BSF fastener grading

I bought some BSF socket head screws from Zoro a while ago. They are advertised as grade 10.9. There are no grade markings on any of the heads.

10.9 is a metric grade, so I'm not sure how to interpret the claim anyway. My understanding was that most self-colour steel BSF screws and nuts were traditionally Grade R, which I believe is a near equivalent to metric grade 8.8. I haven't bought any of Zoro's BSF hex nuts, but I see that Zoro's are either not graded, or described as grade 6 or grade A, sometimes both simultaneously.

I recently asked Thomas Smith fasteners for some information on their own BSF hex nuts specifically:

"Please could you tell me what grades of BSF hexagon full nuts you have available off-the-shelf and which of these grades would best match a supply of self-colour BSF screws I have in various thread pitches and lengths that when sold to me were described as grade 10.9. I have only ever seen graded BSF nuts for sale in grades "R" or "A". My understanding was that R is medium carbon/medium tensile and grade A low carbon/low tensile. Grade 10.9 in metric fasteners is usually considered fairly high tensile, I think."

Their reply:
"10.9 is a metric spec. Gr R is the grade for the majority of BSF Bolts. Our stock is Gr A which again is the common grade for BSF nuts. Never seen a BSF nut in anything harder than EN8. EN8 are manufactured on demand at a significant higher cost. Prices for Grade A are on our shop online."

So my questions are:

Are Thomas Smith essentially saying that the only off-the-shelf BSF full nuts they sell are low tensile Grade A nuts?

If my Zoro BSF screws are genuinely equivalent to metric Grade 10.9 (though how would I know?) then what nuts would ideally go with them and where can they be had? Zoro's own BSF nuts, graded at grade A and grade 6 simultaneously, seem as unsuitable a match for my screws as Thomas Smith's.

When I have the time I plan to use quite a number of BSF nuts with my screws, and I would like the grades of each to be a close match and to be fairly high tensile.

Some clarity on BSF fastener grading generally would be appreciated. There is very little clarity in the marketplace on this topic, from what I can see.

Looks like Thomas Smith is correct in saying that Grade A nuts are the norm.

According to British standard 1083 Grade A nuts are suitable for bolts of Grades A, B, P and R.

Material tensile strengths are :-

Grades A and B :- 28 tons / sq inch

But permitted elongation is different :- Grade A elongation can be up to between 10 % and 14% whilst Grade B permits up to 17%.

Grade P :- 35 tons / sq inch

Grade R :- 45 tons / sq inch

Text of British Standard 1083 can be found here :- **LINK**

https://jl-fasteners.com/pic/1/BS1083%20Bolts_Screws_Nuts.pdf

Clive

Relying on distant memory, but my recall is that, for commercial fasteners, high-tensile nuts are not necessarily "better" than lower tensile. When the assembled fastener is subjected to tensile loading, the weaker material allows better, ie more even, load sharing along the engaged thread. Might be wrong though!

Edited By Clive Brown 1 on 07/02/2023 15:23:04

For 'bar turned' nuts the issue is getting hold of appropriate high tensile hexagon steel bar. here's the relevant page from a stockholder's catalogue. As you can see, the usual 0.445" 0.525" 0.600" BSW/BSF sizes are only available in a leaded free cutting material. I do a lot of work with BSF/BSW era cars and when I have needed proper HT nuts (thankfully very small runs) I have milled the hexagon stock from round using an indexer on my Bridgeport. Stainless hexagon bar is available in BSF/BSW sizes, but isn't suitable for use with anything structural.

On aircraft, high tensile bolts (designated by notches on the periphery of the hexagon head) were often used with standard nuts.

Andrew

I am by no means an expert in this field but I have read that a high tensile nut is usually unnecessary in a tension situation. The core of a bolt being much smaller than the depth of core to exterior of the nut. I stand to be corrected by those that actually know better.

The advice I had from a maker's rep (Unbrako) in the 1970s was that socket headed screws and bolts are not made from 'ordinary' R grade or 8.8 steel, as the stresses in the socket would be inclined to open it out. His advice was 'Socket heads = high tensile'. And he was not trying to sell me anything.

Cheers - Tim

I worked in the motor trade for 30 years and only occasionally saw high tensile nuts and they always seemed to be longer than normal nuts and were usually cylinder head nuts.

I am not sure the increased length has much to do with the high tensile properties.

The proportion of total force carried by each thread of the nut drops off very rapidly as you go deeper into the nut.

See:

http://www.fastenal.com/content/feds/pdf/Article%20-%20Screw%20Threads%20Design.pdf (top of page 7).

With respect to Zoro, it does to remember that they're a clearing house for a large number of engineers merchant's and the like (Cromwell, Erik's, etc.) Like a reverse purchasing organisation of sorts.

So their stock, especially on items which are competitively priced or seem a bit bizzare is often things which aren't moving regularly enough for a supplier and they want to de-stock.

In that vein, it seems likely that your 10.8 BSF bolts were a special item stocked by one of their partner organizations who are no longer selling them (say because a customer stopped making the product they were used in).

DC31k

Early in my career at RARDE I was told that the "long nuts are high tensile" thing was to make it easy to distinguish them from the ordinary breed.

On reflection with the benefit of 50 odd years more experience I suspect that a high tensile nut is generally capable of stretching or even snapping a matching high tensile stud before the thread strips if sufficiently tightened. For fairly obvious reasons the preferred failure mode when someone gets over enthusiastic with a long spanner is a stripped thread in a relatively weak nut leaving an undamaged thread on the stud.

Concerning the proportion of load carried by each thread in a nut I do wonder what happens when the threads distort due to overload as things approach shipping force. Seems logical that the entry thread carrying the most load will distort first pushing some of the load onto the next thread and so on. In principle such effects can be calculated if the thread clearances and pitch tolerance variations are known.

I've long considered that the "first thread carries most load" is a (necessary?) oversimplification in an imperfect world. I'd rather define the "first" thread turn as being the one whose male - female pair first takes up the clearance under load and the "second" thread turn being the next male - female pair to take up the clearance.

Et sequence.

Said pairs may or may not be adjacent or in any defined order. Given that all bolts stretch measurably when correctly tightened there is clearly scope for such re-distribution of load as clearances are taken up. In principle the required maths are easily described but attempting actual calculation thereof is likely to be very, very painful. A hybrid analogue - digital computer would seem the least painful way. Hybrid computers are rather fun, pure digital is basically accounting.

Clive

Thanks to everyone for your replies.

Interesting to read the British Standard, and to see that it approves the use of grade A nuts with fasteners up to grade R.

Something I omitted to mention earlier is that Thomas Smith's reply:

"Gr R is the grade for the majority of BSF Bolts. Our stock is Gr A..."

is at odds with the information on their web page stating that their bolts are Grade R.

Jelly, my understanding was that Zoro and Cromwell belong to the same umbrella company, namely W. W. Grainger Inc.

My BSF screws may well have been end-of-stock items, but I did buy nearly 10,000 of them ranging from 1/4" to 1/2" in various lengths, and the majority of these seem to be still available from Zoro, albeit at prices that are now between 30 and 600 times higher than the prices I paid.

Coming back to the question of grading, I've also been having fun lately buying UNF hex nuts from three well established fastener suppliers. Judging by the orders I've received, not one of these suppliers has been reliably able to distinguish between grade 1, grade 5 and grade 8 UNF nuts, and I'm inclined to think the explanation for this is ignorance rather than them knowing full well the grade of what they're sending out whilst hoping that the customer doesn't.