How accurate are spur gears?

[geneiusxie]

Wouldn't spur gears have a tiny variation in mechanical advantage as they go through different stages in where their teeth mesh? Basically the idea is that as the teeth mesh, they have to slide just a tiny amount, probably around 1% of the tooth pitch, but that also means that both gears change their effective diameter by a tiny amount and that effective diameter goes through that change every time a tooth meshes. Theoretically even helical gears would also have the same problem, but it might be much less distinct because the meshing is more gradual.

That being said, would this make spur gears not recommended for high precision instruments? Would even helical gears be good enough?

 

[Compositepro]

You appear to have some misconceptions about gears. Theoretically the line of action between gears does not change as they turn. Contact between teeth is always on the line of action. There is some sliding and in the real world there are tolerances.

http://en.wikipedia.org/wiki/Involute_gear

 

[geneiusxie]

Ok that makes sense. Sorry I just got lost for a moment there

 

[mikekilroy]

You are correct that there is slight variations as the teeth mesh. But the amount is VERY small when comparing input to output. But, yet it is there.

Actually probably an order of magnitude WORSE inaccuracy is from the required backlash between teeth. THIS is why super high precision motion is done with direct drive (no gears) if possible.

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Remember too that repeatability and accuracy are two totally different animals and effect processes differently.

http://www.KilroyWasHere<dot>com

 

[racookpe1978]

The military (yeah = high dollar! and hard-to-fabricate as well) gears for missile fire control radars used two gears back-to-back to minimize backlash.

The two gears were on a common concentric shaft, but a spring kept a predictable tension between the two. When the drive motor had to reverse the radar's motion, both of the driven gears were always held up tight against teeth on the drive gear.

 

[mikekilroy]

sure there are antibacklash gears; we used them for position feedback in the early days of machine tool servos in the 1960's, but pretty quickly learned they wore out fast due to the teeth friction.

the torque motors we supplied ((supply) to those missiles for guidance and gyros are all direct drive if possible to eliminate all the gear errors.

Even today we are replacing spring loaded 10-20-30hp servo gearboxes with electronically controlled dual antibacklash servos for same reasons.

There is a use for antibacklash gears but their use is pretty unique, not generic as I assumed the OP was asking about.

http://www.KilroyWasHere<dot>com

 

[digitalghost]

Sounds like a mechanical tolerance question to me...

Google or search the internet for "AGMA Fine Pitch Tolerances / Quality Grades for Gears"