Optimum thread angle?

Hey... well written Q.
It is so well written that I think you have actually accurately answered your own question!

The Angle of the thread form and the angle at which it spirals up the shaft are of course two different things. The more threads per inch... e.g the pitch of the thread would give better locking, as if you have say just two tpi it would just slip off like a Bendix on an old starter motor. But obviously infinite tpi is impossible and would take an age to do up a nut , also the threads would be ridiculously fine, which then brings in your point about stripping threads in Aluminium.

My guess is, the various forms and pitches for different diameters and materials is a balance which was discovered empirically and than formalised over the years into the various standards we have today.

The angles and forms they use in lead screws and ball screws may be of interest?

Just realised I'm waffling...maybe you'd better wait to hear from one of the mech' eng' guys who actually know what they are talking about .

<slinks off monitor left, to hide in secret cat nest>

Del

You are right.

The ratio P/d2 giving in fact the thread helix angle (P=pitch & d2= mean diameter) depends on the friction which occurs between the 2 parts and the thread flank angle.

There is NO optimal angle there is only a limit for self-locking or not as it was explained in the thread about this subject. Unfortunately the person who asked the question did not understand the physics of bot tightening.

As long as the helix angle is under this limit the thread will be self-locking. In fact you can find different materials with same or almost same friction level and on the contrary for same pair the nature of the surfaces (lubricants, treatment, a.s.o.) will modify the limit.

Standard threads take into consideration usual friction values mainly for steels.

It is not possible to totally avoid unlocking in use only by choice of the pitch this is the reason there' are so many solutions to keep an assembly tight and which not always perform as the supplier claims since the conditions are different from the ones he tested his solution for.

The adage a rose by any other name is not fitting (: when referring to thread strength fastening aluminium to steel. With the many grades of aluminium available several which have tensile equal to steel the inference of coarse threads used in aluminium to prevent stripping is not because the coarse thread ridges are taller. It is actually due a misunderstanding of torque values relative to increased thread clearance required for application of steel/aluminum thread interference.

Realizing too little clearance results in too much interference itself resulting transfer/adhesion of aluminium thread form material to the steel thread form called marring. The marring is the initial event and when additional torque is applied to the bolt stripping begins, some call it locked but it is weak. Upon removal of the bolt most aluminium thread material will be attached hence a hole stripped of threads.

The fine thread range generally referred to as (SAE) when using correct torque values and thread form clearance result in greater thread form interference area and a stronger fastening than possible of coarse thread.

Anti-seize compound are necessary and torque value adjusted but should be used with caution as ingredients of some compounds can dissolve aluminium same is true for a few tapping lubes.

I had been using steel as heads/caps for 2.500"-17" tie-rod actuators then switched to aluminium for increased production and tool life. The tie-rods were hardened to grade 8 as were the nuts but the tie-rod was threaded SAE into the aluminium head. the fail rate is less than 2% per 1000 actuators.

I agree that you seemed to have answered your own question here, as you understand that lower flank angles = more contact area = more friction. The physical strength needs will dictate the size/pitch/material of the fasteners.

I wish all questions/topics presented in this forum were done with the thoroughness of your post.

Regards,

Ing. Robert Forbus

You are partly right:

in fact friction does not depend on the contact area within some limits of course. From the other point of view as you surely know the contact pressure distribution is not uniform on the threads so that an increase of the number of threads does not lead to a better distribution or to a sensible lower pressure on first thread, the most loaded one.

The flank angle reduction has as effect the reduction of the component which tries to unscrew the bolt after tightening and thus increase the "self-locking" safety. It also increase the same safety by decrease of the friction limit of self-locking.

In general it is better to use coarse threads for strength and fine threads for less loaded assemblies. The fine threads have several draw backs among them the higher flank specific loads which increase the galling tendency at assembly or request for lower rpm in order to avoid local exaggerated heating.

Hi

For special bolt applications the us bolt tensioners on which the pull on the bolt to the required load, screw the nut down by hand (and hand tight) and release the tension.

Even if we use Torque Wrench you don't have the exact load, it is friction depended

The materials are important, (this is quite away of the topic) if you look for self tapping plastic screws (metal screws to be used in die cast plastic parts) they have a very shallow tread teat, a very coarse tread not only to allow the flow of the material, but also to even out the different max load for the materials

You may want to look at http://www.spiralock.com/ they have a unique thread that has a ramp. Look under the technology section.

This answers the question most succinctly. Good find.

While I echo the thoroughness of the theoretical discussions, there are a few missing points. First, can you actually machine according to the required "ideal" parameters? By machine I mean form threads in the "rod" and also make the required corresponding threads in the nut. Tossing another screw into the works, implementing another thread series would be tough. The ideal fasteners would have be to clearly differentiated to prevent misuse with so-called standard hardware. I had one old bicycle that exemplfied the nightmare with metric, American inch, and Whitworth threading. While I only needed two sets of wrenches (US and Metric) the seat post bolt had Whitworth thread. CR4 is good for this type of discussion, but somewhere, reality and practically have to creep in.

"Conversely, if rotational motion (torque) is applied to the screw, the steeper the angle, greater will be the axial force and lesser will be the frictional force."

Torque leads to a tangential force acting on the thread flank. The steeper the helix angle (α = atan(P/(∏*d2) with d2 as mean diameter of the thread) is the bigger the normal force on the flanks and since this is one generating friction the higher the friction. Considering an extreme situation when the flank is parallel to the thread axis (α = ∏/2): there is no axial component since the tangential force is normal to the flank. I consider the same angle as mentioned in your text before : the angle of the thread helix.

If one considers the axial force as the entry vector the situation is the opposite.

This means that your assumptions are totally contrary to the actual situation.

Thank you all very much for your comments and insights which have given me a better understanding of the subject. I appreciate your help.

The most common screw thread form is the one with a symmetrical V-Profile. The included angle is 60 degrees. This form is prevalent in the Unified Screw Thread (UN, UNC, UNF, UNRC, UNRF) form as well as the ISO/Metric thread.

The advantage of symmetrical threads is that they are easier to manufacture and inspect compared to non-symmetrical threads. These are typically used in general purpose fasteners.

Other symmetrical threads are the Whitworth, and the Acme. The Acme thread form has a stronger thread which allows for use in translational applications such as those involving moving heavy machine loads as found on machine tools. Previously square threads with parallel sides were used for the same applications. The square thread form, while strong, is harder to manufacture. It also cannot be compensated for wear unlike an Acme thread.

Basic Size is the nominal size to which the tolerance is applied to determine the maximum and minimum material size.

Allowance is the difference between the design (maximum material condition, MMC)size and the basic size. See Unified Standard Series.

Thread Classes: The different thread classes have differing amounts of tolerance and allowance. Classes 1A, 2A, 3A apply t external threads; Classes 1B, 2B, 3B apply to internal threads. See Unified Standard Series.

Classes 2A and 2B The maximum diameter of uncoated (unplated) class 2A, (external) thread are less than the basic by the amount of allowance. When a coating is intended, the max diameter of class 2A may be exceeded by the amount of allowance. For an internal thread, the minimum diameters are basic whether or not coated (plated)--no allowance is available at the maximum metal limits. See Unified Standard Series.

Classes 3A and 3B are used for closer tolerances than those available from classes 2A and 2B. The maximum diameters of Class 3A (external) threads and the minimum diameters of Class 3B (internal) threads are basic, whether coated (plated) or not--thus no allowance or clearance is available for assembly of components at maximum material condition. See Unified Standard Series.

Classes 1A and 1B are the replacement threads for American National Class 1. They are intended for special applications involving replacement parts, for quick and easy assembly even when the threads are slightly damaged or dirty. See Unified Standard Series.

Coating (or plating) of threads The external threads should not be greater than basic size after plating, and the internal threads should not be less than basic size after plating. For electro-plated parts, Class 2A allowance is sufficient for the plating build-up. After plating the external threads should pass a basic Class 3A GO gage and a Class 2A NO-GO gage. Class 3A does not include an allowance--the class 2A allowance may be used as long as it is adequate for the plating thickness considered. Since only Class 2A external threads have an explicit allowance for coating, other classes both internal and external should have the size limits adjusted to allow for the desired coating thicknesses.

courtesy of efunda

The answer you seek 60°, the angle doesn't change unlike the # of threads per/inch, depth, clearance and other conditions of thread classification.

have you read the question? you sure?

Absolutely, thread angle is the angle of the "V" or the thread form itself. "V" thread is in my experience the best most powerful locking thread form. The "V" is 60° regardless of class, fine or coarse, dissimilar materials no matter the "V" thread form is 60° period.

http://www.efunda.com/designstandards/screws/screws_intro.cfm

Has good depiction of thread form and search able data base worth your time.

But I prefer the Machinist's Handbook and especially the abbreviated version.

What do you think...

Max locking power that does not eliminate thread integrity is found at www.unionbutterfield.com/tech/taps/locking.asp