Monday 6 October 2008

Brassed off

My first attempt at an extruder had stainless steel bearings and a stainless steel drive shaft, more by accident than anything else. I wondered at the time how the bearings would last compared to the recommended brass ones. Obviously stainless steel is harder than brass, but brass should have less friction, so less wear.

The first drive shaft got retired because the bearing lands were off centre. At the end of its life the bearings and lands were still in good condition.

I ran it from 10/07 to 01/08 but up to that point all I had made was lots of HDPE test shapes.

I replaced it with a plain steel shaft that I bought from BitFromBytes. That had the big advantage of being solder-able.

It worked well for a long time but eventually the bottom land on the shaft wore down so much that the pump halves closed together when using undersized filament (2.7mm).

The bottom land has worn down from 3mm to 2mm. The bearings show a little wear but still have some life left in them. This is after running 5lbs of ABS through the extruder.

In the last few weeks I replaced the shaft with a new zinc steel one and switched to brass bearings in an ABS extruder with HDPE filament guide. That worked well until I noticed the pump halves closed together again. When I opened it up I found that the brass bearings had rotated in the ABS, but they had also worn down a lot, considering the short time I had used them.

The drive shaft lands are still fine though.

So it would appear that the best combination is stainless steel bearings and a stainless steel shaft, but I would have to find another way of attaching the nut.

Most metal bearings I have recovered from old equipment are bronze. I don't know how that compares to brass but it seems to be the thing to use.

Maybe it is time to look at ball bearings and an offset shaft like Ian Adkins' design.


  1. Wow that is quite a bit of wear.

    I wonder if the plain steel shaft could be heat treated to harden it.

  2. That might be an idea. I remember by Dad softening clock springs by heating them red hot and cooling slowly and then hardening them again by heating red hot and cooling quickly in water.

  3. The usual material for this application is phosphor bronze. I would expect fairly rapid wear with both brass and stainless. Surface finish on the shaft and on the bearing is important as well and the shaft should be surface hardened if possible.
    Is there much pressure on the bearings - if so needle rollers may be better than ball bearings?

  4. Why don't we use that diagonal threaded rod arrangement like Ian does and then use 0608 bearings locked to the threaded rod with nuts and lock washers? That's what I'm looking to try. I'm sick of bushing wear.

  5. I may steal the diagonal trick but I don't want to change the gear ratio as it is just right with direct drive onto M5. Making it diagonal probably means M6 is the same, but M8 would need gearing down, reducing throughout and efficiency.

    One would think that Ian's design has more torque and much less friction, but I see the same problems in the forums. I.e. a fine line between stalling and slipping, so the extra complexity does not seem to have gained much.

    I have just bought some phosphor bronze stock to give it a try.

  6. My stepdad used to case harden steel with an acetylene torch and something that he sprinkled heavily onto the surface before he started. He'd sprinkle the steel with this stuff and then use the acetylene torch to heat it to bright red to orange heat if higher. He'd keep it there for a while (hey, I was about 8-9 years old at the time)and then let it cool naturally.

    Looking in Wikipedia under case hardening, I'm guessing that he was nitriding the surface. It was cool because he could do it at home rather than at the industrial plant where he worked, so I got to watch.

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