Abrasive 1 1/2 - Spindle Design

I am looking for some input on surface grinder spindle design. I have an Abrasive that I threw a part last year, wasn’t a bad throw or crash, but since have had intermittent surface finish issues. Sometimes I get great finish, sometimes not, can’t tie any specifics, it really seems random.

This is my, or was my, nicest grinder, as far as accuracy, I need it for two potential jobs I am quoting.

Saturday I pulled the cartridge and took the spindle apart to see if I could find any direct issues. The rear most bearing is filthy and packed with black goo that may have been grease or oil once upon a time. Once I disassembled the motor, I noticed that the spindle shaft had about .008 movement front to back on the middle bearing. Problem found, or so I thought.

I have another Abrasive 1 1/2 that I am in process of doing a full rebuild. This has been great to have another to compare to. SURPRISE they are not identical!

Upon further study, there is apparently no “pre-load” as I was expecting the middle bearing to have. This bearing merely sits in its location with no clamping force to it. The front load on the spindle shaft (pushing towards the operator) is caused by 3 springs in a cup that sits in the back of the motor housing.

The spindle shaft then essentially floats, with forward pressure being applied by the springs. Checking my other spindle, the cartridge housing provides .130” clearance in front of the outer race of the middle bearing. My 3rd Abrasive, it you push on the wheel hub, the whole assembly will move about .008”.

Middle bearing is a 7206CG1DUJ74.

Does this sound correct? I don’t have a lot of grinder spindle experience?

Everyday is a learning opportunity:

Preloading with springs
Using springs to apply preload to angular contact ball bearings is common, especially in high-speed grinding spindles. The springs act on the outer ring of one of the two bearings. This outer ring must be able to be displaced axially. The preload force remains practically constant, even when there is axial displacement of the bearing as a result of thermal shaft expansion. For additional information concerning preloading with springs and values for preload force, refer to Preload with a constant
force († page 93). Preloading with springs is not suitable for applications where a high degree of stiffness is required, where the direction of load changes, or where indeterminate shock loads can occur.

https://www.skf.com/binaries/307-12...earings-catalogue---13383_2-EN.pdf#cid-129877

I got the spindle rebuilt with new Barden precision bearings. I am happy to report it is SUPER quiet.

I am not happy to report that the front, plain bearing, seems to get hot. I only have my other grinder to use as a baseline. It can run all day, and will reach about 97, or 98 degrees, and holds there.

The rebuild spindle is slow to heat, I started out with 5 minute increments, on, off, then after the temperature seemed to stay constant through those increments, moved to 10 minutes, on, off. I made it to 15 minutes, when the spindle started getting to 100 degrees.

I have two thoughts on contributing factors.

First, before I turn the grinder on, I can see a bead of oil around the spindle shaft at the bearing. If I rotate the spindle either way, the oil remains. Somewhere as the spindle heats up, this bead of oil disappears. I noticed that my oiler doesn't seem to work quite right, and this is an issue that remains from before I rebuilt the spindle. The oiler is supposed to be set to drip every 5 minutes. Sometimes I cannot get it to drip without unscrewing the oiler. I thought the oiler was defective. If the spindle is off, the oiler works just fine. So something, maybe the clearances, is preventing the front bearing from maintaining its wedge of oil. I think as the bearing loses its oil, it begins to heat up, further contributing to the issue.

Second, I think that however minute the difference, the spindle no longer rides in the plain bearing the way it did before, due to the new bearings. This may contribute to the oiling issue if it is related to clearances?

Any thoughts would be appreciated.

For reference if anyone finds this thread in the future:

Plain Bearing clearances