Ryan, it doesn't matter how long the bar (moment arm) is. You're overthinking this. The SB 75 pound recommendation is just an approximation of about what their experience told them it takes to squeeze out the oil film. As another poster stated, the purpose is to squeeze out the oil film, not put 75 pounds on the bar. Obviously, it's going to take more force to pull upwards on my 13" SB spindle than on my 9. Here's how I do it:
Before you start, make sure your clamp bolts are tight (I like them quite tight to ensure that shims are fully crushed) and your bearing spreaders (if equipped) are properly tightened.
Start at big end.
1) Push down on the bar until your tenths indicator stops moving.
2) Hold for ten to 30 seconds to make sure the oil gets squeezed out.
3) Take indicator reading.
4) Pull up on bar and repeat Steps 2 and 3.
5) Determine difference between the two readings. That is your total diametral clearance that you compare to the SB recommended clearance. Write it down.
6) Go to Small End Bearing. Repeat Steps 1 through 5.
7) Add or remove shims to each end as indicated.
8) Repeat Steps 1 through 7 as often as necessary.
9) When you think you've got it right, run lathe at progressively higher speeds checking bearing temperature and free spinning.
You should also set up your tenths indicator to minimize or eliminate cosine error in the reading.
I have found it useful to cut some 0.0005" shims for this work.
Ed in Florida
Great advice, every single bit of what you said coincides with J. Rozen. I have seen one
factory test bar for sale. One end of it was machined to have a Morse Taper to fit in the
front of the spindle. Some how it was zeroed out and then you did the test. The bar looked to
be about 18" to 24" long. I don't believe it would work on the rear of the spindle that
doesn't have a MT. It was advertised for about $1200.00. Apparently this company supposedly
made these test bars according to South Bend specs depending on what size lathe you own. I
wasted a lot of time and money trying to come up with a bar and scales to pull up at 75
pounds. I believe that some of this discussion on the methods of how to correctly find the
bearing clearance is opinion. Like what was said, just trying to squeeze the oil out.
One will say absolutely 75 or 85 lbs. pull and another will say pull up until the
indicator stops moving. I'm a hobbyist so what I am saying is my opinion. In no way am I
saying which method works the best.
I might try it both ways and select the easiest method that gives me the same results. But
like you all said, as long as the oil has been squeezed out you will have the correct amount
of clearance and go from there. Thanks