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Thread Inserts

Introduction

Many mechanical joints are totally depended on screws and bolts.    This method allows repeated assembly and disassembly with reliability and confidence.    The screws and bolts are generally low cost items with known strength and when used with nuts with similar rating provide highly engineered connections. However when the connections are based on screws tightened into tapped holes there is more need to analyse the strength of the joints.

The preferable failure mode for a screwed connection based on tapped holes is clearly failure of the screw.    This is a replaceable item and a failure is clearly detectable.    The primary risk event is probably in the preloading operation where the screw may by tighted to 70-80% of its yield strength.. A screw failure in this operation is not too serious an event.

If the screw is highly loaded e.g preloaded and is screwed into a material of lower strength than the screw then a detailed assessment of the joint must be carried out to ensure that thread stripping will not occur..    Unfortunately the option of increasing the depth of the tapped hole is of limited value as it has been established that there is no advantage if having a thread engagement in excess of the nominal screw diameter.    The problem is that the load taken by the screw is not evenly distributed. The majority of the load is taked by the first thread and very little of the load is taken by the fourth thread onwards.

The preferred options are to design the joint to based on the use of suitably rated nuts or ensure the screw and parent metalof the female thread,have similar mechanical strengths.

When the design is based on highly loaded /high strength screws fastened into threaded holes in components with lower strengths then it may be necessary to use thread inserts.



Types of Thread Inserts

Helical Wire Inserts (Heli-Coil)

The helical wire inserts inserts, generally identified by the tradename Heli-coil, are made from diamond shaped wire formed into coils.    They are screwed into tapped holes such that the resulting internal profile of the coil is that of the desired female thread.    The tapped hole is created using a special tap .    The insert is over-sized so that it anchors itself into the hole.    Some have tangs to enable convenient install but some do not.    Special tools are required to install the inserts.     When the insert includes a tang , it is snapped off after installation.    The insert can be configured to provide a thread locking duty.

The resulting female threads are strong because the flexibility of the insert provides an even distribution of loads throughout the length of thread engagement.    This flexibility also compensates for variation in the formed thread allowing each coil to carry its share of the load.    The limitation on length of a convenional thread are overcome with this component and the load can be distributed over thread lengths far in excess of that in a tapped hole with no insert.

The heli-coil internal profile is hard and polished dramatically reducing thread wear.

Heli-coil Catalogue

Table showing lengths (in nom.dias) of Heli-coil (Material Spec: AS7245) required to match the screw strength of the material of the female thread with the screw strength

parent metal
MPa
Screw Ultimate strength (MPa)
3004005006008001000
701,522,53,5--
10011,51,522,53
150111,51,522
20011111,51,5
250111111,5
300111111,5
350111111,5

Threaded Bush Insert

This is simply a bush which is threaded internally and externally.     They are simple to install require no special installation tools and do not require special taps.     Some types incorporate Loctite thread adhesive in the external thread.    As with the helical wire inserts these components enaable use of steel bolts with high preload forces with female tapped holes in a comparatively weak material.    They can also be used to repair damaged threads.

A typical threaded insert bush type is the E Z Lok

Shouldered Bush Insert

An engineered bush provided with a shoulder and internal thread. The bush is pressed in from the side opposite to the joint face such that it is prevented from movement by the shoulder.    This component would include features which prevent rotation such as adhesive or knurling. This provides a very convenient and easy to engineer solution

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Relevant Links
  1. Bolt Science..A site dedicated to the Science & Technology of bolted joints
  2. F.E.D.S Screw Thread design .A very useful and informative document- You must read this if you are doing detaile design.
  3. Tribology-ABC Screw Joints metric Lots of very useful notes and calculators
  4. Heli-coil Catalogue Wire thread Insert - the simple method of providing high thread strength with thread locking.
  5. E-Z Lok Screw Bush inserts Useful for providing engineered internal threads and repairing damaged threads.

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Last Updated 24/01/2013