z-axis torque

I tested my x axis with the belt nice and tight, worked great. Same model motor on my Z axis starts messing up. With no power I can turn the motor shaft and get all four threaded rods to turn fairly easily. Without the drive belt attached, the motor and screw shaft turns fine (from the stepper control), as soon as I attach the belt it siezes.
I really don't want to have to take apart the whole damn thing to try putting another motor in there, but I was hoping someone could give my an idea on what my problem could be. I'm pretty sure I have a high enough torque rating on my motors. There doesn't seem to be much resistance at all when turning the shafts by hand. I know for sure I have my motor wired correctly.
I really want to try to get this system to work, but if I can't, I'll put the z axis stuff I have on ebay and replace them with the linear steppers if I can find any in the USA with a decent price.

Please let me know if you can help.
thanks

The motor is most likely not your problem. I am assuming that this is a darwin design. I had the exact same problem that you are having when I built mine. There are a few things that I did to fix it. I believe that the problem it a result of too much lateral force on the threaded rod, which causes it to bite into the corner blocks.

First, I stabilized the top of the threaded rods by securing them inside of bearings which were jerry-rigged to the frame.

I then created an idler-bar which allowed me to tension the belt to exactly the right tension without the need for the z-idler brackets. I did away with those completely. These fixes made the torque required to turn the entire setup, including the motor, about as much as one could do with just their thumb and index fingers.

If this isn't clear or you need any help pm me or email me at redskinsjbs@yahoo.com

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Jay

It turns out it was a torque issue. I'm actually making a wooden darwin. I wasn't using any idle brackets which might have been part of the problem. I took off the top part and the bed and worked my way up testing the amount of tention needed. I had some particle board here that I cut into 16" strips and put holes in them and they are connecting each of the threaded rods making sure that they don't get too close or too far from each other. If I knew this was going to be the case I would have put the pulleys under the little table I made for it. (my motor shaft goes through the table anyway). This way the tention would be perfectly constant because of the drill holes.

So now I have then nice and tight and turning all four rods. I have to remake my pulleys though because they aren't centered properly and don't have a good lip to keep the belt on properly.

Thanks for the idea, that got me started on a new path. I was thinking of using 4 linear actuators (like forest higgs is doing with his X Y axis.) it's a lot easier to setup, but I didn't want to spend any more money for a while. I should have enough to get it to start printing at least. Once I get that far I'll do more and more tweaking to get it to print as good as nopheads (I hope).