CDF Discussion, post Team Update 14

[Mark McLeod]

Here's Frank's description (http://www.firstinspires.org/robotics/frc/blog/week-0.5)

Cheval de Frise

The Cheval de Frise defenses didn’t hold up well during the event, often breaking when hit by robots. This breakage was not revealed during our testing here at FIRST HQ or during the Week Zero scrimmage, and we know why. Turns out, we assembled the defenses we tested using hand tools, but when we got into production mode, we started using power drills to secure the bolts. These drills put more torque on the bolts, stressing on the planks in the defense, make them susceptible to breakage. We’ve redesigned the planks to use a counterbore, rather than a countersink, in the bolt areas. This greatly reduces the stress on the polycarbonate when tightening the bolts, and our new testing here has shown this fix works well. All events will be getting the newly designed Cheval de Frise planks, starting with this week’s events.

[Ether]

Originally Posted by Ether

When the only tool you have is an air-powered impact wrench, everything looks like a lug nut.

Turns out, we assembled the defenses we tested using hand tools, but when we got into production mode, we started using power drills to secure the bolts.

I was joking, but it looks like I wasn't too far off

[Retired Starman]

Originally Posted by GeeTwo

:shudder:

Reminds me of the time (ca 1995) that I pulled on the shoulder of I-10 for a tiny little shimmy that started suddenly. By the time I came to a stop, three of the five lugs on my right front tire had sheared clean off. I was probably a few seconds from using a pine tree as a brake. I don't know what tool they used to tighten the lugs at the shop, but it obviously wasn't the right one. I never used that shop again.

Edit (especially as off-topic):


I believe they were; curious as to the connection between these two. The dealer where I had the jeep towed told me they had been over-tightened. I ran the same vehicle and wheels for another 10 years without the problem recurring.

Now that this thread appears to have run its course, I'll digress into the land of broken lug bolts.

When I first read your post, what came to mind was, "Wonder if this is on a four-wheel drive vehicle"?

When run in four-wheel drive on an interstate, the differential turning of the front wheels can "wind up" the system. The bouncing of rough terrain keeps this from being a problem, as occurs on smooth roads. I've seen ALL the lugs on both front wheels snap at the same time because of this. Don't suppose you had it in four-wheel drive by accident, did you?

Just sayin'!

[ratdude747]

Originally Posted by Ether

When the only tool you have is an air-powered impact wrench, everything looks like a lug nut.

Off topic:

You know that on civilian vehicles you're not supposed to use impact guns on the lug nuts either? At least if you're not using a torque stick (and even then, it's technically good practice to use a torque wrench afterwards).

Otherwise one risks warped brake rotors/drums an/or broken/bent/stripped lug nuts/studs/bolts. Not to be a so called torque nazi, but impact guns can really mess a lot of things up when used to torque things down. They're better suited for removing fasteners (especially rusty stuck ones).

[GeeTwo]

Originally Posted by Retired Starman

When I first read your post, what came to mind was, "Wonder if this is on a four-wheel drive vehicle"?

No, it was a rear wheel drive 1991 Cherokee.

I did a bit of online research, and it appears that the more likely problem for the wheel was loose nuts. Left side nuts tend to loosen farther and spin off, where right side nuts just don't provide enough contact force and put lots of shock force on the studs. While I don't have those studs anymore (though I did find one after I pulled off the road), they had the "beach" look on the sheared face.

As to aluminum, I did find several claims that aluminum wheels were more likely to have this issue than steel, but they were anecdotal or otherwise suspect. Admittedly anecdotal, but I noticed that a few of the aluminum wheels which were thrown happened in snow or had plants without foliage in the background. As aluminum has a higher coefficient of thermal expansion than steel, it seems reasonable that a lug nut tightened on a steel stud against an aluminum wheel in warm weather (or a warm garage) would loosen in cold weather.

I also found quite a few recommendations to re-torque lug nuts after a dozen to a hundred miles after they're removed/replaced. If the nut was tightened a bit off-center, jostling can loosen them. This is essentially the same reason I often stop about two miles down the road and check cargo straps.

[Hallry]

It looks like a CdF just broke in Qual. 1 at the Waterford FiM District Event...

Aaaand they took it out of play again. Bring in the Portcullis.

[MrForbes]

Adding a 1/8" steel plate under the plastic would fix this problem completely, eh?

[GeeTwo]

Originally Posted by Hallry

It looks like a CdF just broke in Qual. 1 at the Waterford FiM District Event...

Aaaand they took it out of play again. Bring in the Portcullis.

Two prototypes work, each followed by a failed production model.

[BeardyMentor]

Originally Posted by GeeTwo

Two prototypes work, each followed by a failed production model.

That tells me it is a process problem, but I guess we already knew that. I am just waiting for them to give up and put the polycarbonate into a steel frame and end the breakage for a while.

[cadandcookies]

FWIW, the CDF at Northern Lights are holding up extremely well with the fix. We have had a broken drawbridge (stress fractures) and many issues with the sally ports though.

[Mark McLeod]

The NE Waterbury CDF repairs gave us no troubles.