pic: Team 3647: Gearbox
 

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That's a beauty, but it's an absolute beast. You certainly won't have to worry about breaking it. If you want to cut some weight, you could use much smaller gears and axles, especially for the first 2 stages.

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The first stage is actually a plastic [Delrin] gear from WCP. Also all the other gears are aluminium so they don't weigh that much to begin with, when I get my hands on some 7072 3/8 Alum Hex we will be using that as well in place of the 1240 steel shafts currently in the box.

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Looks great What's the overall weight?

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With the RS 540 mounted on this, as shown in the image, it should be able to produce the following, assuming ~ 85% efficiency:
Stall torque = 1200 oz/in. = 6.3 ft/lb
Free Speed RPM = 466.

If an RS 550 were used, those numbers would be:
11.34 ft/lb and 536, respectively.

How many uses could I find for a little beasty like this? Let me count the ways.......

Re: pic: Team 3647: Gearbox
 

We are actually planning to switch to a 550, the 540 was just the only motor I could get to test this.

This box is running our tower pulleys, there is an machined pulley that fits on the shaft and is held with a set screw, hence the flat on the shaft.

According to sw it weighs around 1.8lbs, I haven't actually put it on a scale yet.

Re: pic: Team 3647: Gearbox
 

Pfft. There's more weight to be removed :p Not bad - was that including the motor?

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Yes it was including the motor.
Yeah there is definitely some room for weight reductions. Also probably going see about using the plastic FP gears.

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My sense of scale is way off, I thought that was a 700 series motor. What size are the shafts?

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Given the reduction of your gearbox, and how much torque you have at the shaft, I would extremely worried about using a set-screw to transmit power to the pulley. I wouldn't expect it to take more than a few matches for that set screw to start slipping (because of your high reduction I'm assuming this gearbox is being used in a high load application).

The best way to transmit power to and from a shaft is to use hex shaft and hex broached components. I can't see into your gearbox too well, but given that those are WCP gears I'm assuming that your output shaft is hex that's been lathed down to 3/8. I would suggest that you redesign your plate to accommodate a 1.125 OD bearing for your output shaft and use an AndyMark 3/8hex oversized bearing. Or if you don't want to redo your plate and still use that 7/8 OD bearing then I would suggest you mill a keyway into your shaft and broach a keyway into your sprocket, it wouldn't be a good as using hex shaft but it would be much stronger than it currently is.

Edit: I just noticed that you were using WCP's new 24DP delrin gear, where'd you get your 24DP pinion? The only place iv'e found to get a 24DP pinion that isn't brass in to buy SDP-SI carbon steel pinion wire, is that what yours is?

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From their facebook page: It is a pinion from the BB CIM-u-lator. (I asked last night)

http://banebots.com/p/S24P-GMC1-32

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True, set screws should be avoided normally in high load applications, but I wouldn't exactly consider belt pulleys in a tower to be an extremely high load application.

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I just looked at their robot and you're right, it's not a high load application, but it's still more than I would trust a set screw for.

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It's a 500 motor and all the shafts are 3/8 Hex with the ends lathed down to 3/8 round.

The reduction was mainly for low speeds not torque. This runs the pulleys on the tower which is not a high load application, we actually used set screws on the competition robot this season and never had issues. With a flat in the shaft and some lock loctite on the screw it's fine for lighter loads.

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Sorry about the double post but I mis read this before, the weight of the box according to sw is actually 1.08 lbs not 1.8. :D

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Is there a reason you prefer the 1.125 OD bearing over this one: http://www.andymark.com/product-p/am-0439.htm ? I doubt they are going to be experiencing 40 lbs radial on that bearing.

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From AndyMark: "This is a light weight bearing, as it cannot handle over a 40 pound radial dynamic load "

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Without knowing exactly how that shaft is loaded I wouldn't suggest using the 7/8 OD bearing. If they knew that the shaft was well bellow 40lbs load they could use the 7/8 OD bearing, but it's not difficult to design around a 1.125 OD bearing and bearings are not somewhere to cut corners on, I would never use that 7/8 OD hex bearing on a cantilevered output shaft.

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think about the power out put if it were a cim in there

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5310 rpm (1/36) = about 147.5 rpm output (not including efficiency, etc).

No need to go that fast for most applications.

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We fell in love with 3/8" hex last year, well, I did anyways. What is the purpose of lathing the end to 3/8" round?

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Nice looking gearbox!

I have suggestion: the two 15:45 reduction clusters are not, strictly speaking, good design practice. Any one 1 tooth on the 15 tooth gear will contact the same 3 teeth on the 45 tooth gear. This isn't good for gear life and long-term efficiency. Using a 44 or 46 tooth gear instead of a 45 would result in more even gear wear.

Of course this probably isn't a big deal for FRC given the short lifespan of most parts and your intended low-load application.

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Sisk- not to speak for the OP...

...but in the past, we have turned 3/8 hex down to round to utilize regular round bearings. Obviously we prefer to use hex for all power transmission where we can, but it has happened in the past.

One specific example was in our lift for 2010. The lift gearbox utilized a CIM through a series of reductions to spin a pulley to wrap a line which pulled us up. The radial load was most certainly above 40 lbs, which was not a known issue at the time for the AM hex bearings. We destroyed a set of them at one of our regionals, and before the next competition we changed the hex bearings to regular round bearings, subsequently requiring us to turn down the end of the shaft and mill a keyway.

-Brando

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Another small benefit for turning the ends of a hex shaft down to round is that, when you sandwich such a shaft between two plates, you don't need any other hardware to retain the shaft between the plates. A straight hex shaft would need something like snap rings on the outside to be held in place, while a hex shaft with rounded ends would hold itself between the bearings just fine. A minor advantage, but useful nonetheless.

Re: pic: Team 3647: Gearbox
 

Here's a bit of insight of what I went though designing this:

For the stages I turned the shaft down to 3/8 round at the ends so when we used a round bearing they shaft would be sandwiched as stated above and stay in place, which worked out nicely since I didn't need to put in any eclips or collars. Also the 7/8 hex bearing has a very poor loading performance for this gearbox it would have been fine but in general getting good cheap 3/8 round bearings are easier.

The output shaft was also turned down to 3/8 round so I can interface it with our pully https://sphotos-a.xx.fbcdn.net/hphot...84491030_n.jpg.

We don't have hex broaches [wish we did] so just using an set screw on a flat on the shaft was fast and easy. Also since this is mostly a low load application the set screw will be fine.

For our shooter we turned down a 3/8 hex shaft and milled in a key way so we could interface with the 3/8 AM Wheel hub.

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It sounds like you have access to a decent assortment of machine shop tools. If you have access to a surface grinder, a lathe, a Dremel, and an old worm out HSS end mill you can make your own hex broach. Here's how you can make a 1/2" hex broach:

1. Acquire a few inches of 1" or larger hex stock. Drill and ream a 5/8" hole through it, and add a set screw hole.
2. Take an old 5/8" HSS end mill. Cut off the flutes. Mount the shank into the hex tool holder above with about 1" sticking out.
3. Clamp down the hex stock onto the table of the surface grinder.
4. Take a light pass on the surface grinder.
5. Rotate the hex stock to the next face.
6. Repeat steps 3-5 until you have ground the end mill shank into a 1/2" hex profile.
7. Mount the end mill shank into a lathe with the hex profile facing outwards.
8. Put a cutoff tool in your Dremel tool. Turn on the lathe and Dremel, and hold the Dremel tool perpendicular to the lathe. The objective here is to cut a concave dimple with sharp edges into the hex profile.
9. Congrats, you now have a cheap hex broach that will work in soft materials like aluminum or plastic.

You can substitute in different sizes above to make 3/8", 7/16" or other size broaches.

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Wow I never thought you could do that. Might give it a try :cool:
Instead of using a 5/8 endmill would just a stock of Tool Steel work?
Is just getting a good hex on the tool just a lot of time and patience on the grinder with careful measuring?

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Tool still would work just fine. When I did this a few years ago, I used an old end mill because it already had a flat spot to set screw against and because there was a box of end mills with chipped/broken flutes available. :rolleyes:

Grinding out a good hex profile was fairly fast to get it close, then slow and careful to get it right. If you have an indexing head, then that would be even more accurate than using a piece of hex stock as a tool holder for grinding the hex profile into the end mill/tool steel. The surface grinder I had access to did not have the standard machine slots to mount an indexing head to, but rather had a smooth surface magnetic chuck, which is why I just used a piece of 1" or 1.5" steel hex stock as the tool holder.

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Arts method can also work on softer steels, I made one and broached 8 steel sprockets from SDP with no visible signs of wear.
I was also lucky enough to have an indexer I just stock on the magnetic chuck.