Lepel Sheds New Light On Heating Tungsten Filaments for Light Bulbs

The tungsten wires in a light bulb are "as fine as your hair, so they must be processed very carefully," says Daniel Hennessy, Senior Process Engineer at General Electric Company's Cleveland-based Materials Engineering Operation. "It takes extreme heat and an exacting set of induction heating conditions to turn tungsten rod into these very thin filaments."

Last year, to improve the reliability of this process, GE decided to replace its vacuum tube powered RF induction equipment with advanced, solid state equipment. After evaluating several induction heating equipment vendors, GE chose a Lepel LST 34 Solid State unit, capable of delivering 100 kW within a 45 to 450 kHz frequency range. Lepel was selected, says Hennessy, because they were able to deliver the unit when it was needed, at an attractive price and featuring the right specifications.

The Lepel unit incorporates a special coil, designed by Lepel engineers, that operates in an enclosed chamber to provide reliable, repeatable operating parameters. This is important since, Hennessy says, tungsten has complex properties and can be a difficult material to work with.

GE was impressed with Lepel's design capabilities. "Even though Lepel had no prior experience with heating tungsten, their calculations for the coil geometry and power requirements were right on target," Hennessy reports.

The addition of the Lepel equipment immediately solved several problems. The previous equipment had been unstable in its ability to deliver the consistent high power needed for tungsten heating. Additionally, its vacuum tube components were difficult to replace. Of the new unit, Hennessy says, "We're pleased with the high temperatures and overall temperature stability we are able to achieve."

Benefits have included faster throughput, higher quality, and improved product consistency.

"Lepel's LST 34 Series delivers the high temperatures and overall temperature stability needed to process fine tungsten filaments at GE."

Lepel is Making News...

Lepel's highly effective induction brazing capabilities were the subject of a recent article in Modern Applications News. The article explains how a proper coil design can provide rapid, localized heating at joints. The localized heat minimizes deterioration of properties in heat-treated or cold-worked materials. In addition, induction coils can be engineered to provide heat simultaneously at several locations in an assembly.

For a free reprint of the Lepel article on induction brazing, call Mary Keller at (516) 586-3300.

If you're looking for induction heating efficiency, look no further than Lepel's LST Series of RF Induction Heating Power Supplies. These high power, high frequency 100% solid state power supplies offer conversion efficiencies of approximately 90%, compared to 55% for oscillator tubes. They have the ability to provide immediate operation, with no warm-up time or standby power necessary, and require less cooling water.

Another benefit of the unit's solid state design is that solid state components do not deteriorate with time. Therefore, output characteristics do not fluctuate.

The LST's exceptional efficiency can also be seen in its unusual circuit arrangement. In contrast to other circuit arrangements, full power output (85-2000 kW for the LST 31 line, 35-1200 kW for the LST 34 line) is available over a wide range of frequencies (25-225 kHz for the LST 31 line, 45-450 kHz for the LST 34 line) and always at optimum efficiency. The newest LST power supplies are available at 500 kHz and 800 kHz frequencies.

And for increased safety, the 10,000 - 15,000 volts typical for vacuum tube generators is replaced with a maximum of 2,000 volts in the LST design.

One year ago, Lepel opened an Induction Heating Applications Laboratory in Brea, California. Since December of 1993, the west coast lab has helped test and develop innovative induction heating solutions for a variety of manufacturers.

Ray Ariss, Regional Sales Manager, oversees the 2800 sq. ft. California facility. Along with Tony Couzens, Applications Engineer, and a service technician, Ariss coordinates product testing and application assistance for Lepel's west coast accounts.

The Induction Heating Laboratory is available to test product samples for both current and prospective customers interested in brazing, bonding or other induction heating applications. Equipped with the most advanced, high frequency induction heating equipment, including a wall of approximately 25 various sized coils, Lepel can simulate custom applications. For unique coil requirements, Lepel has the facilities, the equipment and the materials to create a custom coil design.

Product samples are tested and evaluated to determine the feasibility of the application. Acting as a technical consultant, Lepel recommends the exact power supply, the power requirement, and the best coil for optimum results.

According to Ariss, "We have no competition when it comes to this lab. None of our competitors can offer as many diverse capabilities in such a convenient setting."

In addition, Lepel stocks a variety of loaner machines to handle customers' increased production rates and temporary needs. With the necessary equipment, parts, and supplies, Lepel provides local service and repair to power supplies.

Ariss invites visitors to take advantage of tours of the Induction Heating Laboratory and have their specific application addressed. Currently, Lepel is planning an Induction Heating Seminar in September. Look for the date in an upcoming newsletter.

If you would like to set up an appointment, call Ray Ariss at (714) 257-1360.

• Carbide Brazing of Steel Punches
• Carbide Brazing of Tool Steel and BeCu for Tapered Cutting Punches
• Brazing Systems for Spraying Technology
• Reflowing Tin on Wire for the Electronics Industry
• Soldering Copper and Brass for a Plumbing Application
• Hardening and Tempering for Turbine Fans
• Bonding Applications in the Automotive Industry
• Coating Pendaflex Parts

Don Blau has a Bachelor of Science in Physics and over 20 years of induction experience with Lepel. He is Manager of Mechanical Engineering.

Q: We are manufacturers of motor rotor assemblies and presently press rotor shafts into the bore of the laminated steel rotor core to form a completed assembly. The laminations of the rotor core are cast in aluminum and made of low carbon, non-silicon, commercial quality, cold-rolled rephosphorized steel. Our shafts are nominally 29/64 inches in diameter with typical rotor diameters ranging between 2 3/8 and 3 1/2 inches, 3 to 3 1/2 inches in length.

We are experiencing excessive failure rates as the shafts are becoming distorted. We are interested in knowing: Would Induction lend itself to reclaiming rotors from bad assemblies? And how can Induction be utilized to make a shrink fit assembly?

A: Typically, shrink fit diametrical interferences of between 0.0002 to 0.0008 inches in motor rotor assemblies will provide a functionally adequate assembly.

Shrink fit assembly of rotors to steel shafts can be easily accomplished using Induction by simply elevating the temperature of the rotor core, for subsequent slip fit of a shaft into the expanded I.D. of the rotor core base.

However, the following considerations and prerequisites need to be satisfied for Induction to provide repetitive quality shrink fits.

Utilizing Induction, rotor core bore I.D. expansion is accomplished by elevating the outside surface of the rotor core to a certain temperature and allowing thermal energy to conduct itself into the core region of the rotor core. The rate at which a rotor can be bore expanded is a function of the energy imparted to the rotor core, which is a product of Power (x) Time. Therefore, expansion of rotor bores requires thermal soaking.

Because of the cast aluminum contained in the assembly, there will be a surface temperature limit to which rotor cores can be heated. That limiting temperature is typically in the range of 850 to 900 degrees F.

Utilizing a 3 kHz Induction power supply, it will require imparting 43 Kilowatts of power into the rotor core for a heat cycle time of 72 seconds in order to heat a rotor core 2 5/8 inches in diameter, by 3 inches long, to a surface temperature of 900 degrees F. The temperature of the core center will nominally be brought to 575 degreesF and rise in temperature at the following nominal rates: 660 degrees F at 10 seconds and 730 degrees F at 25 seconds after expiration of heat.

Assuming a rotor bore diameter of 0.4528 inches at the start of the heat cycle, the bore will have expanded in diameter by 0.002 inches when the bore surface reaches 730 degrees F. This expansion in diameter easily lends itself to a shaft insertion.

For induction to be utilized for shrink fit, rotor core bores and shafts need to be tolerance qualified. Typically for shaft diameters held to 0.4533 inches (+.0003, -.0000), rotor core bores need to be held to 0.4528 inches (+.0003, -.0000) to acquire shrink fits ranging from 0.0002 to 0.0008 inches.

Induction may or may not be a suitable means of salvaging rotor core assemblies where assembly has been accomplished by shaft press fitting. In press fit assembly of shafts to rotor cores, interferences of 0.002 to 0.003 inches in diameter are not uncommon. Assemblies made utilizing Induction can however be salvaged using the Induction shrink fit technique.

Send your technical questions to:
DEAR DON
Lepel Corporation
50 Heartland Blvd., Edgewood, NY 11717