Planetary Gears: A Few More Things

Image courtesy of BFT-Drives.

Before we move on to other topics, here are a few random thoughts on planetary drive design:

1.      Learn the “rules” on numbers of teeth. If you are involved with repairing or reverse engineering parts for planetaries, you will occasionally run into gearboxes with un-equal planet spacing that “violate” the rules. If the designers needed to do that, they also had to deal with imbalance and assembly complications. Just because you can do something does not mean you should.

2.      The gears themselves are easy. Figuring out the planet bearings and how the loads get through the carrier to the next stage is much more challenging.

3.      Do not forget the “reverse bending” situation on the planet teeth. Read and understand the clauses in the rating standard concerning this topic and do not think for a moment that it does not apply to your project.

4.      Be very careful when using rack offset in an epicyclic geartrain. The benefits of customized tooth geometry are impossible to ignore — especially when low tooth counts are on your sun pinion. Just make sure your planet pin locations and internal gear geometry are modified consistently.

5.      Mind your “P’s and Q’s” on bearing selection. Internal clearances, fits, and cage design are critical when everything is rotating.

6.      While we are talking about everything rotating, be very concerned about all the fasteners and lube fittings inside the gearbox. Remember my warning about debris recirculating inside the meshes? Sometimes that debris comes from non-gear parts not being well secured.

7.      Do not underestimate the “recirculating” torque loads that can be present whenever power is split in multiple directions. Be conservative in deciding how many bolts and dowels you need.

8.      No one likes a leaker. Epicyclic designs frequently have large joints that are “under oil” at all times. O-rings are a great way to keep things oil-tight, but only if the joints are properly designed and manufactured. Get a good design guide from your supplier and stick to it.

Each of these warnings came from experience—some of it mine—some of it through observation of others. They are offered here because the hard lessons do not have to be re-experienced to be valuable.

About Charles D. Schultz 511 Articles
Charles D. Schultz is President of Beyta Gear Service and one of Gear Technology's technical editors.

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