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PE and tooth size

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#1 Andy Taylor

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Posted 15 January 2010 - 08:50 AM


I'm adapting my old drive setup to a larger mount.

The mechanics are easy but just need to check with you guys.

I need to order the gears but the question is about tooth size - will a finer tooth reduce PE or does it make no difference... :scratchhead:

#2 Eddgie


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Posted 15 January 2010 - 01:57 PM

I think that in theory, yes, smaller worm teeth would have less PE. The reason is that with any kind of gear, the angle of the contact face changes geometry as it moves though engagement. This introduces very tiny acclerations and decelerations and if the gears are higher frequency, the acclerations and decelerations (in theory) should become smaller.

In reality though, the changes in drive rate from this are MICROSCOPIC. AH, but we are talking about a need to keep periodocity at enormously small tolerances. So yes, I would think that in theory a finer gear has the POTENTIAL to have less PE.

On the other hand, there is a limit to how fine you would want to go. The pressure being exerted on the face of a worm gear CAN be enormous. The contact area is on the order of a few fractions of a square millimeter, and if you attempted to make the teeth TOO fine, even an unbalanced load could distort the worm gear tooth face. A bump could permananly bend the worm gear tooth (not the wormgear shaft itself but the long spiral tooth of the worm gear) because all of the impact has to be soaked up in a tiny contact area.

So, I would think that finer would be better.. Up to a point. Past a certian point, and the gear faces becomes to fragile to support the pressure from out of balance loading or from impacts without becoming permanaly distorted.

Good machining has more to do with INITIAL PE, but in time, as the worm gear and worm wheel wear into almost perfect mesh (which would take HUNDREDS OF THOUSANDS OF REVOLUTIONS.. Think Car differential). For this reason the quality of machining and lapping probably do more to determine PE than anything else because most telescope worm wheels will never pass though a few hundred thousand contact cycles. A high end manufacturer though probably does spend a lot of time lapping a set worm set so that mechanical PE is very low.

Lapping is basically putting a polishing compound on the gear faces and running them in contact with one another. The problem in lapping a worm gear pair is that the worm gear has to make a whole bunch of revolutions to bring the worm wheel around ONCE! So, I would think that the best gear sets would be lapped, but I do not know if this is the case, or if modern manufacturing technology can produce the super-fine faces just by cutting them.

It would be interesting to know, and if anyone has the answer to this (do high end manufacturers lap their worm sets) I would be curious to know.

Will be interested to hear thoughts of others...

#3 Andy Taylor

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Posted 15 January 2010 - 04:33 PM

Thanks for your thoughts...

I'm a camera/lens engineer (six year apprenticeship as scientific instrument maker - and yes - I do a lot of lapping...)but this issue has too many double negatives...

The gears will be stainless steel spur gears driving the mounts worm. I always lap in the worm drive with 1/10 micron diamond paste (using a flexible drive driven by a motor over many revolutions...)

I'm just thinking about short period error v long period...

Yes, I'm thinking finer teeth but a small amount of damage/dirt will (I think) cause biggger problems...

My brain hurts.

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