Hex shafts?

[MattC9]

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?

[Taylor]

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.

[MattC9]

Originally Posted by Taylor

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.

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.

[kramarczyk]

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.

[Chris is me]

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

[MattC9]

Originally Posted by Chris is me

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

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

[Andy Baker]

Originally Posted by kramarczyk

The primary benefit of hex shafting is that there is no key needed. ...

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.

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.

Originally Posted by MattC9

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

We use 2024 and 2021 aluminum for these lightweight applications.

Andy

[Karibou]

Originally Posted by Chris is me

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

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.

[AdamHeard]

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.

[kaliken]

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.

[sanddrag]

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.

[rsisk]

Originally Posted by AdamHeard

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.

Can you recommend some suppliers for 7075 shafts?

[R.C.]

Originally Posted by rsisk

Can you recommend some suppliers for 7075 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

[IndySam]

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.

[kramarczyk]

Originally Posted by Andy Baker

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.

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

Originally Posted by Andy Baker

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.

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.

Originally Posted by Karibou

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).

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

Originally Posted by Karibou

The drivetrain is the one assembly that you can't afford to have knocked out.

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