Lathe spindle bearings and spindle material

[pepo]

Machine tool spindle design is based on a few simple concepts. Get too far away from any of these and you will have problems. The granite with the angle plates with the flat stock is well off the beaten path. I admire your creativity but,the fact you said milling the bores is your plan tells me and anyone else who has learned this stuff the hard way,you need to do some reading.

[mackeym]

Regarding bearings, i found a matched pair of ABEC 7 A/C's (new and cheap) for the chuck end of the headstock. Regarding loading, i plan on staying below the fatigue load limit (and no where near the basic load ratings). For the pulley side, i'm going to use some type of low clearance bearing (which i havn't found yet). NC Cams, regarding your concerns with the 1000lb load on some previous bearings, that load was near the fatigue limit. Correct me if you know otherwise, but as long as you stay below this limit, the bearings should last almost forever (assuming they're lubricated and kept clean). If this is wrong, then the bearing manufacturers incorrectly designate fatigue load limits which is doubtful. You guys probably know this but i'm not talking about the basic load ratings; i'm talking about the fatigue load limit.

Thanks pepo. I'll look more into headstock boring.

[NC Cams]

In order to calculate the life of a bearing, you have to use a load which, if applied 100% of the time and does NOT fluctuate, ever, will cause the part to fatigue. It is important to realize that this is for the CALCULATED fatigue life.

Can, will, should you load a bearing at/to its fatigue load in a one shot basis and could/should/will it live? NO to all the above.

I've seen cases were clients have loaded bearings to high percentages of the fatiugue load and saw the bearings fail almost immediately. Yet, I've also seen where a client has applied a judicious and disciplined break in cycle, a slowly ramped up load and speed application rate and they ran at or near the fatigue limit for longer than the calcs would suggest were ever possible. Why's that?

Because once you do a load life calc, there are then ARBITRARY (and somewhat capricious) rating/derating factors that need to be appled. The take thinks like lube life, contamination, alignment and other variables that can/will/do affect bearing life in to consideration. Sometimes these factors are published but more oftern than not they are in the form of "sage wisdom" that is a tuning tool of the bearing company's application engineering dept.

This is why the bearing suppliers pretty much ALL have the "the customer sould consult our technical dept for specific information and recommendations".

Some of the worst application engineers are those who do their load life calcs via a SWAG of the catalog and mentall apply a simple proportion of the fatigue load to a/the bearing and pronounce it satisfactory. Taking the same ratings and applying the ENTIRE prescribed load/life calc process, I've taken the same figures and same loads and applied the published rating/derating factors and found that the previous "no problem" bearing calc to be quite deficient.

Essentially, the bearings that shouldn't have failed according to the client's calculations lived longer than they should have when calc'd via the factory prescribed method.

IF you have used the ENTIRE load/life calc process that's prescribed by the bearing maker for/of the bearings you choose and feel that you've properly done so, I can't argue with your conclusions. However, if you did a catalog scan and simple compared some arbitrary loads you think you'll be seeing to those published in the catalog, well, all I can say is that my prior experience with this method has not been a glowing history of satisfied, happy self taught application engineers.

Caveat emptor.

[mackeym]

ok, thanks NC Cams for more good info. So basically, you have to go through the entire load-life calc process with all variables considered (which is what i did). My interpretation of the last post is that the bearing generally lives longer than what this "total calc" gives (although there are most likely many exceptions and instances where this isn't the case). If one wants any higher of a confidence level, then you have to talk to the company's technical department.

[NC Cams]

"....If one wants any higher of a confidence level, then you have to talk to the company's technical department..."

In a word, YES, that pretty much sums it up. In reality, the load life involves periods of low load and occasional high load. The calc of a real life load profile tends to be tedious and troublesome. Why? Because there are always assumptions and guesstimations than can affect the load life calc one way or another.

Consultation with a tech dept can ofter uncover a guy who's had experience with an application that can help "reality-ify" the load life calc with an emprically derived modification factor. Or, you can find a slug and not get any real benefit for your call

If you've done a full blown lift calc, use the proper tolerances and fits, you should have as good or perhaps better than the predicted life. If not, then the opposite should also be true.

I wish you well with your project and do hope that you did your calcs well and properly..