In the summer of 2014, I was inspired by FRC Team 971's inverted motor gearboxes and decided to try my hand at designing my own. Upon trying to imitate their initial design, however, I ran into a couple of problems. The first problem was that their design worked really well with their custom sheet metal style of drivetrain, but was difficult to integrate into the West Coast Drive style drivetrain I had intended on designing for. The second problem was that their packaging solution placed the shifting stage on the first stage of the transmission, which would have required types of machining that I wanted to avoid. As a result of these two driving limitations, the idler-gear flip cim gearbox was created.
For those of you who don't know what a flip cim gearbox is, or why it's important, allow me to explain. In FIRST Robotics, the competition these transmissions were designed for, one of the most common limitations is space. Prior to inverted gearboxes, the drive transmission of a standard drivetrain typically took up approximately 9 inches of space per side of the drivetrain, usually in the very center. Ever since the total robot size decreased in 2013, space in the drivetrain has been difficult to find, especially before the smaller control system introduced in 2015. A normal robot width in those years could be anywhere from 28 to 32 inches, and while sizes occurred outside of that range, most teams found themselves sitting within that length. When a robot is 30 inches wide and has two transmissions taking up 9 inches of linear space each, 60% of the drivetrain width is taken up by the gearboxes. The goal of the inverted motor gearbox is to open up more space in the drivetrain by reversing the motor placement such that the body of the motors takes up unused space inside the gearbox rather than space inside the center of the robot chassis.
The first iteration of my flipped transmission was large, bulky, and heavy. It saved near negligible space and weighed more than the non-flipped COTS transmissions available (weight is a huge factor in FRC), but it proved that the concept could be refined. The transmission used 3 motors (since doing so was popular at the time) and required large idler gears to space the motors apart. The idler gears sat on a cantilevered 1/2" hex shaft that spun inside a bearing that was pressed into the outer plate. From there the idler gears connected at a cluster gear on the middle shaft, and from that point on it functioned like a standard dog shifting transmission. It was a good start, but I knew I could do better.
The second iteration was largely like the first, but made a few subtle changes. Most notably, the bearings were moved from the outer plate to inside of the gears, and rode on a 1/2"-20 bolt that threaded into the outer plate instead of a spinning hex shaft, and the inner plate no longer wrapped around the ends of the CIM motors.
Mark III introduced using 1/2" OD, 1/4" ID bearings instead of the previously standard FR8 bearings (1.125" OD, 1/2" ID). This allowed the bearings to be pressed directly into the 1/2" hex bore of the idler gears without any necessary machining or modification to either. This also meant that any COTS gear with a 1/2" hex bore was able to be used as an idler, which greatly decreased the total size of the gearbox by allowing the motors to be placed closer together. Now the limiting factor in total horizontal width was how close the motor bodies got to the gears on the center shaft inside the gearbox, not the availability of 1.125" bore gears.
Mark IV had some minor changes to the lightening pattern on the gearbox plates, but most of the changes happened in the inner gearbox plate, where a change in plate geometry allowed for better support by a single standoff going through the center plate of the gearbox.
Not all iterations are an improvement, and Mark V shows that. While there were a good number of things I did right with it, Mark V was the culmination of a lot of ideas that seemed good on paper but made the gearbox worse overall in practice. Among these ideas were: less support for the motors, reshaped geometry for the inner plate that made it weaker than before, and thinner standoffs holding the plates together. While Mark V luckily never made it onto a robot, it was a good learning experience.
Mark VI was the first of the flipped transmissions to make it onto a robot. FRC Team 1323, Madtown Robotics, one of the teams I mentor, decided that opening up space in the center of the drivetrain would provide advantageous to how we wanted to design our robot that year, and another iteration of the gearbox was born for our 2016 robot. This iteration featured a Power-Take-Off (PTO) mechanism that allowed the power from the drive motors to power another mechanism on the robot, but this feature was later scrapped. One of the many improvements the Mark VI had was the ability to use multiple idler mounting holes on each side to allow the first stage reduction ratio to be easily changed. This provided us with the freedom of changing gears if we thought we were too fast or too slow without changing the entire gearbox.
After using the Mark VI successfully over the 2016 season, Ranjit Chahal, lead mentor of 1323 and owner of West Coast Products decided he'd sell a flipped gearbox based off of the iteration we used on Madtown's 2016 robot. What became of this is the WCP 2 CIM Flipped DS, one of the most popular COTS transmissions on the market as of the 2017 season. The product was iterated from previous versions to be more friendly to teams, offering them more potential ratios via an added third stage, and flipping the pneumatic cylinder as well as the motors, saving even more space for teams. Alongside these improvements, the idler gears were supported by a thin plate mounted to the outer gearbox plate that eliminated inefficiencies found in the Mark VI. The WCP 2 CIM Flipped DS made great strides not only in the COTS transmission market, but also in the long line of iterations that came from the Mark I. For more information on the WCP 2 CIM Flipped DS, please visit the product page on West Coast Products' website: http://www.wcproducts.net/wcp-ds-flipped.