Need general drive train expertise
 

Hey everyone, I'm a rookie member of a team doing research.

On this product's page (http://www.wcproducts.net/wcp-shifter-3cim/), what does it mean by motor pinion select? I see the 11T, etc. down below in the gear ratio charts, but what's the difference between 11Tx2 and 11Tx3?

Also, more general questions:
How do different shifters work?
How does the pneumatics work with the gearbox?
What is spread ratio? Each different spread ratio looks like it has similar overlaps in FPS.

Thanks!

Re: Need general drive train expertise
 

The motor Pinion is the gear on the motor, so you're able to choose from 11, 12,13,14 tooth gears, the x2 and x3 are the quantities as you can get the gearbox in 2 CIM and 3 CIM form.

My team only has experience with the Dog shifters so I can't really comment on Dog vs Ball.

The Spread Ratio is the ratio between your high gear and low gear, the larger this number the further apart your high and low gear are from each other. Which to use really comes down to your teams preference.

Re: Need general drive train expertise
 

Shifters work because the pneumatic cylinder moves the powered gear between two diffrent gears that are both connected to the out put shaft. The gears on the out put shaft have diffrent sizes causing a changing in speed and torque.

Re: Need general drive train expertise
 

Two aspects of the same question. One has to do with the actuation, one with the actual shifting.

Dog shifters have small "dog" that is designed to engage either of two gears that it is located between. Moving the dog out engages the outer gear, moving it in engages the inner gear. The gears have some form of machining that the dog can engage. The upside of this is that anybody can do it with fairly basic machining resources (a mill). The downside is that it can take a split-second longer than a ball shifter to shift, and sometimes a lot of force.

Ball shifters work like ball-lock pins: small balls will pop out into small holes in the right gear when a shaft is moved to the right spot. They're a little newer to widespread FRC use than dog shifters.

Now, on to the pneumatics. Or, should you choose to be so brave, the servo. You'll notice that both types of shifter have something moving in and out, often inside an axle. A short-throw pneumatic cylinder is connected directly to that pin/shaft/whatever you want to call it, and then powered. Move the piston one way, you get low gear. Move it the other way for high. You can use a servo, but it takes longer and you'll want more than one servo per gearbox.


You'll notice that I make no recommendation as to type of gearbox. That's deliberate. First, I have minimal experience with ball shifters; second, some things are a personal preference. Which is actually better is, therefore, left as an exercise for each reader. (Though I do recommend pneumatics, specifically pancake cylinders, over servos for shifting.)

Re: Need general drive train expertise
 

This will give you a general idea of how a ball lock transmission works. This pdf from VEXpro might help as well. In the case of a ball lock transmission, the air cylinder pushes and pulls the shifting plunger (the rod with the protrusion that moves the balls in and out of the gear grooves). In the case of a dog shifter, the air cylinder pushes and pulls the dog gear to mesh with a gear of a different size that has a cutout of the same pattern as the dog. This video might give you a better idea of how dog shifting works.

Re: Need general drive train expertise
 

I do agree with this. They are faster than servos at shifting. My team tried servos in 2012 and they took way too long to shift, I was not on the team at the time but I have heard that it took about two seconds.

Re: Need general drive train expertise
 

I have heard that you are unable to shift on the fly because of that. Is that true?

Edit: Never mind. The Sonic Shifter video I posted higher in this thread shows that shifting on the fly with a servo is possible.
Edit: Never mind again

Re: Need general drive train expertise
 

Not quite. That video shows a free spinning output. Once the transmission is put under the load of a moving robot it will require a bit more force to shift, making servo shifting no longer shift on the fly.

Re: Need general drive train expertise
 

I'd imagine that when the gearbox is under-load it may be a different story. A free spinning wheel puts much less load on the dog-gear than a wheel tasked with accelerating a 120lb robot.

ANNND Andrew beat me to it

Re: Need general drive train expertise
 

Andrew is competely correct. It takes more time. I personally would never recomened servos unless it was impossible to have pneumatics.

Re: Need general drive train expertise
 

I have never met a single person who believed that shifting with servos was a good idea.

Re: Need general drive train expertise
 

Doh. That makes sense.

Re: Need general drive train expertise
 

As far as custom gearboxes go, I like ball shifters better due to their decreased footprint (as there is no dog in between the gears) and their slightly decreased weight. See 192's 2014 gearbox for inspiration.
For COTS options, I believe the WCP 2-cim shifter is a good option due to small size and weight. Their 3-cim shifter is also quite good. I somewhat dislike the Vex options, because the 3rd stage makes the gearbox much larger and it takes up a lot of space to begin with. It's heavier as well.
On the other hand, the Vex version is enclosed, making it lower maintenance. And the space/weight cost isn't all that great.

I didn't see this answered above so:
Spread ratio is essentially the ratio between the high speed and low speed. So a gearbox made for 18fps and 6fps would have a spread of 18/6 = 3. A spread ratio of 3.

Re: Need general drive train expertise
 

The charts for different ratio spreads on the vex website show loaded output FPS. What weight load is vex using? And does the load make a big difference if vex tested with a 100lb load and our robot is roughly 140lbs with bumpers and a battery? Would the weight difference change the speeds?

Re: Need general drive train expertise
 

Weight does have an effect on robot speed. The heavier it is, the slower it goes. You can use the WCP drivetrain calculator to experiment with different robot weights and speeds.

Re: Need general drive train expertise
 

Can you clarify a little bit more as to what you mean by this?

Re: Need general drive train expertise
 

A heavier robot would certainly incur more frictional losses in the drive, and thus would have a lower top speed. Can't say how big the effect is without any data, though.

Re: Need general drive train expertise
 

I'd qualify that by saying that there are plenty of commonly-available servos that can do the job, but they haven't historically been FRC-legal. Digital servos for giant-scale model aircraft would be great, for example. Saving the weight and annoyance of a pneumatic system could make it very worthwhile.

(I've tried shifting a DeWalt-based transmission with FRC-legal servos before. It was terrible.)

Re: Need general drive train expertise
 

Oh, certainly, I've worked with some beefy servos in other applications. I was only talking about FRC-legal hardware.

Unfortunately, FRC-legal servos are so weak that I've yet to actually find an application for them. I'm sure someone, somewhere has used them effectively on their robot, but I haven't.

Re: Need general drive train expertise
 

I know 67 has used a window motor configuration in the past because they did not have pneumatics. Using an opposing cam, it pushes or pulls into high or low gear.

Here is the link to their engineering notebook, I recommend any reads all of them anyways:)

-Ronnie

Re: Need general drive train expertise
 

After writing a front-end to Ether's sims and playing with it a bit, very large swings in weight will have an effect on acceleration from a theory perspective. Torque remains the same, but the mass is larger, thus acceleration is lower. For the same amount of time, it nets a shorter distance traveled. Small changes (5lbs here, 5lbs there) seem to have no discernible difference in acceleration on a FRC field since it's less than an inch or two of difference.

Preview here. Requires Java 8, Win7/CentOS 6+/Who knows what for a Mac. Probably works in Ubuntu 12.04+, but I can't tell what version of GTK it ships with at the moment. The preview is just the latest iteration of layout ideas I'm playing with. For example, I don't think it'll have a web viewer upon release, which is what the current preview has.

Re: Need general drive train expertise
 

Changing the weight will likely affect the rolling resistance of the bot, especially on carpet.

If you change the rolling resistance parameter(s) in the model accordingly, it will affect top speed as well.

Re: Need general drive train expertise
 

When I used to model in Excel, the resistance was modeled something like (1-0.995^n)*<weight> where n = # of loaded bearings. For most robots near the weight limit this falls between 5-6 lbs of resistance since only 4 wheels touch at a time.

This was really so I had some automated way to increase the resistance based upon weight and number of wheels hitting the floor rather than an accurate way to model it. It also provided a correlative model for what happened on our drive trains in 2008 & 2011, where the bearings had some binding due to improper mounting (thus the 0.995 was lower).

Re: Need general drive train expertise
 

FRC0330, back in '05 and I think in '00.

Both were used as latches, and as I recall the one from '00 had some assistance from a spring. (Used to lock a winch with a pin--might have been replaced by a short-throw cylinder.) The one in '05 was rigged to drop a thin sheet of Lexan. Why a thin sheet of Lexan, you ask? It's really nice to be able to deny someone who is trying to jam you against a goal half of their traction. There was no retract mechanism, though.

Re: Need general drive train expertise
 

We used them to make our scale model of Dozer look left or right with his eyes when steering, other than that I've got nothing.

Re: Need general drive train expertise
 

Servos controlled the pan/tilt of our camera in 2007. Tracking the green light over the rack was vital to the robot's success in scoring during autonomous.

2005's camera also had a pan servo, but a combination of factors led to our abandoning vision tracking that year.

There was a servo-controlled release on our 2004 fabric ball-collecting funnel, but we decided to leave the funnel off after our first couple of practice matches. We wanted to focus on knocking the 10-point yellow ball off its pedestal to drop the 5-point balls early.