Originally Posted by Joe G. (Post 1500645)

It looks to me as though this problem can be fully solved by rotating the CIM slightly. Even if there's an alternate means of getting at this screw, it's better to address it the "correct" way at this stage.

A couple extra minutes of CAD work is a couple extra minutes you have to work. A couple extra minutes of between-match maintenance is a couple extra minutes you have to work, a couple extra headaches, and potentially a couple crucial matches where your robot is dead on the field.

Originally Posted by KrazyCarl92 (Post 1500621)

Honestly, I often see the tradeoff between #25 and #35 chain in a drive train application severely underplayed here on ChiefDelphi. The truth is there are pros and cons to each, and teams should evaluate which size chain they use.

It is not uncommon for a robot to weigh 150 lbs with a battery and bumpers and then have a CoF on their wheels of 1.2. That means you'd have 180 lbs of tractive force or 90 lbs per side of your drive train. If your sprockets have a pitch diameter of approximately 2 inches and you have 4 inch wheels, that's up to 180 lbs of force going through your chain! Nothing close to this will usually happen on a WCD in most cases because the center wheels will likely be direct driven and carry the majority of the tractive force, but this depends on your CoG.

#25 Chain Pros:
-Lighter Weight (~1.0 lbs/foot)
-Takes up less width

#25 Chain Cons:
-May require tensioning (higher part count)
-Co-planar sprockets are more critical (more precise machining)
-May come close to pushing limits (have to do the math)

#35 Chain Pros:
-Can probably get away without tensioning (lower part count)
-Sprockets less sensitive to axial position (less precise machining)
-High safety factor on chain working load

#35 Chain Cons:
-Heavier (~2.5 lbs/foot)
-Takes up more width

For the top teams, the extra 10-15 lbs you'd be saving by going with #25 chain is well worth it. These teams have no problem getting their sprockets co-planar sprockets and have experience doing the math to see if they're doing anything risky. On a WCD, the direct drive of the dropped center wheels off of the gearbox reduced the amount of the force that will be in the chain. For teams that may benefit from a lower part count, might have difficulty making co-planar sprockets, and deem the peace of mind of something that will "just work" to be more valuable than the weight savings, an argument can be made for using #35 chain.

I'd estimate that for about 75% of FRC teams, if they were to use chain they should at the very least think about these tradeoffs. What works best for the best teams in the world may work best for them as a direct result of their goals and resources, while it may not necessarily be the blanket best solution for all teams across FRC.

Originally Posted by AdamHeard (Post 1500660)

These chain weights are off. #25 chain is about .1 lbs/ft and #35 is around 2-3x that.

Originally Posted by SerpentEagle (Post 1500649)

When calculating, you must account for the largest diameter component acting on the gearbox, right? i.e. If you have a 6 wheel setup with 4in wheels and 2in diam sprockets, you only account torque loss with the wheels because they have the largest diameter, or am I missing something here?

Originally Posted by BenGuy (Post 1500699)

Shashank, if we were to make this in the near future, you'd have to simplify the part, there is no way that this can be made on the bridge port, this would have to laser cut and we usually can't cash in on our sponsorship benefits from Superb until well into build season. Even if it will weigh a half a pound more, that's fine, just make an iteration that is feasible to make parts for on the mill if we are going to need it in the near future.

Originally Posted by Knufire (Post 1500703)

I've been looking into better optimizing parts for machine time over this off-season, especially gearbox plates like these which sucked up a ton of waterjet time for us. One thing I'm exploring is using unpocketed 1/8" plates with additional standoffs, after looking at 2826's robot.

Originally Posted by KrazyCarl92 (Post 1500671)

You're right, thank you Adam. I meant 1.0 lbs/10 feet for #25 chain and 2.5 lbs/10 feet for #35 chain. That makes the difference in weight on the whole robot only about ~1.5 lbs.



I'm not sure what you mean here. I didn't talk about "torque loss" at all in my previous post.

If you want to know the max amount of tension possible in your chain is, a quick and dirty way of getting there is:

Take your robot's weight. Multiply that by your wheel's CoF on the playing surface. Now multiply that by the ratio of your wheel diameter to your sprocket pitch diameter, making sure you use the same units. Divide by 2 because each half of a tank drive is assumed to support half the weight of the robot.

Tension_max = Mass * CoF * D_wheel / D_sprocket / 2

This will tell you the necessary tension in your chain to make one of your wheels slip when it is supporting half of your robot's weight. Now in reality, one wheel will not be supporting half of the weight, it will be distributed among multiple wheels on that side of the robot. And the wheel that most commonly supports the majority of the weight of the robot will be the one directly driven off of a gearbox on a WCD (your dropped center wheel). And since you have a dropped center direct-drive wheel in a WCD, it is not necessarily a crippling issue to toss a chain.

Originally Posted by BenGuy (Post 1500699)

Shashank, if we were to make this in the near future, you'd have to simplify the part, there is no way that this can be made on the bridge port, this would have to laser cut and we usually can't cash in on our sponsorship benefits from Superb until well into build season. Even if it will weigh a half a pound more, that's fine, just make an iteration that is feasible to make parts for on the mill if we are going to need it in the near future.

Originally Posted by Knufire (Post 1500703)

I've been looking into better optimizing parts for machine time over this off-season, especially gearbox plates like these which sucked up a ton of waterjet time for us. One thing I'm exploring is using unpocketed 1/8" plates with additional standoffs, after looking at 2826's robot.

Originally Posted by SerpentEagle (Post 1500779)

Ben, did you forget that we are getting a CNC mill?

PCNC 1100 Education Package: http://www.tormach.com/store/index.php?app=ecom&ns=catshow&ref=PCNC1100_Package s_Ed1



I'm pretty sure that 1114 has used 1/8 inch plates with bends for many of their drive boxes. I wouldn't think it would be a problem.

Originally Posted by Knufire (Post 1500780)

Yeah, I found plenty of examples of bent 1/8" gearbox plates but we don't have access to a decent brake.

Originally Posted by SerpentEagle (Post 1500781)

EDIT: My bad, it seems that the only brakes online at harbor freight that can bend 1/8" aluminum cost over $200. I'm sure that if you guys use a strong grade of aluminum with frequent standoffs, you should be just fine.

Originally Posted by Knufire (Post 1500780)

Yeah, I found plenty of examples of bent 1/8" gearbox plates but we don't have access to a decent brake.

Originally Posted by MichaelBick (Post 1500785)

There is a strength gain just by using 6061 alloy over the 5052 alloy, which is usually used in sheet metal fabrication for its ease of bending.

Originally Posted by MichaelBick (Post 1500785)

There is a strength gain just by using 6061 alloy over the 5052 alloy, which is usually used in sheet metal fabrication for its ease of bending.

Originally Posted by Knufire (Post 1500703)

I've been looking into better optimizing parts for machine time over this off-season, especially gearbox plates like these which sucked up a ton of waterjet time for us. One thing I'm exploring is using unpocketed 1/8" plates with additional standoffs, after looking at 2826's robot.