The best drive shaft ever (in the picture below)
I used a high strength shaft for my arm on my catapult, but looks like the internal gearing might also need high strength shafts.
Would all the internal gearing require high strength shafts, or did I do something wrong, and is this something obvious to experienced people (meaning am I experienced, 1 year of vex)?
Without knowing exactly how you used the axle I don’t know why it twisted, but whenever there is some sort of high torque transmission it is a good idea to avoid axles, either use a high strength axle or better yet is to screw the gear directing onto whatever it is attached to. However for axles coming directly out of the motor, normal axles can’t twist from just the force of the motor, so if those are causing issues you really need to look into what external loads are being applied to the motors, as you’re probably also wrecking the motors.
If you make sure that regular axles don’t have extremely high amounts of torque applied to them, twisting axles shouldn’t be a problem, and you should make sure it doesn’t happen on future designs.
I would say anywhere you have a hight amount of force required i would use high strength axles… axles are twisted very easily… i know last year we completely destroyed an axle when attempting to make a weird catapult design…
Oh yeah! The old drill bit drive shaft! I asked about a few years ago.
Yes you can twist that guy quite easily. Vex shafts are regular old steel and 1/8" on a side.
https://vexforum.com/t/what-is-the-shaft-made-of-for-max-shear-calc/23146/1
http://www.engineeringtoolbox.com/torsion-shafts-d_947.html
Square shaft calc for the max stress is:
Tau_max = (2/9)* Shear_max bb*b
where b is the 1/8" side (3.175mm)
Then you have to look at how far away the part putting the force on the shaft is away from the thing you want to move. The T_max is the most you can do before you get to permanent deformation (i.e. where you turn your shaft into a drill bit).
Tau_max = T_applied * r / J
T applied is the torque acting upon the shaft, r is distance from motor gear to lifting arm, and J is the polar moment of inertia. We look those up for our shapes.
J = bh/12*(b^2 + h^2)
(or for a square do some math and it simplifies a bit, J = b^4 / 6)
So J is really small for 3.175mm… Much much higher for the high strength shafts.
Tau_max then applies the size of the shaft again b/2 so
Tau_max = T_applied * b^5 / 12
Yikes, Tau_max is getting small!
I believe we did that to 3 driveshafts total during the NbN season. Always entertaining. Until the shaft is in a hard to reach place.
It’s not obvious until you’ve done it yourself, or analyzed the forces in your design/take a materials class.
Long story short, an axle can theoretically only take about 118 inch pounds of torque before it twists.
The only thing that confuses me is that you said internal gearing, which usually refers to the internal gearing of the motors (which don’t use the 1/8th inch axles). Usually, your limiting factor should be the 1/8th inch axle on the motor. This is because you can use circular inserts on any 36t gear or higher (which is needed for your gear ratio). If you use the square inserts, you want to switch to the circular to avoid the force. Additionally, with the gear ratio and circular inserts, it essentially multiplies your torque limit to at least (354 inch pounds) because less torque is going on the axle
Yup, I agree with @DracoTheDragon . If you use the circular inserts on every idler gear in the gear train, you also reduce friction.
Sorry, I meant the gearing in the catapult. So in the gearing close to the actual arm in the catapult, I might have to use high strength shafts?
What I’m planning to do, because we don’t have high strength shafts, is to line the shafts with HS 12 tooth gears and metal square gear inserts. Its not pretty, but the HS 12 tooth gears essentially give you like a half inch shaft
You might be able to use 4 small shafts as a high strength shaft. With the gears, you might still get some twisting between the gears.
I’ve seen this done before. Long before the advent of the 1/4 shaft the Boss Botz, a MN team, did it. I believe it was for a sack attack robot.
HS 12 tooth gears don’t have the holes for inserts :>
I currently don’t know where precisely the twisting occurs on your system
If twisting occurs on the motor axle, then you must use a higher gear ratio to prevent it from twisting. If it’s reaching a non-motor axle, then you first want to use circular inserts. If you start running into bending problems, you want to reduce the spacing between the axles as much as possible(even a half an inch is significant). If that still fails, you then want to use high strength axles.
If you simply don’t have time, you could go straight into high strength axles, but it’s a waste of material
No, no, I know that. I meant a lining the axle with gears AND with inserts. Not inserts in the HS 12 tooth gears
It’ll just twist in between whatever you line it with. Like Kevin said, bolt your gears together to take the load off the axle.
Tip: If you don’t have high strength shafts, just grab 4 normal shafts and they are the exact size as the high strength shafts.
Ok, I’m still not sure if you know what I’m trying to say. The gears would be forced together as if they were bolted together. Imagine a shaft with a line of HS 12 tooth gears. Now each of those gears is likes squeezed next to each other by shaft collars or spacing or something. This way, there is absolutely no room between gears, and it can’t twist between them.
I know what you are describing, but it will still be able to twist between them even if there is no gap. I have seen axles twist between two gears with no gap before as I imagine have many other people who have built compound gear ratios and neglected to bolt their gears together.
The distance from the applied force to the next place down the line is the critical distance for the twist angle on the shaft. So shortening that distance to near zero makes the angle much tougher. Not impossible, just a heck of a lot tougher.
Also, that is why bolting the arms to the gears makes so much sense to truly make it 0 distance of force transfer along just the shaft. The smaller 12 tooth gear side has force still, but the magnified side with the big gear is generally the culprit for twists.
Our bot twisted some of its shafts, even though it had high strength gears on it.