pic: Twisted Hex Shaft

On Thursday, we were testing our robot on the practice field at Sacramento when we decided to use turnbuckles as chain tensioners. Almost immediately after we raised the arm, one of the turnbuckles failed, while the other held. On the side that failed we got this, a hex shaft twisting almost 60 degrees. The side that held had the BaneBots gearboxes roll pins sheared. The machine shop at Sacramento was great, as they came up with a really good and quick fix for our gearbox.

Impressive. A harsh reminder of the kind of force that FRC bots exert.

3/8" diameter, 2024 aluminum?

I’m curious to know just how much torque that took…

In 2008, our arm was able to rotate the keyway of a piece of hardened 1/2" steel keyshaft almost 90 degrees. It’s impressive how an FRC robot can destroy itself!

waht kind of aluminum is the turnbuckle?

1/2" 7075, which is why we were pretty surprised.

Not sure the alloy, but it was some small diameter (bad idea in hindsight)

gotcha 7075 was the number iw as looking for. Just to be nosey it wasnt 7075 -0 right?

I’ve never seen Al hex in anything other than T6

Wow, that is quite a failure for a 1/2" 7075 shaft. Can you explain in a bit more detail the application this was in under failure?


never seen dosent mean never exists. :slight_smile:

most common types of 7075 is 0, T6, and T631. Although there are MANY different types such as T7362 is also a common one.

T7351 Actually.

It was on our arm. Basically we have 2 Banebot’s gearboxes with chain running up to the arm. We have approx. 50 lb-ft of torque (don’t worry, 1:4 chain reduction, only 6.25 lb-ft on each gearbox). We have chain on either side of the arm, and we decided to change the tensioning method to turnbuckles. Almost immediately as we raised the arm, the turnbuckle one side (the twisted side failed) while the other one held. The torque of the arm in addition to tension being held by the chain all released in one instant twisting the shaft, as the tension still held on the other side (on which the gearbox’s rollpins sheared)

thanks for the assist…my brains all scrambled this morning

Whew! thought that was a lead screw

I also think so.