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MagicSat tracking mechanism


RLWP

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I usually have an 'interesting' job on as well as engines and gearboxes. This time it's part of a MagicSat salletite dish mechanism:

 

Assembly.jpg

 

The dish and the cover are missing, I didn't need them for this

 

This is the elevation motor:

 

Elevation-motor-1.jpg

 

The problem the owner was getting was, when the system began to lift the dish, it would stop and show an error code.

 

The dish is attached to the upper shaft in this mechanism. The bronze wheel is driven by the worm at the bottom, which is pegged to the shaft of the motor on the far side

 

Wrom-and-wheel-1.jpg

 

The owner had worked out that, if they turned the worm by hand, it would be free for half a revolution, then went very tight stalling the motor

 

This is the worm:

 

Worm-1.jpg

 

Quite a nice installation, needle roller and thrust bearings each end.

 

The shaft and bearings:

 

Worm-2.jpg

 

I was pretty sure what the problem was, putting the shaft in the lathe confirmed it:

 

http://www.primrose-engineering.co.uk/wp-content/uploads/Bent-shaft.mp4

 

The shaft is bent. The puzzle then is how a nearly half inch shaft could get bent!

 

The solution is that the end of the shaft is drilled for the spindle for the motor. Now, the motor shaft is around 10mm long, the hole in the worm shaft?:

 

Worm-3.jpg

 

over 28mm. In fact, long enough to drill into the worm part itself:

 

Worm-4.jpg

 

That's an 8mm hole in a 12mm shaft. To make it more interesting, there's an undercut at the end of the bearing surface:

 

Worm-5.jpg

 

Instead of a solid 12mm shaft, at this point the worm is supported by a tube, 11.6mm OD with an 8MM bore. Not enough

 

I'm going to bore out the worm and mount it on a new solid shaft - with a shallow hole drilled in the end

 

Richard

Edited by RLWP
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Interesting, and a well engineered piece of kit. But how on earth did the shaft get bent?

 

A worm and wheel mechanism can create quite a separating force between the two parts. Apart from the too deep hole, all the components are very nicely proportioned - including the 12mm shaft and bearings

 

Richard

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Interesting, and a well engineered piece of kit. But how on earth did the shaft get bent?

Didn't Richard just explain that? At one section the shaft is 11.6mm diameter with an 8mm hole leaving only 1.8mm of "meat". The nature of the worm drive is that a force on the dish elevation cog translates partly into a bending force on the worm, clearly enough to bend the shaft. I wouldn't be surprised if a close inspection didn't reveal a circumferential crack at the weak point.

 

Maybe the dish whacked a bridge etc when it was elevated.

Apart from the too deep hole, all the components are very nicely proportioned - including the 12mm shaft and bearings

 

Richard

One wonders if they are all like that, or whether the machinist looked away at the wrong moment on that one

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So, fixing it. First off was to cut off the ends:

 

Worm-6.jpg

 

This gives some idea of the tube supporting the worm, rather than a solid shaft:

 

Worm-7.jpg

 

Parting the shaft off threw up a burr on the inside of the hole, it's actually thinner than it looks here

 

Next up, put the worm in the lathe:

 

Worm-8.jpg

 

I'm using an aluminium split collet here to hold the worm. It holds it true without damaging the 'thread'

 

Then drill and ream the worm:

 

Worm-9.jpg

 

ready for a new 12mm shaft:

 

Worm-10.jpg

 

And the finished component:

 

Worm-11.jpg

 

I decided to bore out the worm completely rather than fitting a stub into one end as this way I can guarantee the ends are concentric

 

Assembly-2.jpg

 

This is the altitude mechanism installed back on the mechanism, next to the azimuth wheel

 

Richard

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Richard, could you have made the shaft a thou or two bigger, heated up the worm and had an interference fit?

I could have made the hole a little smaller and used a push fit, and it isn't worth it. Modern adhesives are very effective for this kind of job

 

Richard

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Interesting and very good work, but to me it seems like a lot of effort to fix what's actually a rather expensive and over-engineered solution for setting up a satellite dish. Considering it usually takes about 5 minutes to set them up manually.

 

Just my opinion for what it's worth.

Edited by blackrose
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Ah, "proper" engineering. These days it can be hard to find someone to do this kind of work. I'm sure it leads to our "throw away" culture.

 

Well done to Richard, and also the owner, who got it repaired rather than simply replacing it.

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I wonder is this is all down to quality control, and a sloppy supplier over drilling the hole?

 

Also surprises me slightly that that the solution was not to contact the manufacturer and have them sorted it, although if nothing else I guess if you have the kit the time to spin the the new parts is actually fairly reasonable.

 

Personally I solve the 'issue' by not having a TV on the boat, but each to the own, and certainly I have no issue with these such devices existing and having seen one in the flesh running with a clear dome, can vouch for the overall high quality of the item.

 

 

Daniel

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The suppliers solution was to sell a new base for around £1000

Richard

They where not up for 'sod off, this is clearly as design/wulity issue of yours, I'll give you £50 for a new worm wheel' then? The gits.

 

How old is the unit, approx usage?

 

Daniel

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I wonder is this is all down to quality control, and a sloppy supplier over drilling the hole?

 

 

Since Rolls Royce nearly downed an Airbus 380 with an oil pipe that was drilled visibly off-centre, nothing surprises me anymore.

 

MP.

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  • 3 weeks later...

Excellent thread Richard, have enjoyed following it.

It's always good to observe an engineer at work.

 

In a different hobby I restore antique amusement machines and regularly use modern adhesives and epoxy putties.

Have you a favourite type/brand?

 

Cheers

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