Balanced Dish

[altis]

Having finally set up my 85cm dish on my Metronic Diseqc motor I'm alarmed by how much it has to struggle sometimes. This is because the centre of gravity of the dish, on an arm, on another arm is far away from the axis of the motor.

It should be fairly easy to design a dish and mount so that the axis of the motor passes right through the CoG - much like a radar dish. Does nobody make one?

 

[gameboy]

Does nobody make one?

Does a radar dish move on an arch?

Think that may be where the problem lies.

 

[altis]

So long as the axis is through the CoG it doesn't matter if the thing is lying on its back - it's still balanced.

 

[rolfw]

The Axis has to be offset from the drive cog, otherwise the dish wouldn't travel the arc, it has to decline at either side of due South. There are some dishes which mount much closer to the armature though, which does of course cause less stress.

 

[Channel Hopper]

Dishes used to be somewhat closer to the polar mount than they are now, and when they were more commonly prime focus designs, quite a few people would add a boom arm and counterweights to reduce the pull of the dish away from the arc.

Now of course its all offset design, with no relation between motor production and dish production, the common component being the clamp arrangement around a 2" pole. That leaves no integration at the design stage to prevent premature failure (wear and tear). In this day and age, many installation companies rely on callouts to existing systems to stay in business, and the wholesalers provide the supply with cheaper and cheaper kit, to remain at the top of the food chain.

Quantity over quality, price is almost without exception, the closing argument with the man on the street.

 

[altis]

I understand a bit more now.

The motor I chose is the Metronic H-H pole mount from here:
http://cpc.farnell.com/jsp/endecaSearch/searchPage2.jsp?newSearch=true&Nty=1&Ntt=motor%2C+h-h+pole+mount&N=401&y=0&showImages=true&Nto=se00595&x=0&Ntk=gensearch

Good price and it looks a very good construction. Not surprising really - after a little detective work I'm fairly certain this is OEMed for them by Jaeger.

However, there's a basic flaw in the design. It can only be fitted 'pin up'. In fact the manual warns against fitting it upside down. This means that the dish is high up and located away from the axis of rotation. On motors that are installed with the pin down, the bend of the pin pulls the dish towards the axis. With this arrangement the axis passes through the dish somewhere in the middle of the back of it - probably near the CoG.

At least I know what to look for next time!

 

[Channel Hopper]

Thats not balanced, or indeed fitted with much degree of longevity. The three attachment points are far too close to each other, causing a very strong lever force which will make mincemeat of the brickwork.

Using standard bolts on a chimney means the first high wind is going to start pulling bricks, especially with the pointing in that condition. There should at the very least be a chimney lashing round the chimney and extended to include the top V supports , just to keep the system from oscillating

The top of the dish protrudes over the ridge, probably as there is no other place to get full arc, but the weakest point ( and the one likely to suffer premature wear) is going to be the main planet gear of the motor.

You need a much stronger system to withstand the British weather on that size of dish

 

[altis]

And I just thought I was being helpful

 

[gameboy]

I've seen a lot worse..

Think it's the 80cm Lidl dish/LNB with the plastic bracket on the back, so not heavy and it's sheltered by the roof and chimney.

Like the fact that you have used safety harnesses (always a good idea).

Don't be too dis-heartened if it lasted over the New Year storms it should be up for a long time!

 

[Simba27]

I have the same dish and motor, mounted in a similar fashion. It hasn't fallen down yet, although my previous one did.O-Ha

Two things I learned from that: Firstly I made a bracket that wraps around and bolts into the side of the chimney, so the pulling force is at 90 degrees to the direction of the bolts. Secondly, I've rested my pole on the small flat area, uphill of the chimney, such that the roof takes the weight and the brackets just have to hold it upright. I still worry about it every time the wind blows.

My motor came factory set with the hardware stops at 45degrees each way. I've adjusted mine to approximately 90degrees and the motor certainly does struggle trying to lift the dish when it's canted that far over. I agree that some sort of balancing would certainly help it.

 

[altis]

Yes indeed, it is the Lidl CO80 dish and LNB. My choices for mounting places are very limited. There is a motorised system up the road that is on a very long poll peering over the roof. This looks ugly and is, in my view, precarious - although it is in a more sheltered position. My Metronic motor can only be mounted on the top of a pole which is why it ended up on on an L-bracket as low down as I could manage. Because of the adjacent roof, there is only room for one lashing and that would have been very high up the chimney. The bolts are M10 chemical fixing and shouldn't pull out - that's the kind of thing that climbers use.

Back on topic...

From my experiences, I would disuade anyone from buying a pin-up diseqc motor. As you can see from the photograph these lead to a very unbalanced set up and will almost certainly cause the motor to wear out quickly. Indeed, the French version of the manual that came with the motor warns agains moving outside +/- 45 degrees with an 85cm dish.

I will invastigate returning the motor and replacing it with a pin-down model. This will also bring the dish much lower - probably below the roof line.

(Hi Simba ;) )

 

[Channel Hopper]

Sorry for giving you such bad news in the last post, with a dish in that location, callouts are lokely to be rather more expensive than one in a garden on a pole. But if youve done it yourself then I commend you (the chemical fixing bolts are however likely to help with brick pulling)

THe larger the distance between the fixing points the lass force there is on the mount and chimney as a whole. Im not sure if this is the only size of 90 degree mounting plate offered, but if there is one that is maybe 1/2m longer then the lower bracket could have been installed at the base of the chimney

A lashing is really the only way to ensure the complete structure is working against the elements, and not just four or five bricks

 

[kungpo]

Interesting; I also got the Metronic motor from CPC, but it looks more like the conventional pole mounted one. I paid about £40 for it.

Seems pretty well made as well.

 

[rolfw]

To add some additional support, a collar and guy wire could be used to secure to the sides of the chimney, using tensioners to tighten and steel corner on the chimney corners. Have had to use this method on one difficult aerial installation and it worked well..

PS. excuse the bad line drawing, having to use a diodgy mouse.

 

[Channel Hopper]

PS. excuse the bad line drawing, having to use a diodgy mouse.

Made in Wales ?

 

[rolfw]

LOL, must type more slowly.