helicoil insert - theory of operation 1

[winpop123]

Can anybody supply a good link for data on threaded inserts? I'm looking for data like pull out strength, details on how they are retained etc. I can't seem to find this on the typical web sites. All I see is hole drill size, tools to use etc.
thanks

 

[dgowans]

http://www.emhart.com/products/helicoil.asp

Typically Emhart has a lot of data on their inserts, although I don't usually spec the Helicoil flavor. I'd be surprised if you can't find what you need there.

 

[winpop123]

Thanks but I've been to that site and don't see anything of any real "engineering" value. (ie) pull out strength, theory of how the helicoil is retained inside the hole etc.
I don't need to be able to defend the use of a helicoil.

 

[winpop123]

sorry last post should read ... I do need to defend the use of a helicoil. thanks

 

[rb1957]

"I do need to defend the use of a helicoil" as opposed to ...

pull-out strength i use is the minimum of the tensile strength of the fastener and the shear strength of the threads in the plate (for me, usually Al)

 

[eromlignod]

There are two main common uses for Helicoils:

1. In Aluminum, or other galling-type metals, where parts will need to be assembled and disassembled often, without ruining the tapped hole over time.

2. To repair a hole where the original threads had to be drilled out because of a broken screw/tap or damaged threads, etc.

Sometimes (often) they are used in softer metals with the mistaken notion that they are somehow stronger. In reality they are no stronger than the outer tapped hole that the insert goes into. In other words, they are as strong as simply using a larger screw with the same threads as the outside of the Helicoil. So if the parts are to be assembled only once, there is no practical reason to use the inserts.

They are retained due to their outward springy character. They are inserted by turning a tang, which tends to contract the spring and let it turn in easily, like on a wrapped-spring clutch. Once the tang is broken off, they remain firmly sprung in place.

Don
Kansas City

 

[winpop123]

Thanks eromlignod.. I was under the impression that the helicoil incorporated a design feature that added an advantage in strength but in reviewing the method of installation closer I see your point

 

[strokersix]

You might add another practical reason that you can use a smaller fastener. This may be advantageous for clearance, accomodating an existing hole in a purchased part, or some other reason over using a larger fastener.

 

[telecomguy]

Found this link by searching the forum for 'helicoil pull out':

http://assist.daps.dla.mil/docimages/0000/86/88/37118.PD4

<tg>

 

[winpop123]

An update to this post. According to the technical information on the Helicoil website, It is not a correct assumption that a helicoil put in aluinum for example does nothing to strengthen it. Reference their techinical publication 68-2 on the web site.
according to their published documents, with the correct length of engagement one can acheive full bolt strength even in an aluminum to steel combination.

 

[arto]

Helicoils distribute the load over a multitude of threads.
Plain tapped holes are loaded on the first 3 threads.
Look at the photoelastic studies of Hetenyi.
{Some of us actually Helicoil by design, not by shop mistake.....& then have to spend hours explaining...
Ah feel yore pain!! ;-) )

"Recoils" are also similar to Helicoils

 

[gfbotha]

In my opinion Helicoils:

1. Prevent galling when assemble/disassemble in softer metals.
2. Increase the effective shear area diameter when used in softer metals.
3. Help distributing the load over more threads without the prerequisite of yielding.

[2 & 3 = means stronger]

Note: Regarding 1st 3 threads normally taking the load... - normally it is not this severe, because plastic adjustment (local yielding) of heavily loaded threads causes the load to be distributed. The statement is true if you screw hard metal into hard metal and loading does not take you above yield. BTW, this is one good reason why nut materials are usually softer than the bolt. Coarse pitch also helps to distribute load.