Asking for comments regarding installation issues of angular contact vs tapered roller bearings.

[homebrewed]

Long subject line, sorry about that. I just changed out the deep-groove bearings on my mini lathe for angular contact bearings and may have damaged the new bearings when I installed them. It appears to me that the headstock and spindle design on these lathes is problematic in this regard, because there's no good way to avoid applying a lot of pressure between the inner and outer races at two points of the installation. Here's the deal, assuming that the old bearings have been removed (and likely trashed in the processes):

1. Install the front bearing on the spindle. This can be done while pressing on the inner race so no risk of damage. The spindle has a step-up in diameter to act as a stop for the inner race.
2. Press the spindle into the headstock. Since the outer race doesn't rotate during normal use there is some necessary clearance between the spindle and outer race -- therefore, the force needed to press the outer race into the headstock has to be transferred through the bearing balls and inner race. Potential for damage here.
3. Press the rear bearing onto the spindle and into the headstock. Like the front bearing, the spindle and headstock are interference fits to the bearing. As in step 2, force must be transferred through the bearing to install it. Also potential for damage, but using a stepped washer with exactly the right step height and diameters might address this. But you need to make what you need BEFORE you tear your lathe apart, assuming your lathe is running well enough to do so.

For tapered roller bearings I'm thinking that the chances of damage are lower, for several reasons. For one thing. the reduced contact area of bearing balls relative to roller bearings favors roller bearings. The other major difference is that the inner and outer races of a tapered roller bearing can be installed separately. So the forces needed to install them also can be separated.

Based on these considerations, it seems to me that tapered roller bearings are the best way to go, despite some minor issues like needing to reduce the spacer lengths because these bearings are slightly thicker.

What are other folks' experiences in this?

 

[Cadillac]

I am replacing the bearings in my headstock now which are taper roller bearings. Yes it is easier. Taper rollers can come in whatever grade your pocket can afford. Looking at a nsk catalog or timken it shows you the forces bearing arrangements can handle. Then the specs for the machines requirements are induced and a compromise has to happen.
What type of arrangement are the bearings you installed? Did you switch from a taper roller to a angular contact Or just replace what was there?

 

[Firebrick43]

angular contact bearings in a matched set have preload built in. Most spindles use 2,3, or 4 angular contacts at the chuck end of the spindle in a back to back arrangement. There is a nut around the spindle that tightens the inner races together and the outer races are trapped in a counter bore with a plate. There has to be positive trap on the outer races. Then the back of the spindle has a deep groove ball bearing or sometimes a presicion straight roller bearing that is allowed to float as the spindle expands/contracts due to temp.

Few spindles any more do so but some use to have the angular contacts mounted apart, as in 1 or 2 up front and a 2nd or 3rd int he back. In this case inner and outer sleeves were used. You should not use a stepped shaft to separate the front and back inner bearings. So a bearing in back to back config is slid on, then an inner and outer spacer sleeve, and then the back bearing, and then the nut. Once again the nut is tightened fully as the preload is set by the bearings but with the spacer sleeves they have to be EXACTLY the same length with the faces exactly parallel. You can not finish cut the spacers on a lathe, they will not do a precise enough job. They are turned and then the ends surface ground. The ID/OD isn't critical as long as it doesn't rub anything but the ends must be accurate or it will affect the preload/runout and destroy the bearings in short order. We measured the spacers on a surface plate with a high end electronic height gauge and would regrind if there was any measurements more than 0.0001" out. You could get away with lapping in a home shop if you had the measuring equipment and really under stand lapping but that is a very long subject in itself. Again the outer races must have trap in the bore of the housing.

Tapered rollers are much easier to design but are not used much as preload is a function of skill (not to much or to little) and have serious speed limitations. They are used more on massive machine tools, very little on small ones anymore at all. They generally take crashes better due to rollers having a lot more surfaces contact compared to a similar sized angular contact area. I am baffled by the spacer comment for tapered roller bearings. Typically they have no spacers and are preloaded strictly by the nut and the mechanic assembling.

 

[homebrewed]

angular contact bearings in a matched set have preload built in. Most spindles use 2,3, or 4 angular contacts at the chuck end of the spindle in a back to back arrangement. There is a nut around the spindle that tightens the inner races together and the outer races are trapped in a counter bore with a plate. There has to be positive trap on the outer races. Then the back of the spindle has a deep groove ball bearing or sometimes a presicion straight roller bearing that is allowed to float as the spindle expands/contracts due to temp.

Few spindles any more do so but some use to have the angular contacts mounted apart, as in 1 or 2 up front and a 2nd or 3rd int he back. In this case inner and outer sleeves were used. You should not use a stepped shaft to separate the front and back inner bearings. So a bearing in back to back config is slid on, then an inner and outer spacer sleeve, and then the back bearing, and then the nut. Once again the nut is tightened fully as the preload is set by the bearings but with the spacer sleeves they have to be EXACTLY the same length with the faces exactly parallel. You can not finish cut the spacers on a lathe, they will not do a precise enough job. They are turned and then the ends surface ground. The ID/OD isn't critical as long as it doesn't rub anything but the ends must be accurate or it will affect the preload/runout and destroy the bearings in short order. We measured the spacers on a surface plate with a high end electronic height gauge and would regrind if there was any measurements more than 0.0001" out. You could get away with lapping in a home shop if you had the measuring equipment and really under stand lapping but that is a very long subject in itself. Again the outer races must have trap in the bore of the housing.

Tapered rollers are much easier to design but are not used much as preload is a function of skill (not to much or to little) and have serious speed limitations. They are used more on massive machine tools, very little on small ones anymore at all. They generally take crashes better due to rollers having a lot more surfaces contact compared to a similar sized angular contact area. I am baffled by the spacer comment for tapered roller bearings. Typically they have no spacers and are preloaded strictly by the nut and the mechanic assembling.

The lathe being repaired is about the lowest possible denominator when it comes to machines. The original bearing set is two deep-groove bearings spaced about 7 inches apart in a fairly skimpy casting (!). There's no way I know of to add additional bearings without basically throwing the whole thing away and starting over. I understand that in terms of load capacity roller bearings are huge overkill in this context. The issue is a hobbyist-level attempt to replace a marginal set of bearings with something better, while not damaging the replacements -- given common limitations to my type of owner, like no hydraulic press and home-made/improvised alternatives that -- in my experience so far -- can result in bearings that start out going in crooked and needing a lot more "encouragement" to be set in place. The "encouragement" part is where substantial damage can occur, so the idea is to separate races to avoid that situation.

FYI, one tapered roller bearing that works in this application costs in the neighborhood of $10. I just checked.

I appreciate your comments but you need to consider the constraints in context of a lathe that, even now, costs in the neighborhood of just $1000. I bought mine, new, for about $600.

My comment about spacers is regarding other parts of the lathe that need to be modified to accommodate the 1.25mm difference between the original bearings and tapered roller bearings. It gets distributed in various ways throughout other components that have to match things like the belt drive pulleys and high/low transmission gears.

-HB

 

[Firebrick43]

My consideration was that any angular contact bearing is going to be damaged unless set up properly.

As to where the spacer goes, it sounded like you wanted to seperate the bearings but now ???? Are you talking OD?, ID?, width of outer race?, width of cone?, width between outer races?, width between cones??????????

 

[homebrewed]

I am replacing the bearings in my headstock now which are taper roller bearings. Yes it is easier. Taper rollers can come in whatever grade your pocket can afford. Looking at a nsk catalog or timken it shows you the forces bearing arrangements can handle. Then the specs for the machines requirements are induced and a compromise has to happen.
What type of arrangement are the bearings you installed? Did you switch from a taper roller to a angular contact Or just replace what was there?

The original bearings were just deep-groove bearings. The arrangement of the replacement angular contact bearings was based on mods performed by owners of similar machines. The fact that the lathe used deep-groove bearings says a lot right there in terms of the (small) amount of preload imposed on them. AC and tapered roller bearings are probably overkill for a 7x lathe, but those deep-groove jobs just offend my sense of what's "right"

 

[homebrewed]

My consideration was that any angular contact bearing is going to be damaged unless set up properly.

As to where the spacer goes, it sounded like you wanted to seperate the bearings but now ???? Are you talking OD?, ID?, width of outer race?, width of cone?, width between outer races?, width between cones??????????

I agree that proper setup is important. A good thing to point out.

Regarding the spacers, they have nothing to do with the bearing install. They are just part of the high/low transmission inside the headstock, to make sure that the spindle gears stay aligned with the gears on the intermediate shaft. They are just affected by the change in bearing thickness, so a minor problem. Except when you discover that _after_ you've torn your lathe down....

 

[Firebrick43]

I have never seen a spindle that you have to press bearings in. Use heat to expand and dry ice/nitrogen to shrink so it will allow every thing to slip together. I don't know really how to help as your descriptions are not very good. Can you take pictures? Do you have a drawing? Can you make a sketch?

 

[homebrewed]

I have never seen a spindle that you have to press bearings in. Use heat to expand and dry ice/nitrogen to shrink so it will allow every thing to slip together. I don't know really how to help as your descriptions are not very good. Can you take pictures? Do you have a drawing? Can you make a sketch?

This guide may help clarify some things. TOT also has a youtube video describing the same mod; he installs the new bearings around 8:30 into the video.

 

[Firebrick43]

Wow, that is a really bad way to have angular contacts without a center spacer.

Any way, I see some of the problems. The spindle shaft should not have to press on (or beat like ToT). You know from the bearings the od of the shaft. Make or buy an OD brass lap and get some diamond lapping paste.

Suburban tool lapping video

Another really good video on bore lapping

To drive the part for lapping it really doesn't have to be that accurate(on center). So rig something up or go to a friends house and use their lathe.
Any way you want a light interferance fit. Literally only .0002" interferance and its almost impossible to get a fit like this on lathe but easy (but time consuming) to lap in. With it very clean an Lap, clean, and measure. It goes slow so you shouldn't overshoot if you use fine paste. Laps keep things on concentric(if they were to begin with) were polish paper will not.

Get some dry ice from town, cool your outer races and warm the housing with a heat gun. The outer races should fall in. Let temps equalize. Heat the inner cone to 225-250 degrees and it should slide on. If you don't want to use your wife's oven, either get a small convection oven from a garage sale or buy the correct tempil stick and use a hot air gun. The tempil stick will melt at its rated temp.

Also ToT showed horrible practice with bearings. Don't set them on an abrasive scotch bright pad, cleanliness is paramount in bearings. We rebuild tools in clean rooms and filter solvent to 2 microns and then do a final rinse of parts with canned brake cleaner.