self tapping screw or ali rivet?

[rosehugh22]

Which is stronger, an 5mm s/s self tapper or a 5 mm ali rivet going into a 3mm ali mast.
Opinion please?

Hugh

 

[greenalien]

Depends on lots of factors, particularly long-term corrosion. Neither are ideal ; stainless screws will quickly corrode if inserted directly into ali, while ali pop-rivets usually have mild steel cores, which also corrode. You could consider using monel-metal rivets, or stainless inserts (properly bedded on chromate paste) with stainless set-screws. Both would be better options than what you are suggesting.

 

[pvb]

As a direct answer to your question, the rivet would be stronger (pullout strength).

 

[PCUK]

The stainless screws won't corrode, it would be the mast that corrodes, but as it is a huge area compared with the screw, the effect would be negligible. As above, rivet is stronger.

 

[rosehugh22]

self tapping screw or rivet?

Thanks for advice. I am installing about 20 composite type mast steps, each of which has five 5mm fixing holes, one of which is awkward to get at with my rivetter and all of which would be tough using monel. I thought to use 4mm ali rivets in each step plus a s/s self tapper in the awkward one. Would a s/s insert be so much better than a self tapper? Or I could tap all the holes.

All ideas very welcome

Hugh

 

[rosehugh22]

sorry - that should have read 4 of 5mm ali rivets in each ste plus a ss etc!

 

[awol]

Beg, borrow or steal a better pop rivet gun and use monel rivets.

 

[VicS]

Beg, borrow or steal a better pop rivet gun and use monel rivets.

+1 I think and set them in Duralac paste.

Two handed riveter rather than lazy tongs

 

[rogerthebodger]

Thanks for advice. I am installing about 20 composite type mast steps, each of which has five 5mm fixing holes, one of which is awkward to get at with my rivetter and all of which would be tough using monel. I thought to use 4mm ali rivets in each step plus a s/s self tapper in the awkward one. Would a s/s insert be so much better than a self tapper? Or I could tap all the holes.

All ideas very welcome

Hugh

I fitted composite folding mast steps some years back and I used ali rivets using a long nose rivetter on the difficult one. If I was to do it again I would drill and tap M5 and fit allan cap screws with corrosion inhibitor.

 

[awol]

M5 thread has pitch of 0.8mm so with a mast wall thickness of 3mm you would have, IMO, insufficient engagement.

 

[VicS]

M5 thread has pitch of 0.8mm so with a mast wall thickness of 3mm you would have, IMO, insufficient engagement.

Would M5 fine be suitable . That has a pitch of 0.5mm but of course the thread depth is correspondingly less

 

[rogerthebodger]

The major load on the fasteners of mast steps is shear so IMHO the pull out load or thread shear load will be relative small.

If this is a concern why not use a stainless rivet nut which would have a higher pull out load plus a greater thread engagement. Also the use of durlac will not be needed if you need to remove the mast step due to the stainless to stainless thread contact.

Vic In softer material you generally use a coarser thread and not finer to improve the thread shear load

 

[wklein]

The major load on the fasteners of mast steps is shear so IMHO the pull out load or thread shear load will be relative small.

If this is a concern why not use a stainless rivet nut which would have a higher pull out load plus a greater thread engagement. Also the use of durlac will not be needed if you need to remove the mast step due to the stainless to stainless thread contact.

Vic In softer material you generally use a coarser thread and not finer to improve the thread shear load

You could always go whitworth i suppose i sometimes use them for tapping into GRP. A lot of steps will have significant tensile load as they tend to want to pivot. such as on these ones:

 

[VicS]

Vic In softer material you generally use a coarser thread and not finer to improve the thread shear load

I thought that was probably the case.

 

[macd]

T I am installing about 20 composite type mast steps, each of which has five 5mm fixing holes, one of which is awkward to get at with my rivetter and all of which would be tough using monel. I thought to use 4mm ali rivets in each step plus a s/s self tapper in the awkward one. Would a s/s insert be so much better than a self tapper?

I asked Vyv Cox (the metallurgist of this parrish) precisely this re the same job a couple of years ago. He said ally rivets would be fine. Why introduce a dissimilar metal if you don't need to? Mind you, 4mm sounds a bit small.

 

[Clyde_Wanderer]

Depends on lots of factors, particularly long-term corrosion. Neither are ideal ; stainless screws will quickly corrode if inserted directly into ali, while ali pop-rivets usually have mild steel cores, which also corrode. You could consider using monel-metal rivets, or stainless inserts (properly bedded on chromate paste) with stainless set-screws. Both would be better options than what you are suggesting.

Definatly Monel revits coated with Duralac, which is used in the aeroplane industry to prevent corrosion between componants of different noblility.
I have noticed the rigger has just fitted my winches on the new mast using 6mm revits.
C_W

 

[greenalien]

He said ally rivets would be fine. Why introduce a dissimilar metal if you don't need to?

There are 2 problems with using ally rivets - first, neither the mast, the steps or the rivets are pure aluminium, that would be too soft - so they are some unknown sort of aluminium alloy. This makes them all dissimilar metals, with the potential for electrolytic corrosion between them.
Second, ally pop-rivets tend to have mild steel cores - the bit that snaps off, but which usually leaves the head inside the rivet - a really good cause of corrosion.
Monel-metal rivets are stronger than ally, and harder to set because they usually have stainless cores. Just needs the right tool - a 'lazy tongs' riveter, not a lever type. Used with Duralac or chromate paste, the risk of corrosion is minimal. I wouldn't want to use mast steps fitted with any other sort.

 

[rosehugh22]

self tappers or rivets

Thankyou gentlemen for excellent advice. I have learned a lot!

If the boat was on shore I would go for monel but I must get the steps fitted whilst it is afloat hence my decision to use ali rivets - 5 of 5mm to each step.
I won't have to worry about corrosion nor to have to fear being catapulted into the cold Atlantic as I set the rivets while on high!
I have already fitted one step to see what it is like and to my unskilled eye it looks very strong - surely it would need a considerable shock load to break the rivets ? and any
danger of them being pulled would apply to monel as well ? And I will always have a good
back up system rigged.
I must now decide how to deal with the awkward hole in each step1

I wish I had stuck to canoe-ing !

Thankyou all again.

 

[JayBee]

......Just needs the right tool - a 'lazy tongs' riveter, not a lever type.....

Why not?

What's wrong with a heavy duty lever type riveter?

Lazy tong types can have a tendency to skid off the work, when the mandrel snaps off. But perhaps that's just me being a bit ham fisted.

 

[emandvee44]

Rivets

There was an article last year in one of the yachting mags (PBO or YM) by someone who installed mast steps on his yacht (all the way up) and he used rivet nuts, being of the opinion they were the safest option. I think he used M5 ss bolts, so the rivets need an 8mm hole IIRC.
After reading the article I googled "rivetnuts" and easily found a UK supplier who would ship to here. I bought some csk Ally rivet nuts, and the rivet nut gun (of course) - looks a nice piece of kit. Comes with mandrels M4, M5, M6 & M8.
My first use of them will be for the burgee staff at the masthead (but let's not drift into a BE thread)

If anone needs more info, ask, and I will look up the supplier.

Michael.