Calling all Mechs! (Torque calculations)

[Jedward45]

Hey,
Here's a challenge for all the Mechs on Delphi:

I'm currently trying to find the correct spring pin size to hold a final stage bevel gear in place. This pin transmits torque from the driven bevel gear, to both a 3" wheel mounted on top of the bevel's hub, as well as to a live shaft inserted through the bevel's hub. (Once again, thats a 3/8" diameter Shaft, within a 1" OD bevel hub, within a 1.25 OD Wheel Hub.) Consequently, the Pin's length would be 1.25", with 2 points of torque transfer (One on each side). The difficulty is that Lateral space to mount the Pin is extremely limited, meaning I need the minimum pin diameter necessary to withstand the torque of the drive train. (with an adequate margin of error)

So, if T=rF, then the radius is .625", but I don't know how to estimate the force.

I know
-the Robot's speed (with a rough overestimate), is 10 feet/sec.
-The robot's weight is 150, but I figure 75lbs maximum on each wheel (4 wheels, but all weight on two wheels in full speed stop)
-Using JVN Design Calc, Robot pushing force is 120lb, and Motor Torque load is 1.9 Nm, Wheel Stall Torque is 14Nm. (sorry I didnt convert)
-Coefficient of friction is 1.4

Once again, I'm looking for the Torque load the Pin will have to endure.
Any help would be great!

[Chris is me]

While I'm too tired to analyze this problem (I'll try again later today), have you considered transmitting torque through some other means, such as a hex pattern or a key?

[EricH]

I'm with Chris that there may be other methods. But, I'm a bit curious about this setup, and here's why:

It sounds like you're trying to power a wheel and a live axle independently from the same gear, which is driven via its gear train. If that's correct, then what's the problem with just driving the axle and using the axle to drive the wheel? A relatively simple key or hex setup will work quite well to do this; you might even be able to do it with a single keyway/key on the axle.

[Jedward45]

Originally Posted by EricH

I'm with Chris that there may be other methods. But, I'm a bit curious about this setup, and here's why:

It sounds like you're trying to power a wheel and a live axle independently from the same gear, which is driven via its gear train. If that's correct, then what's the problem with just driving the axle and using the axle to drive the wheel? A relatively simple key or hex setup will work quite well to do this; you might even be able to do it with a single keyway/key on the axle.

Sorry, I should have spent more time to explain this...

The reason the wheel and axle are simultaneously driven is that the axle also transmits torque to a second wheel opposite the bevel gear set. To avoid the problem of creating a torque nightmare when both wheels pivot (think duelly's on a coaxial swerve), I minimized the distance between the two wheels to less than 1/2". Doing this required mounting the wheel directly on the bevel's hub and building the driven bevel into the wheel itself. Thus the need for pinning...

[Jedward45]

I have,
the problem with both a hex shaft and a key (aside from myriad of nightmares I've gotten from using keys & set screws that ultimately lead me to use pins instead) is that I have no way of creating a hex bore (or key bore) through the steel bevel gear. In addition, the wheel hub (that sits on top of the gear hub) also has to receive the torque load.

[EricH]

You have no way to create a keyway/hex? You mean there isn't a relatively local shop that you can work with that has the proper size broach and an arbor press? It's a 10-minute operation or less per part with the proper equipment.

Given your setup, I'd really try to find that shop.

[Jedward45]

Here are some images of the assembly, hope this clears things up

[sdcantrell56]

Originally Posted by Jedward45

Here are some images of the assembly, hope this clears things up

It looks like you are using a spiral bevel gear as well. Like everyone is saying find a local shop or even a team that has a broach and broach the gear. it's not even a 5 minute process. Better yet buy a 1/2" or 3/8" hex broach and a press and you will be able to use it for many years to come so you won't have this problem. A pin is possible but if you don't have to use it don't.

And if you had your location listed on your profile maybe a local team could help or someone could suggest a team.

[Andrew Schreiber]

Originally Posted by sdcantrell56

And if you had your location listed on your profile maybe a local team could help or someone could suggest a team.

Eagan, MN, USA I've never understood not providing a location but providing a team number...

[Jedward45]

While I appreciate the help, I would like to request that somebody help in my original goal of finding a torque calculation. While broaching would be possible for some components, I would ultimately need to use a pin to connect the drive wheel anyways (see pictures)

[EricH]

Originally Posted by Jedward45

While broaching would be possible for some components, I would ultimately need to use a pin to connect the drive wheel anyways (see pictures)

Not necessarily. There's an old trick that we used back when Skyway wheels were the wheels of choice and didn't have that nice 6-hole pattern available. You build an adapter that goes between the spokes of the wheel and bolts (or otherwise attaches) to your favorite drive method for the wheel

Or you can hex broach the hub of that wheel, use a hex axle straight through, and have a full live-axle setup without the pin being needed at all.

Just because you think you need to have a particular method doesn't mean that that's the only method that will work.

[Jedward45]

Originally Posted by EricH

Or you can hex broach the hub of that wheel, use a hex axle straight through, and have a full live-axle setup without the pin being needed at all.

Interesting... how would you connect the wheel hub to the live axle? (with our drive train constraints, there isn't room to mold the hub around the end of the bevel gear hub.)

[EricH]

Originally Posted by Jedward45

Interesting... how would you connect the wheel hub to the live axle? (with our drive train constraints, there isn't room to mold the hub around the end of the bevel gear hub.)

Let's see...

You could put a setscrew through the wheel hub (and into the shaft a little bit).
You could make the hex shaft just enough longer to put a groove for a snap ring on the outside end.
There are probably a couple other ideas for someone to come up with/suggest.

Pretty much, all you'd need to do with the wheel is keep it from sliding off the end of the shaft. A press fit would also help, but could make quick changes a pain if that issue ever came up.

[JamesCH95]

Taking your assumptions to be correct, i.e.:

uF = 1.4
normal force = 75lbs
wheel OD = 3in =1.5in radius
radius of pin shear = 3/16 (you can compute the larger radius, but the shear forces will be lower)

Max Torque = 75lbf*1.4*1.5in = 157.5in*lbf
Shear force per pin jct = torque/ radius of pin shear/2 = 157.5in*lbf/0.1875in/2 = 420lbf

Or 840lbf in double-shear.

This sounds all well and good, but this pin will need a press-fit between the hub and shaft that it is connecting, otherwise all bets are off. This calculation is incredibly simplistic and does not come near to mimicking the loads your drive train will see. Impulse loads could shatter a hardened pin, the hole drilled out of the 3/8 shaft will weaken it, I don't see this ending well, but I may be wrong.

[sdcantrell56]

Like ericH said. Go with a live axle set up and broach the wheel to match the gear. Problem solved. Pins are much more of a hassle and won't have the same torque transmitting capabilities.