Bridgeport Ballscrew bearings, lock nut and end block questions

[futura]

Hello,

I have a couple of Bridgeport design ballscrews that need end bearings, lock nuts, washers and end blocks and I have a few questions about fitting them with the proper components to meet the specifications of the original design.

I'm using them for a custom application and so far I've been unsuccessfull finding this information on the web.

The shaft's dimensions/description:

Inward from End #1:

- 20mm M25 X 1.5 thread
- 73mm of 25mm bare shaft
- 30mm M30 X 1.5 thread

Inward from End #2:

- 40mm of 16mm bare shaft (two flats on end, handwheel adaptation?)
- 18mm of M25 X 1.5 thread
- 26mm of 25mm of bare shaft

Overall length = 93cm

Questions:

1. Does anyone know of a document, have a link for information, or knowledge on this type of Brigdeport ballscrew installation?

2. Does anyone have a similar Bridgeport ballscrew setup? If yes, please share the bearing lock nut, washer specifications and end block information/designs?

In lieu of a better option I am considering building custom blocks using angular contact bearings:

End #1: two - 7305 single row angular contact bearings back to back each pressed into a custom bearing end block with a cut shoulder to add 40mm of space between bearings on the bare shaft. Load the bearings against the block using two ring type lock nuts (25mm and 30mm) not sure if I require washers between the nuts and bearings?

End#2: one - 3305 or 5305 double row angular contact bearing. Load the bearing inside block against the existing shaft shoulder using one 25mm ring type lock nut.

I apologize for the lack of brevity, I know a diagram might be required to explain things more clearly.

Thanks for taking the time to digest this information, help/suggestions of anykind are greatly appreciated.

Larry

[NC Cams]

Look into buying true ball screw support bearings.

SKF, NSK and Fafnir sell them. They come preloaded already and "bolt in" off the shelf assuming you find a size that applies to your application.

You will NOT be disappointed by doing it this way..... Been there, done that, wasn't disappointed.....

Check out HIWIN as they have drawings for some Bridgeport ball screws on their website....

[futura]

Thanks for the reply.

I've looked at some premade blocks and didn't find any sized to fit the 73mm of 25mm bare shaft between the threads on End #1. The End #2 block isn't hard to find.

I've contacted Hardinge, the company that acquired Bridgeport, it doesn't have this information available. As an alternative I was hoping someone might recognize the Ballscrew and have knowledge of the Bridgeport end block fixture designs to share (types of bearings used and how they are configured). I would make them close to the original specifications.

Thanks for the lead, I'll checkout HIWIN to see what they have.

Regards,

Larry

[futura]

NC Cams - I looked over HIWIN and found some good information. Both ends are fixed.

They recommend a 7602025TVP bearing which is 25mm i.d., 52mm o.d., 15mm height 60 degree contact angle, single direction (do you know what single direction means?). I suppose these bearings can be installed four up to fill 60mm of the 73mm bare shaft between the threads on End#1. That configuration would create quite a high load rating though and maybe unnecessary?

Any additional insight from anyone is greatly appreciated!

[NC Cams]

A review of a A/C bearing cross section clearly shows that the bearing can ONLY absorb thrust in 1 direction - ergo single direction. For bidirectionaly thrust, you have to mount them so that the thrust angles complement each other - ergo in DB or DF configuration.

If you fix both ends of a screw, the trick is to mount the thrust faces of the bearings so they face each other and put the screw into tension in the process. However, you just don't reef down on it. You have to put a calculated load onto the screw so as to maintain lead accuracy...

Thus, if you mount one A/C bearing on one end with the thrust side facing the ball screw threads, the one on the other side has to also face the screw threads. THus the one absorbs thrust in one direction and the other absorbs thrust in the other as they are effectively pulling against each other.

If you mount them in DB or DF, configuration, then all the thrust can be absorbed on one end of the screw. You need to let the other end of the screw float if you do this sort of thing.

You can mount bearings in duplex, triplex and quadraplex to fill up the space and gain huge amounts of axial thrust capacity. You do have have the same number of bearings with the thrust side mounted either back to back or face to face.

[futura]

Great explanation!

I suppose I can alsco choose to DB End #1 and use a radial bearing on End#2 in a "supported" configuration (they show bearings used in a fixed, supported and free configurations). The ballscrew however would not be in tension, any disadvantages to supported condiguration?

[NC Cams]

Life, as is the mounting config of ball screws, is frought with compromise.

If you put the screw in tension and accomodate the lead error that stretching it below the yield point of the material will cause, you will have to create more load via the tool than you already have in the screw via the prestretching. However, you have to know what you're doing with prestretching screw or else you can have junk. you also have to be VERY accurate with fits and tolerances to hold stretch relatively constant - you ain't gonna slap a system like this together.

If you mount one end fixed and let the other float, the farther you get from the fixed end, the springier the screw will get (longer things of same cross section and material deflect more than shorter ones all else the same). Thus, you'll tend to have non linear lead error under fluctuating loads. These are easy to assemble as the fixed end has a preset preload by the bearing supplier and the other end just floats. Sort of a "ball screw for dummies" scenario 8-))

Do the math on thermal growth and you'll see a similiar situation. if the screw and table expand at same rate, the preload of the fixed ends should stay the same and it shouldn't stretch any more or less. Tie one end down and let the rest float, your stacks under temp extremes can become an issue at some point.

Let's be realistic. There is theory and there is practice. MOST single end fixed screws work just fine. I'd suspect that Bridgeports with single fixed end mills had something to do with putting men into space and on the moon. The machine didn't have any brains only those who did the math to make the dwgs and the great guys who made the chips made the parts accurate enough to get the job done.

Ultimately, the person who is designs the system, did the math for the fit and preload calcs and ultimately assembles and uses it has the best knowledge of what the compromises were/are in the machine. He/she can make the machine talk or crab and whine.

Great parts misapplied and hacked up will never perform as well as average parts that were selectively assembled, fit and tuned with care and reason. Hope that answers your question....

[futura]

NC Cams,

Thanks for taking the time to elaborate in more detail.

The process of invention is a bi-directional, dynamic and most importantly unique to circumstances. Perfection is an idealistic and unobtainable goal, used in this circumstance to help maximize precision within the scope of a realistic goal. Solutions are built upon the level of understanding of the intricacies of the problem. Procrastination often results when one knows what one does not know, progress results once such barriers are removed.

You have been very helpfull in confirming, with great explanations, much of what I had only previously assumed. I certainly do appreciate the high-quality of these screws and respect the level of precision and quality of the design required for proper use, however design without experimentation is rarely innovation.

Thanks again for sharing experience and insight which makes this site such a valuable resource. I hope others similarly appreciate your willingness to share knowledge.