Sun in, ring 1 held, ring 2 out. The output is the difference between two nearly equal meshes, which is why one stage reaches ratios a simple planetary needs three for.
Module 1.5 mm, sun 20 teeth, planet 20 teeth, 4 planets, ring 60 teeth. The second stage follows from these.
Ring 1 is forced to zs + 2·zp1 = 60, the only coaxial value without shift.
Gear height 10 mm per stage, outer ring ⌀110.0 mm, carrier plate 5.0 mm tall.
The sun carries a ⌀20 hub on its lower face, 9.5 mm long, which goes down through the carrier and runs in a 6704 seated there, on a ⌀22.8 lip that stands 0.7 mm off the sun so the inner ring lands on the lip and the outer ring runs clear of the gear. That is the bearing taking the sideways load of the train, right where the load is. The motor's ⌀5 shaft turns in a MR105 down in the plate, under the spigot recess, and carries on up into the hub where it is clamped - so the shaft carries torque and nothing else, instead of the sun hanging off the motor's own front bearing.Both start at the size the geometry works out, so adjusting begins from something that fits rather than from zero. The rings share one outside diameter because they stack on the same axis in the same housing; a rim under two modules flexes under load and the bore stops being round. The carrier posts are cantilevers, so it is the plate bending that lets the planets tilt out of mesh.
4 × M3 on a ⌀104.9 circle, cap 5.5 mm thick.
The output ring is the only part that turns with the load, so it is the only thing a load can be bolted to. It is also a ring with planets running in it, which is why the face has to stand proud of them first: level with the planets, a cap is a brake.
Tapped blind into the ring's rim and clearance through the cap, so the screws pull the joint up rather than holding it open. Blind because the ring's underside faces the running gap over ring 1: a screw out the far side would reach the fixed ring. Auto takes the largest of the three the rim will hold with a millimetre of wall each side.
Clearance holes right through the cap: the thread lives in whatever you bolt on top of it, the same way round as the cap's own screws into the ring. What that costs is a limit underneath, because the planets run 1 mm below the cap and a screw standing proud over their orbit meets one. The circle sizes itself inside the free ground over the sun, and says so when the count pushes it out past there. 4 × M4 on a ⌀19.0 circle.
Sun turning at 120 rpm, assembled.
Parts, as exportedOne chip per file. The dashed one is drawn and never written: a bearing is a designation and a fit, and a printed one is how you get a gearbox that binds.
| Reduction | 32.000:1 (reversed) |
| Denominator | -160 stable |
| Ring 2 speed | -3.750 rpm |
| Carrier speed | 30.00 rpm |
| Stage 2 | 24/64 derived |
The denominator is zp1·zr2 − zr1·zp2. One tooth changes it by a large fraction, so the ratio moves in jumps, not smoothly.
A Wolfrom fills the gap a simple planetary cannot: 30:1 to several hundred to one in one stage, at the cost of efficiency.
There is always a shaft. This is its diameter, and everything downstream follows it: the sun is bored to it, and the bearing under the carrier is the one with that bore. Six and eight millimetres are the sizes with the most stock behind them.
A motor shaft comes in from above and this is what stops the sun turning on it. A flat is what a stepper ships with and it is the one a grub screw wants to bite; a key is the answer at real torque. The screw gets a hub to live in, because a radial hole through a plain rim has to come out through a tooth.
Carrier plate ⌀71.20 inside a ⌀44.00 skirt. Both come from 6707, so the bearing drops straight in. The floor of that pocket is a ⌀40.4 lip: inside it the flange is dropped 0.9 mm, so the outer ring lands on the lip and the boss it turns on runs clear of the housing.
Without a boss the plate has to be the bearing bore everywhere, so the bore must clear the posts at ⌀68.0 while staying inside the ⌀87.0 tooth step. That window is often empty. With a boss the plate keeps the diameter its posts need and steps down below them, which is how the part would be turned anyway.
Nominal geometry, for machining or resin. Every surface moves in by this much, so teeth get thinner and bores get wider from the one number. It applies to STL only; STEP keeps the nominal geometry, because a CAD file that carries one printer's shrinkage is wrong everywhere else.
Planets 1 and 2 export separately; on the real part they are one piece, cut with both rims on a common tooth centreline.