Induction Bolt Heating for Tightening and Loosening Large Bolts
Since the early 1990s, induction bolt heating has stood as the most effective and efficient method for tightening and loosening large bolts and fasteners. Despite its proven performance, misconceptions still circulate — from concerns about magnetic fields and metallurgical effects to confusion with “microwave” or “sound wave” technologies. In truth, none of these issues are valid. When properly applied, induction bolt heating is precise, safe, and incredibly efficient.
The Science Behind Induction Bolt Heating
At its core, induction heating works much like a transformer. When an induction coil is inserted into the bore of a bolt, the coil acts as the primary winding, producing an alternating magnetic field. This field induces an electrical current within the bolt — similar to a “shorted secondary winding.” The resulting electrical resistance converts that current directly into heat, quickly and evenly expanding the bolt for loosening or tightening.
That’s the simple version. The more advanced explanation involves an understanding of coil design, flux concentrators, magnetic field coupling, and frequency tuning. The distance between the inductor and the bolt, the coil geometry, and the power supply’s current and voltage characteristics all play critical roles. When these factors are harmonized, the result is maximum efficiency and rapid, uniform heating.
For example:
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A 2” diameter bolt with a ½” bore and 24” length can reach temperature in under 30 seconds.
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A much larger 8” diameter bolt, 50” long with a ⅞” bore, can be fully heated in about 5 minutes.
Choosing the Right Power and Frequency
Machine power and frequency are key to success. At Thermo International, we’ve determined that 50 kW is the minimum power level suitable for most power-generation turbine bolts ranging from 2” to 5” in diameter. Lower-powered machines eliminate induction’s main advantage — speed — and can take as long as traditional torch or combustion heaters.
For larger or specialty applications, such as Inconel bolts up to 60” long or nuclear-grade turbine bolts exceeding 8” in diameter, machines in the 80 kW–120 kW range are recommended for optimal performance.
Equally important is the frequency. Higher frequencies heat faster but have a shorter magnetic field range. Thermo International has found the ideal operating range to be 8 kHz to 12 kHz, which offers excellent efficiency even when the coil and bore aren’t a perfect match — saving time and reducing the number of inductor changes needed on site.
Inductor Design and Practical Efficiency
In a perfect world, each bolt would be heated only between its clamping surfaces — typically between two nuts. In reality, stocking and swapping out inductors for every small variation in bolt length isn’t practical. Fortunately, Thermo International’s systems are designed to perform efficiently even with slight mismatches, ensuring consistent results and faster turnaround times.
This flexibility matters most when working with lower-frequency, high-power systems — precisely the type of systems Thermo International builds and refines for industrial users around the world.
Typical Bolting Patterns
In turbine applications, bolt heating typically starts at the center of each shell half and progresses outward toward the ends. While some manufacturers or site conditions may call for variations, the “center-out” approach is widely used for even stress distribution and reliable assembly.
The Thermo International Advantage
At Thermo International, we’ve engineered what we believe to be the perfect combination of machine power, frequency range, and inductor design. Our induction heating systems feature advanced control technology and wide tuning ranges — allowing operators to achieve faster, safer, and more efficient results across virtually any turbine or flange bolting application.
Thermo International
Advanced Induction Heating Solutions for the Power Generation Industry
🌐 www.ThermoIntl.com | 📧 jeffw@thermointil.com | 📞 973-970-9500