How many threads above the nut?

[chucketn]

On model steam engines, when using studs and nuts to secure things like cylinder heads and bearing caps, how many threads should show above the nut?

Chuck

 

[bazmak]

rule of thumb is half the bolt dia in thread length

 

[goldstar31]

The classic answer is the one that answered an American who wanted to know the Horsepower of a Rolls Royce - which- no doubt the American wished to improve. He got the remark which beat the great American humourist O.Henry's evasive answer- which we all know( or should) and R-R replied 'adequate'

So I've just pulled the top off my new Briggs and Stratton which is 'mis- behaving' and the projection above the flywheel retaining nut is 'as R-R said'.

 

[Hopper]

Not sure about models but in the "real" world, 1.5 threads (minimum) is a common rule of thumb. More than five threads exposed is widely considered "fastener too long".
But I don't think there is a specific standard.
Maybe in Tubal Cain's book "Model Engineer's Handbook" there may be some mention. He gets into that nitty gritty detail stuff pretty deep.

 

[Tin Falcon]

Well chuck I think you need to make them look good and all the same.
Unless you are in a competition what looks good to you. If in a competition what looks good to the judges.
The above answers are certainly correct. And a quick google search shows varied opinions of real world applications anything from flush to a few threads.
Tin

 

[kvom]

I think having them all the same is better than worrying about a number.

 

[modelman1838]

Contrary to what I do myself which is about 1 1/2 threads above the nut, in old engine practice it was custom to leave the stud a little below the top of the nut which would be gradually filled with paint as the engines were repainted many times and this prevented the formation of rust in the thread, unpainted nuts were seldom seen.

Hugh

 

[goldstar31]

I was involved when our T Class submarine sank in the Thames in 1950. About the time that the film 'Morning Departure' was first screened. However, the other accident was HMS Thetis in June 1939 when- or so it is suggested . paint prevented the correct operation of valves. Thetis became HMS Thunderbolt after it was raised.

So you have an example!

Norman

 

[Foozer]

Boeing - Pin length [Go-NoGo] was 1-3 threads.

 

[Wizard69]

I think having them all the same is better than worrying about a number.

Yep! You want to strive for a professional appearance. Frankly the more threads showing the sloppier the machine looks.

 

[Wizard69]

I probably should add that sometimes assembly can be greatly aided by longer bolts. Even threaded rods are used for reach, though in the real world those excessively long bots are removed when the parts are aligned and drawn together. This is my experience at work working with process steam controls.

Assembly issues are also why it is common practice to have a thread or two sticking out, invariably threads will get damaged. Likewise due to chamfering all the thread isn't there anyways. In fact clearing the chamfer is often a indication that the bolt is long enough, a quick visual guide if you will. Finally the other thing to consider is the compression of lock washers, gaskets and the like, you would want the nut fully on the bolt before you start to develop torque.

I don't think there is a universal answer to your question.

 

[Charles Lamont]

Yep! You want to strive for a professional appearance. Frankly the more threads showing the sloppier the machine looks.

I'll second that. Also, a polished (but not too shiny) dome on the end of the stud finishes it nicely.

 

[Blogwitch]

In engineering, we always used one full thread protruding from the top of the nut. That was used for self locking nuts, wire retained, split pinned or just torque tight, so that is what I use on my models.

The easy way to do it is to put two washers under the nuts, one the same thickness as the thread pitch the other being the assembly washer. File thread down to just touching the top of the nut, then remove the thread pitch thickness one. Perfect 1 pitch protrusion on all nuts.

John

Now does anyone have a drawing to make an easy to use tool to put a slight chamfer on the thread end.

 

[Forestgnome]

rule of thumb is half the bolt dia in thread length

That's the rule of thumb for thread engagement of a blind fastener, such as a bolt into a block.

 

[Swifty]

That's the rule of thumb for thread engagement of a blind fastener, such as a bolt into a block.

Sorry, I'm going to have to disagree, always used the rule of 1 1/2 times fastener diameter as minimum engagement.

Paul.

 

[Charles Lamont]

I can't remember where it came from, but the rule I use for tapped holes in materials of similar strength to the bolt or stud, is 7 turns of thread.

 

[Wizard69]

I can't remember where it came from, but the rule I use for tapped holes in materials of similar strength to the bolt or stud, is 7 turns of thread.

Interestingly there is a web site where a guy did FEA on threads, in this case gun barrels, and determined that by 7 threads you have gained all the strength you are going to gain. The roll off from five threads is pretty steep. I kinda wish I could find that site again, it does provide a scientific explanation for this discussion.

Consider that 5-7 threads of standard pitch comes real close to the diameter of the screw, at least in smaller sizes. This may be where the one diameter rule comes from. Of course this FEA is the result of a mathematical model, you still need to consider real world issues such as tolerance and thread quality. The other thing I'm not sure about is the different grades of bolts one can get these days, would a thigh strength bolt benefit from more threads engaged?

Generally at work I go for far more threads than one bolt diameter though. There are other practical reasons to consider. For example if you can drill all the way through you eliminate blind hole tapping problems. Wear and tear do have an impact on thread strength and longevity, more engagement seems to lessen this impact. Often we are taking about dissimilar materials and the reality is threads can be easily pulled out of softer materials especially with better grade bolts.

At the other end of the spectrum I work on injection molding machines with large diameter tie bars. I'm assuming the engineers at one time did some analysis of those tie bars and the associated nuts. Here thread size isn't all that useful to consider, in many cases though the nuts designed for these machines are in the one diameter range or a bit more, for thickness.

 

[Forestgnome]

Sorry, I'm going to have to disagree, always used the rule of 1 1/2 times fastener diameter as minimum engagement.

Paul.

Uum, that's what I said. The op was asking how many threads ABOVE the nut. That's not engagement. When you use a nut the amount of engagement is determined by the length of the nut.

 

[Swifty]

Uum, that's what I said. The op was asking how many threads ABOVE the nut. That's not engagement. When you use a nut the amount of engagement is determined by the length of the nut.

Sorry, I must have missunderstood, I know that people are moving off topic a bit, but you did give the example of a blind fastener such as a bolt in a block where the minimum engagement should equal the diameter of the fastener. I only mentioned that we always used 1 1/2 times the diameter as minimum, this would allow for chamfers on the tapped hole and the end of the fastener.

Paul.

 

[Hopper]

So, it raises the question of how many threads above the nut for a fine thread versus a coarse thread? If a fine thread sticks out 1.5 threads beyond the nut, it will not stick out as far as a coarse thread of the same diameter sticking out 1.5 threads. Which one would be right?