3/8 shaft, is it strong enough.
 

Hi guys, klunk and I are working on a project which appeared on delphi a couple of weeks ago (http://www.chiefdelphi.com/forums/sh...php?p=1072208). I suggested using a 3/8" bolt for the main shaft, but after seeing it in the CAD, I'm not sure that it is strong enough.

Functional View (components in this one are not to scale): http://www.youtube.com/watch?v=U5tHtntT9gU

Exploded View (components in this one are to scale): http://www.youtube.com/watch?v=f9mpx6MWe-k).

In the configuration we have, how much force could be required to pull down the see-saw, before something bad happens to the 3/8" shaft?

Re: 3/8 shaft, is it strong enough.
 

You have a CAD of the file. Most CAD programs have a stress analysis component to them. In this case, you'd estimate the maximum force exerted by the pounding part when it jerks upwards and figure out the torque exerted by your motor. Fix the shaft in the appropriate places and run the program.

Oh, and make sure your material properties are set properly.

That should tell you how much stress the shaft takes. If it's more than the breaking stress of the material, you have a problem. If it's less than 1/2 the breaking stress, you should be good. (Note, in this case you probably want shear stress instead of normal stress.)

Off the top of my head, it *should* be OK, depending on the forces... but there are a number of teams that use axles somewhere in the neighborhood of 3/8" for drive, though admittedly

Re: 3/8 shaft, is it strong enough.
 

This could be done with a 1/4" axle at the center pivot, and a 1/2" axle in the same spot could fail. It all depends.

A big factor is the unsupported length. If you keep the length between supports to a minimum, say 1.5", and that weight on the end is under 20 pounds, you'll be fine with 3/8". That is, the axle can be as long as you need, but the 'business part' should be as short as you can make it without the long arm being too weak. PM me if that's not clear, i'm having a hard time tonight...

Re: 3/8 shaft, is it strong enough.
 

Prior to going to half inch hex shafts for everything we almost exclusively made our wheel axels out of 3/8" Grade 8 hex cap screws we still use them quit a bit when they are correct for the application.

As said above the material and the loading are important in determining if this will work which is why we used hardened bolts.

Pete

Re: 3/8 shaft, is it strong enough.
 

The bending load on the shaft is going to depend on how far apart the load is, from the support bearing. If the two are close together, then you don't need a very large shaft. But if they are far apart (because you're using thick arm materials such as wood, instead of steel, for example) then you might need a larger diameter shaft.

There are no dimensions or mention of the weight or anything, so it's pretty hard for us to be able to evaluate the design....

Re: 3/8 shaft, is it strong enough.
 

Sorry about the lack of details:
The load is cantilevered 5.5" from the last support bearing. We don't have any set weight (we can simply put counterweight in the design), I'm just wondering how much we can safely pull.

Solidworks thinks we can withstand 25lbs with a 2:1 safety margin, but that's hard to believe.

Re: 3/8 shaft, is it strong enough.
 

On one hand, don't rely on intuition to tell if something is good or not; engineers use math to prove it.

On the other hand, work hard to never lose that skepticism; always question the output of a software program, especially if you are not completely knowledgeable about how exactly it comes up with those numbers.
---------

OK, to help you get a better feel for this, try an experiment:
Take a 3/8" steel bolt, clamp it tightly (in a vise perhaps), and whack it with a 12 pound sledgehammer, dropped from about a foot (25 pounds of force, +/- 100%).
Did it bend?

Re: 3/8 shaft, is it strong enough.
 

Hi Guys,

Thanks for the additional support.

I've attached a very rough sketch up of the object to give you further clarity on the scale and materials I'm planning on using.

To paraphrase, the mallet should have some force but nothing too drastic. It's for a visual arts project. It is temporal construct and will most likely never be used again. It's sole purpose is to pound on the block of soil till the block breaks down. The block (made of topsoil) will be formed/petrified with just a mixture of pva and water, so it should be fragile enough to be destroyed by this mallet. What I would like to see and hear is the mallet making a reasonable pound upon contact with the block. That's it.

Thanks again. This place is great.

Re: 3/8 shaft, is it strong enough.
 

here's the attachment...

Re: 3/8 shaft, is it strong enough.
 

Hi again,

I found a Toshiba motor/gearhead the other day - http://www.youtube.com/watch?v=Txu9zKdbOmo and was wondering if anybody is familiar with them.

The details on the labels on both the gearhead and the induction motor are entirely erased...apart from the Toshiba logo and the names of the components. There is only some visible writing next to the gear ratio on the gearhead. It says 1:8 with the space immediately after the 8 scratched off.

Haven't the faintest on how to measure the speed/rpm/torque etc. Had a look at this http://www.electricneutron.com/elect...motor-capacity as an aid and realised it would take me two years to work it out.

Any body familiar with these motors that can tell me exactly what it's capable of just by looking at it? Hopefully I can use it on the above mentioned machine.

Thanks in advance,
K

Re: 3/8 shaft, is it strong enough.
 

its dimensions are almost exactly like this one.

Re: 3/8 shaft, is it strong enough.
 

There are commercially available tachometers for the speed. Typically, you find those with your local R/C pilots or in a hobby store; in this case, you'd probably need to either simulate a second side of the prop or correct the reading for only reading one side.

Re: 3/8 shaft, is it strong enough.
 

The easiest way to measure RPM is to put the video in Windows Media player and go frame by frame. Check how many frames are in a second, then see how many revolutions it makes in a second. Multiply by 60 and you have RPM.

It's a pretty beefy looking motor. I'd say give it a shot. :)

Re: 3/8 shaft, is it strong enough.
 

The easiest way to make sure your design will work as intended, is to test it in the same conditions it will experience whenever possible. If I were you guys, I would overdesign this thing because you are really not heavily constrained (at least not in that aspect of the design).

A 5.5" cantilever is definitely not something to overlook, so I'm glad you are reaching out for assistance.

If you can take a screenshot of your Solidworks Simulation model, and list the relative information (material, load, etc.) I could get a pretty good idea of how much you can "trust" it.

-Brando