I have experience of TRB applications to relate. I once worked in an industrial power transmission company as a summer job and did some time building electric motor driven gearboxes. The ones with TRB's on the output shaft were shimmed to a tolerance of zero to 3 thou clearance. Whenever I've encountered TRB's in wheel bearings , including motorcycles, there is always clearance.
In fact , the only application I know of where pre-load on TRB's has been specified is on the crank mains on Velocette single motorcycles. This is an unusual application though , alloy crankcases that "flex" and move apart when hot. I seem to recall that preload conflicted with advice from the bearing manufacturer but Velocettes research showed that very slight assembly preload when cold resolved a bearing failure issue they were having. Then you have to realise that Velo's were very esoteric machines !!!
You also have to realise steel crankshaft sat in aluminium engine case, coefficients of linear expansion, bearings, for the use of.
The japs were good at developing needle roller mains and big ends on motorbikes, but even then they had to go to oversquare motors, as much for the increased bearing width as anything else.
Even the "tuned" jap engines didn't come close to some of the old english stuff when it came to actual bearing pressures, when you start running long stroke motors (with therefore narrow bearings) at 12.5:1 with mechanical points it really wasn't any problem at all to push way beyond oil technology limits, metal technology limits of course haven't changed.
Tapered rollers (with pronounced "thick" rollers) have pretty much always been seen as "lazy" engineering, eg a "get out" after you fucked up and earlier stage of the design and left too little room to fit (and service) a "proper" bearing.
Nota Bene a tapered roller, comma, bearing, not a tapered, roller bearing.
Even "money no expense" and "safety first" applications where TRBs are carrying passenger rolling stock getting axle loads of 30 tons is really pushing the envelope....
Tell me you've calculated peak momentary loads in a 6/1...
Nota Bene in these railroad applications with a typical 860 mm wheel diameter you have a axle RPM of approx 650 at 120 kmh train speed.
Nota Bene 2,000 hours is considered a useful service life for these very expensive bearings, 5,000 hours with no fretting is something to write home about.
5 yrs / 5,000 hours was Cat's world beating warranty, there are just under 8800 hours in a year.
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By contrast, a plain bearing doesn't have a service life, quite apart from the fact that loads of 30 tons are nothing to a plain bearing, provided the lubricant is maintained at a certain quality and the film is maintained properly there is no mechanical wear.
Plain bearings require more shaft, so everything gets wider, and plain bearings don't do thrust by themselves, but, you can split em, so you can put bearings anywhere, and if you can line bore and white metal there is no limit to the length or diameter of shafting that can be supported by plain bearings.
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There is no way on this planet that a COMPETENT plain to TRB conversion on a genuine lister will come in cheaper than a blueprint (again, most people don't even know what that means) rebuild of the Lister....
because ya gotta blueprint before you can convert to TRB
Sure, a MONKEY can do it cheaper, but not properly.