[klick0]

Ok, i'm trying to determine how to eliminate backlash from the standpoint of the servo or stepper motor. After reading the ballscrew basics a few times, and finding the section on thrust loads, it mentions angular contact bearings as a way to eliminate this thrust backlash that could occur in your driving motor. Which I fully understand and want to eliminate, but after searching for suppliers of angular contact bearings, i'm not finding much that makes sense to me.

mcmaster has angular contact bearings with just look like normal old bearings to me, I understand the whole offset part that if you get 2 then make your own block, but i'd rather buy an assembly already pre-made. So if going through mcmaster, i'm assuming I would buy 2 of these per ball screw and then have to make custom bearing assemblies to hold these properly.

I understand the need to eliminate the thrust load being distributed to my servo motor, and first off, is my comprehension that angular contact bearings are the key? If they are, where is a good supplier of pre-made blocks that would support this. I'm assuming the block would need to attach itself to the ballscrew through some kind of set screw (a series of screws i would assume), but have seen no such setup.

The ballscrew basics post shows one supplier I believe but everything is in millimeters, and www.roton.com (where I was planning on getting ballscrews from) is all in inches of course. Now I know I could roughly translate these, but I attempted that and found nothing that lined up properly. I'm planning on using 3/4" ballscrews for all axises, and need thrust/angular contact bearings inbetween the ballscrew and my servo motors, well, inline with them, as to eliminate any backlash my servo's may cause thrust wise.

Also, on a side note, i've read many good posts about roton's balllscrews, but do people think they are good screws? I'm going with HiWin 15MM linear slides (heavy duty line) for all axis's. I'm planning using roton's ballscrews, but they don't post much for the accuracies of their screws and ballnuts. Their prices please me, but if the quality isn't there i'll have to look elsewhere.

Thanks,
Ross

[HuFlungDung]

I snooped around the Roton site, and see a conspicuous lack of information about the accuracies of their ballscrews, but the price tells me that they are rolled thread, and probably come "backlash free", which means there is lots of backlash which they do not charge extra for

I suspect you'll need to have mounts for dual angular contact bearings custom made. The shaft mount requires more than setscrews, rather, it will require a turned shoulder on the screw and a threaded nut to capture the pair of bearings on the screw. The housing needs to be of such construction that the bearings are also firmly captured, ie, no press fits or snap retaining rings because these are unreliable end location methods.

[NC Cams]

WE chased the backlash deal all over hell and back (look for the Extrak thread that 'explains it) and it can be done.

A/C's take radial and axial thrus and vector sum it in two directions. Axis and radial thrust. The contact angle you choose depends on what you want/need in the way of axial versus radial thrust capability.

15 deg bearings are used more for radial loading as the majority of the thrust vecotr is assubmed radially. As your contatct angle gets higher, your axial thrust AND axial stiffness capability get HIGHER as well. This is why ball screw supports have high contact angles of almost 60 deg - high azial load absorbtion and high axial stiffness.

Then you get into the issue of preload. THe more preload you have, the STIFFEr the bearing. WE chased preload versus contact angle all over hell as well. If you want zero compliance, buy true BALL SCREW SUPPORT bearings and prelod the hell ouf them. These are special made, high contact angle and HEAVILY preloaded. Ours spin like glass smooth, They have drag but not impossible to turn and the accuracy is DEAD ON with nearly 500lbs of preload.

The Extrak thread has the bearing P/N's for the "good" part numbers. You can come up with lower cost alteranteives using 40 deg contact angle A/C's and heavy position preload. True TAC style ball screw bearings are 60 deg and PRICEY but deadly stiff and accruate. Loose/sloppy supports are a waste of time so spend some money and mount the stuff well in solid billet.

Custom bearing preloading is not easy to do DIY. We tried and it is a PITA. Get help from a pro. KAF MFg in Stamford CT does it. I cunno about cost amymore but it is worth the cost. He did mine and I paid like anybody else.

[Oldmanandhistoy]

Hi,
While we have NC Cams attention; could I jump in with a quick question?

Is there anything wrong with using a couple of thrust bearings and one or two radial bearings in place of A/C bearings?

Thanks,
John

[NC Cams]

It is usually esaier to package an integrated A/C than to stack up both a radial and an axial thrust combo.

I"ve also found that a pure thrust bearing (as in a radial needle) is a PITA to prelload. I never learned why but suffice it to say that rolling ball A/C's have lower rotating friction than needle thrusts. I think it has to do with the friction factor that a few large balls with point contatct have as compared to a lot of small rollers have with line contact.

In a word, it is mostly packaging - simply pakaging.

[Oldmanandhistoy]

It is usually esaier to package an integrated A/C than to stack up both a radial and an axial thrust combo.

I"ve also found that a pure thrust bearing (as in a radial needle) is a PITA to prelload. I never learned why but suffice it to say that rolling ball A/C's have lower rotating friction than needle thrusts. I think it has to do with the friction factor that a few large balls with point contatct have as compared to a lot of small rollers have with line contact.

In a word, it is mostly packaging - simply pakaging.

I was thinking using ball thrust bearing.

The reason I ask is that finding 6mm bore A/C’s is not easy if not impossible but 6mm id thrust and radial bearings is not a problem.

Thank you,
John

[lgalla]

The McMaster ballscrews are C7 or .004/ft
Roton's are Tor C10 .009/ft.Apparently you can request .003.
Check http://www.misumiusa.com/ They will machine the ends to your specs.
Larry

[Oldmanandhistoy]

I
The reason I ask is that finding 6mm bore A/C’s is not easy if not impossible but 6mm id thrust and radial bearings is not a problem.

I take it back; I have just found some 4mm,5mm and 6mm bore

[ger21]

Do you have a link for those?

[Oldmanandhistoy]

Do you have a link for those?

http://www.arceurotrade.co.uk/en-gb/dept_21.html

[HuFlungDung]

A flat needle roller thrust bearing is destined for failure under constant load situations. The angular velocity of the races differs across the width of the race, but the needle rollers have a constant circumferential velocity across their length. Something is slipping somewhere and its got to cause friction if the length of the contact area is significantly different (like on small diameter bearings using long rollers).

I've never seen a thrust bearing with tapered rollers, but theoretically, there is likely a set of parameters that would allow the tapered roller to 'naturally roll' around the circle with minimal slippage. But then the damn things would need to be contained, so you'd have end thrust dragging you down

Ball bearings also have the same problem with slippage of the balls in a full contact raceway, and this is why they have particular contact angles designed in. Either define the contact points in the design, or the bearing wears its own clearance. This is why stuffing oversize balls into a worn ballscrew does not produce optimum preload and/or running life.

[NC Cams]

For small diameters, you can often times and literally axially load deep groove ball bearngs and let/shove the balls up against the sides of the raceway - in effect creating an A/C - You won''t necessarily be in a honed portion of the raceway nor have the smoothest operation but it will be a small diameter A/C bearing in effect.

WE did this triick on some low cost overhead fans and the client was satisfied even th0ogh the net result was clearly and effectively a kluged up bearing - it worked and it lived. game over.

Can you axially load ball bearings? yes. do you want to? NO because you can't accurately predict the life due to the level of unknowns (IE lack of full raceway support, lack of full ball contct, etc). Even so, kluged up stuff can and does work, sometimes with astounding results.

[klick0]

Ok, i'm not sure what some of these posts are even talking about eheh. I've determined that I would probably have to make my own angular contact bearings block, some of the links above did goto places that show these in more detail and show basically what I thought would have to be done to build one.

mcmaster, bearingsdirect and a few other places all have angular contact bearings in mm, none in inches, and yet mcmaster and roton have all their ballscrews in inches.. So nothing is matching up. misumiusa.com does have ballscrews in metric, but no prices listed on the site. I'd prefer to order through mcmaster or roton anyway.

The reason I want angular contact bearings setup is to eliminate any thrust movement in the servo motor. This made me think, are there servo motors with built in angular contact bearings that are good? Basically any DC motor (that a encoder can be added to) with little or not thrust deflection would be great. I know motor's already have bearings, and obviously handle some of this, but the one I have I dont' think is 0.003" or anything, probably more like 0.015", which is kinda high, i wanna get that lower.

Thanks for everyones reply so far,
Ross

[klick0]

Ok, I just realized mcmaster has these prebuilt, and for sale on their ballscrews page. I don't know how to get a link to mcmaster, but if you search for ballscrews, or just goto catalog page 1055 you'll see it.

Here is what it says about the End mounting blocks:
"End mounting blocks support a ball screw with angular bearings that eliminate wobble. Made of steel. Each includes bearings, replaceable seals, a locknut, and a washer. Note: Ball screw ends need to bemachined before installing in an end mounting block."

That's preciously what i'm looking for, right, that's a pre-made angular contact bearing assembly. Even has the locknuts and everything. Now granted they are $300 each for the one I want, heck the cheapest is $228, so that about doubles my ballscrews cost, adding $900 (3 axis) to the cost of the ballscrews there.

Those are good ball screws for cnc right, they say 0.004" accuracy, which is more then adequate for my needs. I wonder if the the ballnuts come on the ballscrews already, or if I need to preload them. I know there are articles about that, i'll have to look those up as I have no idea what that entails.

Ross

[lgalla]

Ross,
MISUMI does have online pricing.You must register first.Be prepared to see your angular contact bearings from McMaster being 100bucks and the simple support end 20 bucks.
Their pricing on screws includes the ballnut.
If buying from Roton or McMaster you must have the ends machined.Misumi offers machining to you specs for peanuts.They can offer a complete assembly ready to" roll"right out of the box.Finding a machine shop to machine yer Rotons or McMasters is gunna be big$$$.
Larry

[klick0]

Larry,

Thanks for your reply.

Yes, I did realize a few things after my previous post. I understand now that the end of the ballscrew needs to be machined, and that's the part that would go into the end block assembly. Roton and Mcmaster really don't seem to offer that machining, mcmaster even states that it would need to be machined.

I'm gonna try misumiusa.com, I have to register which isn't a big deal but it looks like there is some activeX component that is windows only, and I run linux, that's why I was hesitant about using their site. Hopefully that's not the case, i'll register and find out soon enough.

Thanks for the info,
Ross

[lgalla]

Ross,
Here is costing I had done previously for grade C7,.004 rolled,72" or 2000mm long 25mm/05.Prices include nut flange and support units.
McMaster,used 1"/.25.Their metric screws are double the price???
$561.00
Misumi
$671.00
MSC Direct,Thompson
$1,112
Roton
$319 no supports in this price.
Misumi is the best for me as the price includes machining.The others cannot offer the service.
BTW what size ballscrews are you after?5/8" are really cheap!
Larry

[ozzie34231]

My question is a bit cross post; bearings and new kind of screw.
Since it's been suggested here that some preload can be placed on deep groove bearings, I'm wondering if skate bearings will work for the rollers in an acme thread roller nut?
I'm thinking 3 rollers, a bearing at each end. The thrust would be distributed among 3 bearings at a time. The application is a CNC conversion of a 14" lathe, z axis.
Thanks,
Ozzie

[Sparky_NY]

My question is a bit cross post; bearings and new kind of screw.
Since it's been suggested here that some preload can be placed on deep groove bearings, I'm wondering if skate bearings will work for the rollers in an acme thread roller nut?
I'm thinking 3 rollers, a bearing at each end. The thrust would be distributed among 3 bearings at a time. The application is a CNC conversion of a 14" lathe, z axis.
Thanks,
Ozzie

I am wondering where you came up with 3/4 inch size ballscrews? It is somewhat normal that the diameter of the ballscrew is smaller than the acme screw it is replacing. IF, again IF.... 5/8 ballscrew would suffice then the hot ticket is homeshopcnc. They have nook XPR ballscrew which is .001 per foot, the cost is about identical to rotons rolled screws. Homeshop will load the nut with oversize balls for just a little extra money which gives about .0005 backlash. That performance cannot be matched for the money. (I tried real hard!)

I got rotron 5/8 screws stock and paid extra to have them preload the nut with oversize balls. I was very unhappy with the results. Out of 3 screws, backlash is .007, .005 and .002. The pitch error is bad also. I really wish now I had bought the homeshopcnc nook screws, they get excellent reports.

[Sparky_NY]

I have read a lot lately about ballscrew bearings. My app is a bridgeport manual retrofit for home/hobby use. I am going to order the rockford ballscrew kit (about $700) which comes with double nuts and adjustable preload, a new yoke and is pretty much a bolt in.

In my reading, and on Rockfords website, 7204b angular bearings are often mentioned as the least expensive choice for a BP. I have read where NC has recommended sending the bearings off to have them ground for preload but I really would like to avoid that if possible. What could be expected with a pair of off-the-shelf 7204b bearings? What about a DIY preload? Its a home/hobby machine, cost is a major factor.

For others.... the 7204b is the size for a bridgeport but the same angular contact bearings are available in many other sizes, 7200b, 7202b etc. So, they maybe a good canditate for anyone wanting dirt cheap angular contact bearings.