Due to the immense versatility and popularity of bolted connections, there is a also a large and constant demand for thread repair services and products. This demand can be observed in virtually any industry and can be broadly grouped into two categories: original equipment applications and repair applications.
1. OEM applications
In modern design, there is a trend towards lightening of materials while still achieving structural integrity of bolted or fastened joints. This requires stronger threads in soft materials such as aluminium, zinc or magnesium than can be achieved by direct taping the soft materials involved. Thread inserts are therefore used to make threads more durable, particularly in applications where comparatively small bolts have to bear high loads and where bolts undergo repeated torqueing and untorquing.
2. Repair applications
In repair applications, the focus lies in repairing threads that have been damaged by over-torqueing or by cross-threading, or that were designed with insufficient safety margin to start with and consequently failed in service.
While it is sometimes possible to enlarge a damaged thread and to use a larger diameter bolt, the more elegant solution – and for design reasons often the only solution – is to restore the original thread and to install a new bolt or stud.
To restore damaged threads, coiled inserts or externally threaded inserts are most often used, though press-fitted solutions are also available for very low strength solutions. For OEM application, designers also have the option to use mould-in inserts, but they are only recommended for use in plastics and not metals.
Coiled inserts (also known as STIs)
Coiled thread repair inserts are similar to a coiled spring and are typically made of 18-8 stainless steel wire with a diamond-shaped cross-section. During installation of the insert, the coils are compressed and attempt to radially expand against the wall of the threaded housing. The resulting tension between the insert and the walls of the housing helps to retain the insert and keeps it from unthreading.
The main appeal of coiled inserts is that they are inexpensive and easy to install. But they lack strength and also create undesirable spreading forces – in fact often more so than externally threaded inserts. A further shortcoming of coiled inserts is their limited product range. They are usually available only in three different lengths (1D, 1.5D and 2D) and in sizes from about M2 to M39 (UNC size size 1 to 1.5″).
Externally threaded inserts
Unlike coiled inserts, externally threaded inserts are made from one solid piece. Like all repair inserts, they are easy to install. They are often self locking, for example by a pre-applied external coat of adhesives that activates upon installation. The adhesive ensures that they are pressure tight until about 41 MPa (6000 psi) up to a service temperature of about 150°C (300°F), at which point the adhesive will start to disintegrate.
A Full-Torque inserts is a special type of externally threaded insert. Its distinctive feature is its external Spiralhook thread which completely eliminates spreading forces and pulls the surrounding material into the thread, resulting in a very tight and strong fit. Because they do not create spreading forces, they are ideal for high-load applications and for installation in soft or brittle material such as cast iron, aluminium and magnesium alloys. By virtue of their thread geometry, they are pressure tight up to 69 MPa (10,000 psi).
The secret to the superior strength of the Full-Torque inserts lies in the design of their outer thread.
A standard bolt and thread insert with a metric or unified standard (UNC / UNF) thread has a thread angle of 60°. During tightening of the bolt, the insert is pulled upwards and slightly compressed.
This results in a radial outward force on the surrounding material as the thread flank of the insert and the thread flank of the surrounding material are sliding off each other.
The patented Spiralhook outer threads on the Full-Torque insert pull the surrounding material into the threads of the inserts and add strength to the part being repaired.
A Full-Torque insert thus makes the bolt hole stronger than the bolt.
|Attributes||Coil style||Key locking||Self-tapping||Full-Torque|
|Repairs stripped threads||✔||✔||✔||✔|
|Emergency repair solution||✔||✔||✔||✔|
|Stays solidly in place during lifetime||✔|
Full-Torque inserts are developed, manufactured and sold by Lock-N-Stitch Inc. in California, USA, since 2001. They have become known as the strongest thread repair system that exists today and are widely used the world over and they are protected by various US American and international patents.
At QuantiServ, we only use Full-Torque products for all our thread repair and we very much recommend to anyone to use them, whenever a thread needs to be reliably and permanently repaired or reinforced.
Full-Torque inserts are available with many different features that make them suitable for virtually any application.
Thread repair brochures
Click on the following images to download our thread repair brochures: