Spiralock

[Jpbingley]

Hi.

The company I work for sometimes uses Spiralock thread forms in aluminium parts. They thread a hole in the aluminium part and fasten the joint with a steel bolt. I'm not an expert in Spiralock, but from my understanding isn't this not a good idea? The thread form in the aluminium would be ground away when the torque is applied to the joint? Shouldn't a steel insert be used with the spiralock thread?

Also, why use Spiralock over a standard locking wire thread insert? Doesn't the spiralock only work once the correct (higher) torque is applied to the joint? If it backs off a little doesn't the locking no longer work? If you used a standard wire locking insert, the locking action is applied no matter the torque and no matter if the bolt backs off a little?

Thanks.

 

[Jboggs]

An idea may seem unconventional but work acceptably in the particular application. But you have told us nothing about the application. I would suggest you contact the company that produces the Spiralock product. Talk to their application engineers. Fully explain the installation, usage, and hazards of failure. It is their job to know where and how their product works, or does not. Their comments would give your recommendation to management more validity than "some guys on an engineering forum said this is a bad idea".

 

[hydtools]

Your company may have found some advantage to using the Spiralock tap thread form in the aluminum. I believe it is a rounded root thread form.

Ted

 

[3DDave]

The goal of a thread locking system is to retain tension in the fastener. If the fastener is allowed to back off the retainer mechanism no longer has a function; friction retention is not helpful. In particular I've seen cases where thread locking compound worked just great at keeping the body of a bolt in the hole long after the head was sheared off by fatigue because there was insufficient pre-load.

That said, the ability to wedge without tightly controlled mating fasteners strikes me as unreliable regardless of material. Perhaps someone has done a detailed tolerance analysis, particularly for contact compressive stresses.

Dissimilar metal corrosion would be higher on my list of worries. In similar applications it was common practice to place key-locked inserts with polysulfone rubber to protect the aluminum. The main reason for the insert was to increase the bearing area of the thread into the aluminum as well as to increase resistance to cross-threading.

 

[mfgenggear]

spiral lock threads would be acceptable as long as the material stress of the material is not overloaded.

I would fix many large holding fixtures were the base material was cast aluminum
That was tapped. More often than not it was over torqued or over loaded. Then the threads stripped out. I would have to repair them by machine out with o/s drill and tap for o/s inserts. Problem solved.

In certain applications tapped aluminum
May be acceptable to reduce cost.
On the other hand aluminium can easily be
Cross threaded or stripped out.
Where I see this most often are aluminium automotive heads. For the spark plugs and
For the intake and exhaust.
If anti sieze, often than not is not use
Dissimilar metals,and heat will strip
The threads.

For a more robust and reliable I recommend
Steel inserts.

 

[DAVIDSTECKER]

From what I understand the insert increases the amount of aluminum thread engagement, adding some strength.
The coefficient of friction for aluminum is very high and prone to galling.
If this is a part that is dis-assembled and re-assembled on a regular basis then the steel on steel situation is possibly you better option.

 

[EdStainless]

Regardless of the thread form putting steel bolts directly into Al is risky business for both galling and corrosion reasons.
My guess is that they used to use Spiralok inserts and then one day they ran out and realized that they could just use the next larger bolt size.
Or someone was trying to be cheap.
If these are a light load, it isn't cyclic, and these do not get serviced, then you are likely OK.
Going back to using the insert, and torquing to the correct value would provide a much more secure joint.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy