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

 

[Barry1961]

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

Barry1961

 

[MikeHalloran]

I thought Taptites were for plastic.

Maybe I missed a meeting.



Mike Halloran
Pembroke Pines, FL, USA

 

[SomeYahoo]

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.

 

[MSU2000]

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

 

[CoryPad]

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.

 

[Screwman]

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

 

[MSU2000]

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

 

[MSU2000]

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

 

[limct]

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

 

[MSU2000]

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

 

[coughdrop]

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.

 

[MechEng13]

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.

http://www.wabashnational.com

 

[MSU2000]

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

 

[coughdrop]

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

 

[MSU2000]

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

 

[coughdrop]

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.

 

[Screwman]

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

 

[coughdrop]

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.

http://www.textronfasteningsystems.com

http://www.ejot.com

I have successfully used both of the above for many years.
Good luck, John.

 

[MSU2000]

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