#25 Chain

[Jeffrafa]

Originally Posted by galewind

I am looking for a side-by-side tensile strength comparison to determine if it's truly worth making a jump from mcmaster #25 to a #25h change. If I find one, I'll post it here.

It is tough to find good manufacturer data for 25 series chain. Have a look at the table at the bottom of page 1 here.

The max allowable load column is synonymous with max recommended working load. Converted to lbf, it is 165 lbf for #25, 242 lbf for #25H. Average tensile strength is 992 lbf and 1323 lbf, respectively. These values are a little higher than I recall from past research, but at least gives you a side-by-side comparison.

We have used #25 chain successfully in drive and manipulator applications since our rookie year in 2004. I would definitely recommend #25H chain over #25 in all applications, the weight penalty is marginal for the added strength. I generally avoid #35 chain at all costs, due to weight, but if you are having failures with #25, I would say switching to 25H alone will likely be a marginal improvement - try to increase sprocket size as well. If you have the weight, consider #35 for the added reliability.

Drivetrain failures are are the worst possible failure point, as it leaves you useless on the field. Whenever possible, I encourage direct-driving at least one wheel, so that no chain is a single-point-of-failure to half your drive. If you must have chain directly from your gearbox, then I would consider #35, as failures there are unacceptable. This year, with 8WD using 8" pneumatic tires, we chose to direct drive one center wheel, use #35 to the other center wheel, and #25 to the outer corner wheels. The idea is a compromise between weight and reliability - no #25 chain failure will leave us unable to drive reasonably well, as it would only cut power transfer to one corner tire.

I think a huge part of the problem people are seeing is the dramatically higher forces involved with the larger tires paired with smaller sprockets. For years FRC drive trains have been evolving toward smaller wheels and less gearbox reduction, which pairs favorably with transitioning to #25 chain. In James' example case, if you were instead running 4" wheels, the max possible chain tension would drop to 365 lbf, and SF would increase from 1.1 to 2.1. I agree that his scenario is conservative, but I think most teams (running live-axle WCD, at least) are using even smaller sprockets than he calculated. Consider that the largest hubbed #25 sprocket Vex sells is 22T, with a pitch diameter of 1.75".