It seems my linear bearings don't fit...

[mikeschn]

It seems my linear bearings don't fit...

Of course when I measure with calipers I get

bearing id = .495
shaft od = .495

cheap calipers btw...


I haven't taken a hammer to them yet, but trying to press them on by hand, I can't do it.

Maybe my mistake was buying those ceramic coated bearings. I just called and some real linear bearings are on the way...

So what do you guys think about those ceramic coated thingies?

Mike...

P.S. You can see, I am getting the stuff together for my first machine!

[balsaman]

Make sure there is no burrs on the shafting where they were cut. They are designed to be tight. Use a little oil.

Eric

[DJ Morrow]

I change the wheel bearings on my motorcycles and when I do, they are also a very tight fit. I normally need a bearing driver to install them. Put the bearings in the freezer for a few hours. It'll still be tight but oh so much easier to install.

[bcromwell]

Originally posted by DJ Morrow
I change the wheel bearings on my motorcycles and when I do, they are also a very tight fit. I normally need a bearing driver to install them. Put the bearings in the freezer for a few hours. It'll still be tight but oh so much easier to install.

Ummm, Freezing makes things shrink, not expand, he would have to put the rod or rail in the freezer for that to work.
Being that I do a lot of automotive work I commonly use a electric griddle set at 350 degrees, lay the bearing on for about 7-8 min and they will slide right on, just be sure the are all metel bearings, and contain no plastic.

[DJ Morrow]

Yes, I see your point. When I put the wheel bearings in the freezer, they are then inserted inside the hub of the wheel. So, the shrinking affects the outside diameter of the bearing which is where I want it but not where you want to insert a shaft.

[Hobbiest]

Freeze your delrin before tapping for drive nuts, and it'll not only be easier to tap, but it will expand enough upon warming to make travel a bit easier without adding backlash!

[lsfoils]

Um, actually, a common misconception it that the unit shrinks. This is not the case as precived. The outside diameter shrinks but so does the inside diameter. Things should slide on nicely when cold enough. The ratio is very small, however. I have worked with liquid nitrogen to achieve the required fits. @ -390 and below things can seem amazingly sloppy. The diagram kinda shows what acutally happens. The blue represents the original size of any round object. The red isn't necessarily to scale.

_Doug

Uh, 3 sheets to the wind, I'm afraid....

[HuFlungDung]

Its funny, but I apparently had the misconception that he wanted the shaft to slide through the linear bearings. All this shrink fit advice is good for making a permanent fit for a tire on a railroad wheel, though

[Hobbiest]

It is definately a good way to press fit things. In street rods, some brake kits come with aluminum hubs and steel rotors. You put the rotor in the sun, and the hub in the frezer, then they go together easily. This is how the company says to do it too, so I would assume that the inner di. of the rotor expands when heated. It still isn't much with metal, few thou maybe.

[mikeschn]

Hey Guys,

I have a simple little Thomson harden shaft... and an even simpler little ceramic coated bearing. No moving parts at all...

So lets see, I heat the shaft in the oven, cool the bearing in the freezer, lose my case hardness, and wear grooves into the shaft. Nope that's not what I want to do.

Okay, I'll freeze the shaft, heat the bearing in the oven... nah, forget it... I'll just wait for the other bearings to arrive!!!

***UPDATE***

I had all my parts sitting near the window overnight. Here in Michigan it's been quite cold... Do you know that this morning every bearing fit onto every shaft. And they glide and slide wonderfully!!!

Mike...

P.S. So the only question remaining is which bearing is better? The ceramic ones, pictured below, or the linear bearings with the little bearing balls in it?

[lsfoils]

Hi Mike,

Glad to hear it worked out for you! Ah, cool pics and ceramics, too! Those have a certain drool factor in my book.

As far as which is better I'd bet it would depend on which salesman you were talking to. Anyway, I favor the ones you have. I am curious about lubrication. Do you leave these dry? Going to be a lot easier to ensure how clean they stay if thats the case.

Good luck with your first machine!

-Doug

(not even hung over)

[HuFlungDung]

Ah, then it was just a case of the newbie not being able to align the shaft with the hole to start with. When you said it wouldn't go on, I assumed you meant it, and therefore all the shrink fit advice was pointless, because it would have resulted in permanent fit. If a shaft and bushing will slide, its got .0005" clearance or more. It can be tricky to get it lined up though.