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Re: linear positioners

Most cheap actuators are for use up to 600 lbs of force, but can usually 
hold a static load of twice that.  In the case of a 1.2M (4 foot) dish
this will survive 90 mph winds even with the cheapies.

The cheapies are brands like "SuperJack".  Many TVRO enthusiasts hate these 
cheapies, but for a 4 footer they should be fine.  These typically use a 
"acme nut" or a oiled sleeve bearing for the screw drive internal to the 

The better units are "Thompson Saginaw" or "Venture Saginaw" types with ball 
bearing drive nuts.  These actuators can be bought up to 2000 lbs of 

They come in four lengths.  12", 18", 24" and 36".  18" and 24" would be the 
most common if your getting a scrap TVRO setup.

12" == Designed for small KU type dishes, a good choice for your 
application, price about $40-45 + shipping ($20)  Usually rated at 600 lbs
[possible source:  http://www.buydvb.com "smallear"]*

18" == Designed for smaller (6'-7') TVRO dishes, also a good choice 
especially if you can get a e-bay or scrap one.  New $70 + ship,
usually rated at 900 lbs.  [possible source:  http://daveswebshop.com  -- 
this guy is a ham, BTW]*

24" == Designed for 7' to 12' TVRO dishes, usually rated at 1200 lbs acme or 
1500-2000 lbs ball screw.  Probably not an effective choice for
elevation actuator, too big.  New $100-130 plus shipping.

36"  == Designed for 10' to 18' TVRO dishes, harder to get, expensive.  
Ususally rated 2000 lbs+.  When I've seen them they are close to
$400 new plus shipping.

* I've bought from "smallear" before, and their products are good, shipping 
is VERY VERY expensive.  I've never bought from "daveswebshop" before but he 
does have a good selection of stuff.  I have no financial interest in 

I'm assuming you want to put a smallish actuator in place on the declination 
adjustment of a 4' dish that is already tracking the Clarke (gestationary) 
belt ok.  I think a cheap 12" or 18" are will do this fine.  It depends on 
your mount.  First figure out how much travel
to make your +/- 30 degrees and see how much movement you need.  Then you 
will have to see if your mount can physically
accomodate a rod (cylinder) that is 24" - 36" long.  If yes, then you are 
set, if not you'll probably have to fabricate a lever to move the

The motors have to be mounted in a certain direction (the instructions with 
the arm will show that) so the "weep" hole for trapped water works.  Make 
sure you have this downwards.

I hope this is the info you need. $100 may seem expensive, but on a surplus 
mount with a surplus dish, two of these arms can track AO-40 and be much 
more mechanically stable than one $500-600 G5500...

This spring, this exact setup is one of my projects.  So, yup, I've not done 
it yet, but I've thought about it for quite a while...

Fred W0FMS

>From: w7lrd@juno.com
>To: amsat-bb@amsat.org
>Subject: [amsat-bb] linear positioners
>Date: Sun, 9 Feb 2003 09:33:39 -0800
>I am looking for a linear positioner to adjust the elevation of a 1.2M
>dish.  It only has to go from 0 to 30 degrees.  The price of these on the
>research I have done is a bit pricy.  I would appreciate any/all advice
>regarding the use of these devices.  I have never seen or used these
>things before.  I appears to be an efficient and perhaps cost effective
>method of elevation control.  The learning curve never seems to stop!
>Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
>Not an AMSAT member? Join now to support the amateur satellite program!
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Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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