This article has been prepared to provide information in the area of screw thread technology, for creation of stronger and longer lasting threads while designing and producing a component or repairing of component having damaged female threads. In the course of the article, it is explained how helicoil in addition to being designed for female threads, can also provide maximum protection and durability to the fastening bolt and screw; and its superiority over bush fitting and other conventional practices has also been compared.
The need for Helicoil arose over sixty years back when the aviation industry, designing larger and heavier airplanes, had to utilize lighter raw materials for their production.
At first, thread inserts were used for repair of damaged or stripped threads, subsequently due to repeated thread failure, the inserts were installed in original manufacture of engine and other components to ensure protection against repeated thread failure.
Today, Helicoil inserts are used in the original equipment manufacture of such diversified products as aircraft, aerospace, automobiles, motorcycles, tractors, aluminum parts such as heat exchangers, power generation equipment and turbines, satellite and remote sensing devices, radar and telecommunication equipment, diesel fuel injection equipment, medical engineering and industries making component assemblies of soft metals, composite alloys, polymers, etc. Many industries making high technology products that require to perform safely and efficiently under extreme conditions use Helicoil also to salvage the scrapped components.
Why Use Helicoil in Original Equipment
1. Create stronger and longer lasting threads in soft metal component
Helicoil inserts are made from 18-8 grade cold rolled stainless steel having tensile strength ranging from 1,50,000-2,00,000 PSI, hardness between 43-50 HRc which gives a better description of how Helicoil inserts can withstand a lot of assembly stress besides creating a stronger thread in weak materials.
Helicoil in stainless steel can be safely used in continuos operating temperatures up to 35degree C and occasional peak temperature of 425 degree C. Materials like Inconel and Nimonic alloys for higher temperature conditions are also available.
The finish of normal tapped hole is observed to be 8 times rougher than the finish of Helicoil insert. This is because wire is cold rolled in multiple stages to a very tight tolerance and finish and thereby providing maximum surface contact area, greater clamping action and minimum friction between bolt and female thread flank.
2. Helicoil is Anti-Corrosive & Non-Magnetic
Made of 18-8 grade cold rolled stainless steel & manufacturing process ensuring non-austenitic properties, it provides a better resistance to corrosion under extreme atmospheric conditions and operational environments available to an equipment assembly.
3. Helicoil reduces bolt failure, breaking and thread stripping
The possible causes of bolt failure and breaking or thread stripping are over torque, pitch angle errors, progressive pitch errors, high stress conditions and continuous vibrations.
Without Helicoil, it has been observed that in a normal thread assembly a bolt is subjected to an average tightening load of more than 70% over first two threads near to its collar or over a few threads finding a proper surface contact.
The conditions even get worse when normal machined threads are associated with threads having pitch errors and above adverse conditions. Helicoil stretches itself to compensate for pitch angle errors and progressive pitch errors thereby providing a better surface contact and more even load distribution to prevent the bolt failures.
4. Helicoil has a "designed-in" radial & axial elasticity
This allows the shearing load to be transformed into favorable hoop stress or radial load towards the full length of the insert. With result the load gets more evenly transformed over the entire length of the threads of a bolt, thus bolt breaking and thread shearing is significantly reduced.
Helicoil-A Gift to Designers
The above compensation and even load distribution features allow a superior reliability of assembly and without compromising on performance allows the designers to choose a bolts of lower strength, bolt of alternate metals, bolt having lesser length, creating possibility of lower torque requirements for soft metals, strengthening components having less wall thickness. Helicoil inserts are also available with a locking coil for thread locking of screws and bolts to prevent loosening due to heavy or continuous vibrations. Unlike the other methods used for thread locking, the locking type Helicoil is reusable for several bolt opening and tightening cycles.
Helicoils are also available in a variety of materials such as beryllium copper, phosphorus bronze, Inconel X-750, Nimonic 90, etc, with or without special plating and coating like cadmium, silver, dry film lubricants, etc.
Helicoil is Superior to Threaded Bush/plain Bush
Threaded bushes are wound inside the threaded hole like a fastener and plain bushes are press fitted in to a hole. Some bushes may be available with extra locking features. The problem normally encountered with bush is the tendency of getting loose, during bolt tightening or opening cycle, due to vibrations, impacts, stress wear and creep factor, in some cases due to differential heat expansion ratio of bush and original component as co-efficient of heat expansion varies between two different metals.
Moreover, once a bush gives away, the next step is to go in for a larger hole. Bushes being thicker also reduce the remainder wall thickness of a component thus making a component weaker.
Use of Helicoil eliminates most of the given problems as it remains in a compressed state inside the threaded hole and holds tightly to the wall despite variable heat expansion, vibrations and high impact conditions. Helicoil doesnt get loose by repeated tightening or opening of bolt. The Helicoil is installed by a tool which compresses it at First to get installed in a threaded hole after which it sits tightly against the thread flanks, the only way it can be taken out is by using a tool which reduces it in diameter during extraction.
Such phenomenon of reduction of Helicoil diameter may never take place while tightening or loosening of bolts. Also, the worn out Helicoil can be replaced by a new one without making any changes to the pre-threaded hole.
This is why such a reliable technology is preferred by aeronautic and aerospace industries where products are subject to most extreme vibration and temperature conditions and not to forget that many human lives and billions of dollars are always at stake.
Why use Helicoil for Thread Repair & Maintenance
· To achieve all above beneficial features and reliability of assembly
· Helicoil is an effective, economical and quick time saving solution for most of the thread repairing needs of a workshop and small user to a large-scale industry.
· Depend less on skilled expertise, cut on outsourcing repairs, reduce equipment down time and avoid expensive conventional methods in practice.
· Helicoil is an easy to use good technology and you could do it yourself at a low cost.
|Posted : 8/12/2005|