Lead Screw Bearings

[Novah]

On my machine (JET 20" swing mill drill) I have tapered roller bearings on one end of each original 19mm acme lead screws. The bearings are #30203 with dimensions of 40mm outside, 17mm inside and 13.25mm wide. Are tapered roller bearings what other brand machines have or are do they originally come with ball bearings with the Acme lead screws?

The CNC conversions I am seeing on this site use angular contact ball bearings. I have a few questions:

1. Is there a reason to change to angular contact bearing when I go to ball screws?

2. Can't you zero the backlash with the tapered roller bearings? I can't see a need for endplay in the cold state as there shouldn't be a lot of heat build up due to the low speeds it will turn.

3. When I machine the .631 ball screws, the ends seem to want to get small in diameter. My motors will have 640 oz-in and a .500" shaft. I will be going direct drive. How small are the shafts you guys use on your conversions?

Thanks for your help - Dave

[D.D.Machine]

its best with ball screws to fix them to one end and let the other end float in a single bearing,
with 5/8 ball screws I have machined them down to 12mm, 10mm and even 3/8" thrue the years and have never twisted a end off yet.

if you want simple and easy just use doubler anguler bearing mounted in your motor mounts in a 12mm size and keep your shaft as big as you can then bore the motor couplings to fit them.

[NC Cams]

Reply as follows:

1. Tapered rollers have a huge radial capacity along with a reasonable axial thrust capacity - some from the angled rollers and some from the sides of the rollers rubbing up against the flange on the raceway. This is much more that regular ball bearings but more or less depending on the size and capacity of a comparably sized angular contact ball .

Tapers are decidedly cheaper than angular conact balls which is probably why they were used in your application. The tapers would NOT have the lower level of friction that one would expect and get from ball bearings, especially in a preloaded state.

2. Tapers work best with 0.000 to 0.005 axial clearance, the closer to 0.000 the better. However, you CAN run them with a very slight preload BUT the problem is determining what is SLIGHT. Go too far and POOF, they brinnell and fail rather quickly.

IF you slowly adjust them to the point of 0 clearance while constantly rotating the shaft, you can do it. The old trick of "torquing the taper and then backing it off to set it" is a dangerous old wive's tale. You really need to measure bilateral axial clearance while turning the shaft - tighten until you see a TOTAL, BIDIRECTIONAL AXIAL clearance of 0.000" to 0.005", the less the better.

Friction will be higher with the taper than with a preloaded ball even when properly adjusted. REASON: you're literally wedging the rollers between the shaft if you "preload them". Thus, one can actually shove the sides of the rollers up against the wall/shoulder of the raceway. Result: sliding/rubbing instead of rollling friction.

On our BPT table ball screws, the OEM bearings were preloaded 6204's with a theoretical preload of 125 lb. We replace them with 60 deg contact angle true ball screw bearings with 480lb preload. The 480 lb bearings actually turn easier than the OEM's with over 3x the preload.

Re: bearing mounting- putting the duplexed bearings at one end and floating the other end is the easy and efficient way to mount ball screws. However, in doing this, position repeatability as the screw temp rises will get worse the farther you go from the constrained end.

Putting a restraint at either end will give a theoretically better position accuracy as you're containing growth BETWEEN the thrust bearings and the end can't move. However, as the screw expands, the position preload will increase thus the bearing axial loads can increase and something has to give sooner or later.

If you don't have a high temp rise, it really doesn't matter that much. DDMachine's comments about shaft sizing are hard to disagree with.

Doesn't matter if you have ACME or ball screws - either way, you'r dealing with a screw that has to expand and bearings that have to hold axial position.

The issue of ACME vs Ball screw accuracy and friction have already been discussed in other postings.

[Novah]

Thank you, NC for the very detailed explaination of tapered roller bearings. I thought the conversions I had read about had standard ball bearings. When I took my mill apart and saw the tapered roller bearings, I thought maybe my mill, being 20" swing had different bearings from the 16 3/8 or smaller mills and I could use the exisiting bearings. I do have a single standard ball bearing on the opposite end of the screw.

DD - thank you for sharing your success with screws cut down to .375. I am trying to use the existing bearing holders like ARC did in this forum. I think he went down to 10mm on the bore but I had decided that 12mm would be more in line with what NOOK uses in their premachined .631" ball screws. I expect that my coupling bore will be around .468" and your experience with .375 makes me feel confident I will be okay.

Thanks again, to both of you --- Dave

[phantomcow2]

What i see often is a duplex pair.
Thats two angular contact bearings that are ground to fit eachother and eachother only. I've got some of these things waiting to go into my mill, when done properly, they are great.
THe bearings just sitting on the screw allows you to move them all around, you would guess they are low quality at first with loads of play. Then as soon as you clamp the inner races together and most importantly, clamp the outer races together, you have a super stiff and backlash free setup.
Never seen taper rollers in a leadscrew setup before though, sounds good