On Thu, 26 Jun 2008 00:37:41 -0400, "Tom Gardner"
wrote:
I want to turn a 4" OD x 3/8" thick gear 80 to 110 degrees in 100 ms. I'm
thinking of a 4" stroke by 1-1/4" bore air cylinder attached to a rack gear that
would engage the round gear. No, I won't do it with a servo! I'm thinking a
1/2" round rack gear in a tube with Thomson linear bearings and a scallop cut in
the middle of the tube to expose teeth to the round gear. This seems simple and
cheap. In fact, it's so simple it would seem that it should be an off-the-shelf
item! I just assume it would be easier to do with round rack than square rack.
Is this available or did I invent a new wheel?
The gear would just spin back and forth and the air cylinder/rack will have an
adjustable stop to get my 80 to 100 degrees rotation. The round gear will have
a one-way clutch installed because I only want the one direction of shaft
rotation. The shaft will power a wire feed mechanism to feed 3" to 3-1/4" of
wire.
===========
Sounds like another fun project.
Only problem I see is that most off the shelf rack stock is
square, and a round rack will have less tooth contact area which
may not be a concern if the loading is light, unless the rack is
cut on a flat of the shaft.
Can you turn the ends of a square rack round and put some round
extensions on the ends to run in the linear bearings?
Cutting a flat rack on a round shaft should be no problem, but as
the teeth on a rack are straight, it would appear that full teeth
all the way around the shaft could be easily cut on a lathe with
something like an Acme thread bit [14-1/2 PA] possibly with
rounded corners for fatigue resistance. Depth of a 1 mm module /
25 DP tooth is about 0.10 inch. Will 0.30 inch carry the load?
Assuming a 4 inch PD, 3 inches is 85 degrees and 4 inches is 115
degrees.
Another thought is to use a worm gear setup but stroke the worm
back and forth rather than rotating it. IIRC most worms are acme
threads with a DP thread spacing, and this would give you an
single envelopment setup for better contact and longer life.
A 1/2 dia round rack gear indicates a fine DP gear, possible like
a 1MM module [20 degree PA] / 24 DP gear. For examples see
http://americanprecision.thomasnet.c...375fw?&seo=110
http://www.qtcgears.com/RFQ/default....rs/KHK046.html
Let the group know how you make out.
Unka' George [George McDuffee]
-------------------------------------------
He that will not apply new remedies,
must expect new evils:
for Time is the greatest innovator: and
if Time, of course, alter things to the worse,
and wisdom and counsel shall not alter them to the better,
what shall be the end?
Francis Bacon (1561-1626), English philosopher, essayist, statesman.
Essays, "Of Innovations" (1597-1625).