1.4462 vs a4-80

[Josh Danziger]

Hi,

Confession first: I am not an engineering professional----so I realise that my question may get moderated away before it even sees the light of day! But if so I'm not sure where to take it ......
I am the owner of the 1911 Bristol Channel pilot cuter Cornubia. Now sailing Victorian or Edwardian workboats without winches, just with block-and-tackle, is knackering, and I decided that rather than have everyone pre-knackered before they even start sailing by raising the anchor on a manual windlass I would buy an electric one. Lofrans Falkon seemed to look reasonably timeless, and is a suitable size for 31tonne vessel, and we are just installing it now. What rather took me aback was that it is fixed with 4no ~11mm mounting holes (I think 7/16"), with plastic liners taking the size down to 10mm. GIven that the weight is borne primarily by the aft mountings this means that two m10 bolts are doing most of the work ...

This just seemed wrong, and indeed a quick check on some website gave the breaking load in shear of a4 m10 bolts (or maybe set screws) of around 0.8 tonne-force. (Obviously bolts have to be marine grade)

To put this in context, I've seen rule-of-thumb indications that anchor rode strength should be about 25% of boat weight, and this ties in with the ~7-8 tonne-force strength of our 12mm hot-dip galv chain; now it's very poor practice to leave the load on the windlass when anchored (you transfer it to a strong point on the boat), but 2no 0.8tonne-force still seemed very low.

POssibilities I considered were
1)Use some form of HT bolt: A4-80, 1.4462 duplex, 17-4ph, 8.8 hot-dip galv, 10.9 hot dip galv. I couldn't find 17-4ph or 10.9 hot-dip galv off the shelf; and 8.8 hot-dip galv didn't seem to have that much over a4-80 (both have tensile strength >800MPa) , so material choice came down to A4-80 or 1.4462 duplex.
2)Use 7/16 UNC or better still UNF bolts---but this would have meant removing the plastic liners (risking galvanic corrosion) and/or enlarging the holes (but the flanges didn't seem that huge to start with, and I worried about weakening the structure). Obviously I could have enlarged further (eg m12) but this was have magnified my concerns further ... so I decided to stick with m10.

So the eventual decision was to use M10 bolts (not set screws) in a4-80 or 1.4462; but web research showed that 1.4462 has strength ~650-850MPa, and a4-80 is advertised as 800 MPa+. So my first question is what justifies the huge expense (comparatively) of 1.4462 if it's weaker than a4-80 (claims 800MPa+)? The original website that indicated 0.8 tf for shearing m10 a4 gave 2.3 tf for m10 a4-80, which seemed just about enough (given that some load is also taken by the forward 2no mountings as well). And my second question is whether you pro engineers think I am seeing this right.

THanks!
JOsh

 

[mrfailure]

Welcome Josh! That is absolutely an appropriate question for this forum. I don't think I have the expertise to try answering it though. Lovely boat!

 

[Josh Danziger]

Here's a photo with the windlass installed

 

[EdStainless]

The duplex SS (2205 is the common name) is a high strength and very corrosion resistant alloy.
22%Cr 5%Ni and 3%Mo give it better pitting resistance than 316L and much better stress cracking resistance than any 300 SS.
If the threads were roll formed then the duplex bolt will nice and strong, while also being very tough.

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P.E. Metallurgy, consulting work welcomed

 

[Josh Danziger]

Thanks! So if i understand, on day 1 the duplex may be nominally slightly weaker, but give it a few years in a damp bolt hole and the duplex will do better, because of reduced pitting and crevice corrosion, not to mention toughness?

 

[EdStainless]

It will matter what other materials are in contact, but generally yes.
If it is contact with less corrosion resistant materials such as steel, then it can cause accelerated corrosion of them.
Your other option for long service is buy fluoropolymer coated steel nuts and bolts.

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P.E. Metallurgy, consulting work welcomed

 

[kingnero]

A 80 ksi 10mm bolt should be able to withstand approx. 2 tonne in shear, a 7/16 even a bit more.
An 800 MPa M10 bolt (like A4-80) should be able to withstand almost 3 tonne in shear, and duplex is also in this ballpark (700 - 900 MPa).
Something doesn't sound right. Is your analysis correct? Does your pulley mounting also not react in tensile? What is it rated for?
If anything, the plastic liners will help to equally divide the shear forces over the four bolts (assuming some deformation is possible), so that'll probably already help some.

 

[EdStainless]

I spent last night digging through some boxes.
We used to have custom made Monel K-500 bolts made with 12 point heads.
I could only find one.
They were 140ksi min UTS, 125ksi min yield, and 30% min elongation.
And they didn't care about salt water.

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P.E. Metallurgy, consulting work welcomed

 

[Josh Danziger]

Kingnero, i got the 0.8t figure for a4-50 (~80 ksi) from here

https://bssa.org.uk/bssa_articles/shear-and-tension-capacity-of-stainless-steel-bolts/

In particular this table

 

[kingnero]

The tension capacity Pnom is given by Pnom = 0.8 ptb At where:
ptb= 0.7 Usb (U is the tensile strength) or <= Y0.2b (i.e. use whichever is lower)

You already start here with .7 * .8 = 56% of the standardised values.

psb is the shear strength of bolt = 0.48 Usb or <= 0.69 Y0.2b (i.e. use whichever is lower)

For shear, even less...
and then:

Note: These values of shear capacity should be used in conjunction with FACTORED loads. The relevant load factors can be obtained from BS 5950 The structural use of steelwork in building, Part 1 Code of practice for design – rolled and welded sections.

Meaning you'll lose even more of the capacity.


For shear, you have approx. 60% of 500 MPa (See Mohr circle etc.), which is 300 MPa. Times 58 mm² gives you 17.4 kN or 1.7 tonne without any FoS.
Just to have ballpark value...