True

I'm guessing it refers to the thread length where the accumulated shear strength of the thread interferences equals or exceeds the tensile strength of the core – so that if you overtighten you'll break off the head rather than strip the thread?

If so it can only be a rule of thumb and may vary with material and pitch/depth of thread.

During my monkey-days with a motorbike in the 70s, I noticed that I always seemed to be stripping BSF bolts, but breaking the heads off BSWs… 

Edited By Mick B1 on 29/06/2018 10:57:04

+1 to Andrew's 'sort of'!

As a rule of thumb, it's not unreasonable. But there are other factors that make a difference, in particular the fit of the thread. A loose fit won't be as strong as a close fit.

Where strength matters, 'Old Engineer' is sent to make tea while Stress Engineer is consulted. Stress Engineer can do the sums. Taking into account the strength of the materials carrying the external and internal thread, he can calculate the minimum number of threads at 100% engagement needed to develop maximum strength. It's not many, see half-sized nuts. If max load is exceeded he knows if the bolt will break before the internal thread strips , or vice versa. More important, he knows that cutting perfect threads is difficult, expensive, and slows down assembly. Even though they are not full strength, it is usually more convenient to make threads to be a loose fit, perhaps achieving less than 50% of maximum in the case of a cheap general purpose fastener, like those holding my lathe on its stand. Stress Engineer will balance the need for strength against cost.

Considerably more care is taken specifying and making threads for safety critical applications. Aircraft are interesting because they also need to minimise weight, leading to components being more stressed than in ordinary applications. Many aircraft have crashed because the wrong bolt was substituted during maintenance.

I'd guess that on a typical model steam-engine, and certainly most of my crude efforts, a 2x diameter rule is perfectly acceptable. Far from sending for Stress Engineer, I rarely bother even to look up the reference tables that simplify his efforts for ordinary mortals, and instead deliberately loosen thread fits by using slightly oversize tap drills, or reduced diameter rod. I do that because it increases the life of the tap or die while reducing the chance of breaking a tap in the hole. Pragmatism rules.

Dave

Edited By SillyOldDuffer on 29/06/2018 11:28:52

I like Jason's nuts logic. But don't forget to allow for the materials in use. Ie aluminium is going to want more turns. We had a head of the Design Office who used to quote 3 turns for the Al chassis. It was fine for the covers, mostly, but things like heavy vibration for aircraft design proving showed this up seriously.

Haven't heard that but a 1/2" bolt was said to be good to straight lift 5 T

Look at the length and dia of the threaded part of eye bolts for example vs weight of lift

Brave Man !!!

Nuts are about 1 x diameter thick, usually. Some cylinder head nuts etc about 1.5. I was taught 1.5 x diam. is considered max strength but 2 x is a bit of a safety margin.

Also, was taught that tapping into aluminium, use a coarse thread as fine will pull out easier. Some textbook theory is at variance with this but over many years I have seen BMW motorbikes with countless stripped fine threads in cast aluminium on drain plugs, filler plugs, cylinder studs and the like. But Harley-Davidson, who use coarse threads in aluminium, seem to last 80 years and more without stripping out.

Always taught dia x 1.5 min anything else is a waste of effort.

Colin

Plus one for 'Structures' by Prof. J.E.Gordon.

A good read throughout.

One gem I was pleased to find was what he says about the English longbow being made from MEDITERRANEAN yew. Yet that same Mediterranean yew doesn't work in the Mediterranean region because the climate is too hot. My father, a skilled woodworker, always asserted that our English yew was too splitty and windy-grained to be any good for making a bow. He would have been pleased to read the book.

Best regards,

Swarf, Mostly!

Stress Engineer knows that the cross section of a short length of thread is greater than the core diameter of the screw.

You don't need a great number of turns for the thread to be stronger than the thread diameter.

As Jason observes, why should a blind hole require a greater length of thread than a nut expected to take the same load?

Notable exceptions are threading into soft materials or with greatly truncated (under-diameter) threads.

As a full time stress engineer for a number of years, I will number among the sort-of camp. If the materials are similar, and the tapped hole is of a material not less than 60% the strength of the screw, it is true. Steel fasteners into alumium or brass are more difficult. Steel fasteners into wire thread inserts in aluminium or brass are more difficult still. A good rule of thumb for steel into 6082-T6 is 3D engagement makes the screw break first for grade 12.9 or weaker. Wire thread inserts make the 3D reduce to 2.5D.

Regards

Richard.

It's a bit more complicated than that.

There is no doubt that any Yew stave can make a Longbow, but it is the 'character' of the timber that matters.

For many years our academic historians reckoned a Longbow's draw weight to be between 90 -120 lbs. For many years those of us who were exploring the lost & forgotten history of the Longbow were saying it was much, much, MUCH higher & we were proved right.

We are now into a proven history of Longbows capable of drawing 160 -180 lbs. This is significant because it re-writes the history that those academics would have you believe !

A bowman would have possessed several bows of different draw weights. I am in no doubt that most were sourced locally, but there is what we now find the need to call a 'Warbow'. This is a bow with a draw weight exceeding 140 lbs & would possess a range that defies most of what the academics base their theory's on.

Brushing the nonsensical academic historians to one side, the cutting edge of finding our true, forgotten history is now about exploring the regions that this Yew grew & how we went about trading with them !