Minimum clearance for oscillating bushing lubed with grease and recommendation for bushing material

Hi everyone,
I would like to ask if anyone could please offer some advice on the following...
I have a cantilever mounted round steel housing about 1.5" long that I
want to oscillate on a 3/8" OD hardened steel dowel pin. I plan on just pressing a bushing material into the full length of the housing, and then drill & ream the bushing ID to fit over the dowel pin. I need the clearance between the ID of the bushing and the OD of the dowel pin to be as small as possible. I can only lube the bushing once at assembly and then never again.
I would prefer that the maximum clearance between the dowel pin OD and the bushing ID is not over .001" but I may be able to get away with . 002". I would like to use a high pressure grease to lube the bushing and shaft, but how small can my clearance be before it won't allow the grease to go and/or stay between the shaft OD and the bushing ID, or before the low clearance defeats the lube somehow ?
The housing is just oscillated by hand intermittently. Any recommendations for a bushing material, bushing ID surface finish, and lube, would be appreciated.
Presently, I have no choice but to press some bushing material into an existing housing, but in the future, would I be better off making the housing itself out of some pre-hardened 4140 and then using the housing itself as the bushing material ? Would this wear much better than a bronze bushing ? Would non-hardened 4140 or non-hardened tool steel wear better than bronze ?
I would like this thing to last as long as possible before the clearance opens up too far. I also think I need to stay with a bushing and not a rolling element bearing.
Any advice or personal experience with bushing materials, loads, and estimated wear rates would be appreciated.
Thanks John
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Hi, John. I can't really offer any advice, but do have a couple of related questions.
First, what is the environment the unit will operate in ? Is it wet, dry, dusty, chemical vapors in the air? Any need to seal the bushing? If no, then any lubrication will not stay in very long. The exception might be the bronze material with lubrication embedded in it. The name eludes me right now, but I could go out in the shop and find some pieces.
Second, what kind of load will this thing carry? And does the oscillation you mention ever do a complete rotation? If the load is low, use teflon for a bushing and it will wear forever. If the unit never does a complete revolution, any lubrication will end up in one part of the bearing.
Just my thoughts.
Paul
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Oillite bearing bronze bushing, 12-20 Ra with an annular groove that doesn't break the outside edge of the bearing. (For lubriciation storage purposes, mostly). Grease, to me is probably not a good lube for this application, as the temp would never get up enough for it to flow. I would probably go with a very high-tack high pressure oil along the lines of chain lube, or a high-tack gear lube or something like STP engine treatment (STP is used as an initial lube/break-in lube by a lot of engine builders, including me, because of it's high-tack properties).
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On Tue, 15 May 2007 09:29:40 GMT, with neither quill nor qualm,

Just so others don't make the mistake of thinking STP is a lube, it MUST be mixed with oil to be used properly so you derive its benefits. When I was attending UTI in Phoenix in '72, I worked for a used car dealer as a trainee mechanic. The first time I saw the main mechanic rebuild a bottom end, I saw him dip the entire rod bearing in pure STP "honey". Asking him if he knew about the lack of lubrication it provides by itself, he said that he'd "been doing it like that for years." I noticed later that he had mixed it with oil and was putting it only on the bearing surface. Sure enough, the one he'd dipped spun on the guy two states over on his trip back east.
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Thanks for the feedback guys,
In regards to Paul's questions, the bushing is in a very clean dry environment, no dust or dirt, no chemicals, always room temperature, there is a delrin thrust washer at each end of the housing that basically acts like a seal but no real need for a seal other than perhaps to help keep lube in, there's really not dirt or anything external to contaminate the system. The unit never makes a complete revolution, and the oscillation angle is about 33.5 degrees (maximum sweep angle, but many times the sweep angle could be very small, say a total of 10 degrees). The load will be fairly high.
Thanks again guys, John
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Define "fairly high" in terms of PSI and you might get somewhere useful.
Visit page 3772 of MSC's big book in paper or on-line for a brief bearing selection tutorial. Oddly enough, that no longer lists the properties of the bearings they still sell on page 3777 described as "PTFE-filled bronze-lined sleeve bearings", which happened to be the best solution to a problem I had (high load but small clearance to put a bearing in). Plain old oilite will likely do fine, but Vespel is the self-lubricating stuff with the biggest PV number (by a wide margin) attached to it...
McMaster also has a nice selection tool on their website - power transmission, plain bearings, sleeve bearings. They have better and more complete information about loading (P, and V, as well as PV) for each bearing type. Look at "material comparison chart". Selecting the highest rated load (figuring high load, low speed from your description) from their selection chart leads to the same bearing type I mentioned above, under a slightly different descriptor - "Steel-Backed PTFE-Coated Bronze".
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