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firepick1 (localhost) edited this page Aug 20, 2016 · 34 revisions

The H20 gear family is based on a triangular tooth with a gear module of 1. Gears with a module of 1 have exactly as many teeth as their pitch diameter in mm. For example, the 19mm GH19 gear has 19 teeth and a pitch diameter of 19mm.

The dimensions of H20 gears are intended for use with:

  • standard 20mm extrusions
  • NEMA 14 or NEMA 17 motors
  • M5 drive shafts
  • M5 or M8 load shafts

See video...

Function

H20 gears are used to isolate stepper bearings from load bearing forces. Load bearing forces (thrust, belt-snap, etc.) can easily break stepper position synchronization even when using flywheels. For example, gears can transfer motor power to a load shaft having bearings designed to handle extreme thrust loads. Stepper bearings are designed for transverse loads and will freeze up under thrust load. Even extreme transverse loads will paralyze a stepper motor.

H20 gears are most often used to transfer power across parallel axes aligned to the slots in a 20mm extrusion. The extrusion slots are spaced 20mm apart, and H20 gears should be paired such that their tooth count adds up to twice the slot separation. For example a 19mm GH19 gear should be used with a 21mm GH21 gear to transfer power between two shafts 20mm apart (i.e., 19+21 = 2x20).

Relatively Prime

The gear tooth count for a pair of gears should always be relatively prime to ensure that each tooth on one gear eventually touches each tooth on the other gear. With the GH19/GH21 gear pair, 19 and 21 are relatively prime, and that is a working combination. Don't use GH19 with another GH19--they won't span the 20mm space between the drive and load shafts.

A downside of relative prime gear pairing is that the gear ratio is never 1:1. One turn of the drive shaft will never result in a single turn of the load shaft. Welcome to gear ratio math and 19/21ths or 21/19ths.

Manufacture

FirePick 3D printed parts are tested on a Lulzbot Mini using PushPlastic PLA, Lulzbot Cura with Standard Print profile.

Assembly

Or in other words, "Why so many screws?".

In contrast with belt pulleys, which only have two screws, H20 gears have three M3 set screws at 120 degrees. This permits fine centering adjustments. In particular, tighten the set screws repeatedly and incrementally, going around and around the gear. The better you are at this, the less wobble you'll see. Yes. It's an art.

Gear Adjustment

Unlike belts, gears are rigid. That rigidity presents an adjustment challenge in that the gears are fixed in place but should not be too tight or too loose.
Tight gears are noisy and self-destructive. Loose gears are subject to backlash inaccuracies. To address this, we adopt the convention that pinions (or driving gears) are rigidly fixed and true, whereas load gears are flexibly conformant and adjustable.

Since we typically only deal with a simple pair of gears, this convention allows us to to adjust the load shaft away from the drive shaft by inserting spacers to raise the load shaft bearings. The H20 gear system includes an X40SP05 0.5mm spacer which can be used for this purpose. For finer adjustments, use paper shims.

IMPORTANT: H20 gears tend to be somewhat oversize when printed, which means you'll almost always need to shim up the load bearings. If you don't shim up the load bearings, the gears will be overtight and will not line up properly.

Lubrication

Given that 3DP extruder gears are unlubricated, you'll probably not need lubrication. Will it help if you lubricate the gears? We don't know yet--let us know if it works for yo if it works for youu.

Wearing In

Like new shoes, H20 gears require some wearing in. Simply run them at low speed until they sound smooth. You can increase speeds once the gears are worn in.

On a related note, you'll eventually notice white plastic dust in the gear building up over time. You might want to clean that with a wet paper towel instead of breathing the dust in.

Pinion Gears

H20 pinion gears are designed to fit a 5mm slotted drive shaft. Different size STLs are provided for 3D printers with different nozzle sizes and/or extrusion percentages. Use the one that fits. [Print with 80% fill and anneal gear onto the stepper shaft for best results](Anneal PLA).

GH17

The 17mm GH17 gear is a small, powerful workhorse:

Other pinion gears

Although intended for drive shafts, pinion gears can also be used as load gears on either end of a load shaft.

Load Gears

An H20 load gear has no inner taper and may flex slightly at the non-attachment end. The lack of taper allows the gear to be placed anywhere on a load shaft. Load gear identifiers end with L8 for 8mm shafts and L5 for 5mm shafts.

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