Hex Shafts and Bearings

[Tom Line]

We're potentially going to use hex drive shafts this year to get away from keys (that always seem to get lost at the worst times...).

I know other folks have used them. Can you run the hex shaft directly in the ball bearings without fear of point-loading the inner bearing race?

[sdcantrell56]

While it might be possible to do that for a while it is not a proper practice. Most teams turn down the ends of the hex shaft on a lathe so that it is round. There is also one other method which involves broaching bushings to the size of your hex and then putting that on the shaft to go in the bearing

[IKE]

I have always seen them turned down to round which is pretty easy with a 3 jaw chuck on a lathe, and some patients. If you are turning steel, it may be better to use HSS rather than carbide, because carbide will sometimes chip.

That being said, the point loading probably isn't a big worry to the bearing, but soft hex stock may yield at those points and thus be sloppy similar to a shaft that is several thousandths too small.

If you are switching over to hex, you will also want to have a broach and press to make custom interfaces.

If you only have a 4 jaw chuck for your lathe, some options might be:
Buy a 3 jaw (this could be cost prohibitive depending on lathe.

Make a round insert and then broach a hex. Cross drill and add a set screw.

Use square stock instead of hex. (you will also need a square broach). This does not seem to get used in FIRST as much so expect replacement parts to be custom.

[Jeff Pahl]

Originally Posted by IKE

If you only have a 4 jaw chuck for your lathe, some options might be:
Buy a 3 jaw (this could be cost prohibitive depending on lathe.

Or if your lathe will take 5C collets, get a hex collet the right size.

[Stephen Kowski]

http://www.chiefdelphi.com/forums/sh...aft+ bushings

check out page 2 of this thread paul and aj have some suggestions on hex shafts, especially post #26 from aj if you are worried about the point loading

[bmarick]

hey im wondering what are the advantages to this switch? i'm confused on why you would change other than the the key getting lost at the worst time. which is why i always make sure we get extras .

[Cory]

Originally Posted by bmarick

hey im wondering what are the advantages to this switch? i'm confused on why you would change other than the the key getting lost at the worst time. which is why i always make sure we get extras .

A hex is superior to a key in pretty much every respect. You nailed one reason. No keys to lose. Another reason is that it's a pain is keys often don't fit into keyways easily, or over time the key can seize the wheel to the shaft (or sprocket or whatever you've keyed to a shaft). Keying a sprocket also introduces a major stress riser on the hub. Every sprocket failure we have ever had (and I believe the same is true for 968) occurred on the keyway of the sprocket.

Hexes mean assembly is way faster. They just slide on and off way more easily than any keyed interface I have ever used. It also is better at transmitting torque, as you have a vastly larger surface area to deliver the same amount of torque-this eliminates the stress riser associated with keyways.

If you have the ability to use hex shafts and hex broached gears/sprockets/wheels, it's an absolute no brainer, in my opinion.

[Andy Baker]

Originally Posted by Cory

A hex is superior to a key in pretty much every respect. You nailed one reason. No keys to lose. Another reason is that it's a pain is keys often don't fit into keyways easily, or over time the key can seize the wheel to the shaft (or sprocket or whatever you've keyed to a shaft). Keying a sprocket also introduces a major stress riser on the hub. Every sprocket failure we have ever had (and I believe the same is true for 968) occurred on the keyway of the sprocket.

Hexes mean assembly is way faster. They just slide on and off way more easily than any keyed interface I have ever used. It also is better at transmitting torque, as you have a vastly larger surface area to deliver the same amount of torque-this eliminates the stress riser associated with keyways.

If you have the ability to use hex shafts and hex broached gears/sprockets/wheels, it's an absolute no brainer, in my opinion.

I totally agree with Cory. One additional advantage is that the hex automatically centers the thing (gear, hub, sprocket) it is driving when torque is applied.

Andy B.

[IndySam]

Originally Posted by Andy Baker

I totally agree with Cory. One additional advantage is that the hex automatically centers the thing (gear, hub, sprocket) it is driving when torque is applied.

Andy B.

Wouldn't it be cool if some small Indiana company would offer some bearing adapters and bearings for our applications?

[Dick Linn]

Take a piece of appropriately sized aluminum tubing or bushing. Taper the end of a scrap of hex stock slightly, lube it and pound it through the aluminum tube, thus forming a hex. Spin it and use a file to turn down the outside until it is round and fits the bearing of your choice. Of course, that means using a bearing of sufficiently large size to leave enough material at the points of your homemade custom-forged hex adapter so it stays together. You could probably get it done with a wood lathe or drill press.

[Cory]

I think everyone is making this more difficult than it needs to be. While I understand these solutions are probably geared towards teams who do not have a lathe, the best and easiest way is to use a lathe and turn down the ends.

I would guess that 90%+ of FRC teams could find a machine shop willing to do so in less than an hour of calling up shops. Just for reference, when we do this it takes no more than maybe 5-8 minutes to face a shaft to length, and turn both ends down to fit into a bearing. So you're talking about maybe an hour tops to do all of the shafts in your drivetrain. I really don't think you'd have a difficult time finding a shop to do an hour of work for you (especially since it's all manual and involves virtually zero setup time).

[IndySam]

Originally Posted by Cory

I thin everyone is making this more difficult than it needs to be. While I understand these solutions are probably geared towards teams who do not have a lathe, the best and easiest way is to use a lathe and turn down the ends.

I would guess that 90%+ of FRC teams could find a machine shop willing to do so in less than an hour of calling up shops. Just for reference, when we do this it takes no more than maybe 5-8 minutes to face a shaft to length, and turn both ends down to fit into a bearing. So you're talking about maybe an hour tops to do all of the shafts in your drivetrain. I really don't think you'd have a difficult time finding a shop to do an hour of work for you (especially since it's all manual and involves virtually zero setup time).

Unless you want the bearings in the middle of the shaft.

[ajlapp]

method which involves broaching bushings to the size of your hex and then putting that on the shaft to go in the bearing

The method mentioned above is the cheapest and fastest way to pass a hex through a round bearing. Oversize your bearing id to accept a bronze bushing with suitable side wall. Then broach the bushing to use as a sleeve for carrying the hex.

You can also find something like this. Though I do not know of a source that stocks these.

Some notes about hex shaft from a former "hex only advocate." In applications where backlash and precision are a concern, say in an arm joint assembly, hex shafts can be less then desired. A broached hole is quite accurate, but hex stock has uncontrolled dimensions and may provide a loose fit.

Milling your own hex onto your shaft will allow you to control the fit precisely, but this requires more machining capability and diminishes the main 'ease of use' advantages.

For drivetrains, where lash is not a huge deal, hex is the way to go......

Also, precision ground keyed stock is readily available and keyway broaches are cheaper than hex broaches.

Good luck.

[bmarick]

some of that shot over my head, but that makes sense

[Dick Linn]

Hey, someone want to contact these folks and find out price/availability/sizes of their hex shaft adapters? Might be a cheap solution.

http://www.bnb-industries.com/Roller...%20Bearing.htm

See middle of page.

Stock Drive Products has round sleeves with a hex hole:

www.sdp-si.com/D790/PDF/D790C04023.pdf