[Swede]

Sorry, no I don't have the truth, but I AM still looking. :rainfro: The "Truth" I seek (sounds vaguely Zen like) is this: Are those oh-so-cheap "ABEC-7" skateboard bearings that you can find in nearly any sporting-goods store REALLY ABEC-7? I get this weird vibe that it is all B.S. and is simply technobabble designed to get some teenager to part company with his $$.

A pack of 10 ABEC-7 metric bearings goes for $29 at our local Oshman's sporting goods store. The same ABEC-7 bearing from Boca Bearing online goes for $16 apiece. If truly ABEC-7, they'd be a great source for ways and lightly loaded ballscrews.

I know this is nothing new, and guys have been using skate bearings for years for CNC projects, but does anyone know if they are genuinely ABEC-7? Any way to actually test them? If one has a set of pin guages, and a good mike, one could check them dimensionally, but the dynamic aspects of the radial bearing is tougher to check.

[kong]

Funny you should bring this one up, I am in need of some 8mm id bearings, and they are seemingly difficult to find over here, so I was considering using skate bearings for my leadscrew ends. I will be interested in your findings! The thing is, I take it they won't stand a lot of axial force, so I'll probably use as many bearings as I can squeeze on the end of the screw.

[HuFlungDung]

What is the radial clearance of these skate bearings? Are they really wobble free?

[kong]

A quick google to understand what you are on about Hu, revelaed a supplier of "your average skate bearing". The info on the site claimed a radial clearance of "C5" or 0.0005 - 0.0008".

[Ferenczyg]

Well, the only things I could state are:

1.- all the skating ballbearings I've seen show 608Z or 608ZZ and the ABEC rating on the flange, so the rating is from factory not from packager.
2.- I've never seen ABEC bearings on a sports store over ABEC-5, but this can be a local issue
3.- ABEC-3 are about 1$ the piece, ABEC-5 about 1.5 $ the piece, at least where I've seen
4.- Answering Hu, yes, they are absolutely wobble free, but absolutely only means 'as far as I can measure'

And if I am not in a mistake, If they say are ABEC-X and sell as ABEC-X but they're not ABEC-X, you can sue them..

Fer

[HuFlungDung]

C5 clearance is quite a bit. Most electric motors use a C3 clearance, and they are fairly loose, that is, you can make the inner race wobble quite a bit, yet the bearing is considered perfectly okay. This design is so it can run hot and not self destruct.

So depending on what kind of load you want your skate bearing to carry, it may or may not be what a person would want.

[buscht]

Here's an example.
http://cgi.ebay.com/ws/eBayISAPI.dll...671672169&rd=1

[ESjaavik]

I don't think the biggest problem will be the bearing. My Bulgarian made crapbox of a woodworking machine uses ball bearings on a steel rod for its table movement. They are arranged in a triangle and there are one suchsetup at each end. They have in a short time worn down into the steel rod and rides like on a bumpy road, binding even on the smallest speck of dirt. So at least 2 things are obvious to me:

They need to be totally enclosed, and have felt wipers at both ends.

They need to run against a flat, not a round rod.

The ways they run on need to be case hardened. But my hardening furnace can only take objects up to 50cm, so I can't do itmyself. The rods are supported at 2 places each, so a through hardened rod would not be a good idea. I checked the price of square rod 30x30mm in semi-hard carbon steel too, and it is not cheap! So I may try PTFE pads riding on the current slides. Or ballslides from eBay. I've seen them at prices below the just mentioned rod. Especially when someone offers slide without blocks, they go for almost nothing. Very few take the chance they can get matching blocks for a good price.

Kong: What you need for the ballscrew ends are angle contact bearings, or if you don't run them at high speed use ball thrust bearings. They can't be used at high RPM because the centrifugal forces will make the bearing bind. Bicycles use angle contact bearings at several places, maybe they couldbe a source of cheap bearings?

[John S.]

Don't get confused by the term ABEC
There is the standard ABEC and there is Acme Bearing Exporters, China

[InventIt]

Originally posted by John S.
Don't get confused by the term ABEC
There is the standard ABEC and there is Acme Bearing Exporters, China

That's funny!
I remember a teacher in high school, many years ago, telling us how Japan renamed one of it's citys to "usa" This way they could stamp "Made in usa" all over every peice of junk they exported over here. I don't know honestly if that's true, but I do remember the teacher saying it.

[JFettig]

I skated for a little while, and I beleve these abec ratings are correct, That is how good they are as you get them, you get what you pay for though, you buy cheap abec 7 bearings, they dont last long, I have a friend who has gone through like 20 in about 2 years of skating.

and yes, there are Abec 7, I have heard of abec 11 bearings too.

"Don't get confused by the term ABEC
There is the standard ABEC and there is Acme Bearing Exporters, China" lol thats a great one.

skate bearings in this application are probably very promicing, if you get good ones you wont have any slop in them.

Jon

[Ferenczyg]

Ok, but really we are not underestimating the fact that the abuse we will put on the bearings on a diy cnc are much less than the abuse put on them when skating? So this seems to me at least..

Fer

[buckaroo]

Can someone outline the general parts of a 3-axis CNC mill (both xy table and gantry style) that would warrant using angle contact bearings?

I haven't found a bearing usage designers guide anywhere -- which type of bearing for which type of running forces, if someone has a reference to share, that would be great too.

Marc

[JFettig]

You really dont need angle contact bearings, and I would say, dont spend $500 on them

I dont know how angle contact bearings can be all that much better than regular ones, if your really conserned turn some sleeves for the bearings with your groove.


Jon

[doug6949]

I'm probably going to get some flak for this but here goes. Abec7, and in most cases, angular contact bearings on a router table are much like carbon fiber air scoops on a $1500 car. If you examine the tolerance limits for abec3 and compare them with the limits for rolled screws you will find the abec3 limits are several orders of magnitude below those of the screws. The bearings don't even figure into the problem.

So, what if you are using class 5 screws? Yes, the bearings make a difference. However, if you are using class 5 screws on a machine that is built from aluminum, not thermally stabilized, not scraped in and not designed to minimize the effect of Abbe errors then you are back to the carbon fiber air scoop analogy.

We have had excellent results with generic taper roller bearings preloaded with Belleville washers. Set the preload just slightly above your maximum axis loads. For float/fixed ends just put the tapered pair at one end and a roller bearing at the other. If your end supports and frame are rigid enough you can put one tapered bearing at each end of the screw and leave out the Belleville washers. Preloading a long screw in tension will raise it's critical speed (resonance) and not adversely affect precision.

You can also use simple deep groove ball bearings instead of tapered roller or angular contact bearings. The trick is to derate them according to manufacturer's axial load specs, meaning that you must use bigger bearings. The only real advantage to this method is that you can buy them permanently lubricated and sealed and thus eliminate seals from the housing.

Doug

[trubleshtr]

It truely depends on your specific needs, low speeds heavy radial loads then yes caged bearings, or spherical bearings, or even needle bearings(no axial loads for long periods here or else....). heavy axial loads only? then thrust bearings are a good choice.
High speeds with both axial and radial loads then angular contact are recommended, it's what they are designed for, the steeper the angle between the outer race and bearing the more load it can handle, if you need to support heavy loads or pulsating loads then you can multiply the number of angular bearings used, keeping in mind they need to be orientated correctly to be effective, angular contact bearings placed in the wrong direction of the load will seperate from the inner and outer race under heavy load. If the speeds are not excessive then tapered rollers as Doug mentioned are fine, as they can handle both heavy radial loads as well as moderate axial loads. Again it depends soley on your intended application, consider speed, and direction of the forces involved. As far as Skateboard bearings with a 'C5' rating I would not recommend them for high axial loads, skate boards support the load directly above them (radial) and although you may side load the bearings shortly when sliding your board sideways, this movement is very short lived and if I remember correctly back when I "skated" one had to "unload" their weight from the board to initiate the sideways movement I don't think they would handle the regular and constant abuse from a cnc...

[doug6949]

I concur with your comments. Tapered roller bearings are not suitable for spindle applications due their lower speed restrictions (though they are often used anyway in low end mill/drills and lathes). An oil bath doubles their speed capacity over packed grease. Lead screws generally run well below the limit, even for packed grease.

An interesting application of tapered roller bearings is the lower end of the Mercruiser outdrive. One TRB is used for forward thrust. Reverse thrust and axial loads are taken by two ball bearings. These things run upwards of 3000rpm. They get away with that speed because the housing is water cooled.

Doug

[Hobbiest]

Sounds to me like too much thought is being wasted on this subject. Yes the tolerences are not divine, but read the tolerences on some comercial linear components sometime, grab a tissue, and cry. The high cost is because (IMO) theyare already built for you. I am not saying that is bad! Use whatever you can be happy with on your first machines, and then upgrade. Just my two cents. Sorry if I have offended anybody.

[glenfraser]

I got my 8mm skateboard bearings from a company in norwich.
If u need there website come phone number ill post it here for you.

Glen

[Hobbiest]

By the way...skateboard bearings from a skate shop, and the same from Kmart, are not the same. Even if the ABEC is the same, many times the quality is different. The more expensive bearings that you get from a shop, are more expensive because they are made to handle abuse. I have skateboarded since I was 5 (24 now), and can tell you first hand what a grenaded bearing looks like! As has been stated elsewhere, the difference between the rigors of home CNC, and skateboarding are immense. They aren't eve in the same league...not even the same sport!
Cheers