Why not to shift?

[Gregor]

Other that cost, what are some downsides to shifting gearboxes in the drivetrain (either COTS or custom).

We are considering shifting this year, and while I can find numerous threads on the benefits of shifting, I can find relatively few on the disadvantages.

Additionally, what are some pitfalls to avoid when using shifters? What are some useful things to know.

[wilful]

If you do decide to use shifting gearboxes, be sure that you have easy access to the shifting mechanism. We had this problem with a summer robot and had to cut holes in our bellypan (solid sheet of 1/8" aluminum) at Calgames this year.

As to disadvantages, they take up more space and weight because of the shifting mechanism and extra gears

[F22Rapture]

Added complexity. Servo-based shifting is slow and somewhat unreliable. Pneumatic shifting is much better, but if you have no other pneumatics on your robot the compressor is going to be a giant power suck. It's also easy to pick the wrong gear ratios and end up with a low ratio that's too low and a high ratio that's too high.

There's also the issue of training your drivers to use it properly.

Ideally, pick a gear used only for pushing and climbing (mounting the bridge, etc.) and another for regular driving.

[Gregor]

Originally Posted by F22Rapture

If you don't keep track of your gears, you may find that you're going slower than you would be with a single gear.

What are some ways to counter this? How do teams that to manual shifting train their drivers "how to shift."

[F22Rapture]

Originally Posted by Gregor

What are some ways to counter this? How do teams that to manual shifting train their drivers "how to shift."

I actually edited it to be a bit more clear. Done properly, that shouldn't really be an issue. The most successful shifting teams I've seen primarily use one gear for pushing and the other for driving, but don't really switch otherwise.

[BrendanB]

A downside to shifting is the added complexity of your robot and maintainence. More moving parts, more components, etc. Another area with increased chance of failure if not done properly and the last place you want failure is your drivebase!

We have had a issues in the past two seasons with our pistons slipping out of gear either limiting us to either low, high, or neutral. Thankfully our team has never experienced neutral in match but it has happened during drive practice. We use AM Supershifters and we haven't found much of a remedy aside from checking the pnuematic mounts every few matches but once you release the air from the system its not hard for the AM shifters to switch gears. If we use shifters again in the future we'd look into better ways to get around this problem like using springs to keep the piston in low gear when their isn't pressure in the system and redo some of the mounting for a more permanent/solid fit.

We did have an issue during 2011 where the shifter was sent with an extra washer resulting the one gearbox being compressed when assembled. It resulted in the jags on one side being overworked and shutting down at various points in the match. It was hard to chase because we took apart that shifter so many times each time it was tighted differently. Our first event it happened once a match but during our second event it was happening every 5 seconds. Lesson learned: make sure you know everything about your shifter what belongs and what doesn't!

When using shifters especially COTS you really need to make sure you need them. For teams who make their own transmissions it is easy to customize your ratios and mountings for better speeds to fit your strategy and save weight.

Of the robots I've worked on we have used supershifters three times:

2010 (1519): We used supershifters mainly to have high torque and low speed for crossing the bump and defense. They were very handy to have we spent most of our matches in low gear but high was great for quickly transitioning from one goal to another to get around defenders or get another balls. Super low speeds were used to get balls off the wall and line up with the goal.

2011 (3467): Our design as a rookie placed defense as a top strategy with our minibot and low row arm. We had a nice 6wd and planned to use shifters for defensive pushing matches. IMHO bad move. It was hard for our driver to transition from low to high speeds easily to adjust for what was happening on the field. Low gear was too slow to keep up with opponents and instead we played in high gear to keep up with faster robots and once we got into a T-bone push we'd were supposed to switch to low. I believe our driver kept just kept it in high gear to keep up some quick machines like 177, 40, 131, 33, 1718, 118, etc. We could have done the same with toughboxes and used the weight elsewhere but since defense was our priority it served us well. If we replayed 2011 I would push for single speeds dependent on our strategy.

2012 (3467): Used them for crossing the bump, balancing, and lining up on the bridge. I know a lot of people asked why you would need high gear but we made use of it by quickly going to the other side to grab balls before heading back and spent most matches collecting balls in high gear. Great choice for our robot.

It really comes down to your robot as a whole and the field you play on when decided whether to shift or not. While most powerhouse teams have two speeds they aren't required to win events or do well on the field. Using single speeds is one way you can keep your robot simple and focus energy on other areas.

As for when to shift it depends on why you put those shifters on. Was it to push? Was it to cross something that required low speed? How high is your center of gravity and what speed is safest for when your manipulator is up in the air?

[Tristan Lall]

Shifting gearboxes are cheap this year, and the stupid cylinder restrictions are long gone. That means the set of circumstances where a shifter would not be beneficial is exceedingly small.

That said, the main disadvantage for a competently built shifting gearbox is the fact that it inherently spends some of its time out of gear before resuming under power. (CVTs don't have that issue, but they're quite rare in FRC.)

Think of it in terms of a diminishing return: for each ratio, you add complexity and unpowered time every time you accelerate and shift through the gears, but gain output efficiency (by running your motors more optimally). At some point, the losses of time equal the gains in acceleration. Consider the situations in which your robot will be expected to perform, and estimate whether you're running into the practical limits of the design's utility.

Also, don't underestimate issues like being in the wrong gear and running out of air, or synchronizing two shifters.

(For my own amusement, I enjoy having two speeds and six drive motors very, very much. Incidentally, it's a good combination for a lot robots in a lot of games.)

[George C]

If you're concerned about pneumatic shifters losing air, use single acting cylinders. They're air powered one way and spring powered the other. If you lose air, you won't be stuck in neutral. They're also lighter, cheaper and require less plumbing. McMaster part number 6498K195 works perfectly at $18.02 each.

[Nemo]

Factors favoring single speeds:

1) You might be able to use a one stage gearbox instead of a 2-3 stage to get better efficiency. That gives you the same benefits as using more powerful motors, except you don't have to draw the extra current and produce the extra heat.

2) Single stage gearboxes are lighter by a pound or more.

3) Pneumatics weigh several pounds, so you have significant weight savings available if you're not using pneumatics for anything other than shifting.

4) Single speed boxes are thinner, and you could have certain situations where that becomes important. For example, you can fit a battery between the CIM motors sometimes, but using thicker gearboxes could eliminate that possibility depending on the setup.

5) Cost is a factor, of course, but consider that it's even more of a factor if you're on the edge of being able to make a practice robot. Now that components are cheaper, more teams will hopefully look at practice robots. When you have to buy four of every transmission, the $110+ difference between single and double speeds turns into $440+, which is a decent chunk of your second robot.

6) It's easier to make a custom single speed transmission than a custom two speed transmission. That has benefits, but the one that I'll mention is that you can more easily make a direct drive robot with a custom gearbox that mounts to your frame in some convenient way of your choice. Direct drive can increase your reliability and decrease complexity. It can also make it possible to drop another reduction stage (see item #1).

After your game analysis and main strategy lock, ask yourself this:

Is there any reason why you will need a low gear for the extra torque?

If you definitely think you'll need a low gear with more torque, then shift. If not, a single speed will do you just fine.

[LeelandS]

Coming from one team who does shifting almost every year to another that has never had a shifting robot, here's my take.

Pro, it significantly simpler. I'm not very mechanically inclined, but I could look at the drives and understand exactly what was what, what it was doing, where it came from and where it went. I could even maintain them a bit, which I could never do on my old team. Weight is also a factor, especially when you consider the pneumatics required for instant shifting. 67, I believe I read, had window motor powered instant shifting, which obviously still eat up weight.

Cons, hands down, was versatility. Especially from a coaching standpoint, it was really nice have the ability to shift to low gear for power. I can't count how many times I wished the robot had shifting, either when interacting with other robots or tackling the bridges. If the robot had shifting this year, I don't doubt they would have done much better. That's not to say shifting adds to success directly, but having the added ability is very helpful in matches.

Personally, I would like to make a push for shifting in the future. Buying shifters is out of the question; the team simply can't afford them. But if we can develop a solid, custom design that suits our resources, I'd be thrilled.

Just my take.
-Leeland

[BJC]

Shifting is an interesting solution to an interesting problem.

First, what is the problem that teams are trying to solve by shifting?

-My answer would be that, "we want to go fast and push hard." This stems from needing to go fast to get somewhere on the field and pushing hard to get through someone in your way while trying to get there.

A single speed transmission needs to maintain enough torque to not trip its breakers while pushing. Most of the time that much torque is not needed while driving around on the field. However, when it is needed it better be there or the robot's breakers will likely trip.

A two speed transmission still needs to maintain enough torque to not trip its breakers while pushing. However, using it's second (higher) gear it can drive around most of the time at a faster speed that it would not be able to safely push in.

Of course, there is more then one way to skin a cat, peel an orange, or make your drivetrain go really fast without tripping its breakers.

Another potential answer to the same problem is to add motors to the drivetrain. Their isn't any law that says a drivetrain has to be four CIMs. By increasing the overall available power one can gear the robot higher (improving its speed) while maintaining the same level of torque.

Another solution is decreasing the overall drivetrain weight. While the robot can be geared faster and not trip its breakers the overall pushing force of the robot decreases because there is less normal force on the wheels.

Now what if we used a shifter, had 6 motors, AND decreased the overall robot weight?

Just some things to think about.
Regards, Bryan

[waitwhat?]

Our team has never used shfiters in the past. That being said, we have focused a lot of our driving in the past two years on defensive strategies. Keep in mind the decision to be primarily defense was probably the first design decision that the team made. From there the desired gear reduction was picked.

We have found that if you pick the right gear ratios, you can be just as maneuverable and still pack a solid amount of pushing power.

Also, we have been playing around with the idea of "shifting" in a differant respect. Rather than shifting gears in the transmission, last year, we implemented a dynamic lift into our drive system. The robot was an 8 wheel drive, with the center four wheels dropped 1/8". This yielded a very agile drive base. But when we got into a bumper to bumper situation with another bot (which happened frequently, as we were a defensive machine) our driver could hold a thumb button, dropping the outter four wheels 1/8" giving us a true and level 8 wheel drive. Once the thumb button was released, the outter wheels would lift back to their home positions automatically.

Although this probably doesn't provide as much pushing power as shifting to a lwoer gear, it did give us better traction and would stop a spin out from happening instantaneously. The drops were actuated via two PG71's and two one inch sections of threaded rod.

Just another method of making your drivetrain more dynamic with out integrateing gear shifters.