|Andrew Moyes 1||14/07/2021 21:17:53|
|146 forum posts|
This follows on from the interest shown in my Kennedy hacksaw restoration in the 'Hoover motor lubrication' thread. I hit a snag with replacement of the main bearing and the following is the method I used.
The shaft is ¾” diameter and the bronze bearing is 2” long. I purchase two 7/8”OD Oilite bushes x 1” long, thinking it would be a straightforward job to press the old bush out and new ones in. I then found that Kennedy didn’t follow normal engineering practice by reaming the hole. The hole is left as-cast with a pronounced taper. Their solution was a bush stepped on the outside and driven in from the wide end. The bush fitted the bore with line contact only around two rings, at the narrow end and in the centre.
I reamed the bore 7/8” but only half of the bore cleaned up. I pressed in the first bush and retained the loose bush with Loctite. I cleaned the outside of the loose bush with acetone and used an oil-tolerant grade of Loctite. I also machined a mandrel to align the two bushes, making sure no Loctite came into contact with the mandrel!
For the shaft, I used ground silver steel that was 0.5 thou under nominal size. The two Oilite bushes were also 0.5 thou undersize as supplied. The pressed-in bush contracted a further 1.5 thou.
Reaming an Oilite bush to size is not recommended because it smears the porous bronze and destroys its self-lubricating properties. Instead, I used a technique recommended in an old booklet by a bearing manufacturer. That was to press through a precision ground hardened steel ball. Mine was spot-on 0.750” diameter but due to spring-back of the bush, the shaft still did not enter the bushes. I heated the ball in stages with a blowtorch and used the drill press to force the ball through. It eventually enabled me to get exactly the fit I wanted with a highly polished, truly circular and porous bore.
|Nigel Graham 2||14/07/2021 22:54:03|
|2263 forum posts|
I think that burnishing to size by pressing a ball through is still used in some production work, but you may find that doing it to an Oilite will squidge its surface closed as you point out a reamer will.
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