Drive train problems at Peachtree

[JeffersonMartin]

During our last qualification match, we noticed our drive train was a little sluggish. After completely removing it, we found the problem.
The spacer between our wheel and sprocket was wrecked. It had been melted, shattered, and one face was completely sheared off. We took a wheel from our practice bot and had the whole drive train locked down before alliance selection.

[kingbrandon14]

Exactly what cause it to be damaged so badly? That's crazy

[JeffersonMartin]

We don't really know what happened. It ran fine through all of Palmetto and until our last few qualification matches at Peachtree. We only noticed because we had to fix another problem on the drive train.

[T^2]

Is that a VersaHub? If so, I'm sure the IFI guys will want to hear about it.

[Mark Sheridan]

Are those polycarbonate? Any chance loctite got near them?

[JeffersonMartin]

They are VersaHubs. They are made out of nylon.

[ttldomination]

To provide slightly larger picture:

We used the VEX high traction wheels (6") with the versahub setup (as recommended on the versahub product page). The product in the image is indeed a versahub (1/2" bearing pilot).

Our DT is geared for about 9 FPS (slightly slower than the KOP) using 6" wheels. We generally get into quite a few pushing matches, but it's all within the expected scope of FRC gameplay.

Before one of our matches, we noticed a couple of the screws on the wheel assemblies (sprocket + versahub + wheel) were loose. Due to time constraints, we played one match and then proceeded to perform a full service on the DT (checking all wheels, tightening all chains, and adding loctite to all bolts on the wheel assemblies).

Upon on removing this particular assembly from the shaft, the entire assembly fell apart (literally) to reveal sheared screws and the versahub as you see above.

We're just happy we caught this before elimination rounds, where there's no telling how long this would've taken to become disastrous.

- Sunny G.

[Abarth15]

The loctite may be your issue there. I know for a fact Andymark wheels are weakened by loctite. I can't say if this is true for the versahubs but it is something you may want to take a look at. Perhaps consider nylock nuts or locking washers as an alternative.

[JesseK]

I'd predict that the bolts weren't properly tightened during some matches. This would cause the Versa hub holes to wear. The larger wear holes over time causes friction in later matches as the drive train is shoved from forward to reverse quickly, which would then cause this type of deformation. The sheared bolts are the evidence of the root cause -- when everything's tightened down properly, #10 bolts will not shear in a FRC drive train*. Supporting Anecdote: 118 supported their entire 140-lb robot driven by a 6-motor crab-drive in 2008 on 8 #10 bolts.

We used nylocks on our wheels every year from '08-'12, and never had an issue with bolts loosening or shearing. This year's an exception since we're using a WCD with a live axle and AM Performance wheels, hex bore (so no hubs/flat sprockets).

*This is potentially not the case in one of the rarest of use cases -- high traction full-speed impacts of two robots that hit each other square-on without backing off after impact. Teams 48 (Delphi ELITE), 68 (Truck Town), and 245 (Da Bears?) all have good experience with heavy high-traction defense, so they may have more insight.

[Lil' Lavery]

Originally Posted by ttldomination

Due to time constraints, we played one match and then proceeded to perform a full service on the DT (checking all wheels, tightening all chains, and adding loctite to all bolts on the wheel assemblies).

Did you have loctite on these bolts previously? Not all forms of loctite are safe to be used on or around various non-metal materials.

[ttldomination]

Originally Posted by Abarth15

The loctite may be your issue there. I know for a fact Andymark wheels are weakened by loctite. I can't say if this is true for the versahubs but it is something you may want to take a look at. Perhaps consider nylock nuts or locking washers as an alternative.

Originally Posted by JesseK

I'd predict that the bolts weren't properly tightened during some matches. This would cause the Versa hub holes to wear. The larger wear holes over time causes friction in later matches as the drive train is shoved from forward to reverse quickly, which would then cause this type of deformation. The sheared bolts are the evidence of the root cause -- when everything's tightened down properly, #10 bolts will not shear in a FRC drive train*. Supporting Anecdote: 118 supported their entire 140-lb robot driven by a 6-motor crab-drive in 2008 on 8 #10 bolts.

We used nylocks on our wheels every year from '08-'12, and never had an issue with bolts loosening or shearing. This year's an exception since we're using a WCD with a live axle and AM Performance wheels, hex bore (so no hubs/flat sprockets).

*This is potentially not the case in one of the rarest of use cases -- high traction full-speed impacts of two robots that hit each other square-on without backing off after impact. Teams 48 (Delphi ELITE), 68 (Truck Town), and 245 (Da Bears?) all have good experience with heavy high-traction defense, so they may have more insight.

Originally Posted by Lil' Lavery

Did you have loctite on these bolts previously? Not all forms of loctite are safe to be used on or around various non-metal materials.

To address all of these, we did not use loctite on these wheels. The VEX assemblies are held together by the manner of threads in the actual wheels into the captive standoffs (as described on the product webpage).

- Sunny G.