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Model Forum / General / Models / April 2008Steel/bronze bearing
What sort of surfaces are best for steel/bronze bearings? I thought to polish the steel shaft as high as I can (shiny!), but leave the bronze bearings as-bored, ie a little groovy. The bearings compress, so after wearing in I can adjust them back. They are 22 mm and 18 mm dia, will run at up to maybe 10,000 rpm, lubricated, sideways loads only (it's the mill main shaft, with an integral ER20 chuck on the end, for a BCA). Is this a good idea? Any ideas what kind of lubrication to use? Any suggestions? Looking for better-than-a-tenth (~1.5 microns) tir. Ta, -- Peter Fairbrother >What sort of surfaces are best for steel/bronze bearings? > [quoted text clipped - 12 lines] > >-- Peter Fairbrother If you believe Tony Griffiths', The shaft should probably be hardened, but barely tempered. Then either lapped to perfection. or cylindrically ground. The bronze bushes should then be scraped to fit as well as possible, then tightened down a bit to get the desired lack of play. Alternatively, haven't tried this on mine yet, but have wondered about it... To get the desired speed, twice what the original was rated for, stick with the hardened and ground spindle, but use a couple of 7/16x5/8x5/8 needle roller bearings for the original spindle or larger for you one. That could manage up to 25,000 rpm before flying apart. Basically, I don't think you will get both the required speed and TIR without either air bearings, forced lubrication or needle rollers. But I'm usually wrong :-) Mark Rand RTFM >> What sort of surfaces are best for steel/bronze bearings? >> [quoted text clipped - 15 lines] > If you believe Tony Griffiths', The shaft should probably be hardened, but > barely tempered. Then either lapped to perfection. or cylindrically ground. I don't know what the new shaft is made of, it started life as an ER-20 straight shank collet chuck. It's a stone bitch to work, gummy (while still being plenty tough and hard) and work hardens in an instant, but I have eventually managed to get two good surfaces on it - which are pretty **** hard. I'm at 600 grit diamond, and they are flat within 2 microns (and concentric to well within 1 micron, and within 1.5 microns of the ER-20 cone) - just wondering how much further to take the surface? All the way, I guess. > The bronze bushes should then be scraped to fit as well as possible, then > tightened down a bit to get the desired lack of play. I'm making new bushes as a) the flats on the shaft are the wrong size and b) I want to keep the old bearings, shaft etc untouched. I don't know how one scrapes bushes - with a curved scraper? > Alternatively, haven't tried this on mine yet, but have wondered about it... > To get the desired speed, twice what the original was rated for, stick with [quoted text clipped - 5 lines] > either air bearings, forced lubrication or needle rollers. But I'm usually > wrong :-) I could come down on the speed I suppose - I had planned to fit a 350W or 450W up-to-3200 rpm variable speed DC motor to the head (the bit which holds the bearings, not using the pulleys and belts) with a choice of 3:1, 1:1 and 1:3 gearing. I have a 56,000 rpm air spindle though, which can be fitted, and which would do for the high speed stuff :) - I don't know how powerful it is but it takes a 2kW compressor to drive it - or I could balance the spindle (when I have got my balancer working ...). Just need a rotary table, and I've got 6 DOF ..,. :) -- Peter Fairbrother > Alternatively, haven't tried this on mine yet, but have wondered about it... > To get the desired speed, twice what the original was rated for, stick with > the hardened and ground spindle, but use a couple of 7/16x5/8x5/8 needle > roller bearings for the original spindle eh? The original "glass-hard" spindle is (very slightly under) 7/8 at the fat end, and 3/4 at the thin end. -- Peter Fairbrother >> Alternatively, haven't tried this on mine yet, but have wondered about it... >> To get the desired speed, twice what the original was rated for, stick with [quoted text clipped - 7 lines] > >-- Peter Fairbrother I will admit to not having disassembled mine yet (too many other machines ahead of it in the rebuild queue) and took the dimensions off Tony's website without checking his numbers.... It's been a hateful day at work and I've spent most of the evening building a new mailserver to reject most of the couple of thousand spams that arrive at home every day. Bloody excuses. Anyway:- Those sizes give limiting speeds for needle rollers of between 12,600 and 13,600 depending on which 7/8" bearing was used. Mark Rand RTFM >>What sort of surfaces are best for steel/bronze bearings? >> [quoted text clipped - 30 lines] > Mark Rand > RTFM As an addition A narrow groove (preferably spiral) in the bearing would help to carry your lubricant (oil or grease).  SignatureNeil reverse ra and delete l Linux user 335851 > What sort of surfaces are best for steel/bronze bearings? > [quoted text clipped - 8 lines] > Is this a good idea? Any ideas what kind of lubrication to use? Any > suggestions? Looking for better-than-a-tenth (~1.5 microns) tir. In an engine you'd have a number of similar situations. 1) The crankshaft pins and bearings would be pressure fed with oil and surface finish on both would be as fine as possible. Lubrication would be entirely by hydrodynamic wedge separating the running surfaces. 2) In the little end of a fully floating conrod the only lubrication would be from whatever got splashed up there. The pin would have an ultra fine finish and the bearing would be either honed to a cross hatch finish or roller burnished. 3) The finish in the gudgeon pin bore of a splash lubricated aluminium piston is normally roller burnished. I've also seen honed finishes but only in aftermarket pistons where perhaps the manufacturer didn't have the ideal equipment. In other words lubrication, or the absence of large amounts of it, doesn't seem to impose a requirement for the bearing to have an oil retaining surface if that's what you're concerned about and I'd recommend a fine honed finish. I suggest that leaving it as bored wouldn't be ideal and the bearing contact area would be much reduced. The shaft would always be fine polished. SignatureDave Baker Puma Race Engines > In an engine you'd have a number of similar situations. > [quoted text clipped - 17 lines] > finish. I suggest that leaving it as bored wouldn't be ideal and the bearing > contact area would be much reduced. The shaft would always be fine polished. Thanks. You mention roller burnishing - any idea of how strong the spring has to be? Of course that1s a bit like asking how long is a piece of string, so suppose an 18 mm dia roller with a 3mm radius on the edge working on something like en24T - any idea? Even a ball-park figure would help. -- Peter Fairbrother > Thanks. > > You mention roller burnishing - any idea of how strong the spring has to > be? None at all I'm afraid. It's not something I've ever looked into. SignatureDave Baker Puma Race Engines |
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