Author Topic: Does this axle look overstressed? (Read 5020 times)

frank26080115 · « **on:** February 01, 2008, 07:30:44 PM »

Does this axle look overstressed?

http://franksjunk.thespudmall.com/axles_good_q_mark.png

It's actually going to have tank tracks, wheels are hockey pucks (the foam variety maybe, but if I use ordinary pucks I'll drill holes in them to reduce weight)
Almost all of that is 1/8" thick aluminum, the axle bolt is 1/4" diameter, and I'm using washers in hopes of lowering the stress.
Solidworks says what I have already modeled is 3 pounds. The robot will carry 2 drive servos and 2 head servos, electronics, and a 7.2V 3000mAh battery, plus all the other hardware to mount everything, just so you get an idea of mass.

It's currently just a all terrain explorer robot. (GPS + compass, range sensor, camera)

SmAsH · « **Reply #1 on:** February 01, 2008, 07:49:29 PM »

well you cant really tell because its a cad...so ya. depends really on how much the chassi weighs.

HDL_CinC_Dragon · « **Reply #2 on:** February 01, 2008, 10:26:22 PM »

Quote from: SmAsH on February 01, 2008, 07:49:29 PM

well you cant really tell because its a cad...so ya. depends really on how much the chassi weighs.

And also any payload you might be putting on it other than its own structure. What is the MAXIMUM weight you expect to use it for? Will you be hauling a gun turret? A 6 pack of root beer perhaps? Whats the weight of the battery and electronics combined?

SmAsH · « **Reply #3 on:** February 01, 2008, 10:31:04 PM »

something i forgot to add...how thick is the axle?

HDL_CinC_Dragon · « **Reply #4 on:** February 01, 2008, 10:32:06 PM »

Quote from: SmAsH on February 01, 2008, 10:31:04 PM

something i forgot to add...how thick is the axle?

Quote from: frank26080115 on February 01, 2008, 07:30:44 PM

the axle bolt is 1/4" diameter

Thats plenty strong for a bot of that size. The 1/8inch chassis would give way before the axle... depending of course on the design of the chassis and its ability to spread out its weight evenly.

Also, to reduce friction as much as possible, I suggest using sleeve bearings on the insides of your wheels(the pucks). They have incredibly low friction if you get the right ones.

SmAsH · « **Reply #5 on:** February 01, 2008, 10:35:40 PM »

lol sorry i didnt read it properly...and if the bot is thinner than the axle....lol i'd bet it would give way before that axle did anyway.

frank26080115 · « **Reply #6 on:** February 01, 2008, 10:55:59 PM »

I was going to use a brass tube, the inner diameter of a 1/8" NPT brass tube is a perfect fit (0.269" according to mcmaster)

SmAsH · « **Reply #7 on:** February 01, 2008, 11:57:47 PM »

well brass isnt that strong i dont think...or maybe it was soft copper. oh well. if you want anymore info please tell us more about the robot and maybe make a tutorial

frank26080115 · « **Reply #8 on:** February 02, 2008, 12:26:26 AM »

I have my choice of galvanized steel, black iron, or brass, brass is what people normally use for bushings, so why not brass?

Is there any place where I can get cheap ball bearings with a inner diameter of 1/4" ?

And yes, I am writing a full tutorial, it's going to be hell writing it though

SmAsH · « **Reply #9 on:** February 02, 2008, 12:40:38 AM »

ok i researched and found brass is good because it is self lubricating but is weaker than steel so you just replace the brass when the lube runs out

ed1380 · « **Reply #10 on:** February 02, 2008, 12:56:05 AM »

brass isnt self lubricatin. just many brass bushing have lubricants on them. graphite being one of them

dunk · « **Reply #11 on:** February 02, 2008, 04:06:16 AM »

Quote

Does this axle look overstressed?

if you are worried about it find a way to attach the bar at the outside end of the axles to the rest of the chassis so the axles are supported at both ends.

dunk.

SmAsH · « **Reply #12 on:** February 02, 2008, 04:09:45 AM »

Quote from: ed1380 on February 02, 2008, 12:56:05 AM

brass isnt self lubricatin. just many brass bushing have lubricants on them. graphite being one of them

ohh rite sorry.

frank26080115 · « **Reply #13 on:** February 02, 2008, 10:16:05 PM »

Good news! I found four 3/16" ball bearings and a bunch of gears I can use for the power transmission.

Bad news... they are 3/16" ball bearings, I need 1/4", so I'm sticking with the pipe for a bushing

Latest, totally accurate model
http://franksjunk.thespudmall.com/current.png

ed1380 · « **Reply #14 on:** February 02, 2008, 10:27:34 PM »

looks nice.

is that solidworks 08 btw?

frank26080115 · « **Reply #15 on:** February 02, 2008, 10:30:16 PM »

Trumpkin · « **Reply #16 on:** February 04, 2008, 12:56:52 PM »

it looks like you are using servos, if so, what kind are you using?

frank26080115 · « **Reply #17 on:** February 04, 2008, 02:28:08 PM »

hs475hb

Trumpkin · « **Reply #18 on:** February 04, 2008, 08:18:38 PM »

you might need something with more tourque

frank26080115 · « **Reply #19 on:** February 04, 2008, 09:02:19 PM »

I know I might, but the motor factor calculator says I'll be ok (calculated with manufacture specs, but I'll be running them on 7.2V so I'm sorta sure it's gonna work). These guys feel like they have the muscle too.

I know it sounds like I'm saying "Just ****ing watch me" like an arrogant ass but I already have these servos and I don't want to spend too much money so I'm recycling parts.

I might need to swiss cheese the aluminum a bit, and the side guard may be made with plastic and a 3/4" wide aluminum spine.

Backup plan is HS-645MG, $42 each, that will be 4 weeks worth of lunch money though, but better than paying for good gearmotors + speed controller + gear hubs.

tjpark1111 · « **Reply #20 on:** February 04, 2008, 10:48:03 PM »

i thought servos fry if you give them over 6 volts?

airman00 · « **Reply #21 on:** February 04, 2008, 10:54:01 PM »

Quote from: tjpark1111 on February 04, 2008, 10:48:03 PM

i thought servos fry if you give them over 6 volts?

no , just their lifetime lessens

frank26080115 · « **Reply #22 on:** February 04, 2008, 11:02:27 PM »

What's the typical lifetime anyways? HS-475HBs are not terribly expensive to replace once every 4 months.

The motor inside sure as hell would not go bad for a long while, if the circuitry dies from over-current or something, I have a little solution usually known as a L298.

I'm confident these servos would meet the bottom line.

airman00 · « **Reply #23 on:** February 05, 2008, 06:20:20 PM »

it will probably last longer than 4 months @ 7.2V , dont worry about it at all frank

frank26080115 · « **Reply #24 on:** February 05, 2008, 08:26:04 PM »

A motor RMF calculation is attached to this post, using manufacturer's specs and a estimated 5kg mass (think 2.8x digikey catalogs) with 75% efficiency

Gertlex · « **Reply #25 on:** February 07, 2008, 04:52:47 PM »

Brass is nice because the friction between two types of metal is generally less than that between two pieces of the same type of metal.

You should probably put spacers between the drive train plates (i.e. bars running in between them)

Cheap bearings can be found on ebay.

paulstreats · « **Reply #26 on:** February 07, 2008, 06:36:54 PM »

I would try and connect the outside guard rails to the main body to take some of the stress

frank26080115 · « **Reply #27 on:** February 09, 2008, 08:22:36 PM »

need a laugh?

meshes perfectly! 3d modeling paid off

Admin · « **Reply #28 on:** February 16, 2008, 05:14:33 PM »

Im a bit late to comment but . . .

The axle isn't overstressed, but the chassis where the axle is connected is definitely overstressed.

Just take dunks/paulstreats advice to solve it.

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Author Topic: Does this axle look overstressed? (Read 5020 times)

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