Vex Shift Rod through 3/8 Hex Shaft?

[KohKohPuffs]

In CAD, the shifting rod has a diameter of 0.187 inches. So I'm wondering if custom shifting shafts can be made out of 3/8" hex shaft... or is that way too small to be considered safe?

[cbale2000]

Originally Posted by KohKohPuffs

In CAD, the shifting rod has a diameter of 0.187 inches. So I'm wondering if custom shifting shafts can be made out of 3/8" hex shaft... or is that way too small to be considered safe?

Are you referring to a ball shifter, dog gear shifter, or something else?

If it's a Ball shifter I would think probably not since the plunger inside the shifting shaft would likely be too big to accommodate 3/8" Hex. Can't speak to the possibility of doing it with a dog gear shifter though.

IMO It would probably be easier to adapt a 3/8" hex onto the output end of a Vex gearbox than it would be to make a custom shifter for it. Not sure if that helps for your specific situation though.

[KohKohPuffs]

Originally Posted by cbale2000

Are you referring to a ball shifter, dog gear shifter, or something else?

If it's a Ball shifter I would think probably not since the plunger inside the shifting shaft would likely be too big to accommodate 3/8" Hex. Can't speak to the possibility of doing it with a dog gear shifter though.

IMO It would probably be easier to adapt a 3/8" hex onto the output end of a Vex gearbox than it would be to make a custom shifter for it. Not sure if that helps for your specific situation though.

Should have made that clear. I'm referring to the dog shifter rods, and in my newest CAD project the usage of 32dp gears from WCP and the 20dp 14t gear from VexPro requires the use of 3/8" hex. Perhaps your idea of a 3/8" hex adapter might work, except that I'm a bit concerned with how much space I have for it.

[R.C.]

Originally Posted by KohKohPuffs

Should have made that clear. I'm referring to the dog shifter rods, and in my newest CAD project the usage of 32dp gears from WCP and the 20dp 14t gear from VexPro requires the use of 3/8" hex. Perhaps your idea of a 3/8" hex adapter might work, except that I'm a bit concerned with how much space I have for it.

You could make your own rod @ 3/16 diameter or something around the .15 diameter in steel.

[asid61]

+1 on using a smaller custom rod.
Using a 3/8" hex shaft could be possible if you used custom Dog stuff, but ballshifting would be very tricky. On the plus side, you could use the 3/8" x 7/8" flanged bearings, which are very compact, for your dog gears.
Using two different pitch gears on a shifter seems interesting but also risky.

The main thing I would be worried about if you reduced the rod diameter is the little shifty bit (4-40 screw in WCP dogs). SW stress analysis could make sure it's safe.

[InFlight]

Use 1018 cold rolled steel. Stock bar is available in 0.125 and 0.1875 sizes.
You will need to have bearings in the pneumatic shift interface. Sticking with standard fractional sizes will make it easier to source bearings (R2 and R155).

** Edit- you could look at available Metric shaft as well. **

[KohKohPuffs]

Originally Posted by asid61

+1 on using a smaller custom rod.
Using a 3/8" hex shaft could be possible if you used custom Dog stuff, but ballshifting would be very tricky. On the plus side, you could use the 3/8" x 7/8" flanged bearings, which are very compact, for your dog gears.
Using two different pitch gears on a shifter seems interesting but also risky.

The main thing I would be worried about if you reduced the rod diameter is the little shifty bit (4-40 screw in WCP dogs). SW stress analysis could make sure it's safe.

My applications for using a shifter rod in a 3/8" hex shaft is for a brake for an arm gearbox. I would prefer to use COTS parts for the sake of machining time and because I know through experience that those work perfectly well without failure (as much). My main concern is if there's a limit to what size hole you can make in a shaft for the shifting rod to go through. Of course, I'm not going to make a hole in a shaft and leave only 0.01" of wall thickness, but I'm wondering if there is a general rule to follow.

[asid61]

Originally Posted by KohKohPuffs

My applications for using a shifter rod in a 3/8" hex shaft is for a brake for an arm gearbox. I would prefer to use COTS parts for the sake of machining time and because I know through experience that those work perfectly well without failure (as much). My main concern is if there's a limit to what size hole you can make in a shaft for the shifting rod to go through. Of course, I'm not going to make a hole in a shaft and leave only 0.01" of wall thickness, but I'm wondering if there is a general rule to follow.

In general, hollowing out the center of an axle doesn't cause too many strength issues. IIRC 1/2" thunderhex is 92% as strong as standard 1/2" hex even with the hole through it.
Solidworks Simulation can help you calculate what effect the same torque has on the original 3/8" hex compared to the new one. Or you can plug in equations you find online for the strength of a tube versus a rod in torsion.

[AdamHeard]

Originally Posted by KohKohPuffs

My applications for using a shifter rod in a 3/8" hex shaft is for a brake for an arm gearbox. I would prefer to use COTS parts for the sake of machining time and because I know through experience that those work perfectly well without failure (as much). My main concern is if there's a limit to what size hole you can make in a shaft for the shifting rod to go through. Of course, I'm not going to make a hole in a shaft and leave only 0.01" of wall thickness, but I'm wondering if there is a general rule to follow.

A dog gear doesn't make a great brake. It takes quite a bit of force to remove under torsional load compared to more traditional brakes.

[KohKohPuffs]

Originally Posted by AdamHeard

A dog gear doesn't make a great brake. It takes quite a bit of force to remove under torsional load compared to more traditional brakes.

The gearboxes I designed for 649's shooter arm this year had a brake component, which utilized an AM shifter block with the rod and a custom part which acted as the dog. It was basically a plate that shifts against a urethane brake pad. I didn't see the thing in action very much, but people tell me that it worked very well during the competitions and has never needed maintenance (not too sure about never failing).

I guess this would counter that a dog gear doesn't make a great brake. Compared to a traditional brake, not so sure. I don't really know what traditional brakes look like (... is it a bike brake?).

In general, hollowing out the center of an axle doesn't cause too many strength issues. IIRC 1/2" thunderhex is 92% as strong as standard 1/2" hex even with the hole through it.

Despite a horrific history final coming up tomorrow, I did some work today, and currently I'm expecting to make a 3/16" hole in the hex shaft for a custom shift rod with a diameter of 1/8". At some point I will run the shaft through Solidworks Simulation with these dimensions.

[R.C.]

Originally Posted by KohKohPuffs

Despite a horrific history final coming up tomorrow, I did some work today, and currently I'm expecting to make a 3/16" hole in the hex shaft for a custom shift rod with a diameter of 1/8". At some point I will run the shaft through Solidworks Simulation with these dimensions.

That is a lot of clearance for the shift rod. Why not make the hole bigger or the shift rod larger?

[AdamHeard]

Originally Posted by KohKohPuffs

The gearboxes I designed for 649's shooter arm this year had a brake component, which utilized an AM shifter block with the rod and a custom part which acted as the dog. It was basically a plate that shifts against a urethane brake pad. I didn't see the thing in action very much, but people tell me that it worked very well during the competitions and has never needed maintenance (not too sure about never failing).

I guess this would counter that a dog gear doesn't make a great brake. Compared to a traditional brake, not so sure. I don't really know what traditional brakes look like (... is it a bike brake?).



Despite a horrific history final coming up tomorrow, I did some work today, and currently I'm expecting to make a 3/16" hole in the hex shaft for a custom shift rod with a diameter of 1/8". At some point I will run the shaft through Solidworks Simulation with these dimensions.

What you described is a traditional brake. It relied on contact force and friction, not the interference of a dog in a mating part.

[roboruler]

If this is an off-season project/design you might have enough to try some other methods of braking such as a disc brake.

[KohKohPuffs]

Originally Posted by R.C.

That is a lot of clearance for the shift rod. Why not make the hole bigger or the shift rod larger?

If there's a lot of clearance, wouldn't you make the hole smaller?

Originally Posted by AdamHeard

What you described is a traditional brake. It relied on contact force and friction, not the interference of a dog in a mating part.

Oh... I guess that gearbox I made had a traditional brake after all. Silly me.

Originally Posted by roboruler

If this is an off-season project/design you might have enough to try some other methods of braking such as a disc brake.

I've seen a lot of disc brakes in Recycle Rush for the lift mechanism. However, for my intentions I'm not sure if I would be able to fit one in the gearbox I'm making, even if it is a custom-manufactured part. You are right though. Offseason is a great time to explore new concepts and designs: something I failed to do last offseason by designing WAY too many drive gearboxes and drivetrains.