Taptite screws 4

[MSU2000]

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

 

[Screwman]

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

 

[rorschach]

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.

 

[MSU2000]

rorschach,

Good point, I'll keep that in mind.

MSU2000

 

[hasse]

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.

 

[Tunalover]

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

 

[CoryPad]

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.

 

[MSU2000]

Corypad and Tunalover,

Thanks for clarifying.


MSU2000

 

[MSU2000]

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

 

[TVP]

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?

 

[CoryPad]

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.

 

[MSU2000]

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

 

[MSU2000]

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

 

[TVP]

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.

 

[MSU2000]

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

 

[Tunalover]

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

 

[wes616]

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.
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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.