Mendel Bearings question

I was thining about all the bearings required for the Mendel, and I was wondering why we're using roller bearings on a fairly low mass/low force machine instead of plastic linear bushings (Igus parts for instance). I've had the prices come out pretty competitive, and one bushing would replace three bearings plus much of the associated steel hardware. Some variants run on steel without issue (most prefer aluminum).

Any thoughts? I'm curious to hear from those who were more involved in the design.

I'm not involved in the design, but I did just buy a pile of 624 bearings. I'm quite stoked about them.

Some of the advantages I've found so far with them - they fit perfectly on a 4mm diameter bolt, so there's all sorts of different thing you can do with them - pinch wheels, axles for gear trains, pivot points, you name it. Lots of neat applications. Plus they are pretty cheap, I think mine cost $1.40 CDN each.

The biggest advantage has to do with the Mendel design though - with separate ball bearings (not roller bearings, btw), you can control the number of constraints on your axis. Using linear bearings, any moving axis becomes quickly over-constrained. The X carriage on the Darwin becomes over constrained if you add linear bearings - that means you have to have your guide rods very carefully aligned to prevent jams. If you've got great industrial manufacturing tolerances, this is no problem, but if you're rapid prototyping things in your apartment, over constraining things is a big problem.

They could have swapped out some of the bearings for linear bearings, such as the 360 size of the Z axis, but that then increases the number of kinds of bearings you need, which is bad.

Ed describes the constraint issue very clearly on the second video on this page:
[objects.reprap.org]

Plus, you can adjust the tension on all the Mendel linear bearings - handy if you've got non standard, or worn parts.

Here's a shot of a Mendel Vertical Bearing 360 I printed out and tested - it's shockingly smooth. I almost want to abandon my second Darwin and just jump straight to a Mendel, but the Darwin is almost finished.


Edit - also, my Darwin uses printed plastic bushings. They have a lot more friction, wear fairly quickly, are sloppy, and tightening them is a bit tricky - it involves melting capa and squishing it into the bearing.

Edited 1 time(s). Last edit at 10/15/2009 07:06PM by Wade.

That explains a bit. Industrial block and rail and other designs align and have a single floating axis to get around this on most light duty machines. Metalworking is a whole different world, and yes, that's where dovetails, hand ground ways and gibs, and a whole lot of trial an error come in to get the accuracy and stiffness correctly.

I just wish they had been able to use 608 bearings as those are about 1/3 the price

I'm probably going to order parts for a Makerbot as Darwin parts are not immediately available. I've been peicing together the non-specific hardware up until now. I have a wife imposed time limit on this.

Maybe the design can be reworked to incorporate the cheaper bearings.

The 624 bearings are $99 per 100, see [www.vxb.com]. That's pretty cheap. Now, if only MakerBot would carry Mendel hardware packages... They'd have the volumes to get prices down to reasonable levels.

-Geert

GeertB Wrote:
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> The 624 bearings are $99 per 100, see
> [www.vxb.com].
> That's pretty cheap. Now, if only MakerBot would
> carry Mendel hardware packages... They'd have the
> volumes to get prices down to reasonable levels.
>
I think they're too far into Makerbot to do that kind of switch at this point. Nothing to stop you from doing that, though. :-)

---

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Hell, there are no rules here - we're trying to accomplish something.

Opportunity is missed by most people because it is dressed in overalls and looks like work.

Thomas A. Edison

There wouldn't need to be a switch. Just a couple of extra products. Given that there is a lot of overlap (extruder parts, electronics, stepper motors, rods etc), there might not be much involved to increase sales volume.

-Geert

I haven't ordered them yet so I don't know how respectable they are, but I was able to find a place selling smaller quantities of the 624 bearings in the US for almost as cheap as the UK supplier:

[www.thebigbearingstore.com]

Not as cheap as $99 for 100, but you won't have 52 extra bearings On the other hand, it would be cheapest for pairs of people building the machines to order the lots of 100 and split the costs (plus the extra shipping of the 50 to the second party).

624 bearings are not readily available in New Zealand, and will need to be ordered in, which I plan on doing, and they WILL be expensive, so expensive I may have to do two purchases within my finances to get all of them for a Mendel

I wonder if the bearing are offset from each other (so the bolt heads do not collide) if you could switch back to using normal skate bearings when are generally available every where?

wesjanik Wrote:
-------------------------------------------------------
> I haven't ordered them yet so I don't know how
> respectable they are, but I was able to find a
> place selling smaller quantities of the 624
> bearings in the US for almost as cheap as the UK
> supplier:
>
> [www.thebigbearingstore.com]
> /624-dsh-2RS-624-dsh-ZZ-Radial-Ball/Detail
>
> Not as cheap as $99 for 100, but you won't have 52
> extra bearings On the other hand, it would be
> cheapest for pairs of people building the machines
> to order the lots of 100 and split the costs (plus
> the extra shipping of the 50 to the second party).

I ordered my 624 bearings from thebigbearingstore.com, they shipped fast, and they seem to be pretty high quality.

it seems like you could buy round nylon stock and cut it too the same dimensions as these bearings and they would work about the same. You may have to protect the inner diameter from any exposed threads that would damage it as it turned though.

Personally I don't like the idea of increasing the bearing count as they represent vitamins, nylon is used for many different things and can be easily foraged. This is espeacially true in this case as somone would have to find many identical bearings to make this configuration work.

I need bearing Question

The 608 bearing is more bigger than 624 but is more cheaper.
In Argentina, small bearings are very rare and expensive
I paid 1,25 ARP each 608 zz ( 0,32 US$ )