Helicoil question

[n653ds]

I want to remember you can "gang" Helicoils to give a deeper thread (i.e. two 3/4" long inserts to give approximately 1-1/2" thread). Am I remembering correctly?

Steve

[John Stevenson]

Yes but why?
Any thread length greater than 2D isn't any stronger.

John S.

[ronsmith100]

And D is?

[chipeater]

diameter

[ronsmith100]

and the diameter is?

[JCHannum]

Helicoil supplies short, medium and long inserts. They are 1X, 1.5X & 2x the diameter.

Diameter is nominal fastener size.

[chipeater]

The diameter of the bolt/screw. What John is saying is that a thread depth greater than twice the diameter of the fastener adds little in the way of strength.

Here's a more precise treatment:
(2d is close enough for me, thank you)

2.3.1 Minimum length of thread engagement for screw breakage

This is where the tensile strength of the material must be employed. under the assumption that the stripping tension factors are not the same, the following calculation process is realized:

Keeping the acceptable shear tension in mind, the strip off diameter must be determined. The acceptable shear tension for the screws and nuts are constructed so:

t mB = bB*RmB t mM = b M*RmM.

The opposite relation to acceptable shear tension result from the material factors of the screws and nuts through:

a B = tmM/(t mM+tmB) a M = 1-aB.

In the middle of the theoretical thread lies the pitch diameter d2 between the Thread tips. The measure-ment from the pitch diameter d2 to the tip of the thread is H/2. Half of the diameter is transformed to the value
D d/2 = H/2-HB/2
although
HB = a B*P/tan (a آ°/2)
when using different material values, such as breaking point e.g. yielding points.

Upon including of the pitch diameter d2, the value 0.5 = tan (a آ°/2)*H/P and the factor a B have the strip off diameter

dt = d2 + (0.5-aB)*P/tan (a آ°/2).

As the pitch here is included so varying length of thread engagement for the standard and fine threads will be as a result.

The breaking load of the screws results to
FBr = RmB*As.
With this strength the strip off cylinder’s recommended shear surface becomes
At = FBr*(1/tmM+1/t mB)
Under the influence of the strip off diameter, the minimum length of thread engagement becomes
mmin = At /(dt*p ).

At this length of thread engagement, "equilibrium" exist between the loaded thread levels and the stress cross section tension of the screws. in order to avoid stripping, the minimum length of thread engagement must be increased. An increase of about 5 % is recommended.

From:

http://www.hexagon.de/dose/dose1e.htm

[ronsmith100]

WOW! all this information and I still don’t have the answer I am looking for. I want to know what diameter is the screw or bolt he is using?
The gentleman stated he wanted to put two helicoils back to back to get a thread length of 1-1/2 inches. That is all I can read. I do not read anything where he mentions the diameter of the screw/bolt
If you don’t know what the diameter then this makes no sense to me:

"Yes but why?
Any thread length greater than 2D isn't any stronger."

How did you guess that 1-1/2†was more than 2D?

Ron

[John Stevenson]

Mind reading

[precisionworks]

Steve,

I've often stacked two heilcoils end-to-end. Just a matter of inserting the first coil to depth, breaking the tang, and inserting the second coil (and break the tang).

It's good practice to run the bolt through the double (or triple) insert to make sure it threads in well. There is no way I can imagine messing this up as each coil is a continuation of the coil installed before it.

------------------
Barry Milton

[ronsmith100]

<font face="Verdana, Arial" size="2">Originally posted by John Stevenson:
Mind reading</font>

Oh! dang me! I shoulda known by now.

[ibewgypsie]

"Mindless reading"??



David

[n653ds]

Geez, I didn't want to stir a hornets nest. I thought it was a simple questionbased on what I wanted to remember, but the early 70's were pretty rough on the ol' brain cells, sometime the memory plays tricks.

What we have is some 3/8-24 holes (OEM) that are called out as 1" full thread (hey, we didn't design 'em, we are just contracted to fix 'em to the drawing). However, we have some 1/2" long 3/8-24 Helicoils on the shelf. We tried ganging a couple but ran into problem when the bolt threaded to the inner Helicoil - the second insert wanted to gall at the starting thread. I thought I had ganged them the past, at least I was pretty sure I had (damaged memory cells again).

The length in the original post was a "for instance", just a point for conversation. I agree 2 X dia is all you really need, but like I said, we didn't design 'em, we just fix 'em per the drawing.

And now back to the regularly scheduled insanity.

Steve

[JCHannum]

Steve, the problem is that the Helicoil will shift in position somewhat. A single unit will adjust itself when the fastener is installed. It is when the fastener passes into the second that a problem might occur, the fastener may push it down, making the threads out of pitch.

It may be possible to hold things in place with a light application of Loc Tite removeable thread locker.

If the fastener is to be removed and replaced, a solid insert would be a better choice.

[tattoomike68]

Helicoil is junk and so is anything with a Helicoil in it.

junk,Helicoil = POS.

end of message.