pic: 2011 "Nova" Swerve Drive (Front)

[Jedward45]

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[AdamHeard]

I'd be very worried about those bevel gears, what DP are they?

Also, the steering sprocket setup, is that one solid piece?

[Jedward45]

Originally Posted by AdamHeard

I'd be very worried about those bevel gears, what DP are they?

Also, the steering sprocket setup, is that one solid piece?

The DP is 32. They are a Boston Gear set. The Mounting Distances are up to spec with Boston Gear's technical drawings. Also, the hub of the Gear inserts into the wheel assembly.

The steering sprockets sit on a 1" OD Aluminum tube (3/4" ID). This tube is machined from a 2" diameter Aluminum rod because at the base of the 1" tube, a 2" mounting plate attaches to the rest of the chassis (Dark grey).

Hope that clears things up

[=Martin=Taylor=]

Originally Posted by Jedward45

The DP is 32. They are a Boston Gear set. The Mounting Distances are up to spec with Boston Gear's technical drawings. Also, the hub of the Gear inserts into the wheel assembly.

The steering sprockets sit on a 1" OD Aluminum tube (3/4" ID). This tube is machined from a 2" diameter Aluminum rod because at the base of the 1" tube, a 2" mounting plate attaches to the rest of the chassis (Dark grey).

Hope that clears things up

Wow. I once spent a long time designing a drive just like this. I have to say, this is a very good design!

I gave up on my design because I was worried about turning the modules. The very closest I was able to move the wheels was 4" wide. The effectiveness of a swerve is limited by how fast the modules can be rotated, and with only the window motors for rotation I figured this might be difficult with such wide wheels.

My next and final swerve design was a 45* module like in the 1625 teaser. In the end, this design wound up being more complicated to make then a conventional module and was only slightly smaller so I abandoned this too.

Its cool seeing all the new swerve designs on CD. I wonder how many teams will use swerve next year...

[Peyton Yeung]

Originally Posted by =Martin=Taylor=

Its cool seeing all the new swerve designs on CD. I wonder how many teams will use swerve next year...

I don't know if we will use it next year but team 45 has started thinking about adding swerve to the list of drive trains we can construct.

4wd
6wd
tread
ball drive
swerve?

[Tristan Lall]

Originally Posted by Jedward45

The DP is 32. They are a Boston Gear set. The Mounting Distances are up to spec with Boston Gear's technical drawings. Also, the hub of the Gear inserts into the wheel assembly.

Are they hardened? And is the gear's thickness drawn to scale? They seem rather undersized for a final drivetrain stage. I wouldn't proceed with those particular gears unless you've calculated the gear strength to verify the design—they're just too unlikely to be adequate.

Also, you didn't mean to include a differential, right? (I seem to recall a team having a design similar to this, but with differentials on all wheel pairs. Might have been 1140 around 2005.)

[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.