OK Folks. Here comes chapter 7. I’m basically done with all the mechanical setup, and wiring. The following will include some harsh engineering language so please proceed with caution.
I was doing some adjustments to make sure my Y gantry is parallel to the bottom transverse beam. While doing so I realized my tray is tilting to the left (viewing from the build side). I started to measure the distance from the edges of the tray to the bottom transverse beam and found out the difference between left and right sides is ~3 mm. After some thinking and further measurements I found out that the nylon spacer that holds the V-Bearings (it’s on the left side) is compressed. I removed it and measured the height. It was 2.91 mm. This is with the load removed... As you remember these are ¼” height, so they should be 6.35 mm. I measured one unused spacer and it’s indeed 6.35 mm high. So I did some calculations below. I used 5 and 10 Nm of torque in the calculation, because I do not exactly know how much I have exerted. I know I used a short handle ratchet, and I know it must have been 5 to 10 Nm. Also the nut has a nylon insert which will have an additional 1-1.5 Nm prevailing torque (this will decrease the torque). So the real number must be somewhere between 5 and 10 Nm.
So As you can see @ 5 Nm of torque the M5 thread will generate 5269 N of clamp force, while @ 10 Nm it’ll double - 10538 N.
The original Hadron Bot BOM recommends using 94639A352 from McMaster. This is a 6/6 Nylon spacer. This is an unreinforced Nylon.
http://www.engineeringtoolbox.com/engin ... _1225.html recommends 2-3.6 GPa for Elastic Modulus. I used 2.5 GPa in my calculation. The following calculation was made for a hollow cylinder in simple uniaxial tension. With all these plugged in the compression (u) of the nylon washer will be 0.42 mm @ 5 Nm and 0.839 mm @ 10 Nm of tightening torque. So this is @ the location where the nylon washer sits. To find out the deflection @ the end includes some additional math. Here is a drawing that includes all the dimensions and notation:
As you can see the deflection @ the left end of the tray (x1) can be as high as 1.8 mm (@10 Nm torque) and on the right end (x2) – 1 mm (@10 Nm torque). The total misalignment (X) can be 2.8 mm – close to what I measured.
Considering all these I made two more calculations with aluminum and steel for the spacer. As you can see in worse case scenario the same max misalignment (X) will be 0.1 mm for Aluminum and 35 micrometers for Steel (both @ 10 Nm of tightening torque).
OK. OK. I know it’s too much math for someone, the conclusion from all these are that using a nylon spacer will put the tray into a misalignment by default. Even if you have a torque wrench and you can control the tightening torque it’ll compress the spacer. Some can say that by suspending the hot bed on springs one can level it by adjusting screws that mount it. Sure you can do it, but I just simply hate to make something with a design flaw like that. Besides, plastic creeps. With compression on it creeps even more. With heat cycles (the darn thing is under the hot bed!!!) it even creeps more, so over time you’ll lose clamp force and your tray will start to wobble like a Hawaiian belly dancer, while you're sittng there and thinking why the darn thing is not printing as it should.
You can use the famous 92510A562 aluminum spacer for 83 cents or the 92415A742 steel spacer for 84 cents. The BOM calls for 8 pieces.