Manual backlash relief in modelling a helical planetary ring gear

[ironrobot]

The team wants to create a helical planetary gear arrangement for driving a turret on which they want to mount a crane-like arm for their robot. We have access to a gear geometry tool in fusion 360 that can create helical gears, but it can't generate helical planetary rings. We saw a video that shows how you can create a regular helical gear to subtract out of a cylinder to create the ring. But you have to manually add the relief needed to allow some backlash. Given that we are planning to 3D print these gears, are there simple rules of thumb about how to add the correct relief? That's kind of what the video suggested, but without giving out those rules. This isn't a super high precision application. But strength is welcome.

Even if they had access to gearotic and didn't have to do it manually, I think this topic would be good for the team to understand.

Also related to this, are there any concerns about driving just one planet gear? Normally the sun gear is driven. If we even have a sun and other planets, they would only be in this system as idlers to help stabilize the pressures on the driving planet gear - to ease the side load on the motor shaft. But if we can find another way to deal with those loads, then removing the sun and other planets would be a goal just to keep things simpler.

Any and all advise welcome!

Karim Virani
Coach for FTC teams 6832-Iron Reign, 3734 - Imperial Robotics, 15373 - Iron Core, and 15375 Iron Golem
ironreignrobotics.org

[ironrobot]

The process we are experimenting with is shown in this video. https://youtu.be/5a1w9daIybc The ring gear is made by beefing up a normal gear as an inverse of the ring to cut out of a larger cylinder. This is because that tool can't calculate internal gears.

At 17:12 to 19:45 https://youtu.be/5a1w9daIybc?t=1032 he talks about using a negative backlash to fill out the tooth profile. But he doesn't say why he used that value.

Then he talks about extending the tooth so it will cut a deeper root. The rule of thumb he states is to take the module and multiply it by a number in the range of .25 to .5, depending on how much clearance you want. I don't know if this just practical advise or based on theory.

Then around 29:45 https://youtu.be/5a1w9daIybc?t=1785  he shows the inverse ring gear subtracted from a cylinder to make the internal tooth ring gear.

Then he extrudes another cylinder as a cut to barely clip off the teeth of the ring gear.

This all seems to be pretty artfully done and maybe not so much mathematically. Just interested in your reactions to this method. I'm assuming gearotic can do this precisely.

[ArtF]

Hi Karim:

  Interesting project. Gearotic wont allow a helical planetary. It will only do helicals
as gears. At time of design it wasn't though making one would be easy enough to
make sense designing it, though I may have to revisit that in light of all the 3d
printers.

  When it comes to backlash we used to adjust shaft position slightly away, but the technique
in gearotic is to shorten the width of one gear so its backlash is proved by that clearance.
When I 3d print a gear for example, Ill typically set the tooth width to .49 for a 2% clearance.

  When it comes to using only one planet in the ring, I guess its a matter of the strength of that
shaft, it might appreciate the support of the other planets for torque force. In the end there
is no right or wrong, just does it work well.

Art


Art

[ironrobot]

Thanks for the advise. We might default to a standard non-helical for the first attempt. I'll let you know how it goes.

But it would be great for gearotic to support this. And support internal bevel gears too. Combine them and make internal bevel helicals! That's what we really want.

You are right - 3D printers have gotten to a point of maturity and capability where they can enable entirely new thinking. Every high school has access to them. Better funded robotics teams are regularly printing in metals or composites.

Here is an example of an interesting gearing system that we are also going to experiment with. It's for something called a swerve/differential drive. Two motors let you both drive the wheel and steer it by mixing the inputs. The next evolution of this would incorporate herringbone outer gears and maybe internal bevel gears for better alignment stability.

[ArtF]

  Very cool design. I like the dual toothed wheels. The world is changing rapidly
it seems. Ill give some thought and take a look at the code to see if there
is anything I can do about helical ring gears.. Gearotic is getting long in the tooth
and its base is more complex than it should be which makes such mod's difficult at
times.
  Perhaps Ill get up the energy some day to put out a Gearotic 4 to allow such things..

Art