Single vs Multiple Speed Gearbox

[Type]

In the past, my team has just focused on trying to do alright at events. Recently, our team has kind of moved up and have become better, so we have been looking at more types of drivetrains. Ive looked online and here comparing different gearboxes but I haven't seen much about why a two speed gearbox or shifting gearbox is better than a single speed gearbox. Could anybody tell me what the advantages are and if it's worth the complexity and space it takes up? We have been looking at the WCP DS just because we've heard that it's probably one of the best gearboxes for FRC.

[Jay O'Donnell]

In general, if you go faster with the motors we have access to, a pushing match might cost you by popping breakers. A 2 speed gearbox allows you to go pretty fast, but then shift to a much lower speed when you're in a pushing match so you don't pop a breaker.

Hopefully someone who isn't typing this on their phone quickly between classes can explain better than myself.

[Hgree56]

I'm just going to say, our team's best drive train had a 3cim ball shifter on each side. This was in 2014 so we used high gear for speed and low gear for pushing matches.

[gerthworm]

Very much depends on the game, your strategy, and how effectively your driver can use both speeds.

For Stronghold, our bot had two-speed gearboxes on the drivetrain. We mostly stayed in low gear, which had the ratios optimized around pushing, fine maneuvering (shot alignment), and defense crossing. We also had a high gear for sprinting across the field. From our post-match data gathering, it looked like our driver only stayed in high gear ~5% of the time, but it did help us win a few races to boulders.

Additionally, we put some protection logic in software to monitor the current draw from the battery while in high gear. If it exceeded a threshold for too long, we'd force the bot into low gear regardless of driver command.

For strategies & games where you don't have a reason to need both, it's probably not worth the complexity (and weight and size). Recycle rush is an example of a game where it probably didn't make sense for most teams to do two-speed.

Advantages:
-- Two speed ranges reduces some tradeoffs during drivetrain design
-- Potential for getting fine maneuvering, pushing power, and speed all at once
-- Gives drivers more options for during-game strategy

Disadvantages:
-- Heaver, larger, more expensive, more points of failure, more complex software
-- Robust operation usually needs pneumatic system, or fairly chunky servos
-- Requires more input from driver (unless you write some beastly software)

[Lil' Lavery]

In my experience, I've found very little absolute need for two speed gearboxes. In the seasons I've been on teams that have had them, the driver very seldom felt the need to shift gears. Based on that, I've largely pushed for my teams to stick with single speed transmissions to save money, time, weight, and complexity. That being said, that is based on my experience, and I know there are others who have very different perspectives on this issue. I have never had the chance to try out a dual speed transmission with any sort of automatic shifting code, nor do my teams get the countless hours of driver practice some top tier teams do. Either of those may lead towards more effective uses of a shifting transmission.


The one thing I will mention is that shifting transmissions essentially mean you're also using pneumatics. While it's possible to servo or motor-shift, it's incredibly rare and generally has worse performance. I know 67 has some experience in shifting without pneumatics (as they avoided pneumatics entirely for a long time), but even they changed towards using pneumatics to shift.

[techhelpbb]

I agree with the others. It depends on your strategy. In effect you have have lower speeds with more torque or high speeds with less torque. You can even create a higher speed that wouldn't allow the robot to break inertia and switch into it from the lower gear (like an automobile's higher gears).

If you just want to hold ground, there are other ways to achieve this than using drive train torque. You could simply plant your robot like a backhoe does to improve stability while digging. One could even rig a ratchet or something like that so they can only advance in one direction till they loose traction on the carpet. If you increase the surface area on which the additional support touches the carpet you can create a situation where another robot up to the legal weight would probably trip their breakers or loose traction before your robot moves. Be kind of fun to watch a team that usually just plows things out of their way find their 'unmovable' object.

[Jonathan Norris]

For the majority of offensively focused robots, getting in pushing matches is just bad strategy, avoiding defense should be your priority. In my experience acceleration is just as important as top speed, I'd take a 6-CIM 1 speed drive over a 4-CIM shifter. And in most cases prefer the simplicity of a 4-CIM 1 speed over a shifter.

[D.Allred]

We used 2 speed gearboxes in 2014 and have been single speed every other year.

2014 seemed like a good year for using 2 speeds, but ironically ended up in low gear most of the year. Bottom line, the driver needs lots of practice if you end up manually shifting. Our driver was comfortable in high gear at the shop practicing one-on-none. Things got a bit more chaotic on the field three-on-three, and he didn't feel in control at top speed.

The other design consideration is acceleration and distance. Top speed might be less important than good acceleration over a short distance.

David

[messer5740]

We had a single speed gearbox this year and played a TON of defense. Even though we did not have very high torque, we still played great defense by gaining speed and ramming the opponent before they made a shot. Other teams we had competed with used two speed gearboxes, but they mostly stuck to high or low gears and did not venture out of the driver's speed and torque comfort zone. It all depends on how your driver uses the different setups.

[Richard Wallace]

Originally Posted by Type

Recently, our team has kind of moved up and have become better, so we have been looking at more types of drivetrains.

Your team certainly has moved up! WMRI should be a lot of fun next month.

Let's get together there a talk drive trains. I think the WCP DS is a great place to start the discussion, and have considered it the strongest COTS alternative since it came out.

Like your team, mine has yet to run a shifting gearbox in competition. Maybe 2017 will be the year.

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edit: Jay's explanation (above) is spot-on. A 2-ratio gearbox lets your driver go faster unopposed, and shift down to deal with resistance. How fast and how far down are the interesting details. I like the DS because it offers quite a few options.