Advise for Galvanized Steel

[artdutra04]

Originally Posted by DELurker

...

from matweb.com...
1018 steel versus 6061-T6 aluminum versus ASTM A526 galvanized steel

I could never bring myself to use aluminum for a shaft. It is too weak (comparably) to take that shock and torsion loads, as well as being more prone to unannounced catastrophic failure over steel (steel bends and then breaks ... aluminum has a much smaller deformation period before it snaps under the same loads). I'd rather use a steel shaft so that I could press-fit it into a bearing. The surface properties of aluminum just aren't good enough (at least for me) for rotating or direct-contact sliding.

I would agree that 6061 alloy aluminum is a poor choice for shafts, but 2024 alloy aluminum and particularly 7075-T6 have excellent material properties that are competitive with many steel alloys. These two aluminum alloys are much harder and have significantly higher yield strengths than 6061.

I've been using 2024 and 7075 for shafts on FRC robots since I was in college in 2010, and have never had one fail. Plus, I absolutely love machining 7075 aluminum on mills and lathes. With good carbide tools, you can machine it like a hot knife through butter, as it does not have any of the galling issues that plague softer aluminum alloys. It's my second favorite material to machine after Delrin.

// Note: I didn't directly link to the Matweb pages for 2024 and 7075 since apparently the links are tied to cookies and 404 after a while.

[magnets]

Originally Posted by yash101

I wouldn't say "significantly stronger" but yeah, it can be stronger per unit of weight.

Well.... 7075 is twice as strong as certain steels, so yeah, it is "significantly stronger"

[magnets]

Originally Posted by DELurker

I disagree, although I will point out that stainless steel (302, 303, 304, 316) is a specific and (typically) less durable subset of steels (without getting into hardenable stainless such as 440C). The 3x price differential on stainless makes it less than desirable for teams to work with in the raw, aside from the additional tooling needed to properly work it. However, I will support my original contention with the following links and tables...

Reference #1
Reference #2

from reference #2...
........................................Aluminium. ................Steel
Density, ρ kg m−3...............2,700......................7 ,800
Young modulus, E N mm−2....70,000....................210,000
Shear modulus, G N mm−2.....27,000....................81,000
Poisson ratio, ν.....................0.33........................ 0.3

from matweb.com...
1018 steel versus 6061-T6 aluminum versus ASTM A526 galvanized steel

I could never bring myself to use aluminum for a shaft. It is too weak (comparably) to take that shock and torsion loads, as well as being more prone to unannounced catastrophic failure over steel (steel bends and then breaks ... aluminum has a much smaller deformation period before it snaps under the same loads). I'd rather use a steel shaft so that I could press-fit it into a bearing. The surface properties of aluminum just aren't good enough (at least for me) for rotating or direct-contact sliding.

You're not looking at the right alloys. AndyMark steel parts are 4140 stainless with a yield strength of 60200 psi, but 7075 aluminum (vex pro parts) have a yield strength of 78,000 psi. Also, I don't really follow the shaft argument. Any 3 CIM gearbox that I've ever seen uses an aluminum shaft. Same with our climber.

[DELurker]

Originally Posted by artdutra04

I've been using 2024 and 7075 for shafts on FRC robots since I was in college in 2010, and have never had one fail. Plus, I absolutely love machining 7075 aluminum on mills and lathes. With good carbide tools, you can machine it like a hot knife through butter, as it does not have any of the galling issues that plague softer aluminum alloys. It's my second favorite material to machine after Delrin.

I will totally agree that aluminum, especially 7075, machines really well. However, we have a tough time getting anything but 6061 locally. One of our former mentors went out and got some cheap aluminum from a different alloy in the 6xxx series that machined like garbage. Regardless of speed, feed, or cutter age, it rolled instead of cutting. It reached a point where I would have preferred to be cutting gummy stainless (AL6XN).

However, I digress. We've bent a number of aluminum shafts and torn up the surface of others in bearings, so we're gunshy about going that route again. I'd be interested to know what installation and mounting techniques you use for the aluminum shafts.

Originally Posted by artdutra04

// Note: I didn't directly link to the Matweb pages for 2024 and 7075 since apparently the links are tied to cookies and 404 after a while.

Whoops. Didn't realize that. Sorry. The short form was that the materials listed matched the chart in the post for properties and that the galvanized was not significantly different from the 1018.

[DELurker]

Originally Posted by magnets

You're not looking at the right alloys. AndyMark steel parts are 4140 stainless with a yield strength of 60200 psi, but 7075 aluminum (vex pro parts) have a yield strength of 78,000 psi. Also, I don't really follow the shaft argument. Any 3 CIM gearbox that I've ever seen uses an aluminum shaft. Same with our climber.

That's possible. However, 4140 is not stainless (under AISI and ASTM coding). It is a steel which, among other things, is excellent for shafting.

[DonRotolo]

We're all shafted now and then.

We've digressed significantly. I think the OP has a better idea of how to work with Galvanized Steel, and aluminum versus steel probably deserves a thread of its own. Or?

[jman4747]

Originally Posted by DonRotolo

We're all shafted now and then.

We've digressed significantly. I think the OP has a better idea of how to work with Galvanized Steel, and aluminum versus steel probably deserves a thread of its own. Or?

I don't really have a problem with a materials science class in a thread now and then.

Also on the steel vs 7075 shafting thing, personally reducing drive train inertia is a priority and also reduces overall weight. Design wise I like to go a little heavy in the lower frame and keep all the scoring stuff up top light. Coupled with lightweight drive train parts and as few as possible. I can't tell you how much time I spent trying to get a 2" colsin to work so I'd only need a 4:1 of the CIMs.
PS I really like the torque/speed combo from 8:1 on 4" wheels. PSS really dislike drive train inertia.

[yash101]

This isn't as bad as the "Driver Station Power" thread. That started out with legalities of marine batteries in the driver station/battery cart, but diverted to microcontrollers, the propeller chip and comparing and contrasting it's functions.

At least this thread is still on topic.

[Ether]

Originally Posted by jman4747

Also on the steel vs 7075 shafting thing, personally reducing drive train inertia is a priority... really dislike drive train inertia.

What do you mean by "drivetrain inertia" in this context? Are you concerned about the moment of inertia of the output shaft of your gearbox? And if so, why?

[jman4747]

Originally Posted by Ether

What do you mean by "drivetrain inertia" in this context? Are you concerned about the moment of inertia of the output shaft of your gearbox? And if so, why?

I mean to say the total mass of all my moving parts and the friction between them to increase the efficiency of the drive train itself.

[Ether]

Originally Posted by jman4747

I mean to say the total mass of all my moving parts and the friction between them to increase the efficiency of the drive train itself.

OK. Friction is a big issue in an FRC drivetrain. Inertia, not as much. They are entirely different things.