Need replacement screws for 1.2mtr Channel Master offset

Snake Plisken

Hi guys,

Could someone advise the types of screws I need to replace my very rusty screws that hold the lnb bracket to the support arms and also the lnb clamp screws also, can I source them locally like in B&Q? Any help greatly appreciated

Turok

this,are usa type screws,you can let make them,in your iron factory,i think it is imposible to,get exactly the same measure it!
and,then look,what diameter the screws must have
cloning the winding of the screws is not easy,but not imposilbe,my nabour has iron factory,he can make nearly scew for me,but its to expensice in low quantenties!

ps:welcome on our board

greetings to the desert,from the cold snowy alps!

pedro2000uk

you can replace any of those easily at a B & Q or local hardware store or steel stockholders/engineering supplies trade store etc.. take samples with you and get rust proof - not dear.

wod

just looking at a booklet i have with my channel master dish the bolts are m6 x 30mm for the feed arms and m6 x 20mm for the two lnb clamp bolts

Snake Plisken

Great thanks guys, I thought there might have been an american type thread on them, it took me about an hour to get them off rusted through oiled them up and eventually got them off, for such a great dish you think they would have put more thought into the nuts and bolts

Topper

Obviously just cheapskates these days , I thought stainless steel were de rigeur for sat use

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Formerly toppervte33h
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Turok

why you think americans have always other standarts,and difrendt scew types ,then all other countrys?
is there any special reason for this fact?

pedro2000uk

Stainless steel has certain characteristics that might not be so ideal for satellite use. It distorts a lot more than carbon steel with temperature changes for one.. SS has a much higher coefficient of expansion coupled with a lower heat conductivity compared to the galvanised steal used on most satellite dish brackets that could distort anything they are used on- including the face/ lnb arm/ etc..

It can also have a lower fracture threshold too rather than being stronger - depends on the SS but basic 316 and the stuff they make threaded bar & bolts out of can. We keep threaded bar in stock to knock up large U bolts if needed, we used to use SS but it would fracture at the bends (unless you heated it- then you are doing something else including potentially losing the rust proof).

We just use good quality rust proof nuts/ bolts (galv etc.. ) not SS.

Topper

Obviously the ones that have been on my dish since the previous owner put then there in 1984/85 are the strong ones.

With most metals that are bent without heating it into a malleable state first the cohesive bonds break down at molecular level rendering the material weaker, I'm sure the o/p was talking about nuts screws bolts and washers not exhaust pipe clamps and dish brackets

In short then if it doesn't rust it will do the job

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pedro2000uk

You can bend carbon steel threaded bar 90 degrees without heating with no significant loss of strength for the job- custom U bolts for dishes / motors etc...... you'd have to keep bending it back and forth to get it to show a fracture... you just wouldn't do it with SS threaded bar without it cracking.

Using stainless nuts/ bolts on a steel satellite dish etc.. isn't a good idea- they will expand (& contract) differently to the steel they are bolting in temperature extremes ... no big deal on a basic bit of engineering but on satellite dishes you could lose x% of signal in hot or freezing weather as a bolt become x% looser or tighter.

GreenmanBelg

Some might laugh but on a dry day soon I intend to smeer vaseline on the nuts and bolts of my new installation just to slow down corrosion.

Any comments?

Regards JD Belgium.

Snake Plisken

actually my Dad does the same and have to say it works his are in better condition then mine, its also strange that some of the screws seem to rust up sooner then others, for example the hex head bolts holding the flange to the lnb 3 are rusted badly and 3 are like brand new, go figure, actually has anyone got the specs on those bolts??

Topper

We will have to agree to disagree on this Pedro since all metals expand albeit as you stated at different rates, you appear to be implying that a design engineer builds in a tolerance to guarantee a specific amount of expansion over a specific temperature range for the bolts, I have never seen any dish specification supporting this, nor any available calculations on applied torques and the effect of using a different metal. As it is your trade perhaps you have so please let me read it as I am interested. The theory would only hold water if it was indeed true that the coefficients of expansion are different and according to this table the coefficients of expansion of all ferric metals including stainless steel are the same up to a specific temperature. So I would be extremely grateful if you could point me to some scientific evidence supporting this theory, since it is actually only the bolts we are talking about I am not able to understand the mechanics of this leading to signal loss. My brother is a metalogist I will have to quiz him at christmas

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pedro2000uk

We've worked with various stainless in some extreme environments and one of the biggest headaches is it's distortion under heat differentials - as for a low 'threshold' temperature, the link didn't work but you would be telling me something - we've worked with much greater temperature differences and seen what SS does in the extreme and applied the logic that it's therefore not a good idea to use stainless because any tiny movement on a dish is ...er a lot..30000 miles away, we have enough things that effect signals without adding in another variable - is that ss bolt pulling the lnb arm off a fraction of a degree when it's freezing or red hot. On a scale of one to whatever.. it's in the area of not a good idea... but if you can prove that SS won't expand (0%) at the temperature ranges an exposed dish is subjected to .. I'm receptive.



Hang on... is that link Canada... don't they make some really duff sat gear.. no @@*# wonder.

Topper

The link works fine for me Pedro if it does not work for you how do you know it is Canada??

The whole premise of your argument is flawed you are working from your 'experience' and what others have told you but not producing any scientific facts apart from your opinion to back it up. I try to work with scientific facts whether gleaned in Canada or otherwise. Correct me if I am wrong as you are the one with the meter not me, let's take focal length, from my limited experience + - 1mm movement of the lnb neck in the holder back or forward will impact very little on both signal strength and quality if you are in the sweet spot. Well the coefficient of expansion of SS is 6 microns per mm per degree F or 10 microns per mm per degree C As no dish operates in temperatures above 100 degrees C this gives a max possible figure of which is 0.001 mm expansion per mm. This will hardly hardly make much of a difference will it if 2mm movement does not make a difference. If you were talking about a very long bolt measure in cm's I could understand that expanding sufficiently to distort something but not a small 10mm screw.

The coefficient of expansion is the same for both metals.

The statement you make below you know is incorrect, I have never remotely suggested that SS does not expand of course it expands as stated in my previous post at exactly the same rate of expansion as mild steel or cast steel..... nobody has said that it does not expand apart from you in the statement below issuing a challenge knowing that it cannot be met as the criteria are incorrect

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pedro2000uk

Morning Topper... you've been busy... just let me get some coffee and I'll reply...

the link didn't work it just stayed waiting.. I'll try it again...

..... SS 'has' a higher coefficient of expansion and lower heat conductivity than carbon steel ... either that or a whole engineering industry have cocked it up... seen it in practice far too many times and spent too much time dealing with anti-expansion techniques for SS ...... if SS has the 'same' ...0% difference, at low temperatures ... is what I meant by 0%

If ss bolts on the back of an LNB arm expand or contract x% more than the carbon steel they're bolting together... and you amplify that slight tightening or loosening effect down the length of the lnb arm to the lnb .. and then to the dish face ... .001mm turns into what over 1.5m ?? (depends where the pivot point is and if the LNB arm is braced too) and then up to the satellite..... If SS bolts are used on the carbon steel dish/ polar mount too ... and the day heats up or freezes ... and you drop x% signal off a weak sat and lose it.. I think it's just common sense to keep that out of the equation with everything else that effects the signal.

2mm can be a mile on an LNB's FL/ skew and angle of aim to the dish on a really difficult sat. Where the real sweat spot is on any LNB is one of life's mysteries... they're all different... I could get rude here and compare it with a certain part of the female anatomy and how they are all different... but I wont do that.



EDIT... _http://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html without spending hours (which I haven't got) one quick Google and this link shows 316 SS v Steel's coefficiant of expansion.. the other part is heat conductivity... multiply the effect and you have a... er.. bi-metal strip thing going on..

Topper

Ok with all due respect Pedro this is a satellite hobbyist forum and based upon a comment made by me because my 24 year old dish is fitted with ferrite stainless steel screws we are getting to the point where we appear to be beginning to annoy each other.

So for the benefit of the forum and this thread this will be my last post on the subject

Where 1.5 mtrs comes from and the mysterious amplification effect I have no idea, 1.5mtr could come into the equation if the screw was 1.5mtrs in length as the expansion of the screw would be 6 microns/mm/degree C over a length of 1500mm ( 1.5mtrs) at 100degrees C would produce an increase of 1.5mm over its entire length. In this instance the screw is no more than 10mm long thus how the amount of expansion can be multiplied up or amplified by 1.5mtrs escapes me.

Bimetallic principles have been in use for over 40 years in an electric kettle switch for example however, the metals involved have to be thin enough and bonded back to back to allow any movement to take place. In the case of a screw of one material bolted into another material the overall effect will be to increase or decrease the applied torque on the bolt head dependant upon which is the dominant material used.

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pedro2000uk

Don't ever get annoyed.. it's just healthy use of our respective knowledge, experience etc.. nothing wrong with that...
Here is a link to the other part of the equation... lower heat conductivity of SS v Steel.. _http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html put these two together and SS is a sod for distorting under heat changes and well known across the engineering industry (what's left of it).

Where the 1.5m comes from is 7-800mm from the back of the dish to the LNB (down the LNB arm and up to the lnb then calc shortest distance point to point... the effective length) and another 7-800 mm back to the dish face, mainly if it's braced.

A .01mm movement at the back of the dish on ss bolts pulling on a mild steel bracket. .2mm from the pivot point and then amplified over a 7-800mm lnb arm can move the lnb 4mm up or down and even a bit sideways... double it if you include back to the dish face with a second pivot point if it's braced ...

If you look at an analogue clock face and watch the second hand it demonstrates that amplifying of a small movement at the centre to a larger movement at the outside.
I was involved in the design, build or had built, installed and serviced carbon and stainless steel projects. You get used to the lengths you have to go to deal with SS's propensity to expand & distort above steels'. Extra expansion bellows, expansions loops, exp' bolts, rollers, spring floating and special design techniques at fab/ welding level to limit SS distortion as it's being made etc... temperatures were much higher but the tolerances were nothing like the fine tolerances you routinely work with on satellite.

There is a company that is selling a stainless steel dish.. if it's .4 or .5mm - I say that would buckle if you get a hot sun and a shadow on the dish face = very hot and colder area.

Turok

most comon screws,of todays dishes,and diseqec motors,come from china,and has often miserable quality,best think ,you replace it,with others,and never install them!

is galvaneising a good way,to protect scrws against rust,or is there any better solution?for long time rust protection?

GreenmanBelg

Vaseline???

Regards JD Belgium.

Turok

dont know,never try it!?