On 2008-10-02, Pete C. wrote:

Wayne wrote:

Pete C. formulated the question :

[ ... ]

Come to think of it, are you sure it isn't a dual/multi lead screw?

[ ... ]

I didn't even know about multi lead screws until I researched
a mini acme rod for a small stepper motor. Wasn't an even number
of TPI. If you look on the end, do you see 2 starting points?
What's the purpose of them?

I'm not real sure the advantage of multi lead, but they do exist,

The advantage is that you get more motion per turn.

The disadvantage is that you don't get as much force
multiplication.

The reason to use a multi-lead thread in place of a coarser
standard thread is that you don't have to cut as deep into the rod (thus
weakening it) -- the threads are shallower for the travel per turn.

The primary example that I have here of multi-lead thread (three
starts) is the leadscrew used to raise and lower my garage door. (Hmm
.... another advantage is that it provides three locations in which you
can snap the nut back into engagement after disengaging it for manual
motion of the door. With a coarse single-start thread (which would
require a larger diameter shaft and a larger aluminum extrusion in which
the shaft moves) you would probably have to jiggle the door up or down a
little to get it to snap into engagement.

and
yes looking at the end you do see the multiple starts. I seem to recall
that the TPI number counts all the leads, so the turns per inch is the
threads per inch divided by the starts.

That sounds reasonable -- though I think the term fro the
latter is "the lead".

Enjoy,
DoN.

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