Best method of clamping 2 pieces of aluminium together?

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The unit in which is fitted the engine control on our Jeanneau (5 years old) is a pear-shaped aluminium casting, in two pieces, that clamp round a stainless steel post next to the wheel.

The paint (baked on) has mostly come off (complained to Jeanneau inside warranty period but no interest or help) and now I am doing something about it! It turns out that there are 6 threaded bolts (sitting in a threaded hole in one side) holding the two sides together and as you can imagine, with the seawater around, they were seized pretty solidly. I got 4 out with effort but the last 2 have had to be drilled and smashed out! (I won't bore you with the pain and agony of trying to drill in a confined cockpit space using a battery hand drill).

I am now going to have the 2 aluminium sides re-sprayed but the question of how to rejoin them once complete is to be decided. I will not put threaded s/s back in - obviously, but what would be the best way to remake the join, that needs to be robust enough to take the working of the morse control?

Drill out the holes and use a nut and bolt of some material that wont react with the aluminium? Put a plastic sleeve in and use s/s nut and bolt? Ideas please?
 
if you reassemble them with Duralec on the threads there will be a fighting chance that they will not seize. Also put a bead of silicon sealer between the mating faces to keep water out.
 
I suggest you use Stainless Interscrews as the threaded portion? You will have to drill out the threaded portion to take the outer diameter of the interscrew, they are straight knurled so can be knocked in or pulled in with a bolt. Then use stainless socket head cap screws to attach the other half. cover them with Duralac or equivalent when you fit the screws.
Also why paint? it may be better to get the aluminium halves anodised to the marine spec(although no idea what this would cost).
Just checked and look at Avon stainless and Marine Interscrews - they do several styles but the marine are only M4 and M5 threads. The other styles don't have the straight knurling to hold in place.
 
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PetiteFleur said:
I suggest you use Stainless Interscrews as the threaded portion? You will have to drill out the threaded portion to take the outer diameter of the interscrew, they are straight knurled so can be knocked in or pulled in with a bolt. Then use stainless socket head cap screws to attach the other half. cover them with Duralac or equivalent when you fit the screws.
Also why paint? it may be better to get the aluminium halves anodised to the marine spec(although no idea what this would cost).
Just checked and look at Avon stainless and Marine Interscrews - they do several styles but the marine are only M4 and M5 threads. The other styles don't have the straight knurling to hold in place.
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Stainless and aluminium??? Even with Duralac!
 
I know little about this but aluminium bikes are held together with 7075 aluminium bolts (which I think are quite pricey, but will be available from your nearest decent bike shop)). I was going to look at these bolts for marine application but they are low down on the list.

Jonathan
 
What is the consensus of opinion re 'normal' aluminium nuts and bolts at sea? My thought was this could be a solution but salt water and ali are bad friends, aren't they? Is there such a thing as a marine grade ali nut and bolt - I haven't found one yet?
 
For painting, my current favourite is zinc spray etching primer, overcoated with black rubberised spray paint as it's easy to respray dings (Halfords is convenient, but not cheapest)
I would use s/s bolts and nylock nuts with nylon sleeves and washers with a bit of silicon grease.
 
I am not surprised it is causing problems. The trouble is not just the stainless steel bolts, but that it is clamped around a stainless steel post. That is lot of stainless steel electrically connected to a relatively small piece of aluminium in a wet environment.

The best you can try and do is to isolate the aluminium from the stainless steel as much as possible. Duralac is very good. Plastic washers under the stainless washers help. I would also try and isolate the stainless pole. Using thin flexible plastic chopping board material is good for this purpose.

I would not use aluminium bolts, but if you do, one of the 6 series aluminiums have much better corrosion resistance than 7075. Aluminium 6061 bolts are difficult to find, but are available. 6061 is a marine rated aluminium.

See here:
http://www.aluminumfastener.com/images/catalog.pdf

I guess it is a bit late, but I would have anodised the aluminium sides rather than painting them. This will last much longer. Anodising can be done quite easily on small parts like this. You can even DIY.
 
I'm interested - is there any evidence, actual evidence rather than anecdotal comment, that 7075 is materially different to 6061? If they are used on bikes they will, in the UK, be subject to salt water every time the bike is used in winter. I have never heard there are issues or do people with bikes not get them wet.
 
Hadenough said:
Stainless and aluminium??? Even with Duralac!
Click to expand...
It'll work fine - the stainless interscrew will be fixed in the Al. bracket and even if it eventually corrodes it won't come out. Then the stainless screw into the interscrew. Good idea to insulate the bracket from the stainless post, particularly with plastic from a milk bottle!
 
Neeves said:
I'm interested - is there any evidence, actual evidence rather than anecdotal comment, that 7075 is materially different to 6061? If they are used on bikes they will, in the UK, be subject to salt water every time the bike is used in winter. I have never heard there are issues or do people with bikes not get them wet.
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It is not commonly appreciated that aluminium has very different properties depending on the alloying metals.

The 5 series aluminiums have by far the best corrosion resistance. These metals are used for aluminium hull building. The best of 5 series like 5083 is basically inert in seawater, even without any protective coating.

The 6 series have considerably worse corrosion resistance, but they can be extruded and thus formed into complex shapes, which is not practical for the 5 series. Most sailboat masts are constructed of one of the 6 series aluminiums like 6061, but this is generally then painted or anodised to increase the corrosion resistance.

The 7 series aluminiums are very strong and therefore used in some aircraft and bike parts. Unfortunately, they contain significant amounts of zinc and copper. In a marine environment the zinc acts as an anode and erodes, leaving the aluminium weak. They have considerably worse corrosion resistance than the 6 series.

The above is just a summary. The aluminiums are grouped in these series, but there are still significant variations within the group, especially regarding corrosion resistance.

There are a number of problems associated with aluminium corrosion on 7 series bike frames, although the aluminium is generally heavily protected with paint and they don't have the constant exposure to salt water. There is a lot of debate in bike circles if 6061 or 7005 is better for bike frames. The greater corrosion problems of 7005 frames is a factor. This is despite 7005 being one of the more corrosion resistant 7 series aluminiums. It only has less than 0.1% copper (verses 1.2 - 2% for 7075) and slightly less zinc at 4 - 5% (verses 5.1 - 6.1% for 7075).

The properties of aluminium alloys is a complex subject. It can be a great material in the marine environment, but the right grade of aluminium must be used in the right way.

Personally, I would be very wary of using 7075 on a yacht.

The application for 7075 is limited anyway. It is my understanding that 7075 should not be welded because of the subsequent risk of mico and stress cracking (although I believe it may be possible with exotic welding technics like electron beam welding) so you are unlikely to see this grade used often.
 
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noelex said:
I am not surprised it is causing problems. The trouble is not just the stainless steel bolts, but that it is clamped around a stainless steel post.
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I do wonder about designers! The whole aluminium box for the control, has rubber or plastic 'washers' at top and bottom to, presumably, keep the ali pod away from the mounting pole. Except, that inside! it is very firmly bolted to the same pole, making a no doubt excellent metal to metal contact - hence my dramas with the bolts!

From your various helps, I intend to drill out the holes to 'over-size, and fit a stainless nut and bolt right through both halves but putting plastic washers under both the bolt heads and the nuts, with Duralec plastered on the central threaded section which should, if I get it right, be floating and not touching the ali anywhere anyway.
 
I'm not questioning the theory - I'm wondering has there been any practical evaluation.

G70 chain is described as being brittle and susceptible to hydrogen embrittlement - in practice - none of these dire warnings has eventuated (when in use as an anchor chain). Anchor shanks made form low tensile steels are are predicted to bend like cheese. People even suggest that concave anchors are disaster waiting to happen. But at the end of the day - there is little practical knowledge or experience behind the rhetoric, in fact concave anchors are proven very succesful - and bent anchor shanks???.

Repeating the mantra is good therapy but hardly useful - if we want to move forward we might better look at actual hard won experience before simply repeating, possibly , flawed information.

7075 might not be acceptable - but until its proven otherwise I have an open mind. I certainly would not condemn it, outright. It might have restricted application, it might be perfect in some marine applications - but unless we are willing to try - we will never know (a bit like G70).

Jonathan
 
I thought I'd add to my rant on those who make suggestions based on minimal application knowledge.

I recall a long time ago when we bought our first reel of dyneema on being warned it was neither UV nor abrasion resistant. At the time Rothman's (this shows how long ago it was) had removed the outer sheath of reefing lines to save weight but only where the lines were inside the mast or boom. What a joke - now its been tried and tested we find dyneema is one of the most UV and abrasion resistant bits of cordage. We are still using (I think one halyard and a few sheets) some of the same dyneema we bought then - it was an excellent investment.

But to refocus

Anchor Right in Australia introduced an alloy version of their steel Excel. The fluke is a welded 5083 alloy and the shank, which is removable, is 7075. The shank sits in a pocket of 5083. The anchors have been available for about 7 years. Our model, 8kg and equivalent in size to the 15kg steel model, has seen good service over 5 years and is stored in a bow anchor locker. There have been no reports, of which I am aware, of shanks corroding and we see no corrosion of our shank nor where our shank abuts the 5083 pocket. Because 7075 alloy is strong and Anchor Right beefed up the shank thickness the alloy version of the anchor must be as strong as the steel version with its high tensile shank and certainly as strong as the original Bis80 shanked version.

If the mantra described above, post No 14, was correct our anchor, or its shank, should by now be a pile of dust

I am currently trying to arrange testing of 7075 fasteners in a marine environment so as to add some further application knowledge.

For further information on the alloy Excel contact Chains, Ropes and Anchors in New Zealand who will have a comment as a retailer.

Jonathan
 
Possibly not heavy duty enough for your clamping application BUT have you looked at or tried using NYLON nuts and bolts - corrosion free - self "lubricating" non seizing etc I'm assuming the load exerted is a shearing action on the bolts as opposed to a pulling on the thread action ?

we have them on some of our work boats for equipment retention purposes (light loads) and they seldom work loose or shear - of course easily stripped if you get carried away tightening them up but we have found benefits IE I don't have to fill out countless work permits and forms and be on my hands and knees with a TIG welder trying to get the stainless studs out of an aluminium casting for an hour

Just a thought
 
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