[bpratl]

I just installed a ball screw (Thomson .750 x .200) to my G0602 to enhance the ELS conversion and eliminate the troublesome half-nuts misalignment and friction. So far it works great; but I am concerned about the .007" backlash, which to me seems excessive. What should be the maximum backlash one could expect, from a new ball screw and ball nut? Both items were purchased for McMasters-Carr. Bob

[G59]

Is it the ballscrew or the support and preload bearings at the ends that weren't properly adjusted?
As for how much backlash, if it was a new assembly, it should have a certificate that came with the ballscrew that tells you the expected backlash and maximum lead error.

[jalessi]

Bob,

Have you tried using a dial indicator to see if the ballscrew is shifting longitudinally?

I see I am late to the party, G59 is already on the money.

Jeff...

[bpratl]

Is it the ballscrew or the support and preload bearings at the ends that weren't properly adjusted?
As for how much backlash, if it was a new assembly, it should have a certificate that came with the ballscrew that tells you the expected backlash and maximum lead error.

The ball screw is supported by adjustable thrust bearing, at both ends, that I can set for zero backlash. The ball screw/ball nut did not come with any certificates or data sheets so I am not sure what to expect for backlash.

[bpratl]

Bob,
Have you tried using a dial indicator to see if the ballscrew is shifting longitudinally from side to side?
Jeff...

Jeff, the backlash was tested using a dial indicator on the carriage which tracked within .0001" with the readout on the ELS controller when going to +1.0000" and I would read .9934" on the return travel or -1.0000"
I also get .007" reading when I zero out both gauges and move the carriage back and forth. I also had a dial indicator on the lead screw which showed no displacement. Thanks Bob

[G59]

What series of ballscrew did you buy? Rolled or ground and what is the C rating.
Some rolled ballscrews have less than 0.001" backlash. Precision ones usually 0.00025" or less. It depends on the C grade.

[jalessi]

Bob,

Found this on page 44:

Non-preloaded ball nuts come standard with no more than 0.007” backlash

Jeff...

[G59]

Well that explains it.

[bpratl]

Well that explains it.

I guess you are right, so I guess that I three choices, live with the backlash by programing out the error, purchase a pre loaded ball nut or try to minimize the backlash.
Is it worth trying oversize (+.0001") balls or add another piggy back ball nut shimmed for minimum backlash. This is my first experience with ball screws and I that they had less than .001" backlash. Live and learn.
Jeff thanks for the chart. Bob

[gcofieldd]

I would try going up a size in balls. Measure the balls and order the next two larger sizes off of ebay...try this guy:

toolsupply on eBay

[bpratl]

I would try going up a size in balls. Measure the balls and order the next two larger sizes off of ebay...try this guy:
toolsupply on eBay

Thanks, I'll give this a try and see if I can get the backlash down to <.001" as a preloaded ball nut is not in the cards right now. Bob

[arizonavideo]

You can get to near zero backlash with the right balls.

I do have a bunch of different sizes. Perhaps a 2 thousand balls altogether. If the screw is not too large to ship I could pack it for you if you want to pay the shipping and perhaps $10.00 for the balls.

If not with .007" backlash I would go .0015" to start and see what you get.

[bpratl]

You can get to near zero backlash with the right balls.
I do have a bunch of different sizes. Perhaps a 2 thousand balls altogether. If the screw is not too large to ship I could pack it for you if you want to pay the shipping and perhaps $10.00 for the balls. If not with .007" backlash I would go .0015" to start and see what you get.

Thanks for the offer; but I just picked up the next size balls. This morning I measured the balls and they are all .1238" so I picked up .1250" +-.0001" and I will give them a try.
I will post the results when I finish. Thanks again. Bob

[MuchMach]

Glad to see a fairly recent thread on backlash. I am looking for a recommendation on thrust bearings for the X and Y axes of a Bridgeport Series 1 milling machine.

I purchased the milling machine used a couple of years ago. It came with ball nuts/lead screws in the X and Y axis. Both axes had a backlash of approximately 0.010" and I decided to try to minimize it. The lead screws were refurbished and bigger bearing balls were installed. When the refurbished lead screws were put in operation I noticed a backlash of only 0.0025" but a horrible grinding noise mainly in the Y axis; I assumed the problem was with the lead screws and I sent them back to be checked. No problem was found with the lead screws and it was reported that they had a backlash of 0.0001". It was suggested to me to check the thrust bearings. I did not think that they were the problem since I replaced them soon after I bought the machine; I bought the bearings from ICAI Online and I think I paid something like $35 for each one x 4 = $140; recently I found out that ICAI Online also sells similar bearings but labeled "precision" for approximately $200/pair.

As I read threads about this type of issue it became more apparent that the bearing could really be the problem. To make a long story short, among the possible replacement bearing that I have found there is a price range (for the four bearings required) from $32 to $1000. Yes, I already can hear the comment, "You get what you pay for..."

The X-axis lead screw is supported by a pair of thrust bearings on the left side (the pocket for the bearings is 47x32.5mm) and one floating radial bearing on the left. The Y-axis is supported by a similar pair of thrust bearings as the X-axis at the front end and the ball nut at the other. I wonder if someone could make a practical recommendation on which thrust bearing to use since there is such a big price range among them. This is what I have found so far,

Timken or FAG #30204 taper roller bearing, 20x47x15.25 mm - as low as $32 for all four. These bearings are used in the BMW motorcycles; they don't seem to have seals.
NSK #20TAC47B angle contact bearing, ABEC 7, 20x47x15 - this is top notch but is pushing $1000, set 4.
Nachi #20TAB04 angle contact bearing, ABEC 7, 20x47x15 - this is also top notch and I can get them for $550, set 4.
Replacement bearings from Hardinge (p/n 1060203 = RM 204KT4) approximately $200 ("precision" set from ICAI online)

I don't think I need ABEC 7 bearings; I thought ABEC 3 or 5 would be good enough but I was told that these are special order. For me, a total backlash of 0.0005" would be more than adequate. Which thrust bearings will help accomplish this at minimum cost?

I am tempted to try the 30204 bearings but wonder if the lack of seals would be a problem. Thanks.

[bpratl]

My ball screw is supported at both ends by 201 FF (12 x 32 x 10 mm) bearings of which one end has an adjustable bearing plate that takes up all the endplay. This way I can get away with $5.00 eBay sealed bearings and have 0 ball screw end play.

[Eldon_Joh]

Timken or FAG #30204 taper roller bearing, 20x47x15.25 mm - as low as $32 for all four.

i would use them. biggest problem is tapered roller bearings have a bit of stiction due to the axial thrust on the ends of the rollers. however you don't need to preload them very much so this should not be a problem.

[MuchMach]

Eldon_Joh, I can see your point with the rollers; they drag on the end surfaces. Thank you for your comment.

I am a little concerned about not having seals to contain the grease and to keep them clean . What do you think?

[Eldon_Joh]

Well they drag on their end surface until a grease film is built up. I found an instruction manual for a certain vehicle's differential that said torque the preload nut until the torque required to turn the pinion is 25 foot pounds! but once it starts moving only 2 foot pounds is requried to keep turning it. So I suspect with a moderate preload and any standard 18 degree taper, you should see the same ratio.

Anyhow, how hard is it to make your own seals?

You deffinitly don't want grit in the bearings, that will totally ruin the feel.. if you do climb milling by hand grit in the bearings could be a big problem.

I have no comment on Timken vs FAG, but I do know that you can see the difference between a 6$ 6203 bearing and a 1$ 6203 bearing.. the cheaper bearing has 100 times as many slag inclusions in the steel when viewed under a good quality stereo microscope.

[arizonavideo]

The grinding sound is not from cheep bearing but other things.

If the screw is not truly square it will make a bunch of noise. You should be able to turn it by hand. Loosen one mount and I bet it may run smooth.

I have re packed a few Rotons and they are not as good as the China C7 screws. I do have a extra 3' one here ready to go with a re packed nut.

Good end bearings would be any a/c or TRB made in Japan. I would not even go that far, the better China bearings are OK too. I think anything more than $10 per bearing is too much. Going to a high grade is silly for our use.

[MuchMach]

Eldon_Joh, nice to hear from someone that seems to know what he is talking about. I ordered four (4) roller bearings #30204 from McMaster (p/n 6677K54) at around $32 each. These are made by Timken in the USA and are ABEC 1.

I decided to try these bearings because their cost is quite manageable and, most importantly, because there is something about their dimensions that intrigue me. The precision contact bearings I have found have exact metric dimensions; something like 20x47x15 or 20x47x14. But these bearing are 20x47x15.25. When your stack two of these you get a total height of 30.5 mm and the packet of the milling machine is approximately 32.5 mm; there is a lip 2.5mm high in the aluminum retention ring of the thrust bearings which leaves 0.5mm of interference. I don't think this is a coincidence! I have read of peole using 14mm bearings and they have to add a spacer (e.g. Smalley washer). It seems to me that the Bridgeport engineers designed the milling machine around this 15.25 dimension.

I have some very nice felt I used for my surface grinder; I guess I could use to improvise seals for the milling machine. I also have several types of rubber to make seals. Wish me luck...hahahahaha. Thank you for your time.