pic: 2011 "Nova" Swerve Drive (Front)

[Jedward45]

Currently the gears are baseline steel.

As you probably know, selecting the Bevel Gears was by far the trickiest part of this design. If the mounting distances are too large, the wheel separation makes rotation of the module impossible. On that end, if you happen to know where I might find a hardened, spiral bevel gear set that fits near the specifications for under $120, I would greatly appreciate it....

Also, how exactly do you calculate gear strength anyways?

[Tristan Lall]

Originally Posted by Jedward45

As you probably know, selecting the Bevel Gears was by far the trickiest part of this design. If the mounting distances are too large, the wheel separation makes rotation of the module impossible. On that end, if you happen to know where I might find a hardened, spiral bevel gear set that fits near the specifications for under $120, I would greatly appreciate it....

My first instinct would be to check QTC (part of the Designatronics group familiar to many via their SDP/SI division). Although I realize it's not the ratio you were depicting above, you should probably look at this page, specifically SBS1-4020R and SBS1-2040L to get an idea of what's available, and what it might cost you. Those teeth are induction hardened to Rockwell C 48, which in steel, correlates well with a tensile strength of about 1 632 MPa (237 000 lb/in², or really strong). The teeth are 1.0 module (or 25.4 diametral pitch), which I'd say is about as small as you'd want to go for a final stage of a drivetrain—and even then, only with a hardened gear.

And by the way, if you need more clearance for the pinion (too wide), you might consider dishing out the wheels facing the centre of the gearbox (so that the pinion and gear sit in a depression in the wheel, sort of like a typical west-coast style wheel with the spokes on one face).

Originally Posted by Jedward45

Also, how exactly do you calculate gear strength anyways?

Typically, in FRC applications, gears fail because the teeth are overstressed. (Other failure modes are possible.) The important characteristic is therefore the bending strength of the teeth.

There are a few ways of evaluating this, with varying levels of complexity and accuracy. The Lewis-Barth method is traditional and conservative, and requires relatively few parameters. AGMA has another method that takes fatigue and contact stress (another failure mode) into account, as well as a whole slew of other design factors—but you probably won't know what values to assign to them without some sort of basis for comparison. Given that bevel gears are a bit of an unusual system for an FRC application, I'd avoid relying on rules of thumb alone (so don't just take my word for it).

For an introductory reference, see this. Check out the literature provided by the manufacturer. Also, there's this: a whole book about gears, with a good explanation of the Lewis method. For more about the AGMA method, I recall that any recent edition of Shigley's Mechanical Engineering Design should include a chapter (written for upper-year mechanical engineering students). You might find this a convenient resource for equations and examples. Definitely search the Chief Delphi forums for other resources.

[Hawiian Cadder]

I suggest looking into worm gears, they can get you a lot more reduction in that space, and I bet it will be easier to find a set that small. with a worm gear you may be able to remove the cimplebox entirely.

[Tristan Lall]

Originally Posted by Hawiian Cadder

I suggest looking into worm gears, they can get you a lot more reduction in that space, and I bet it will be easier to find a set that small. with a worm gear you may be able to remove the cimplebox entirely.

I'm not generally a fan of worm gears for drivetrains (see here, at page 30 for a discussion of low efficiency, especially as the reduction increases). However, seeing as you're only replacing a CIMple Box (4.67:1), and maybe part of the 4:1 bevel gear set (since I found a 2:1 set that might work for you), your total reduction is only around 10:1. With a well-designed structure and high-quality worm gear, you can probably achieve 90% efficiency on that reduction, which is quite competitive with spur gears. You'll probably need a stiffer structure to make it happen, though (alignment is critical with worm gears).

[Jedward45]

Originally Posted by Tristan Lall

I'm not generally a fan of worm gears for drivetrains (see here, at page 30 for a discussion of low efficiency, especially as the reduction increases). However, seeing as you're only replacing a CIMple Box (4.67:1), and maybe part of the 4:1 bevel gear set (since I found a 2:1 set that might work for you), your total reduction is only around 10:1. With a well-designed structure and high-quality worm gear, you can probably achieve 90% efficiency on that reduction, which is quite competitive with spur gears. You'll probably need a stiffer structure to make it happen, though (alignment is critical with worm gears).

Out of curiosity, with those 2:1 Bevels, the 5:1 Cimple box, and 3" wheels, what would a good FPS estimate be?

[Hawiian Cadder]

Originally Posted by Jedward45

Out of curiosity, with those 2:1 Bevels, the 5:1 Cimple box, and 3" wheels, what would a good FPS estimate be?

According to the JVN mechanical design calculator;

7 feet per second after frictional losses.
196 LBS of pushing force with rough-top tread.

[Jedward45]

Originally Posted by Hawiian Cadder

According to the JVN mechanical design calculator;

7 feet per second after frictional losses.
196 LBS of pushing force with rough-top tread.

Is it possible to reduce the CIMple box further, I was hoping to get right around 10 FPS

[lemiant]

Originally Posted by Jedward45

Is it possible to reduce the CIMple box further, I was hoping to get right around 10 FPS

You mean reduce it less. And I would just decrease the reduction on the bevel gears.

[Chris is me]

With all due respect, if you can't figure out gear ratios, you really should not be designing a swerve drive.

[Jedward45]

Originally Posted by Chris is me

With all due respect, if you can't figure out gear ratios, you really should not be designing a swerve drive.

With all due respect, I didn't post myself on this website, I posted my design. If you have a comment or criticism about that, I'd be happy to hear it. But if you only want to discourage me from experimenting and learning, I'd prefer if you kept that to yourself.

[Aren_Hill]

From looking at it, you have a fairly robust looking module with most of the issues attended too, aside from that bevel gear looking tiny.
What i'd be concerned with in this module is the amount of power necessary to steer those wheels (depends on the number of modules and what motors are steering).

Also depending on your steering arrangement some method of angle adjustment between the steering sprocket and the module may be necessary.

Do you have the machining capabilities to make that one piece bracket and securely mount it to the vertical support?

It also looks rather huge, but i like small compact packages if you look at our recent swerves.

Chris's statement has some merit, and I've learned to like when people are straight up blunt with me, as it makes things much easier to gauge and get a realistic evaluation of your current state.

I would suggest listening to his advice and getting a firm grasp on gear ratios and the efficiency of various methods of power transmission, this will only help your design. Exercises like this are a chance to develop further, and as has been pointed out you could use some work in the gearing arena.

Don't immediately think someone is attacking you when in reality they're just trying to bump you back to the path you've strayed from. He gains nothing from tearing you down.

[Jedward45]

Originally Posted by Aren_Hill

From looking at it, you have a fairly robust looking module with most of the issues attended too, aside from that bevel gear looking tiny.
What i'd be concerned with in this module is the amount of power necessary to steer those wheels (depends on the number of modules and what motors are steering).

Also depending on your steering arrangement some method of angle adjustment between the steering sprocket and the module may be necessary.

Do you have the machining capabilities to make that one piece bracket and securely mount it to the vertical support?

It also looks rather huge, but i like small compact packages if you look at our recent swerves.

Chris's statement has some merit, and I've learned to like when people are straight up blunt with me, as it makes things much easier to gauge and get a realistic evaluation of your current state.

I would suggest listening to his advice and getting a firm grasp on gear ratios and the efficiency of various methods of power transmission, this will only help your design. Exercises like this are a chance to develop further, and as has been pointed out you could use some work in the gearing arena.

Don't immediately think someone is attacking you when in reality they're just trying to bump you back to the path you've strayed from. He gains nothing from tearing you down.

Thanks for the advice. As far as my previous comment is concerned, I understand the need for blunt criticism. I understand that Chris was trying to explain I need to better understand gear ratios, and I agree with him. However, I'm slightly taken back that anyone in FIRST would discourage someone from trying something new, especially when those new and difficult things are a source of the knowledge so vital to their own creation. Moreover, the criticism wasn't followed by an attempt to teach me the skills I need, or even show me where I could find the materials to teach myself.

As far as the module is concerned:
-The module is designed to be tied (via 25 chain) to the other three modules. (All I know is a banebots motor was used to rotate, but I wasn't in charge of that system, so I don't know how much power was needed. We also used 1" wide wheels, and rotating wasn't a problem)
-following some earlier advice, I'm working on interfacing a more robust and larger set of bevel gears.
-We do have access to the machining necessary to create the lower chassis.
-The size may be deceiving, but the wheels are only 3" in Diameter and 1/2" wide. I don't know if that still constitutes a "large" design, but if it does, I'd sure like to see Winnovation's swerve design

Thanks!