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H20
The H20 gear family is based on a triangular tooth with a gear module of 1. Conveniently, gears with a module of 1 have exactly as many teeth as their pitch diameter in mm. H20 gears have dimensions suited for use with:
- standard 20mm extrusions
- NEMA 14 or NEMA 17 motors
- M5 drive shafts
- M5 or M8 load shafts
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).
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.
Or in other words, "Why so many screws?". Belt pulleys 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. If you are the casual sort who never ties your shoelaces, you probably will end up using only set screw. The rest of us will shake our heads at you. Use all three.
Unlike belts, gears are rigid. That rigidity presents an adjustment challenge in that the gears 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 a X40SP05 0.5mm spacer which can be used for this purpose. For finer adjustments, use paper shims. H20 gears tend to the generous side of plump, so this simple adjustment is usually all that is needed to adjust gears for minimal noise and backlash. Never over-tighten gear assemblies--keep them somewhat loose while you adjust the inter-gear distance with shims.
Given that 3DP extruder gears are unlubricated, you'll probably not need lubrication. Will it help? Who knows?
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.
An H20 pinion gear such as has an inner drive shaft taper that narrows, captures and centers the end of the 5mm drive shaft.
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.