pic: 254/968 Gearbox - Destructive Testing Results

[Travis Covington]

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[Eldarion]

Good job! I think this is one of the first times strength testing has been performed on a FIRST robot powertrain?

Wow. I knew that it was strong when I played with it... But not THAT strong. Props on a seriously sweet design!

[Rich Ross]

Not all teams have a gearbox that is also a senior project though. Its cool to see this information. I seem to remember travis saying that it wouldnt be to practical to manufacture at the same level/cost as an AM shifter. Thats too bad. I'd love to throw a couple of those on my robot.

[MrForbes]

Originally Posted by Travis Covington

- 170 foot pounds of torque before failure on the output shaft. That's the equivalent of a higher powered 4 cylinder engine you'd find in your car!

Kind of...except that transmission output shafts are subject to the torque from the engine, times the gear reduction in low gear. Which is why automotive transmissions have larger output shafts than input shafts.

Interesting tests, thanks for sharing! It's a nice transmission, but not nearly as cost effective as the AM shifters as noted. Unfortunately real life generally means economic considerations are top priority for a design.

[Travis Covington]

Yes, automotive torques are measured at the wheels in a ~1:1 gear or at the crank... However, I will stand by my original point that 170 foot pounds of torque at the wheels is still more than a honda civics engine can output. Correction: (without a transmission)


I am not saying this will withstand that torque, as there is no factor of safety present, I am simply giving a basis for comparison for those who see these results as just numbers.

Also, with regards to cost effectiveness, that's not always true. This project was based around a model of weight being the utmost concern, and cost being forth of fifth down the line. Also, weight can become a cost savings itself. When it costs ~$3,000 to $5,000 per pound (estimate) to send a sattelite into space, that weight savings sometimes can create an economic savings greater than the added cost of manufacturing.

[Travis Schuh]

Wow! That is one strong tranny.
Were these results close to what you were able to calculate?

[Travis Covington]

Originally Posted by Travis Schuh

Wow! That is one strong tranny.
Were these results close to what you were able to calculate?

More or less. We didn't concentrate too much on these forces, as the gear teeth would strip long before any of the shafts or webs would fail. We spent considerable time calculating bending strengths on the gear teeth to make sure they would be okay.

As such, yes, these were very similar to what we calculated and predicted. Some components had an initial 10X factor of safety in them, but due to the design, the shape and size couldn't be changed much and further weight wouldn’t have been easily removed. Since the components didn't weight much at this point anyway, we didn't feel that a design change was worth while or necessary.

[Travis Schuh]

Originally Posted by Travis Covington

We spent considerable time calculating bending strengths on the gear teeth to make sure they would be okay.
As such, yes, these were very similar to what we calculated and predicted.

How were you able to calculate the strength of the components?

Thanks, Travis

[Travis Covington]

Originally Posted by Travis Schuh

How were you able to calculate the strength of the components?

Thanks, Travis

For gear tooth strength, we used the Barth revision of the Lewis bending forumula. For other strengths, we used other bending eqns, torque eqns, and shear strength eqns depending on the part we were analyzing.

We also did finite element analysis on all of the components to verify that our initial calcs were correct.

Here is an example of the FEA for the 80 tooth gear. These parts were subjected to two 20 KSI loads on the gear teeth, ~ 90 degrees apart.

Stress Distribution





Displacement

[Arefin Bari]

So, how much are you guys selling it for next season?

[EricH]

Originally Posted by Arefin Bari

So, how much are you guys selling it for next season?

I think the real question is, where can we find the plans/instructions so we can make it better? (Is that even possible??)

[Travis Covington]

Originally Posted by EricH

(Is that even possible??)

Always! "Continuous Improvement" is the future!

[MrForbes]

(first part removed, I think we cleared that up)

Originally Posted by Travis Covington

Also, with regards to cost effectiveness, thats not always true. This project was based around a model of weight being the utmost concern, and cost being forth of fifth down the line. Also, weight can become a cost savings itself. When it costs ~$3,000 to $5,000 per pound (estimate) to send a sattelite into space, that weight savings sometimes can create an economic savings greater than the added cost of manufacturing.

In the real world, cost is a major factor affecting design. The 254/968 robot, and space missions, are notable exceptions. AndyMark sells a lot of transmissions, because they have a very cost effective design.

[Travis Covington]

Yes, torque is increased as the gear reduction increases... of course. I think I added a little too much there at the end with "transmission or otherwise" My ASSumptions were based on engine output torque, neglecting that fact that you have the ability to increase that torque in lower gears. Sorry.

I think the confusion here stems from my failing to explain that this rating I quoted is the "specification" value. Usually these horsepower and torque ratings are measured on an engine or vehicle dyno in a gear close to 1:1, or simply at the flywheel if on an engine dyno.

So no, this shaft wouldnt handle the torque at the wheels in 1st or 2nd gear on a honda civic. However, most people wont say their honda has a torque of 400 ft lbs because that is entirely dependent on the transmission and gear reduction, as you mentioned. And it would in fact be strong enough to handle the torque coming out of the engine.

I should read more carefully next time.


Also, I should add as I was somewhat upset by the other comment... we did do a cost analysis and mass production process and material selection. As the quantities get higher (very high), these gearboxes actually do have the ability to be cheaper than a steel alternative. Material costs become negligible, and the decreased machine run time and tool wear begin to make up for the costs added by anodizing and additional lightening. As manufacturing engineering students, we were far more concerned with these issues than we were the mechanical engineering based calculations.