FEA on Gearbox Plates

[SerpentEagle]

Might be a dumb question but what forces should be expected on the plates in a simple wcd gearbox while running under stress?

My goal here is to be able to run accurate FEA tests on parts that I design, and be able to adjust the design accordingly. Thanks!

[asid61]

Assuming that your gearbox is direct driving the center wheel, here's a list of forces:
1. upward force from the effect of gravity on the wheel shaft
2. forces between gearsets (look up calculating the force of a spur gear)
-These translate into forces on the bearings, and thus the holes in the gearbox plate.
3. forces on the sprockets going to the front and back wheels (although this might cancel out, I'm not sure).

There might a couple more but those seem to be the major ones.

[SoftwareBug2.0]

Originally Posted by asid61

Assuming that your gearbox is direct driving the center wheel, here's a list of forces:
1. upward force from the effect of gravity on the wheel shaft
2. forces between gearsets (look up calculating the force of a spur gear)
-These translate into forces on the bearings, and thus the holes in the gearbox plate.
3. forces on the sprockets going to the front and back wheels (although this might cancel out, I'm not sure).

There might a couple more but those seem to be the major ones.

I can think of a couple more things missing, I'm not sure that they should be called minor:
1) Mounting of the gearbox
2) Force from the wheel in directions other than vertical.

[pmangels17]

These seems to be the main forces in my mind too. Others that you might consider are the weight of the motors, which are presumably cantilevered off of one of the plates, though this is probably negligible. What FEM software are you using? Be careful that you are getting a good analysis and choosing the proper type of element when creating your mesh. Also, remember that FEM models are almost always overly stiff, so your analysis might not yield an exact result.

[Bryce2471]

Originally Posted by asid61

Assuming that your gearbox is direct driving the center wheel, here's a list of forces:
1. upward force from the effect of gravity on the wheel shaft
2. forces between gearsets (look up calculating the force of a spur gear)
-These translate into forces on the bearings, and thus the holes in the gearbox plate.
3. forces on the sprockets going to the front and back wheels (although this might cancel out, I'm not sure).

There might a couple more but those seem to be the major ones.

Having done a minimal amount of FEA on gearbox plates for comparative purposes, this is a good starting point. A couple forces that I would add are the cantilevered weight of the CIM motors, and the clamping force caused by the bolts through their standoffs.

An important thing to remember when going down the path of FEA is that it can only help you prepare for forces in the mechanism that you can accurately predict.
For something like a gearbox, it is impossible to predict all of the forces that it will undergo, so it is not a good idea to make your design purely off of the FEA results, and it is a good idea to keep a safety factor of 5 or greater.

With that said, I think FEA is an incredibly valuable tool, because it can teach students about the way stress and strain work in materials without having to build and break mechanisms. I plan to hold a CAD class for our team this year that will include some FEA training for this reason.

[asid61]

Originally Posted by SoftwareBug2.0

I can think of a couple more things missing, I'm not sure that they should be called minor:
1) Mounting of the gearbox
2) Force from the wheel in directions other than vertical.

I agree with force from the wheel, that's definitely a thing.
Mounting the gearbox I'm not sure I would count as a major force as that's on the face of the plate, but it can't hurt to add it.
Look up the "split line" tool to ensure that the forces from the bolt are applied only in a small spot.

[Cory]

I think it's great for people to learn FEA, but you'll likely run into a number of issues:

1) garbage in, garbage out. If you've never done this before, your input parameters will likely be wrong. You will be fed a result that shows you some pretty colors and you will be tempted to believe the answers are true. This is even easier to fall victim to if you don't have intuitive experience/a way to do some hand calcs to verify you're in the realm of possibility. It's best to learn on simpler bodies with simple force setups so that you can verify what you're seeing is what you should be getting, via a hand calc.

2) You can't accurately model the effects of gameplay, because we can't quantify all the forces the robot sees. As a result you'll need a huge factor of safety, which ties into...

3) It's really just not worth it from a time and effort standpoint to do FEA on something like a gearbox plate during build. Just build it beefier. Some things might be worth it, like sizing a shaft or other simple bodies with limited forces being directly applied to them, but a gearbox plate gives so little relative savings by being more aggressive with pocketing that it's not worth it.

[ThaddeusMaximus]

Originally Posted by asid61

Assuming that your gearbox is direct driving the center wheel, here's a list of forces:
1. upward force from the effect of gravity on the wheel shaft
2. forces between gearsets (look up calculating the force of a spur gear)
-These translate into forces on the bearings, and thus the holes in the gearbox plate.
3. forces on the sprockets going to the front and back wheels (although this might cancel out, I'm not sure).

There might a couple more but those seem to be the major ones.

Point 3 may cancel out in best-case loading but it does not in worst-case loading. (think: one wheel off the ground) (Actually, it doesn't truly cancel out in best-case loading because a moment is still produced in the vertical axis since sprockets are offset...)

Point 1 may need to be treated as an energy rather than force. It's definitely an impact in a game like stronghold...

For point 2 be sure to account for pressure angle. 14.5 deg means it isn't huge but it's there.

Another force: Turning scrub is a force, depending on the gearbox application.

[SerpentEagle]

Thank you all for your advice!

[Mechvet]

I had a few of the previous comments selected to quote and reply, and then I made my way down to Cory's post.

Perfectly worded, excellent advice. Understanding the parameters around what's actually being calculated while using FEA is critical towards using it correctly.