Quote:
Originally Posted by KrazyCarl92
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.
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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?
We have little experience with chain, negative experience really, and have used belt drives since at least 2014 if my memory is correct, without any problems (15mm, not 9mm as they snapped during a match in 2014). Are there any real benefits to using chain on a WCD opposed to using belts instead? The only thing compelling me towards chain is increased flexibility in design, less space consumption, and ease of replacement.
In terms of weight, I would actually be compelled to keeping #35 chain as even though it will weigh more, the CoG benefits would outweigh the weight benefits. After all, the drivetrain is the last subsystem you want to fail, so, from my PoV, #35 chain would be a good precaution to take.