SOHC
Thanks for clearing up the matter of what type bearings your power hammer has. In light of it having cast iron bearings, I'd suggest the following:
1. measure the shaft journals with a micrometer, taking two readings at 90 degrees to each other, and several sets of those readings along the length of the journals.
From those readings, you can then make a kind of 'map' of the shape of the journals.
2. Using a telescoping gauge & the same outside mike, take a similar set of readings with the bearing caps in place, bolts snugged down. Again, you can then make
a 'map' of the bearing bores.
What these two sets of mike readings will tell you is:
-are the shaft journals still round
-are the shaft journals worn to a taper
-what the largest and smallest diameters of the shaft journals are
you will get the same information about the bearings.
The other item to check is the condition of the surfaces of the bearings and the shaft journals. If the shaft journals are worn to the point that they are deeply scored (grooved) or visible 'necked down', then the repair must address the shaft. Before the repair to the shaft is done, the same check of surface condition in the bearing bores has also to be checked.
Since the power hammer operates at relatively low speed and is not a precise machine tool, I'd say a running clearance of about 0.0015-0.002" per inch of shaft diameter would be about right. Thus: if the shaft journals were 1.5" nominal diameter, about 0.003-0.004" of clearance would be what I'd shoot for. There is a fine balance here between having too tight a bearing clearance for the application vs too loose a clearance. As you correctly state, you do not want the shaft flopping around in the bearings. With the application being a power hammer, you have some 'unbalanced reciprocating mass' (to wit: connecting rod, hammer block, springs, etc) aside from the impacts of the hammer hitting home on the work or tooling. Too much clearance and the shaft journals will slam up and around in the bearings. Too little clearance and the bearings will run hot, particularly when the impacts of the hammer squeeze out any oil film from them. I am giving you what, in engineering, is known as a "WAG" (wild ass guess) as to where I'd set the bearing clearances.
Getting back to the condition of the shaft and bearings: if the shaft is worn to the point that the scoring looks like a coarse thread and/or is necked down visibly at the journals, the shaft should not be run in the hammer. I am guessing the crank disc is solidly on that shaft, not about to come off easily. A repair that would likely work is to build up the worn journals using TIG welding (I would avoid flame spraying due to the impact loading the shaft journals see). A careful TIG weld buildup of the worn areas done a little at a time, and possibly peened with air needle gun will minimize distortion from weld stress. After that, the journals can be turned to correct diameter in a lathe.
As for the bearings, I'd look inside them. If you see plenty of undamaged smooth bearing surface, say about 80%, even with some scoring, the bearing should be OK. I'd run a heavier bodies oil, such as an ISO 68 or thereabouts, and possibly add a 'tackifier' to it like "Lucas Oil Extender" or "STP" (both of these are sold in US auto supply stores). If there is a lot of scoring such that there is not a continuous smooth and round bearing, and if they scored more than a maybe 0.010" deep, I'd look to line boring as I noted in my previous post. If you go with line boring, you will need to build up the shaft journals and turn them to match the new bearing inner diameters. Let's say, for example, that the bearings were originally 2.000 inch nominal bore. With some wear and scoring, you determine that 1/16" of metal has to come out of the bearings to restore a continuous smooth, cylindrical running surface. This opens the bearings to 2.0625" nominal ID. You then turn the shaft to get rid of the scored areas and do a weld buildup, adding maybe 1/8" of metal on each side of each journal. So, a shaft worn to maybe 1 31/32" (1 /32" approximate wear off a 2" nominal journal) winds up with a skim cut to get rid of the scored areas. This brings the shaft down to maybe 1 15/16" diameter. The ends of the turned-down areas have to be cut with a radius'd tool and no sharp corners. The welding is done using either TIG or MIG, and the as-welded diameter is 2 1/8". This gives enough meat to turn new journals at 2.0625" nominal, maybe a couple of thousandths under that. Shimming the bearing caps does the rest as far as setting the bearing clearance.
Before line boring the bearings, make sure to have plenty of shims under the bearing caps. This gives you adjustment to take up wear in the future.
If the bearings are only lightly scored, scoring you can barely catch a lightly-dragged fingernail in, I'd forego the line boring. Instead, I'd opt to do a light scraping on the bearings. All you are looking to accomplish is to break any sharp ridging on the scoring and take off minimal metal. I'd take something like a lathe parting (cutoff) tool blank with a nice sharp squared edge and use it as a hand scraper, holding it with both hands between thumb and index fingers and drawing it towards yourself on a kind of diagonal motion within the bearing as you work it around the inner circumference. You hold the bearing caps in a vise. Make sure to match-mark the caps as to which bearing they came off, and which way they face when mounted on the hammer. Use a prick punch or letter/number punches for this. Paint or marking pen marks tend to get obliterated during working on machinery.
When you get done giving the bearings their light scraping, polish them with something like a 'Scotchbrite' pad, then wipe with clean paper towels and solvent such as mineral spirits or automotive brake cleaner. Cast iron gives up 'free graphite', which is one of the beauties of it as a material. As such, it has some tendency to 'self lubrication' when used for bearings. As such, also, it tends to blacken your hands and wiping cloths if you are cleaning the bearing after scraping or polishing with an abrasive pad. I'd avoid using abrasive cloth or sandpaper.
