12mm rods and bearings. first machine question

[srmaietta]

Hello all, I have a question regarding these bearings:
http://www.vxb.com/page/bearings/PROD/Kit7442

A bit of background first.. Im building my machine with 12mm rods and bearings. I cleaned the grease off the rods for installation, and I think foolishly tried to install the gantry onto the Y rods as one piece (big, heavy, akward, hard to line up) and with no lube. Eventually I got it on, but the movement was very sticky/choppy. I added some light oil to the rods, and the movement freed up substantially, to the point where I could push it and it would glide all the way to the end of the rails (24" X and Y travel)
BUT there was a grainy or rumbling feel.
Eventually I took it off to tighten some things and tried reinstalling it, but got it racked and one of the bearings spit out a few of its balls. So I need a new bearing.

My questions:
Should the motion be completely smooth with no feeling of knocking or grind or steppiness?
Is there a way to check these bearings for damage?
What type of lubrication is recommended for a setup like this?

I am going to get a new bearing, but if I messed up a couple I'd like to know and order them all at once.

thanks for any tips or guidance!

~Steve

[mar999]

more info needed on machine very gray.(how big)(rail length) etc

[ger21]

Are you using precision ground hardened shafts? Are they unsupported, or fully supported?

[srmaietta]

The rods are precision ground and supported every 6" or so. My x and y are approx 22" and I'm using 3/8 acme 2 start. 200 oz/in steppers.

So are bearings like this supposed to be silky smooth? I imagine they are supposed to be super smooth, especially when trying to move them in such tiny stepped increments.. Do I need to get in there and align everything to the thousandth for smooth performance?

any info on this type of bearing, experiences, stories, etc would be greatly appreciated!

thanks
~Steve

[Oldmanandhistoy]

Originally Posted by srmaietta The rods are precision ground and supported every 6" or so. My x and y are approx 22" and I'm using 3/8 acme 2 start. 200 oz/in steppers. So are bearings like this supposed to be silky smooth? I imagine they are supposed to be super smooth, especially when trying to move them in such tiny stepped increments.. Do I need to get in there and align everything to the thousandth for smooth performance? any info on this type of bearing, experiences, stories, etc would be greatly appreciated! thanks ~Steve

Hi,

These types of bearings should run very smoothly. Run a bearing along the rod in your hand if it is not very smooth then you have a faulty bearing.

If you still have the bearings that popped out you can pop them back in again. I have done this with out problems using long nose pliers.

Alignment of the rails is very important and even more so with supported rails; so I think there will be your problem.

John

[ger21]

Yes, alignment is very critical. Mine move effortlessly, and actually have some play in them, but actually bind very slightly at the far end of the travel on my Z axis.

[Oldmanandhistoy]

Try slackening off one rail on your X axis and run the gantry down the length of its movement. If it runs smoothly move the gantry to line up with each fixing point of the slackened off rail and retighten. If you still have problems then it would be down to the rails not being level so you will need to shim (or other method) under a rail to set them level.

John

[srmaietta]

thanks for the tips guys, Im buying the replacement bearing soon (lost the balls when they flew) and Im going to drill out some of these holes a few thousandths oversize so I have some play to get things lining up perfectly.

any tips on how linear bearings are to get and stay lubricated?
related note.. howabout spray graphite on acme leadscrews?

thanks again!
~Steve