Constantly rising oil prices, global warming, alternative energies – buzzwords that we hear almost every day. Even when this excess of catchphrases leads to saturation, the energy-hungry world cannot commit itself to seek new alternative and regenerative energy sources.

In addition to solar and water energy, wind energy is probably the most significant current energy source. It is clean, inexpensive and constant. But how can this inexhaustible energy be harvested? The present article should illuminate some partial aspects of this question and thereby provide a glimpse of the technology of modern windmills. The goal is to answer the question: What does wind energy have to do with induction hardening?

Fig. 1. Possible applications of induction surface hardening within a windmill[3]

Wind Energy Plants

Modern wind turbines are effective because they work with moderate revolutions. A single 1.5-megawatt (MW) installation produces 2.5-5 million kilowatt-hours of power annually, depending on location. Thus such a plant can supply more than 1,000 four-person households with energy or replace 90,000 tons of brown coal in 20 years of operation.[1] The largest wind turbines have rated powers of 5 MW. They produce up to 17 million kilowatt hours of power annually. Thus, a small wind park can already supply an entire small town with power.[2]

In addition to the tower of steel and/or concrete, the main component of a wind turbine is the so-called machine house (gondola). The power-producing components of the wind power plant are found inside of this. Figure 1 displays the main components of the machine-house interior for which heat treatment is needed.

Fig. 2. Azimuth adjustment of the machine housing[4]

Main Bearing and Rotor Blade Bearing
The task of the main bearing (hub bearing) is to secure the rotation of the rotor-blade hub. The diameter of this bearing is dependent on the output of the windmill. Usual diameters lie in the range of 2,000-3,000 mm and should in the future be scaled to a range of 4,000-5,000 mm for 5-MW installations.

The rotor-blade bearing serves to support the rotor blades. Depending on the occurring loads, bearing types include ball and roller bearings.

Horizontal Pivot Bearing (Azimuth Adjustment)
All modern wind power plants are automatically tracked by active systems with azimuth motors. The wind direction tracking is assured by means of hydraulic motors or electric motors (azimuth drive; Fig. 2). The wind direction is determined using sensors. The machine house is arranged according to the wind with up to eight drive motors by means of a gear ring.

Fig. 3. Electrical pitch adjustment of the rotor blade[4]

Blade Angle Adjustment
The vast majority of windmills are currently pitch-regulated wind power plants. These rotate the blades around the longitudinal axis in order to control the power of the installation. Every blade can be rotated singly and thus used as a brake. The blade-angle adjustment system serves to precisely set the blades in the correct position and also to bring the blades into a secure position in an emergency. The blade-angle adjustment can be realized by means of several designs. Electrical blade-angle settings are used for large wind power plants (rated power of 500 kW and higher). In this connection, partial gearings and electric motors are used (Fig. 3).

Hub and Generator Shaft
The hub shaft has the task of transferring the low rotations and the high power of the rotor blades to the drive. In contrast, the generator shaft transfers the high speeds, which are transmitted by the drive, to the generator. Both shaft types are currently made primarily of heat-treated steel.

The Drive
The vast majority of windmill manufacturers use drives that alter the speed and the torque between the rotor and the generator. The rotor shaft rotates slowly with a very high torque, and the generator rotates very quickly with a low torque. The rotor speed of a megawatt wind power plant is dependent on the fast running number and lies in the range of 6-20 rotations per minute (the bigger the installation, the longer the blades and slower the rotor speed). In order to achieve a high degree of efficiency, to be able to adjust to the network frequency – 50 or 60 Hz – and to reduce the size of the generator, the generator speed must be much faster than that of the rotor shaft. The speed is transmitted using the drive. There are different drive construction types:
  • Pure spur-gear drives are currently only cost efficient for very small windmills. They were primarily used in old windmills up to a rated power of 500 kW.
  • Planetary drives are built with three different gear-wheel types. The slow shaft, which is coupled to the rotor shaft, is called a center gear. The sun gear lies in its middle, which is coupled to the fast shaft of the generator. The two gear wheels are connected by means of three (or more) planetary gears. The planetary carrier can be stationary or mobile.

Fig. 4. Solutions for induction gear hardening (“m” corresponds to the minimum possible module for the process)

Applications of Inductive Surface Hardening

Two applications of inductive surface hardening are hardening of large gear wheels and surface hardening of bearing rings.

Gear Wheel Hardening
The dimensions of the gear wheels and sprockets that are used for the listed main components of large wind plants lie in the range greater than 500 mm with modules greater than 4 mm.

In addition to case hardening, the inductive surface hardening of gear wheels is also offered. Principally, three different processes are used in regard to the induction hardening of gear wheels. Figure 4 provides a schematic overview of these processes. In addition to gear-wheel dimensions (tip circle diameter, tooth width, etc.), which serve as selection criteria for the plant arrangement, the module is an important criterion for the respective suitability of a certain procedure. The selection of the converter frequency is geared toward this module. The frequency decreases with increasing module size. The spinning gear-hardening process (encircling inductors) is technically not reasonably presentable.

Fig. 5. Macro sections of individual tooth gap hardening (gear wheel with helical gearing)

Tooth-By-Tooth Hardening
This process is used with a multitude of less-highly loaded wheels for the cost-effective and thus low-warpage increase of the permanent roller hardness. Depending on material selection, 80-100% of the roller hardness of the case hardening is achieved. The tooth base permanent hardness, however, is reduced by up to 20% compared with a wheel, which is only hardened through.[5] The cause is the hardness-zone runout in the proximity of the root circle, whose own tension profile affects the permanent hardness of the tooth like a mechanical chamfer.

Fig. 6. Coil for gap-by-gap hardening

Gap-By-Gap Hardening
With this procedure, the described chamfer effect is prevented on the root circle since the complete tooth gap is roughly hardened like a contour. The unhardened zone between two teeth lies in the range of the tooth point (Fig. 5). Thus, values for the permanent roller hardness of 80-100% and for the tooth-base permanent hardness of ca. 80-85% of the values can be achieved with case hardening.[5]

Depending on the converter frequency, the operationally secure utilizability begins for modules greater than 6 mm, or 5 mm in exceptional cases. Thus, tighter requirements are placed for precision of the inductor guide and the partial apparatus (coupling distance between coil and workpiece partially only a few tenths of a millimeter). These are assured in today’s plants using elaborate measurement systems and CNC-controlled inductor follow-up guides. The overall warping of the sprockets can be reduced using complex hardening strategies. In this way, for example, individual teeth are not hardened consecutively, but rather different gear-tooth parts alternately. Figure 6 shows a hardening head used for gap-by-gap hardening in scanning mode.

Fig. 7. Schematic of a roller bearing[6]

Large Bearing Ring Hardening
Based on its construction, a ball or rolling-body bearing consists of one outer and one or two inner rings, which are to be hardened. Figure 7 displays a schematic design of a large roller bearing.

Roller-bearing rings and roller bodies are heat treated in order to increase surface strength. This heat treatment serves in part to increase the roller hardness and improve the wear resistance. The heat-treatment procedures that are used for larger bearings are currently case hardening or through-hardening. Both procedures, however, have grave disadvantages in regard to a further scaling of the ring dimensions.

With case hardening, depending on the required hardening depth, the exposure time for the carburization can last hours up to days. After the carburization, the parts are typically quenched in a salt or oil bath, subjected to a hydrogen expulsion and annealed in a separate oven.

With the through-hardening procedure, components are hardened through the entire cross-section. The exposure time in the hardening oven is in the range of hours.

With both procedures, mass and form changes lead to increased distortion, longer process times for the subsequent work steps and higher production costs and energy usage. Moreover, conventional hardening installations for these large parts are technically limited and inefficient.

These disadvantages could be overcome by using inductive surface hardening. The main advantages of the inductive surface hardening in contrast to case hardening and through-hardening include:
  • Lower investment costs
  • Lower process (energy) costs
  • Quality monitoring of the process on the individual component
  • Possibility of integration of the hardening process in a production line by reason of the good capability of automation
  • Reduced treatment times
  • Decreased distortion of the components
It is, however, also faced with disadvantages:
  • Complex plant technology
  • Elaborate inductor developments
  • Partially very complex process technologies
  • Material selection and material development
  • Until now partially unknown values of the usage properties
Based on principle, three procedures for inductive surface hardening should be distinguished. They are scanning with soft zone, scanning without soft zone and single-shot hardening (Fig. 8). The procedures and the plant concepts connected with them are explained in the following section.

Scanning with Soft Zone

The scanning mode is the most widely used procedure and is successfully used for pivot bearings (ball bearings). The disadvantage of this is, depending on the process, a soft zone (seam or slip) emerges.

This does not have a negative effect on pure pivot bearings, since the loading regime can be selected in such a way that no loading or only a minimal loading occurs in the range of the slip zone. In the case of rotary applications, this soft zone can lead to a premature failure of the bearing. Therefore, it is not allowed for roller bearings. Both horizontal portal installations and special machines are used as machine concepts. Very often, however, vertically or diagonally placed machines are used for these procedural variations. The advantage of this concept is decreased space requirement as well as the advantageous flow conditions and the arrangement of the workpiece.

Fig. 8. Solutions for induction hardening of large bearing rings; Fig. 9. Principle of ring hardening without soft zone (seamless hardening)

Scanning Without Soft Zone

Inductive seamless hardening, which is patented by EFD Induction[7] with inductors in opposite directions, avoids (like the total surface hardening procedure) the formation of a pronounced soft zone and can be used for very large rings.

The principle operation of this procedure is shown in Figure 9. In the first step, a warming of a restricted ring section occurs with two inductors (step 1). After the hardening temperature is achieved in this area, quenching is initiated and both inductors simultaneously harden the unhardened ring areas in opposing feed directions. Corresponding to the ring dimension, the end section is pre-warmed either simultaneously or delayed with a further inductor (step 2). Before both scanning inductors reach the end section, this inductor is removed from the end section (step 3). After the end section is brought to the hardening temperature with the aid of the two feed inductors, they are removed and quenching occurs (step 4). This procedural sequence makes it possible to achieve a hardness structure in both the front and end sections without a soft zone. However, it should be noted that the adherence to a multitude of procedural parameters is required.

Single-Shot Hardening

So-called single-shot hardening also prevents the formation of a soft zone. It is, however, only expedient for diameters up to 2,000 mm. For a ring with a 2,000-mm diameter, a converter power of approximately 2 MW must be installed. The costs for converters and inductors as well as a quenching apparatus increase significantly with increasing component size, which makes the procedure inefficient for the production of large roller bearings. Furthermore, large quenching-agent quantities and a comprehensive periphery are required.


Wind power as an energy source has a high future potential. However, the technical solution required for it presents a great challenge. Inductive surface hardening presents a solution to the questions connected with it (if also a smaller contribution). Thus, increasingly more large components such as gear wheels, sprockets and bearing rings of large wind plants will surely be surface hardened with the aid of inductive heating. Single-tooth hardening procedures are primarily used for gearings. For bearing rings – depending on the requirement profile – the tracks are hardened by means of scanning with and without soft zones or with smaller dimensions in single-shot mode. IH

For more information: Contact Dr. Hansjürg Stiele, EFD Induction GmbH, Lehener Strasse 91, Postfach, 426 D-79004 Freiburg, Germany; tel: 49-761-8851-0; e-mail: Mark Andrus, EFD induction Inc., 31511 Dequindre Rd. Madison Heights, MI 48071; tel: 248-658-0700; e-mail: