Roller Bridge Joint for QR B2
I am convinced again and again that sliding joints are evil and waste of power. Rolling joints rock and roll! To this bridge-joint you can attach whatever extruder(s) and/or hot-end(s) you like: The bill of materials: One piece of plastic 30x30x10mm (can be…
And extrusion is comparatively cheaper than hardened rod. (/me ponders the possibility of a rodless printer)
It looks a little heavier than a sliding carriage, but the box tubing probably makes up the difference. How does the rigidity of the box tube compare to 8mm stainless?
Is this not a kind of sliding roller point. looks like the bars slide in the plastic somewhat.
@Dale_Dunn I’ll let @Shauki answer that in depth but I believe it’s down to the rigidity of the piece being mostly defined by the outer edge, with the centre not contributing much. Makes sense if you think how rigid thin walled tubes are.
Well done I bought some square tube last week to trail that idea. Beaten to it never mind at least I know it works now.
Right, I’m aware that the box section has good bending properties, but I’m curious if that offsets the lower elastic modulus of aluminum vs stainless.
Aluminium guide rails WILL NOT LAST, they will set up fine, but 2 things will happen;
- The coefficients of expansion between frame, bearings and rail will send everything out of register.
- Once out of register they will wear very quickly.
V slides and bearings, very stable and very accurate say 8-15 microns. Check out Hepcomotion.com, GV3 size13 or 18 instrument bearings and yes, I do work for them, but there are other companies out there.
@Tim_Page-Brown and your saying your rollers and v-slide dont ware. 1 metre of 15 x15 tubing less than £5 in the uk.
So with bearings and hardware all in £15.
So its a case of cost for some of us.
Forgot to add I can always print some rubber boots for the bearings thus no wear to the tube
OK, here’s what I wanted to know about rigidity:
Deflection of beam with point load W at center of length L:
=(WL^3)/(48EI)
Aluminum**************
E=69 GPa, 10^9N/m^2
Ix=492 mm^4, 4.92 × 10-10 m^4
EI=33.9 N*m^2
Area=3.610-5 m^2
Density=2.710^3 kg/m^3
0.097 kg/m
Stainless**************
E=180 GPa
Ix=201 mm^4, 2.01 × 10^-10 m^4
EI=36.2 N*m^2
Area=5.010-5 m^2
Density=7.510^3 kg/m^3
0.375 kg/m
The box section is a little more rigid, even with the much more elastic aluminum, with a weight saving of a couple hundred grams for a half meter beam. Not bad at all.
Intuitively, I didn’t think the box section could make up for the weaker material. A good illustration of why intuitions should always be backed up with engineering calculations.
@Shauki , I did some Googling and came up empty looking for hard anodized aluminum 10x10x1. Can I pester you for a link to a source?
I did find some 10mm carbon fiber square tubing.
@Nigel_Dickinson oh, they wear, but not the way you think, dependent on loads, bearing life of say 20,000km is not a problem before noticeable tolerance issues.
I don’t have time today to dig up the Hertzian contact stress formula and compare the result to aluminum fatigue numbers. The formulas aren’t hairy, if someone wants to look it up and work out how long the surface of these tubes will last.
@Tim_Page-Brown reading you comment its to much sales patter not enough thought.
Some of us are old school engineers who build things that work using experience… And we manage to do this without using formulas.
Its not bearing life with v-slot but roller material. Doubt that’s 20000 km.
But like I said cost is a factor for some of us. Money no object I’d have makerbeam or openbeam for my ideas.
I was thinking one could use the stepper’s max torque and the radius of the pulley to find a maximum force that could be applied, whatever the acceleration of the whole system turns out to be. Add some for preload on the bearings to take slop out, then compute a ballpark figure. Anyone have a spare sophomore laying around?
Someone should probably look at the weight of a hot end as well. I wonder how the force of gravity on a Kraken compares to the force a stepper can apply. One or two E3D V5 hot ends hardly weigh anything. I don’t remember (and can’t look up now) what acceleration I’m using on my Prusa, but some quick Googling shows defaults of only about 0.3g. Really? Even with whole extruder on the carriage, I thought more should be possible. @Tim_Rastall , what acceleration can you do with your Ingentis?
Definitely worth a look if it turns out the naked bearings will damage the aluminum.
I’m still considering sliding Delrin AF, too. That stuff is mighty slick. Or Rulon, etc.
At the end of the day you get what you pay for, I appreciate that my suggestion is over £100, and that most people are doing this for fun. From my profession point of view, bearing life, precision and moment capacity are key.
@Tim_Page-Brown well precision yes I’ll have that with this system. Bearing life…same as yours maybe better depending on source.
And then we have cost…thing is value vs performance comes into this even at the top end 3D printing.
More expensive isn’t always best.
Would something like this solve the possible wearing problem?
http://m.alibaba.com/product/1618859649/plastic_coated_bearing_delrin_deep_groove.html
As opposed to polyurethane.