[Kevin Kolodziej]

Team 1675 used 8 belts on our 2014 robot. This was our second experience with belts. Our first use was in 2011 on our roller claw (I don't remember the ratios but they were 15mm HTD belts from Gates on fairly small pulleys). This year we used 8 belts on our robot:
Drive: (4) VexPro 150t 9mm belts, 30t pulley on each end. 6 Wheel WCD Gearboxes with 2CIM shifter, 15:1 / 5.13:1 ratios, 4" wheels. Center pulley was a double pulley, versablocks with cam tension on the outside wheels.
Arm Shoulder: (2) Gates HTD 239t 15mm belts, 18t pulley to 60t pulley. 18t pulleys were driven by 100:1 versaplanetary with 775 motor and additional 18:24 gear reduction. Versablock with cam tension on the shoulder joint. 18t pulleys were on a shaft supported by drive gearboxes.
Roller Claw: (2) VexPro 150t 9mm belts, 30t pulley to 42t pulley, 4" Versawheels on intake. 30t pulley driven by 15:1 Versaplanetary with BAG motor and bevel gears. Versablocks with cam tension on intake wheels.

We experienced 2 belt failures during the 2014 season, both on the drive. On our practice bot, one of the drive belts was shredded (messy, frayed break). The second failure was on our competition bot on Friday afternoon at our second regional. One of our rear drive belts snapped (perpendicular clean break as far as I can remember).

We believe the first failure was due to a pulley failure. The inside flange of the pulley in the gearbox (the one with bolt belts wrapping it) popped off. When the belt walked to the edge of the teeth, it shredded on the sharp edge. We believe the second failure was due to over-tensioning of that particular belt.

After talking with Aren Hill at Vex about our pulley failure, we took a careful look at our alignment and determined that our belts were not straight and that was putting stress on the flanges of the pulleys. I believe it is common practice for smaller timing pulleys to have the flanges pressed on so any misalignment is likely to cause this failure over time. After replacing spacers to achieve better alignment, our walking issues seemed to disappear, however, we still had 4 more pulley failures (total of 5) over the course of the season (3 total on practice bot, 2 on competition bot, both at the gearbox and at the wheel). The failures always seemed to occur on the same half of the drive, which leads me to believe we had an inherent flaw in our fabrication causing a misalignment that we could not detect. For what its worth, however, we did notice that at least one of our drive axles was always towed in towards the gearbox when the belt was tensioned (not overly tensioned). We believe this was a flaw in either the versablock at that wheel or the bearings or some combination of the two. We disassembled the block and bearings and reassembled and the problem persisted. We did not have additional bearings to try swapping out. This was not the corner that we saw our pulley failures in, however, and it did not give us any noticeable problems.

We had no issues with our printed pulleys on our roller claw.

We had some limited belt slipping on our shoulder joint, but this was due to the bottom shaft flexing as the drive gearboxes flexed. A brace between the two gearboxes would likely solve this.

As for loads on the belts, our estimated pushing force was 170 lbs, and the 30t belts on our drive I think had about a 1.5" diameter, so potentially one belt could have seen approximately 250 lb of tension if we were in a pushing match, plus the pre-tension. The shoulder belts saw some shock loads as our arm bounced around a bit, but I couldn't begin to estimate that load. Perhaps the most surprising, however, was on our roller claw. Though generally under very low loads (lightly tensioned, not much load from the ball), there was a match when the screw from the versablock backed out and interfered with the spokes of the wheel (VexPro DT 4"). The belts held up just fine as the motor pulled the wheel through the screw, creating a notch in the wheel from the head of the bolt and bending the 10-32 bolt.

As mentioned above, our drive belts shared a common pulley in the gearbox, and this is where our initial failures occured. When we popped a flange off at this pulley on our competition bot, it was actually the outside flange, not the inside flange, that came off, but the belts did not walk off the pulley.

Final thoughts: We love belts. There's a few things we learned this year about proper tension that we will implement in the future. One disadvantage is when we popped a flange or snapped the belt on the drive, the only way to replace the internal pulley or replace the belt was to remove the entire drive gearbox from the robot, which also involved removing the belts for our shoulder because of the shared mounting. After having to do this a few times, we got pretty good at it and had it down to a 30 minute repair, but this doesn't work for elims. Simple solution to this is to move the output pulley to outside of the gearbox, but that does create other design challenges. But I certainly do not miss trying to put a masterlink on a chain in a cramped part of the robot, upside down! We are going to explore using the printed pulleys in more applications as well. Tooth wear was practically zero and having the flanges not pressed on is a bit more forgiving for alignment issues. I was initially concerned with our choice to use 9mm belts on the drive, instead of 15mm, but the larger pulleys (30t instead of 24t or 18t) seemed to be the right combination for use with the 9mm belts.