Hex shafts?
 

Hello I was recently talking to someone about hex shafts and i was wondering where to get them. I was looking around McMaster and this is what I found. Is this ok to use or are there other hex shafts out there that I should look at that are designed for this.

but a few questions about hex shafts,

Are there any tricks that go into using hex shafts VS key shafts?
Have you had any problems with hex shafts?
What are the benefits of Hex shafts?

Re: Hex shafts?
 

I assume you've already looked at what AndyMark has to offer.
For what specific purpose are you using the hex shafts? If we know more about your problem, we can offer better solutions.

Re: Hex shafts?
 

yes I have I was wanting to make a drive that used hex shafts (a 6/8wd) i was going to use this to drive the main wheel but i was looking to drive the corner wheels as well with hex, plus if i wanted to make an arm drive by hex shafts, or a fly wheel.

Re: Hex shafts?
 

The primary benefit of hex shafting is that there is no key needed. Which means no key to fall out at an inopportune time and no keyway to cut which saves machining time. It is also an alternative if you don't have the equipment to cut a keyway, like a mill. Yes, you could get shafting with a keyway already in it, but then you need to stop the key from moving too far.

The only difficulties that I have encountered with hex shafting are difficulty with finding mating parts (bearings & hubs), and shaft tolerances. AndyMark has largely solved that first problem for 1/2" & 3/8" shafts. Shaft tolerances can still be a gotcha if you are not paying attention. For example, the shafting you link to is 1/2" +/-.005" which means it's actual size may be anywhere from .495-.505". An AM hex hub (AM-0096) has a hex bore of .501-.505" per the web site. If your hub comes in on the low side and the shaft on the high side it won't fit. (.505" shaft going into a .501" bore) Something with have to give to make that fit. Alternatively, McMaster 6607K43 is also a 1/2" hex, but with a +0 -.008 tolerance for actual size limits of .492-.500" It should always fit in the AM hex hub.

I find it easier to locate mating hex shaft in steel, but the decision between steel and aluminum should really be based upon your load requirements.

Re: Hex shafts?
 

Be sure you get steel or 7075 aluminum, rather than 6061.

Re: Hex shafts?
 

McMasters doesn't have 7075 they only have 6061 or 2021, im going with the 2021 if we do get it

Re: Hex shafts?
 

As always, Mark has wonderful advice. We have noticed that our aluminum hex shafts are often tight fits into our hex hubs. This is exactly due to the reasons Mark lists above. We have found that the best and easiest way to take 1 or 2 thousandths off of a side is to carefully and evenly use a belt sander on each of the 6 sides of the shaft.

Also, another reason why it is beneficial to use a hex shaft is that the torque load is now distributed across 6 different points of contact (the six corners of the hex shaft) as opposed to the concentrated point of the keyway.

We use 2024 and 2021 aluminum for these lightweight applications.

Andy

Re: Hex shafts?
 

I have to second that. We used hex shafts in our 6WD in 2008 (which was the first year we made the switch from keyed shafts after what I believe was a sheared keyway in 2007, same drivetrain), initially made of steel. We brought aluminum replacements to our first competition to try and save a little on weight. I don't know what kind of aluminum we used - Mark, you might know - but we ended up with an axle that had to be sheared off and pounded out of the wheel (if I remember correctly, it got bent during a match).

Every bot is different, but make sure that you know exactly what kind of performance you need from your shafts, including any mishaps that may happen in a match setting. The drivetrain is the one assembly that you can't afford to have knocked out.

Re: Hex shafts?
 

You can run other alloys in lower load spots, but in drive I would recommend 7075. We had no issues in 2010.

Don't buy metal from mcmaster, buy from a real supplier and you can get 7075 hex for very cheap.

We use hex on everything, its so much nicer than keyways.

Re: Hex shafts?
 

We have typically run 6061 hex on our drive train with fairly little issue. Again as people have mentioned before, understanding the loading is of the up most importance. Our drive shafts are doubly supported (we do not cantilever) which puts the loads well within 6061's range. So bottom line "do the math"

Only issues we have had was a some galling issues between the alum. shaft and the alum. wheel. Usually only occurs when the hex was too tight of a fit to begin with.

Re: Hex shafts?
 

I've bought 7075 1/2" hex from Fry Steel for REALLY cheap. We're talking about $18 for a 12 foot piece. They may be able to ship it, call them. I think I recall it being about a thousandth or so oversize. Nothing a little bit of sandpaper and a few minutes couldn't fix.

Re: Hex shafts?
 

Can you recommend some suppliers for 7075 shafts?

Re: Hex shafts?
 

Depending on your area, it may vary.

But we get material donated from Valley Iron and Ryerson here in Fresno, CA. Very friendly people and have pretty good prices.

Coast Aluminum is also another big one that many people use.

-RC

Re: Hex shafts?
 

We were hesitant at first to try hex but have used AM hex products for two seasons with great results. Even flying over the humps in 2010 we never had a failure.

Once you try hex you'll never go back.

Re: Hex shafts?
 

I have been known to sand them down as well. Be prepared for that shaft to get warm.

Thanks Andy, I don't know how I forgot that one. I must have repressed it. For those that haven't experienced it this post should give you the idea.

It was 6061-T6. Let's just say I won the bet on thier survivabilty.

What are the three most important subsystems on a robot?
Drivetrain, drivetrain & drivetrain.

Re: Hex shafts?
 

Coast does not stock any high strength aluminum.

The best place on the west coast for 7075 of any profile is Fry Steel, in our experience.

Tolerance on the hex is a constant battle for us as well. We either need to broach the mating piece multiple times or use a 3M deburring wheel to knock a little off the flats of the hex. We've tried using an oversized hex broach but it's unpredictable whether you end up with a nice fit or a sloppy fit and it does you no good when your mating piece came stock with a hex in it.

Re: Hex shafts?
 

Thanks for all the responses guys. But on buying the hex no where in Austin stocks hex in 7075 it's just 6061. But what were some problem y'all had with the 6061 hex? (sorry for the limited respons I am away from my comp. right now)

Re: Hex shafts?
 

http://www.onlinemetals.com/merchant...p=2&top_cat=60

Here's an online place to buy some 7075.

-RC

Re: Hex shafts?
 

During the middle of the 2010 season we switched our transmission output shafts from keyed to hex due to numerous keys sheared the previous three years. We will never go back to keys, and you will probably see us use hex in other applications also.

Actually, now that I think about it, we used a hex shaft on our custom worm gearbox this past year for the tower. No problems at all from the hex, but some other things in the gearbox were problems.

Re: Hex shafts?
 

Fry Steel will ship via UPS at quite reasonable rates.

Re: Hex shafts?
 

We have found that most fit problems with Hex shafts come at the corners. We always turn our hex shafts on a lathe to cut the corners off just a little and 90% of the time that fixes our fit problem.

Re: Hex shafts?
 

They bend and shear far too easily for our liking. We also managed to strip the hex of the shaft (!) in a drivetrain this year.

Re: Hex shafts?
 

This past year we used steel hex shafts in our drivetrain. normally I would use 2024 aluminum but weight wasn't an issue for us this year and the steel is very easy to slide bearings and hubs onto. Our 2010 robot had the aluminum and had no problems with bending or breaking going over the bumps.

Re: Hex shafts?
 

We have the same issues. We found that we can order various tolerances on the hex from McMaster last year. Beware what the fine print says when yoou order from them. Some of the hex they carry comes in as much as .005" over.

Also I have to agree with Paul that 90% of our issues are actually the corners caused by a worn broach.

We have almost exculsively used hex shafts since we first tried them in 2008 and I can't imagine going back. The y are just way too easy to make compared with keyway. If we key anything other than motors its a rare occurance these days.

Re: Hex shafts?
 

To elaborate, the material property that relates to strength in this case is called the yield stress. This is the point at which the material suffers a permanent bend. For things like a shaft, bent = no longer working properly.

Simplistically speaking, a shaft material with twice the yield stress will be able to take twice the load. This assumes that the size and support of the shaft remain the same. Changes to those are really important, but are a subject for another thread.

For some of the materials mentioned in this thread, here are the yield stresses as found at www.matweb.com

Aluminum - 2024-T351 - 290 MPa or 42000 psi
Aluminum - 6061-T6 - 255 MPa or 37000 psi
Aluminum 2011-T3 - 262 MPa or 38000 psi
Aluminum 7075-T6 - 462 MPa or 67000 psi
Steel 1018 - 370 MPa or 53700 psi
Steel 4140 - 675 MPa or 97900 psi

Be sure to compare the numbers for 7075 aluminum and 1018 steel to understand why folks are talking about 7075.

Re: Hex shafts?
 

How do you put bearings on hex shafts? Is there a good source for bearings with a hex ID or do you simply turn portions of the shaft down and put them in standard bearings?

Turning the shaft down seems like it would limit possible configurations by forcing you to put bearings only on the ends of the shaft. (e.g. It would be impossible to have a Wheel-Bearing-Bearing-Sprocket configuration as in many west coast drives.) Is there a way around this limitation or do you just live with it?

Re: Hex shafts?
 

AndyMark sells hex bearings

Re: Hex shafts?
 

Both. Andymark's hex bearings are a fairly recent thing which makes working with hex a lot easier.

Not simply. Some teams go from 1/2 hex -> 1/2 round -> 7/16 hex in order to put bearings in the middle of the shaft. Step hexing is pretty complex though.

Re: Hex shafts?
 

Who does that? Wouldn't it be significantly easier to install a sleeve instead? (I envision using a narrow needle bearing to make up for the added thickness of the sleeve—this would even work on an aluminum hex shaft, because the steel sleeve protects it against wear.) Even if you had to retain the sleeve to avoid sliding, it would still be far easier.

Re: Hex shafts?
 

254/968 have commonly done that. Its a super nice part and makes sense if you have the resources to do it.

http://www.chiefdelphi.com/media/img...3db741ff_l.jpg

Output shaft, but same concept.

973 last year was even able to have a student do it on manuals.

http://www.chiefdelphi.com/media/photos/35999

We (973/1323) even did the same setup for our arm this year. Its not that hard of a part, the output shafts are a bit trickier.

-RC

Re: Hex shafts?
 

I should have guessed. They're among the few teams that can prioritize gearbox compactness, low part count and ease of maintenance to a very high degree. The tradeoff is that each of those shafts has to be set up on a mill's indexing table (with tailstock?) and a lathe—each with multiple precision passes.

Re: Hex shafts?
 

We have had all of our custom hex shafts made for us by a sponsor who uses a swiss screw machine. The most complicated part (the gearbox output shaft, with 3 unique hex sizes, 3 diameters, and 2 snap ring grooves) is made in one operation in less than 2 minutes per part. These parts are relatively simple compared to what these machines typically produce.