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Questions about felt in bronze bearing

RCPDesigns

Hot Rolled
Joined
Sep 3, 2014
Location
Atlanta GA.
I'm rebuilding my early 1900s Cincinnati Cutter Grinder and things have been going great up to now. When the machine is running it literally pumps the oil out of the reservoirs and slings it across the shop. I have the felt gaskets/washers in the places where the illustration says they should be but it does very little. I've attached picture of the bronze bearings and I have a question about the hole where the oil enters. There are oil "entry" holes, one on each side. I believe there is one on each side such that the part could be made interchangeable. There are more holes on the bottom of the bearing and those were filled with felt when I took the head apart. The parts manual shows them being filled with felt and that is rather obvious. My question is should the two "entry" holes also be filled with felt? This would slow down the oil entry. I tried it this way and it appeared to greatly reduce the oil consumption though the spindle did get a bit warm.

Thoughts?

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Joe Michaels

Diamond
Joined
Apr 3, 2004
Location
Shandaken, NY, USA
I'd suggest trying to run the spindle with felt plugs in the oil entry holes. Some experimenting with different densities of felt may be needed to get the right rate for feeding oil to the bearings. The grinder spindle and its bearings are a very basic design, so I will add my two cents to this discussion. This type of spindle/bearings is typically brought to a 'running heat' before grinding is started. The reason for this is to allow the bearing temperatures to stabilize and this has the bearings expanded (and clearances closed a bit) to the size they will remain at during operation of the grinder.

As a rule of thumb (sorry about the pun), if this type of bearing is too hot for you to comfortable to keep your fingertips (or the back of your hand if your fingertips are callused), then the bearing is too hot to be safely run. This type of bearing needs to come to a 'running heat' and that is what I'd call a "comfortable warmth". The reason the oldtimers who taught me about this sort of work used the backs of their hands was thus: the belief was their fingertips and palms were too callused to get a good sense of bearing temperatures. With today's 'temperature guns', checking bearing temperatures is an easier and more accurate matter. However, to get to a finer point on this: the temperature gun or your hand is only sensing the temperature at the outside of the bearing assemblies. If we turn our mindset to the era when your grinder was designed and built, the 'measuring' of bearing temperatures by 'feel' was likely all that was used to set the bearing adjustments. Nowadays, there would be drillings in the bearing head casting and RTD's (resistance temperature detectors) would be tapped right into the bronze bearing metal or at least in hard contact with it. On powerplant hydroelectric turbines (60" diameter shaft journals, tilting shoe babbitted bearings similar to some grinder spindle bearings) after we'd do any bearing work, we'd always do a 'heat run'. We'd monitor the bearing temps via the RTD's and get a plot on it. When the bearing temps 'flat lined' on the plot, we knew where we stood as far as adjustment of clearances and our scraping jobs. Of course, we had set the clearances using 'Porta Power' rams and dial indicators, so we were not going on bearing temps alone. Still, even with a good clearance setting, it can happen that a bearing does run hot. This can be due to a variety of factors, not the least of which are lubrication (type/viscosity of oil used, and rate at which oil is fed to the bearings).

Whenever I work on plain bearings, whether in a small lathe headstock, steam engines, or anything that comes my way ('retired' since 2013 from the powerplant), I always do a heat run. I work up from slowest speed to maximum speed, running with no load on the machinery. Once things feel stable and the heat feels right to me, I then move to a heat run under load. On a machine like a plain bearing lathe, I take successively heavier cuts on steel. I admit that I have never worked on a high speed plain bearing grinder spindle such as yours, but it is one of those things us engineers call "elegantly simple", and the rules and principals for plain bearings still apply. The difference is that, for a grinder spindle, a much tighter running clearance is needed than for something like a steam engine or hydro turbine or even a lathe.

I would suggest using an oil made for high speed plain bearing spindles (Velocite ?). As for setting the clearances on the bearings, if you have a manual (which seems to be the case from the cross sectional 'cut' you posted), maybe the recommended clearances are posted there. The felt in the entry hole should serve as a metering device to keep that thin-bodied spindle oil from passing right out of the bearings as you describe. If no clearance values are handy, I'd suggest starting with a fairly loose clearance (checked by dial indicator and pry bar such as a hardwood hammer handle, applied under the pulley). I do not know the diameter of the spindle journals, but if you went with maybe one-and-one half thousandths of an inch as a starting adjustment, that would let you make a heat run. I may be way off base with this figure of 0.0015", a WAG (wild-ass-guess) on my part and probably a safe starting point. If the bearings, after maybe 10 minutes running at speed/no load are at the same temperature as when you started the spindle turning, they are too loose. Take up on the adjustment nuts, maybe a few 'tenths' and try another heat run. Note the ambient temperature in your shop and the starting temperature of the bearings (or at least the grinder head holding those bearings, as close to the bearings as you can get with your temperature gun or your hand). It is normal to get some temperature rise in the bearings, and it is wanted as it brings the bearings from a cold clearance to a tighter running clearance.

You do not show the spindle journals so no idea as to their condition, likewise for the running surfaces of the bronze bearings. The oldtimer's test was to drag a fingernail lightly over a journal if it appeared scored. If your fingernail catches on the scoring, the journal should be stoned. This can be done by setting the spindle in a lathe and using an "Arkansas Hard" small oil stone. The stone is held at about 45 degrees to the journal and is worked over the journal with light oil as it turns in the lathe. This will break any ridging on the journal and not take off enough metal to affect concentricity and roundness of the journal. Similarly, drag a fingernail inside each bearing and if you feel ridging, polishing with a very fine "Scotchbrite" pad followed by thorough cleaning with solvent could not hurt. I am hesitant to suggest what I call 'giving the bearings a shave' to address ridging. We do it on larger bearings with the edge of a parting tool blank, or the edge of a piece of power hacksaw blade ground to a good straight edge, end corners radius'd. A light diagonal shearing motion as the scraper is pulled over the the surface of the bearing is used, and is done in a cross-hatch pattern. This breaks ridging, and is followed by the polishing with the Scotchbrite. The result of these processes is to just clean up ridging and scoring, not to remove enough metal to affect roundness or concentricity. Breaking the ridging (if it is present) will enable the bearing and journal to better maintain that 'wedge shaped oil film' over the length of the journal and bearing. It will also let the bearings be set up with a little tighter cold clearance. We are talking about adjustments measured in tenths of thousandths, and an adjustment might only be a couple of tenths at a time. Again, the heat runs are what (at least in my opinion) are the final arbiter as to whether you have the bearings adjusted properly. I call it 'sneaking up' on the final clearance adjustment. As you work towards the final clearance adjustments, you are moving in the tightening direction. However, if you wind up with a bearing running too hot, you need to loosen the clearance adjustment, flood it with oil and let the whole grinder head cool back to ambient temperature before attempting further adjustments. The process is kind of a one way street in that regard.

As I've said elsewhere on this 'board, I am an oldtime engineer and machinist, and self describe myself as a 'proud stubborn dinosaur'. I am not scientific in my approach to your inquiry. In the era your grinder was designed and built, as I wrote earlier in this post, a lot of the bearing adjustment was done using the most incredible instruments and tools of all: our minds, bodies, and the senses we were blessed with. Add "common sense" to the list of senses and you are there.



As a ball-park value, you might want to shoot for about 120 degrees as the maximum stabilized temperature
 

RCPDesigns

Hot Rolled
Joined
Sep 3, 2014
Location
Atlanta GA.
Wow... Joe that really helps! The path you describe is one that I've been trying to navigate down but was unsure some of the details and, frankly, lacked a bit of confidence that it was the right way to go. I now have a much clearer way forward and it feels much more like something I can do. I was unaware of the 'running heat' idea but that makes perfect sense. I think I've stumbled into having it just about right. Thanks again for your reply and, really, all your posts in general.
 








 
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