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Taptite screws
4

Taptite screws

Taptite screws

(OP)
I have a taptite screw breakage, before reaching it's seating position, on my bolted joint. I've used all the recommended hole dimensions for optimum joint connection. I've also reduced the screw driver speed considerably (600rpm). The taptite screw has already made of high strength steel with wax on it too. The screw goes in to aluminum die-casting.

if the breakage happened before it bottoms out, therefore its seating torque (maximum torque) hasn't been met. Would shorter screw fix this?. in other word, does shorter  length of engagement of thread forming screw reduce the seating torque(maximum torque). I can't see this relation on top of my head.

Please help
ME2000

RE: Taptite screws

Many tapping lubricants that work well on steel work very poorly on aluminum.  Is the wax meant to be used with aluminum?

Barry1961

RE: Taptite screws

I thought Taptites were for plastic.

Maybe I missed a meeting.

Mike Halloran
Pembroke Pines, FL, USA

RE: Taptite screws

I'm more concerned about a steel screw in an aluminum casting.  Hope you don't ever want to get it out after it corrodes in place!

Shorter length would give you less surface area and less friction, reducing the drive torque.  That may be your solution.

RE: Taptite screws

(OP)
Taptite is for metal. The screw is clear zinc chromate finish and Since it is thread forming screw, serviceability isn't an that great which by design.

Gensetguy, So you are saying that the torque drive increases as the length of engagement increases?

Thanks
ME2000

RE: Taptite screws

2
Screw length should have little influence on the joint.  If the screw fractures before seating, you are exceeding its torsional strength.  Perhaps the screw has too low of strength, or perhaps the frictional stress is too high.  The frictional stress is dependent on the dimensions, materials, and lubricant.  It seems you are attempting to address each of these issues.  

Perhaps you can provide the screw size, actual screw material and strength, and the applied torque that is causing fracture.  With this information, we may be able to help identify design or materials deficiencies.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: Taptite screws

Is it a drilled or cored hole?  One thing that often happens with Taptites into cored holes is that the hole size is worked out for the mouth of the hole and no allowance is made for the draft angle and as a result the hole get too small at the bottom, resulting in excessively high drive torque.
I have dealt with Taptite screws for many (too many) years.  Usually the causes of breaking during installation are hole size related.  as Corey said, you are exceeding the torsional strength of the fasteners.  The thread forming torque should not be nearly this high during installation, so something is creating abnormally high drive torque.  The RPM will contribute but it is a much bigger influence in plastic (frictional softening).
 
The first thing that I would do would be to double check the hole size (top and bottom) and then check that back to the Taptite hole size chart (not the same as a tap hole chart).  Also; make sure that you are not bottoming out in the hole, that would create a blind hole failre situation.
Have the heat treatment on the fasteners checked to make sure that they are to print.

My money is on the hole size being off.  It happens very often.

Dick

RE: Taptite screws

(OP)
Cory,

Screw size is 4-40 x 3/8 taptite 2000 w/short point, made out of alloy steel with case hardening heat treat. It is Pan Torx drive head. The actual torque that caused the screw to broke is unknown. I know that the seating torque was set to be 11 in-lb and prevailing torque is about 5-6 in-lb. When I used manual screw driver, I could go all the way to seating torque with no problem. It is when I used electric driver, when I experienced breakage. This breakage happened 1 out of 8 holes patern from 10 samples and they are random. My last attemp on this is to reduce the length of the screw.

PS. the bore hole is drilled .101 diameter and the screw has wax on it already and we are still experiencing 10%breakage.

Thanks
MSU2000

RE: Taptite screws

(OP)
Screwman,

It is drill hole and it is set to the recommended size .101 from taptite manufacturer handbook. I do not think that the breakage was caused due to bottoming out of the screw. (The blind hole is .375 and the thread engagement is .300). I will check for the heat treatment spec. The thing is it is random, I supposed I can do experiment reaming the blind hole 1 thou at the time to see what's going on.

Thanks
MSU2000

RE: Taptite screws

MSU2000,
It sounds a little bit not right, as far as I know, a taper hols is usually required for a taptite screw. I have some experiences on taptite and I only have cast hole (you may want to for it as cost saving idea).

For breakage issue, you should look into these areas;

- the geometry of the hole, diameter (hole opening and the bottom) and the depth, as taptite is a "thread Cutting" screw, sufficient relief should be given for the chips

- the geometry of the screw, major and minor diameters.

- the hardness of the screw and maybe the breaking torque test (you may check BOSSARD website for test procedure and specs)

Good luck to you.

Best regards,
ct

RE: Taptite screws

(OP)
Limct

You are right about the cost saving feature of tapered hole. Infact we are incorporating that in the production part. I followed the recommeded hole size from the taptite manufacturer. I will look for the website you suggest. I appreciate for the info.

I'll keep you guys posted, I'm about to do some testing with shorter screw while keeping existing parameter intact.

MSU2000

RE: Taptite screws

MSU2000, something feels wrong about this. I like screwman have also worked for too long with Taptites. I am assuming that we are talking about Tri-lobal thread forming screws here?
I'm a little rusty on my imperial units but, 11 in-lb seems excessive! I convert this to 132lb/ft? 97N/m?

A little tip, it's definately worth using a cast hole, Taptite have a data sheet with recommended draft tapers etc. but, you must be aware that the Torque required to drive the screw into the cast hole is in the region of 15% greater than for a drilled hole.
There are 2 reasons for this, firstly the tapered hole is clearly a snug fit to the profile of the screw (the draft taper is critical), secondly the HPDC process creates a hard skin on the cast surface which is more difficult to thread-form than the drilled hole. Obviously the drilled hole removes the hard skin.

Good luck, let us know how you get on.
John.

RE: Taptite screws

coughdrop

Wouldn't 11 in/lb be less than 1 ft/lb and just under 1 1/4 N/m?



Alan M. Etzkorn  
Reliability Engineer
Wabash National Corp.
www.wabashnational.com

RE: Taptite screws

(OP)
To be honest, that 11 in-lb of seating torque was based on my experiment. I know I need at least 5-6 in-lb torque to drive the screw in ( this is so called prevailing torque, I suppose) and this based on experiment also. Now, I tested up to 16 in-lb before I started to screw head snapped off. Then I took 65% of that maximum torque of 16 in-lb to get the joint to clamp securely, while exposed to 10 G's. That was 11 in-lb .

MSU2000

RE: Taptite screws

Sorry guys! I said I was rusty on Imperial measures! MechEng13 is of course quite correct.
I have encountered sheared off heads with Taptites in the past, after all of the hole dia/depth checks were verified, it turned out to be the "snatch" effect of the clutch on the electric torque driver.
Just remembered another cause for shearing - Motorola casting which was Alochrome pretreated for painting! Here in the UK Alochrome has now been replaced by environmentally friendly alternatives which don't give the same "skin hardening" effect. Are your castings pretreated for corrosion protection?
What alloy are the castings? if they are high silicon (AlSi12) this can cause problems if the foundry is casting around the eutectic range, which can lead to random instances of silicon grain growth. You said your problem was random?

John

RE: Taptite screws

(OP)
Coughdrop,

The material is Aluminum die casting alloy 383

I believe the casting isn't pre-treated for corrosion protection.

The thing is, when I used manual screw driver. I didn't break a thing. It is only when I used electric screw driver and yes it is random. I tried to align the screw to mating hole by having removable locating pin in several holes location and it didn't help either.

What do you mean by "snatch"?

MSU2000

RE: Taptite screws

MSU2000,

OK, so the material is perfectly suitable for Taptites (AlSi10Cu2Fe) in fact it is very close to many of the castings we produce and fit Taptite screws to.

So, you are drilling the hole only 0.075" longer than the screw length.
Is this the depth of the full dia' or to the drill point? Sorry it seems an obvious question but....
Is this the specified screw length?
The actual length of screws varies realistically by up to 0.050".
We design the hole dia' depth, to be greater than the screw length, by whatever the diameter of the screw is. This allows room for deformation of the material.
Remember, there is no swarf, this is thread forming.
Cheers,
John.

RE: Taptite screws

MSU2000,
I have been traveling and couldn't get to my reference books.  Now I'm back in the office and can look at my materials.  The info that I have on hand says that you should expect to have a thread forming torque of about 2-4in# and a seating torque of about 11 in# seating torque.  The torsional strength of about 20 in#.  So you should be OK with the torques you are using.
Have you checked the clutch on the drive gun to be sure that it is functioning correctly?  They can get worn and give erratic results.
For a die cast hole I would use a .105/.102 top of hole and a .099/.096 bottom and a draft of about 1.5 degrees.

Given your description you should have a robust application and I am really surprised that you are having problems.  This is a strange one.  
One thing that an earlier poster mentioned is thet you would need to allow room for chips and that is NOT correct.  Taptites form their threads and don't cut them so you don't need to allow chip clearance.

Dick

RE: Taptite screws

MSU2000,
Sorry I didn't answer your question about what I mean by "snatch". Screwman mentions checking the clutch on the drive gun, if this gets worn it can momentarily overload (ie higher than set torque)which we call "snatch".

I have come across 2 isolated incidences in the past of "cheap imitation" taptite screws. The symtoms you have fit this criteria - if you look at the thread form, of say 20 screws, under a magnifying glass, you should see a nice crisp thread form, if the form varies or is rounded then these are "cheap imitations". The only way to be certain of the pedigree is to go to manufacturers direct.
www.textronfasteningsystems.com
www.ejot.com
I have successfully used both of the above for many years.
Good luck, John.

RE: Taptite screws

(OP)
Coughdrop,

You brough a good point about the hole depth. I haven't checked the actual dimensions on real part. I'd be surprised if they can't hold the depth of one hole to be the same as the rest (it is CNC set up). I found out also that the scews are made overseas. So, this might relates to the "crisp" thread that you talked about, but I'll make sure I check that.

Screwman,

Thanks for the answer and verification of my parameters.

Question to everybody.

1. For hole dimensions of the die cast that Screwman mentioned, Is that independent to the depth of the hole. I was under assumption that the dimension you have is for a specified length of engagement (.224" for this 4-40 taptite screw per manufacture spec).

2. Would having longer thread of engagement require higher seating/prevailing torque. Also, the longer the thread engagement, the more heat created from friction drive, therefore driver speed/lubrication is critical. The manufacturer said that that the length of engagement is MINIMUM 2-2.5 times diameter of the screw. That means I can go as long as I want to. This doesn't seem to make sense in my head. Please correct me if I'm wrong in saying that above.

Thanks
MSU2000

RE: Taptite screws

MSU,
Good questions.  The hole sizes (top & bottom) that I gave you are for a .224 deep hole.  The key if going with additional length of engagement is to make sure that the hole size at the bottom of the hole doesn't exceed 100% thread engagement (smaller than .0958) because that will cause a big spike in the drive torque.  We generally calculate the hole size on die cast holes to be at the nominal desired hole size at a depth equal to one half the effective lenght of engagement (subtract the lead threads from the overall engagement.  This gives the best overall results.

The minimum length of engagement is the length needed to assure screw breakage as a failure mode.  Less than that will generally result in stripping as a failure mode.  You are correct that you can as long as you want, but it will result in a higher driving torque and no appreciable increase in performance.  I generally say to go 2.5 - 3 dia. of engagement.  In my opinion anything over that is just adding cost to fastener and not providing a performance benefit.

There are some good taptite screws made overseas by licensed suppliers who have all of the proper tools and specs and there are some that are of very poor quality.  
Here is the link to REMINC who is the licenor of Taptites. It has all of the licensed mfg. globally.  I would stick to this list to get consistent parts.

http://www.taptite.net/

Dick

RE: Taptite screws

In my experience, the torque limiters in powered screw drivers are very innacurate and can vary by as much as +/-20%. 4-40 screws are quite sensitive to overtorque because of thier small size. I've routinely broken 4-40 standard-thread screws when assembling them with thread locking PEMnuts using hand screw drivers, therefore I hardly ever use 4-40 screws on my assembllies unles I have to. I use #6-32 screws, they are much more tolerant of overtorque.

RE: Taptite screws

(OP)
rorschach,

Good point, I'll keep that in mind.

MSU2000

RE: Taptite screws

My excperience of using taptites into die-casted parts isn't encouraging. All parameters involved is this specific case is creating an extremly small process window for the assembly operation. My advice is to invest in a Atlas Copco Tensor screwdriver or similar more advanced equipment, this was our only way out.

RE: Taptite screws

Folks-
In case this was not brought forward, Taptites are used for plastics OR metals (typically nonferrous metals).  These are trilobular, self-threading, and (most importantly for electronics applications) they do not strip away any material; they simply make their own threads by coldflowing the material in the receiving part.

Do a Google on TAPTITE and TEXTRON will pop up.  CAMCAR/TEXTRON is a major mfgr holding the patent rights on some aspects of TAPTITE screws.


Tunalover

RE: Taptite screws

Textron Fastening Systems is one of the world's largest fastener manufacturers.  However, they only have a license to manufacture Taptite screws.  The patent holder is Reminc.  Here are the Internet sites:

http://www.textronfasteningsystems.com

http://www.reminc.net

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: Taptite screws

(OP)
Corypad and Tunalover,

Thanks for clarifying.


MSU2000

RE: Taptite screws

(OP)
Gentlemen,

To continue the discussion. Like I mentioned earlier, the prototype testing yield to 16 in-lb of max torque before the head snapped, therefore I set the seating torque to be 11 in-lb (65%).

If the screw head snapped before it bottoms out, it leads me saying that I'm overtorquing it. How could I overtorqued it, when I know that the electric screw driver could only go 11 in-lb which is significantly lower than its maximum torque of 16 in-lb.

Anyone take on this?
MSU2000

RE: Taptite screws

The problem statement should not state "overtorque".  It should state something like "head fracture".  Now, areas to address head fracture include the following:

1. Variability in torque-tension relation such that 16 inch pounds is not a statistically significant number, and therefore 11 inch pounds is not conservative.  Do you feel that friction, material properties, and other geometric features are properly controlled and understood?

2. Eccentricity of screw axis to hole axis during tightening.  Eccentricity introduces bending stresses.

3. Underhead radius.  This can lead to bending stresses in the head.

4. Quality of scrw manufacture.  Are there surface defects in the underhead transition such as cracks or folds?  Cross-section the fastener and etch it in hot acid-- is there a problem with the grainflow pattern that would initiate premature failure?

RE: Taptite screws

What is your definition of bottoms out?  If the head has not seated, then your screw threads are binding (insufficient allowance).

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: Taptite screws

(OP)
TVP

1. I should mention that I was using manual screw driver to come up with that number while it was using the electric driver that we experienced head fracture. Could you elaborate more on your first sentence please?

2. We tried to control any miss-alligment as best we can, but I undertand your point.

3. Screw has underhead radius.

4. I obtained certificate of conformance of the screw from our vendor. Visually, It looks like we can believe that the certificate of conformance is to be true.

Thanks
MSU2000

RE: Taptite screws

(OP)
Corypad,

My definition of bottoms out is when the underhead screw is seating flush to the top piece part providing clamping pressure to the bottom piece part thru threads generation.

MSU2000

RE: Taptite screws

MSU2000,

The pretension in the fastener is highly dependent on friction.  Friction of the entire system will have a statistical distribution, with the distribution hopefully being normally distributed and well-controlled.  If friction is not well-controlled, then using torque as a parameter for tightening becomes extremely difficult because for any given input torque the resulting pretension in the fastener varies significantly.

RE: Taptite screws

(OP)
TVP,

Yes, thank you for you clarification. Since the screw is waxed and the hole is drilled, I am assuming that the friction is controlled. I couldn't imagine that 11 in-lb is too much of the seating torque.

MSU2000

RE: Taptite screws

MSU2000-
The cored hole tolerances are critical.  I've been using Y14.5M heavily lately on castings for controlling size and positional tolerances of cored holes and can provide you with tips.  I'm at HUGHES NETWORK SYSTEMS.

Tunalover

RE: Taptite screws

MSU2000,

 I'm comming in late in the discussion, so I don't know if you have answered this already. Have you verified that the electric screwdriver is actually giving the set torque.

Wes C.
------------------------------
There are no engineers in the hottest parts of hell, because the existence of a 'hottest part' implies a temperature difference, and any marginally competent engineer would immediately use this to run a heat engine and make some other part of hell comfortably cool.  This is obviously impossible.

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