H-M Supporter - Gold Member ($25)
- Dec 20, 2012
During my quest to eliminate the excessive runout on my RF-31 Mill/Drill I determined that the spindle bearings were a major contributor. As you can see, these bearings have seen better days. In addition to being filthy, they have not been maintained and have worn much earlier than they should have. Rather than find a direct replacement I wanted to use sealed/lubricated for life/maintenance-free angular contact bearings because the construction of the head on the RF-31 provides multiple areas for ingress of dust, dirt and chips that will eventually end up on the exposed upper spindle bearing. This is not good for longevity so I wanted to eliminate the possibility.
Surprisingly, this doesn’t appear to be a common mod as information on this subject turned up little. However, one of our very own HM members, Canuck75, did this exact job. I contacted him and he encouraged me to go for it and suggested that I provide a detailed write up for the benefit of the other RF-31 owners on our forum. This post is to honor that request.
The bearings we used are from FAG, a German maker of precision bearings:
Upper: FAG 7206-B-XL-2RS-TVP
Lower: FAG 7207-B-XL-2RS-TVP
· These bearings have the exact same dimensions as the stock bearings but are each 1mm thinner, which is a minor issue I’ll touch on later. Both are rubber-sealed on both sides and lubricated for life.
· They are single row angular contact bearings so they must be used in pairs as we are doing here in order to sustain the bi-directional axial loads a spindle experiences. These bearings have a 40° contact angle, which means they can sustain large axial loads while handling very high radial loads as well.
· The XL designation means the raceways have an improved geometry and are precision-honed to improve their load bearing capacity.
· The TVP suffix means the cage is made from glass fiber reinforced polyamide, not metal.
· They are accuracy class P5 bearings, the equivalent of ABEC 5. They are probably much more accurate than the spindle and quill are.
Cost will vary with your source and luck. I was able to find both brand new for under $100 but retail for them is somewhere around $300.00 or so.
I contacted FAG for instructions on installation procedures, preloading specifications and their recommended break-in procedure for these specific bearings. Here is what they had to say:
Normally, in a spindle, you have a break-in routine to distribute grease because the bearings are usually un-sealed. But these bearings are sealed and lubed for life, and more robust than your average spindle bearing, so there is no need for that. The newer x-life products (-XL suffix) have such finely honed raceways that there is no measurable burnishing of the surface finish. [Therefore] no special run-in required.
There are a dozen variables that could influence the running temps of your bearings, and we know virtually nothing about your system. So it is not possible to predict what temps your bearings might see. But the upper operating limit of your bearings is limited by the cage, seals, and the grease, and should not exceed 100 deg. C. A good target would probably be 75-80 C max. More than anything, you should expect to reach a steady-state temp within about 30 minutes. If you do not reach steady state, or if you rise above these temps in a short time, that might be an indication of excessive preload.
As I said, these bearings must be installed in pairs; they can be installed either back to back or belly to belly, meaning the inscribed bearing designation on one face can face in or out but both must be facing the same way – X/X or O/O but not X/0. Supposedly, X/X is slightly stronger (inscriptions facing out).
FAG emphasizes in their literature that installation forces should be applied only to the race being pressed to avoid brinelling (the permanent indentation of a hard surface) of the precision-honed races. Therefore, pressing adapters that contact only the race involved is wise. They also advise the use of an arbor press, not a hammer. (Sorry, I know us hobby guys love to use our hammers but this is not one of those times.) I used a Dake hydraulic press for this project and it worked fine, albeit with less tactile feel than an arbor press would give.
The press fits are only about 0.0002” so there isn’t a lot of force involved. However, alignment and the application of force to the right area is critical so I made pressing adapters from solid aluminum rod as I had trouble finding pipe of the appropriate size – that’s a lot of boring! The largest adapter is 2.8” OD.
The adapters on the left and center are used on the lower bearing. The one on the right is used on the top bearing and engages both inner and outer races at the same time; this is fine according to FAG since the two races are the same thickness and pressure applied to both races at once will not stress the internal bearing surfaces. Each adapter is sized to just fit the width of the race it is intended to contact.
Remove the quill from the head. This is a simple job that involves removing the depth stop, quill return knob and quill return spring along with the spring anchoring screw and quill alignment pin. Be sure to lock the quill before you remove the pinion shaft. Once the pinion is removed, catch the quill as you unlock it. The locking cotters can remain in the head.
With the quill removed from the mill, remove the little cap overlying the lower bearing. Mine is aluminum and is a right hand thread that comes off easily.
Now the spindle can be pressed out of the quill from the top. This leaves the lower bearing still on the spindle and both bearing races in the quill. The upper bearing just lifts out and off the upper spindle.
After pressing the lower bearing off the spindle the next step is to remove the races from the quill.
The lower race comes out with some judicious tapping with a 3/8” steel rod and hammer behind the race. There are two access slots in the quill casting to allow a good angle for this. Tap alternately on both sides until the race releases.
Once everything is cleaned, the bearings can be installed. The manufacturer makes a big deal about cleanliness when installing their bearings. I wiped each contact surface with alcohol until the paper towel came away clean and then used a microfiber towel before I blew the area out with air and applied a light coat of oil. The bearings were unwrapped just before installation.
Here, the smallest adapter is being used to press the inner race of the lower spindle bearing in place:
Then the lower bearing outer race is pressed into the lower end of the quill:
Once the lower bearing is installed, the upper bearing can be pressed onto the upper spindle. The upper bearing is supported underneath by the aluminum adapter that rides on the rim of both the inner and outer races. I applied force to the lower bearing rim with the large adapter and got the upper bearing in but had to also apply a little follow up pressure on the spindle nose to be sure both bearings were bottomed out.
Again, these bearings are installed back to back so that their inscriptions face out.
Recall that I said these bearings are 1mm thinner than the tapered bearings they replace. It turns out that this matters because the preload nut has a limited amount of downward travel. At this limit there is about a 1mm space between the bearing and the tabbed washer so the bearing cannot be preloaded. Therefore, a spacer with the dimensions of 1.4” OD X 1.2” ID X 2mm thick was made from 6061-T6 Aluminum to take up this space. Aluminum is fine for this purpose – it just needs to solidly take up space between the inner bearing race and the tabbed lock washer. You can just see it under the tabbed washer in the pic below.
The castellated tabbed washer is placed on top of the spacer (Note: the castellated washer has a downward-pointing tab that must fit in a groove in the spindle) and the preload nut is threaded on – LEFT HAND Thread; the beveled edge of the preload nut faces the bearing. As these bearings do not have a specific preload value I just snugged the first nut very firmly with a spanner and a spindle wrench and engaged one of the washer tabs. The second nut is firmly tightened down on top of the first one and the spindle is complete and ready for installation in the mill. The Luminar 28mm spindle wrench held the spindle solidly while tightening the preload nut and I can recommend it.
I checked runout at the bench with the quill clamped down firmly. Runout was less than but close to 0.0001” TIR. The quill was then installed and the bearings were run at 1800 rpm for ½ hour.
I checked the temps directly at each spindle bearing with an infrared thermometer to be sure that preload wasn’t excessive. Both bearings leveled out at 41.9°C at 25 minutes and then the temperature remained stable.
Run out was checked with a Compac 215GA inside the R8 taper after the bearings were hot; it was 0.002”TIR. While this was disappointing I knew the spindle was running pretty true so the runout I was seeing had to be due to the drive sleeve bearings, which it was. Runout after the drive sleeve bearings were changed dropped to less than 0.0001” TIR. (I’ll discuss the drive sleeve bearings in another thread.)
Overall, this is a very simple maintenance step that will reduce runout and the need for bearing maintenance for the useful life of the bearing. Once the pressing adapters were made it was done in under 5 minutes.
Once again, my thanks to Canuck75 for doing the legwork to find the correct bearings!