Machinist

[Lawrence London <lfljvenaura@gmail.com>]

fine threads versus coarse threads - Mechanical engineering other topics - Eng-Tips
http://www.eng-tips.com/viewthread.cfm?qid=72123

joel47 (Mechanical)

(OP)

22 Sep 03 10:47

What are the pros and cons of using fastener with fine treads vs coarse threads?

2

Cockroach (Mechanical) 22 Sep 03 11:58

Typically, this is a design issue and decided upon because of the limitations in geometry of the piece.

For example, given a thin walled tube subjected to internal pressure, length of engagement becomes problematic since there is not enough wall to support the minimal number of threads to meet a particular safety factor.  The alternative is to vary threading pitch rather than attempt wall build-up schemes (threadolet, raised shoulder, etc) which could add cost or make the piece dysfunctional.

A possible downside is commercial availability of the pin.  Depending on the size of screw used, you may be entering a specialized fastener or hard-to-get component.  The alternative is to turn your own or special order the piece, again driving up cost.

Perhaps others can comment on additional pros/cons.  There are many, but these considerations are the most common.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

4

unclesyd (Materials) 22 Sep 03 12:57

I'm going to have to get on "Cockroach". He left out the most important factor in choosing between fine and course threads.  The fatigue endurance limit on fine threads versus course is approximately 25%-30% higher. That is way you see a lot of set studs with the fine threads set in.
The best reason not to choose fine threads is that they are so easily cross threaded. Fine threads in some alloy fasteners should be avoided.

ccw (Nuclear) 22 Sep 03 13:12

See "Machinery's Handbook" , Industrial Press Inc. -/Threads and threading/ Unified Thread Series/ Thread Series / Fine-Thread Series:/ for additional answers on where fine threads are suitable.

3

CoryPad (Materials) 22 Sep 03 13:35

The information from Cockroach and unclesyd is correct.  In addition, from Handbook of Bolts and Bolted Joints by Bickford and Nassar:

"Coarse pitch is generally recommended for routine applications.  Such threads will have greater stripping strengths when used with weak nut or joint materials or when used on larger diameter fasteners."

"It is easier to tap brittle material if coarse-pitch threads are used.  Such threads are also easier to use in most cases: easier to start, faster rundown, etc."

"Fasteners with fine-pitch threads can have higher tensile strengths because the thread root and pitch diameters - and therefore the tensile stress area, A_s - are greater than they would be for a coarse-pitch thread on the same nominal diameter. [...]  They also resist self-loosening under vibration or shock better than coarse-pitch threads."

Regards,

Cory

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Mandrake22 (Mechanical) 22 Sep 03 15:28

Fine threads also can provide 10 to 15% greater clamp loading than coarse.

ie3 (Industrial) 24 Sep 03 08:55

One of the important issues here is also assembly. Coarse threads assemble considerably faster and easier. If designing for high speed assembly, consider this.

unclesyd (Materials) 24 Sep 03 10:07

I might add,  a little ding on a course thread is a big ding on a fine thread. Out comes the thread file.

For those that are interested go to the following for a dissertation on bolt threads and I mean dissertation.  I’ve never seen bolts threads/stress  presented like this.
Click on the help file and after that the "errors in text books".  

http://www.boltplanet.com/default.asp

I wish I would have had this when I stripped the studs and blow the heads of my flat head Ford engine.

crun (Mechanical) 9 Oct 03 00:46

An easy way to get a clear mental picture about the clamping force of fine threads compared with coarse is to think of the thread helix angle as being the inclined surface of a wedge. A long thin wedge can easily be hammered in to lift a heavy weight (fine pitch thread) while a steep wedge requires much more effort to hammer it in under the same weight.
The flip-side is that you have to insert a fine wedge a long way for a little amount of lift, while a steep wedge lifts more quickly. (see the "faster run-down" comment from Corypad)
So for high clamping force use a fine thread, but for general "bolt 'em together" work a coarser thread is more practical.
I assume we are talking metric threads here by the way, imperial thread systems (BA, Whitworth, UNF, UNEF, Brass threads etc. can be a real trial!)

ornerynorsk (Industrial) 9 Oct 03 09:46

Fine thread will generally gall up easier due to the closer running fit and because most people do not use a torque wrench, but fine thread is a must when higher fatique and strength properties are required.

christoph (Structural) 10 Oct 03 00:10

A couple of minor corrections to this otherwise excellent thread.  unclesyd and ornerynorsk, the fatigue strength of coarse threads is higher than fine threads, due to the smaller (sharper) root radii in fine threads.  (By the way, metric threads have higher fatigue strength than imperial threads due to their superior proportions and improved root radii.)  crun, the effort to torque a standard bolt or nut (provided its bearing face has already made contact) is greatly dominated by friction, so the slight mechanical advantage of the inclined plane of fine threads surprisingly causes virtually no decrease in installation torque.

CoryPad (Materials) 10 Oct 03 10:21

christoph,

You are correct that fatigue is a function of root radius.  But, according to V. Kagan in ASTM STP 1236 Structural Integrity of Fasteners:

K ∝ (P/R)^0.5

where

K = stress intensity factor
P = pitch
R = thread root radius

Using M10 x 1.5 (R_min = 188 µm) and M10 x 1.0 (R_min = 125 µm) as examples:

K_p=1.5 = 2.82
K_p=1.0 = 2.83

Fatigue crack propagation rate (da/dN) is a function of stress intensity:

da/dN = C K^m

For martensitic steels, m ~ 2.25, therefore:

da/dN_p=1.5 = 10.34
da/dN_p=1.0 = 10.37

Which is less than a 1% difference.

Also, according to ASTM STP 1236 & 1391, thread root radius only affects stress intensity factor if a/D < 0.02, so once the crack depth exceeds 2% of the minor diameter, neither pitch nor root radius matter.

The way that thread pitch really affects fatigue life is due to the fine thread's larger stress area.  This allows higher preload, which reduces additional bolt stress from external forces.  Also, the larger stress area will require more cycles for a crack to progress across it, assuming equal crack propagation rate.

Regards,

Cory

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snaketray (Mechanical) 10 Oct 03 13:33

One aspect not mentioned is torque accuracy. Fine threads will increase torque accuracy up to 300 percent over course. An important factor when considering high pressure seals.

unclesyd (Materials) 10 Oct 03 14:01

Adding a little to "CoryPad’s" excellent post.

There are several other aspects to initiation of a crack and that is the actual shape of the root radius. That is the point that the radius is tangent to the flank of the thread.  I don’t have any current data but the old numbers are.  Tangent at a minimum of 83 1/3 % of the thread depth up to 179,000 psi fasteners.  Special fasteners have a  profile with a tangent at 75% thread depth which will not gauge out but will work in standard nuts and holes.  This point of tangency is the reason for the different thread forms in bolts and nuts.  Blending the corner helps a lot.

I would say 60% of the fasteners failures investigated that were caused from fatigue failure in the threads had a metallurgical or mechanical notch in the root regardless of the thread forming process.  This was both with fine and course threads.  But overall we had less trouble with the fine thread if it was a set-in stud or a bolt.  We actually used fine threads on the set-in half and course on the other half of studs on a very high pressure cyclic service.


Actually the best advice is: “Tighten the fasteners beyond their working load and keep them tight!”  
A quote from Mr Eltoon McBroom; Sports Car; January 1966

earnest (Mechanical) 13 Oct 03 10:52

Its the tolerances!

Here is my attempt to cut through the confusion and concomitant resort to cookbook formulas and facile rules of thumb -

We are comparing the strength of externally threaded fasteners with a: coarse pitch and b:fine pitch threads.
What are we holding equal?
The material & finish, of course, the flank angle and the Major Diameter. Why? because bolts are sized by major diameter: it dictates what they can pass through etc.

Now you can clearly see that the shear-stressed cylinder defined by the ROOTS of the male threads is  larger on the fine-threaded fastener than on the coarse-pitched one. That is why the fine pitch bolt is stronger - fatigue and otherwise.

What's the catch? Tolerances. If tolerances are fixed, as pitch gets finer, the diametral clearance becomes a larger percentage of the thread height and less of the flank is contacted. Too fine and the thread just strips.

I observe that Automotive fasteners are getting finer-pitched. Presumably because their fasteners are being made to finer tolerances.



 

HomeMadeSin (Mechanical) 17 Oct 03 13:40

Awesome post and replies!  I was going to post the exact same thing.

How would cold-flow of the clamped materials affect the selection of fine versus coarse.  For example, my preference for plastic parts is thru-bolt design (more industrial parts - not consumer goods).  In our case (we manufacture plastic pumps), we've seen loss of torque (and yes, we use calibrated torque wrenches) in coarse fasteners.  I haven't fully evaluated the cause yet (temperature variations, cold-flow, or vibration).  

I realize fine threads are better for vibration, but could you also reduce the effective torque to minimize the effect of cold-flow using UNF?  Nylock nuts are relatively expensive.

CoryPad (Materials) 17 Oct 03 15:04

HomeMadeSin,

For your application (through bolted plastic components), the thread geometry and preload themselves are irrelevant since they must pass through the bearing surfaces to create the compression stress in the plastic parts.  The compression stress determines the magnitude of the creep strain and creep rate that the plastic exhibits.

Regards,

Cory

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HomeMadeSin (Mechanical) 17 Oct 03 17:03

CoryPad:

Thanks for the input.  However, I would expect that for the same reason fine threads resist loosening via vibration better, you could reduce the clamping force or compression stress and achieve the same anti-vibration level as coarse threads.  Because in our particular situation, using o-rings for sealing we don't need as much compression as you would with flat gaskets.  

I'm trying to boil it down to the difference in pitch and how it affects the tendency to loosen the nut, apply compressive force, etc.  We could lower the torque with coarse threaded fasteners (to achieve less compressive stress and therefore lower creep rate) but risk the vibration issue....

unclesyd (Materials) 21 Oct 03 21:17

Ran across this site going through some of my old stuff.
It's quite interesting except a little hard to navigate. Look at the videos

www.surebolt.com

TimtheToolMan (Mechanical) 27 Dec 04 02:25

One key factor in keeping fasteners from self loosening is lubrication. The lower the coefficient of friction the greater the tendency to self loosen.

Also, machining fine thread in harder materials is easier and less likely to break taps with CNC machines then when machining coarse threads.