made with very high precision. The carriage also needs to be very well constrained, because any wobble of the rack relative to the gear introduces backlash (or binding). Moreover, you also need to keep the rack and pinion well lubricated lest they wear out prematurely.

Given that belt and pulley work well enough, I don't see why you'd need to move to rack and pinion. The main advantage of rack and pinion is that the rack isn't elastic. The maximum length of a belt system is limited by its elasticity, but given we aren't building meter-long 3D printers anyway, that advantage of rack and pinion does not apply.