Backlash

[Cory]

Originally Posted by vivek16

Where can I get flanged bearings cheap? Is there a place to get them in bulk for a reasonable price? I need 3/8 and 1/2 ID

I would rather not do complicated milling after the initial cutting.

It does seem a lot easier just to do flanged bearings. I guess the low-on-funds-ometer is going off again

thanks, Vivek

EDIT: Also, what kind of reamer? Flute style, length, etc.

I'm not sure where the cheapest place to get bearings online is. We get them from a local place.

You just want a straight flute chucking reamer. HSS is fine. No need for carbide.

[vivek16]

Do I NEED bearings like AM uses or are cheaper ones from the local hardware store fine?

thanks, Vivek

[sdcantrell56]

Honestly the best place I have found to buy bearings is on ebay. The store is VXB wholesale I believe and they have very cheap prices compared to most places.

[vivek16]

I might just stick with the local hardware store's bearings since they are cheaper. With the run time of a FIRST bot I doubt there will be problems. It is not really a high load situation anyways.

Anyhow, I never did get an answer on how much backlash to put in. I was thinking like .01"? The gear teeth are .1" long and the machine we are using to cut the plates will have .003" tolerance at worst.

thanks, Vivek

[sdcantrell56]

It is ok to put in none and then just run them in. At least I'm pretty sure it's ok. It is not the "best" method but it accomplishes the same goal and you don't have to worry about to much backlash and power loss.

[sanddrag]

I would speculate that you want your center to center distance increased by at least the amount of tolerance of your machine to ensure your gears do not bind. If your machine can only hold .003, I'd say either look for a better machine, or increase your center to center distance .005 over nominal. I'd be weary of going with any more than that though.

For the bearing fit in aluminum, you certainly don't want any more than .001, with something in the range of .0005 being optimal. More than probably .0015 and you'll start to excessively load and potentially even damage the bearing. There are formulas to calculate this, but this is a basic rule of thumb based on experience.

Make sure you use precision ground ABEC rated bearings. If you don't, you'll certainly have problems. Unground and/or non-precision bearings are not rated for anywhere near the speeds, loads, or tolerances of precision ones, by a factor of up to several-fold. I too would recommend through holes and flanged bearings. The most common bearings to use in FRC gearboxes are FR8ZZ and FR6ZZ. These can be obtained from a variety of sources, including Small Parts at about $4 each, under part numbers BRF-06 and BRF-08. There are also cheaper sources. Check local suppliers.

[vivek16]

yeah, our machine does atleast .003" (better from what our mentor tells me). We are trying out a waterjet from our sponsors for the first time. They list the tolerance as .005" on their website but my mentor assures me that they can do much better than .003". I made my previous designs for .002" of backlash but it is no big deal to change.

Ok, I will look into flanged bearings. They certainly would make life a lot easier. (Come to think of it, I have half the bearings I need from a toughbox I [slash]demolished[/slash] err... modified )

thanks, Vivek

[IKE]

If you really want to learn what the right demensions are use a machine design book (norton or shigley) or you can buy an old machinist Handbook. Since you are using spur gears any old addition should have the right info.

As far as tolerance goes, almost any mill that has functional lock on it can hold a tolerance tighter than 0.005 learn how to work against slop and practice.

That being said our team made AL side-plates for an AndyMark gearbox. It took two tries. The first shot I merely explained what to do with the students. They made a couple big set-up errors and the plates wouldn't work. I worked closely with them to make the second set and it worked out better.

Rule of thumb on press fits 0.001" per 1" diameter. This has almost always worked for me in the past. Gearboxes are a good test of skill. They hate being too tight and will destroy themselves if too loose.

[vivek16]

I hope to learn more about machining over the summer. Our mechanical team knows how to run the mills but to different extents (and we use a digital readout that holds .0005" of tolerance, although not sure how accurate it is).

I will probably switch to using those flanged bearings.

I am using hex key turned down at the ends for all of my shafts and was wondering how good of an idea it is to turn down hex shaft like that. I would think that all those edges banging against the bit can't be good for it (although if I came in from the side that would be solved. This is what I have done before but the AM hex shafts already had an end turned down onto them.).

thanks,

-Vivek

EDIT: Is an arbor press absolutely necessary for press fit bearings? Could the same effect be achieved without too much trouble with a vice or some blocks of wood and a hammer? I am not against getting one, it seems like they would be quite useful. It is just that we can't really afford luxuries right now .

[sanddrag]

A suitable vice, a couple pairs of hands and eyes, and some aluminum plates can make a suitable press.

Turning hex shaft into round is common and acceptable. I've done it many times without issue. Just feed slowly.

[DonRotolo]

You can make a pocket in your gears on a lathe, and tolerances to .001 or better should be easy with just a little care. .001 interference in aluminum is a pretty tight fit, .0005 for a 1.5" OD bearing would be plenty. You can fiddle this a bit with sandpaper or a file.

For clearance between teeth, you must be sure there is no negative clearance (too close together). The higher the speed, the closer the tolerance, the harder the material, the closer they can be. For moderate speeds (1000 RPM-ish) in aluminum, .0005-.002 is fine, be sure to run the gears in for several hours with light lubrication, in both directions and light load.

When making intermittent cuts (like turning hex to round) don't use carbide bits, regular tool steel will manage that better. Carbide tends to shatter, especially the ones with the little triangular inserts.

An arbor press is convenient, but not required. The reason to avoid using a hammer is the shock loads being transmitted. Better use a vice or some clamps with metal blocks than a hammer, but if unavoidable, go slowly with the hammer.

Cheap bearings are OK for low speeds (600 RPM-ish) and sloppy tolerances. ABEC bearings are great for higher speeds (10,000 RPM) and longevity, as well as tighter tolerances. I buy bearings on eBay all the time, but if you have a bunch of useable bearings, use them.

Old trick if the pockets are accidentally a loose fit: Make a plug for the hole from scrap metal, then drill three (or more) evenly-spaced 1/8" or so holes in the edge of the bearing pocket, with the holes protruding maybe 0.010 into the pocket (and hole plug). Fill the 1/8" holes with metal rod, file them down on the inside a little bit to get your press fit.

Don

[vivek16]

I believe the only bit we have ever used for our lathe is a carbide. There must be some tool steel ones laying around somewhere...

That is a pretty nifty trick about the hole plugging! Never would have thought of that.

Our mill has a digital readout that goes to .0005 (with the last digit being either 5 or 0). Now that is 25 times what I estimated the tolerance to be (I believe the digital readout is a Bausch and laumb). I will have to find out the actual tolerance next time I go to our workshop.

thanks, Vivek

[Andy Brockway]

If you want an alternative to flanged bearings you can make your own support for the bearing out of button head screws. Here is a picture of how I do it. http://www.chiefdelphi.com/media/photos/16362.

[vivek16]

Originally Posted by Andy Brockway

If you want an alternative to flanged bearings you can make your own support for the bearing out of button head screws. Here is a picture of how I do it. http://www.chiefdelphi.com/media/photos/16362.

That sounds like a pretty good idea. Would you recommend threading the aluminum itself or doing a through bolt? It seems like just threading the aluminum should be plenty strong.

thanks, Vivek

[RyanN]

Originally Posted by sdcantrell56

It is ok to put in none and then just run them in. At least I'm pretty sure it's ok. It is not the "best" method but it accomplishes the same goal and you don't have to worry about to much backlash and power loss.

With steel, I would say that would be fine, but with aluminum it would wear out fast. Another idea... what about brass bushings.