[TheForge] Re: UHMW hammer guides

[Tom Troszak]

Message: 4
    Date: Sun, 06 Apr 2003 21:59:29 -0700
    From: Ian <ilerner2@shaw.ca>
    To: TheForge <theforge@mailman.qth.net>
    Subject: [TheForge] Air Hammers
    Reply-To: theforge@mailman.qth.net

    Instead of using brass bearings for the hammer and slider, has anyone 
used
    UHMW ...

Dear Ian,

I have made dozens of power hammers using UHMW for guides.  The most 
important thing is that the steel side has to be smooth at lease a 60 
microinch finish or better.  I have built some hammers that strike 
30,000,000 blows per year, and the brass guides had to be replaced every 
three or four months as the brass needs constant oiling, and most guys oil 
about once a day. The good thing about brass guides is that even after 
100,000,000 blows, the rams still looked like new.  When the brass guides 
were replaced with UHMW, there was no appreciable wear on the UHMW after 
even six months.  However, in a solid fuel shop, (coal or coke fires), or a 
place where you do a lot of grinding, microscopic particles of grit get 
"bedded" in the plastic forming a "lapping tool" which actually eats the 
steel. there does not seem to be a good cure for this, but the grooves in 
the steel do not seem to impair the performance, they just look "bad".

Brass guides can be tightened up to a very "crisp" alignment with only a few 
thousanths play and still slide well, UHMW guides always feel a little gummy 
by comparison. You have to make a fairly tight fit, almost pinching to get 
rid of the "wobble" but the UHMW is so slippery that it works well. The UHMW 
has virtually no structural strength at all, so in any application it needs 
to be "backed up" structurally some way. UHMW is very spooky stuff. An 
unsupported bar will deform from it's own weight, yet it is virtually 
indestructible in many applications, but rough surfaces will erode it fairly 
quickly.

Another plastic that I have used successfully for hammer guides is 
oil-filled nylon, also called Vecton (and other names I'm sure)  It is olive 
green in color and is structurally much stiffer and tougher than UHMW.  I 
have used it in fully automatic applications where no lubrication is 
allowed. It is not usable for food grade applications, however. You just 
"prime' it with a little swipe of light oil as you assemble the machine and 
then it runs for years with no further oil.  The steel side needs to be 
smooth, as with UHMW.  Believe it or not, regular nylon works pretty well 
even without constant lubrication, but I would not recommend it for high 
speed applications.  I discovered this by accident when a supplier 
accidently send plain nylon instead of the oil-filled stuff, the hammer ran 
great anyway.  It is more expensive than brass, but very cool stuff.  The 
caution about dirt applies here as well. It is not as slippery as the UHMW, 
but perhaps a little more slippery than bare brass.

I hope this is useful.

Tom Troszak

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