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I understand most forums are run by self -styled experts( e.g. bobistheoilguy)who have little to no actual knowledge.
The note that 'oil may damage the insulation' is certainly true of older insulations such as rubber and waxed paper, I suspect it is not true of modern polyurethane based enamelled wires and plastic insulation - indeed oil immersion for cooling coils and other electronics is common and I have used old engine oil for this (tip - it's a poor choice at RF... better to use an ester like Midel...).
So, what is the construction of this old motor ? (how old - I have a cotton and paper insulated pre-war machine under my lathe, and that works just fine on 20/50 oil, (I still have about 95 % of the oil left in the container as takes a quick 'bloop' a few times a year )but I am careful that it only goes onto the bearings.) The idea of sintered bronze (as opposed to the solid cast bearings pre about 1930) is that the oil holds to the surface, so it sort of 'wicks in' to the structure of the metal (a sort of absorbent sandstone of the metal world at a microscopic level) To that end the surface tension and viscosity of the oil at the running temperature are the key aspects.
I suspect the choice of a detergent or not is not so relevant in a bearing, as the idea of adding them is really for an engine to keep the carbon in suspension until it reaches the oil filter.
If you are worried you are using the wrong oil, you can check to see if the bearings heat up after a few minutes of idle running - when correctly oiled the shaft should be full supported on an oil film and run cool- - I'd expect it to stand several hundred PSI (estimate of the shaft to bearing contact area may be tricky without dismantling) of side load before the oil crushes out and it starts to complain by squeaking and getting hot.
You can certainly get non-detergent oils, at a premium, sold for old motorbike enthusiasts and so on, but unless this is a priceless heirloom, I'd try the common stuff first.
Now waiting to be shot down by others.
These online forums are amazing. Here is what I have found out. Fram oil filters - useless(they do have a lawsuit for the filter media collapsing). But they make filters for Honda. For the last 20 odd years I have been told never to use Quaker State oil as it makes sludge(Castrol never does). but a friend of mine was told to use Quaker State in his Toyota because the 'waxes' protect things on start up. Now Pennzoil(owned by Shell) is second rate because their oil is made from natural gas. One bloke said his car makes a noise after 3000 miles. I always thought oil did not 'wear out.' It just got dirty.
What lathe do you have?
Your machine is relatively youthful, my lathe is a 1921 or 22 Drummond B (early pre-cursor to the one that later became myford but belt driven and open gear train) but the counter shaft and pulley wheel for the flat belting is older though it has at some point been re-bored for ball races. Sadly in an odd imperial size. The motor is an unrelated 1908 induction job with a centrifugally switched reluctance delay starter winding, and goes with a fair crack of the switch and a blue flash worthy of a dodgem car when up to speed, and it is time to disengage the starter. I do suspect 50Hz may not have been the design frequency. The rope drive passes through two holes in a 1960s desk as the motor is beneath and the lathe above. The inrush limiter is an early 2000s attempt by me to stop it firing a B32 on start up.
Not a sintered bearing to be seen on that machine, all plain.
Welding up a decent guard for it all is on the list of things to do, as it does occasionally fire off the flat belting or the rope if things get a bit hard going, which is a touch un-nerving.
It is not exactly a classy installation, but it keeps proving itself very useful, and is easily repaired.
Hobbymat MD65 owner.
1930s Portass 3 1/2 inch "Super" belt driven, plain bearings
1950s (?) "Rodney" stand alone miller, ball races
Old lathes and mills are quite affordable here. The curse is moving them. 1500kg of 60 year old iron costs the same to move as 1 year old iron!
Now to develop something simple to produce in my dotage to keep the wolf from the door - no pensions in private sector USA.
Your odd size bearings may still be available here in the US. If you need a set give me a shout.
I still find it hard to believe that a UK or US company cannot use Far Eastern castings to make a decent lathe as alternative to the 'all-Chinese' offerings. I am surprised India has not started to compete in this market.
I still have my Einhell branded mini lathe tucked away under some shelves but haven't used it for years. It required new headstock bearings plus some work on the tailstock before I could get satisfactory cuts. I also have a Proxxon FD150 for very small work and as a get me out of trouble when the Hobbymat is in milling mode and I need to turn up a spacer for something.
What sort of things are you intending to make? I try to repair broken things and also make small internal combustion engines.
I have some synchronous electric clocks and the manufacturers recommended using engine oil to lubricate the bearings with (the gears run dry) but this happened to be the single grade engine oils sold in the 1930s to the 1950s. Modern synthetic multigrade engine oils are unsuitable as they contain all sorts of detergents and additives that can damage the mechanisms and stain brass.
Anthony Brooks:They are a bit too fluid for clock pivots but are ideal for sintered bronze bearings as they soaks in well.
Arran. You might be able to purchase some rifle oil for your clocks. It has few additives and evaporates without leaving a film. Sewing machine oil is probably the same.
There was a graphited lubricant called oildag used to lubricate selectors in Strowger telephone exchanges that is ideal for lubricating small mechanisms with. Is it still manufactured?