Ultimaker-style gantry stiffness/friction normal?

I built an ultimaker-style gantry. Without the belts, the printhead moves around pretty easily. I can push it with one finger and it will roll for a little while due to inertia. As soon as I add the belts, the printhead becomes stiff and will not roll around very easily at all. Is that normal?

I know that I should expect it to become stiffer after attaching the belts because now the force I apply has to rotate two M10 rods, but I wouldn't expect it to become that stiff.

I tried to search for a video of the ultimaker stiffness, and all I found were these:
[www.youtube.com] (at 1:25)
[www.youtube.com] (at 2:13)
[www.youtube.com] (at 0:21) - in the comments, he says it’s apparently the expected stiffness.
[www.youtube.com]

It looks pretty stiff, so maybe its normal. If it is normal, then was I wrong to assume that this gantry style's advantage was that it could zip around really fast and easily? It seems like the X and Y motors have to work extra hard just to get the printhead moving. I can see how this gantry style lowers vibrations though. But I digress... Is the stiffness normal???

Edited 1 time(s). Last edit at 06/26/2015 10:57PM by jessicabrenner.

(normally I'm used to things becoming stiff after removing belts, not after I put them on)

It might be that your belts driving either end of the rods are slightly out of phase, causing the rods going thru the carriage to not want to cross at exactly right angles. if you can loosen one of the pulleys on one end of one of the shafts, run it back & forth by hand, and then re-tighten the pulley, this may relieve the binding.

Does it feel like scraping, or just smooth resistance? Is it hard to string the belts? If the belts are strung loosely, can you see them tighten more on one side than on the other?

LoboCNC, I've measured the distance between the edge rods and the parallel one that crosses in the middle. I measure the distance between them in two places - on either side. I make adjustments until they are the same (to make sure the rods are perfectly parallel). And I repeat this for the other dimension. Then I move the carriage around a lot and repeat the measurements to make sure nothing changed with motion. Using this method, I'm pretty certain that the rods are all parallel and that they cross at 90 degrees.

epicepee, It feels like smooth constant resistance. No jerkiness or scraping. Just much more resistance than I'd expect. What do you mean "string the belts?" The Y belts are nice and tight. The X belts are longer and both are about the same tightness as each other, but less tight than the Y belts. I can easily pull the X belts upward 40 mm unlike the Y belts.

Typically, with an over-constrained system like you are building, it doesn't take much mis-alignment at all to get significant binding. You probably wouldn't be able to measure the misalignment. The usual way to relieve the binding it loosen up some element of the system (like one of the pulleys on one of the shafts), work things back and forth to jostle that element into exactly the right position to eliminate the binding. Then re-tightened whatever it is you loosened once it is in the ideal position.

Have you tried doing this - loosening one of the pulleys, moving it around, and then re-tightening?

Once you get it operating as smoothly as you can, then driving it back & forth a bajillion times with an exercise g-code program will cause the linear/rotary bushings in the carriage to wear ever so slightly which will further relieve any binding. I built a printer using a similar crossing-rod carriage drive and went thru similar issues.

Edited 1 time(s). Last edit at 06/27/2015 06:49PM by LoboCNC.

Loosening pulleys, moving around, then retightening has helped tremendously in a few cases. Thanks for the tip LoboCNC. What also really helped was to correct my alignment errors, oops!

Quote
jessicabrenner
(normally I'm used to things becoming stiff after removing belts, not after I put them on)

hahaha no one even touched that. it was funny