Why no bearing on lead screw top?
I can only guess the design for the z axis omits bearing in order to reduce price? I don't even have my MK3 yet but this seems like an important oversight. I am tempted to remove the armature / shaft form the steppers and turn the end down for a pilot bearing, and modify the z axis to accommodate.
Re: Why no bearing on lead screw top?
Although I’d normally agree with you about having the bearings at both ends, in this case it would sort of “over constrain” the assembly. If there is any misalignment in the printed parts you’d be running into all sorts of binding issues. The fact that z axis is riding on linear bearings on both sides in addition to an extra bearing would cause some grief especially since those other bearings would need to be precisely located so they’re exactly parallel to the linear rods.
Re: Why no bearing on lead screw top?
On most printers in the i3 style putting bearings on top of the leadscrews will introduce Z wobble problems.
Some people assume bearings will give stability, but if the leadscrew is not perfectly straight then constraining it on the bottom and the top will force the misalignment to be given to the component traveling the length of the lead, meaning the X axis assembly. Which result in the Z wobble people try to avoid.
I also am waiting on my mk3, and have only printed with a pretty cheap clone for the past year. (got it at March 1st of 2017) and on that machine i could definitely notice that constraining the leadscrew on both ends is a bad idea. As for the mk3, being that it used integrated leads instead of those flex couplers, will result in much better results. I'm sure of that.
So i agree with Matthew completely.
Re: Why no bearing on lead screw top?
The smooth rods and the bearings constrain the x axis. The leadscrews simply provide the movement mechanism in the z plane. Tying down the leadscrews will mean z axis binding if they and both smooth rods are not perfectly parallel top to bottom, which is not likely. For this reason the leadscrews are allowed to float, fixing both ends of them is a bad idea.