Under normal operation, gears undergo significant contact and bending stresses. Heat treating the contact surface of gear teeth increases their strength against these stresses and lengthens their workable life.
The crane industry uses four typical types of heat treatment, depending on the application of a gear: through hardening, induction hardening, flame hardening and carburizing.
Read on to learn about the benefits and drawbacks of each type of heat treatment for specific crane applications.
Through Hardened Gears
This process creates a relatively uniform hardness throughout a gear’s steel structure. Through hardened gears either start out as blanks or as nearly finished gears that are baked in a furnace until reaching a predetermined temperature. They’re quenched in a water or oil bath to cool.
After heat treating, the gear or blank is cut to final dimensions. Compared to other hardening methods, through hardened gears generally have a very low hardness, normally 27-33 HRC (Rockwell C). Please note: the term “through hardening” is somewhat of a misnomer as the hardness of the material does decrease as you move deeper into the part.
The lower hardness does not allow the gear to sustain very high surface pressures, however, through hardened gears exhibit quite good impact resistance and bending fatigue strength.
This heat-treating process is common in older gearboxes of low-duty cycle cranes, as it is easy and cost-effective to manufacture. Through hardened gears are still a viable solution for low-duty applications.
Induction Hardened Gears
An induction hardened gear has a hard outer casing and a softer internal core. As its name implies, induction hardening uses induction heating to treat the surface of parts. This can be done all at once on an entire component, using a large coil, or the process can be completed tooth-by-tooth. Once the gear or tooth is heated to a set temperature, it is quenched in a water or oil bath to cool.
This process results in a surface hardness generally between 54-60 HRC, with an internal hardness like that of a through hardened gear. Only the surface of the gear is treated, because if the gear was through hardened to the same hardness as the outer case, it would be very brittle and fail quickly in service. The improved surface hardness resists higher contact stresses, and the softer core offers excellent bending fatigue strength and impact resistance.
On smaller gears, the induction hardening process can be controlled very tightly to ensure that surface hardness and depth of the final component are ideal for the application. However, variances in hardening depth occur in larger gears with large teeth and a quite small diametral pitch number.
Depending on the quality of gear that is being produced, and the design of the gear, gear grinding may be required after the hardening process to ensure the gearing meets initial specifications. Induction hardened gears are relatively cost-effective and can be used in a variety of low and high-duty applications.
Flame Hardened Gears
Flame hardened gears are like induction hardened gears in respect to the heat treatment process and hardness results. A flame torch applies heat to the outer surface of the gear teeth. Then the gear is quenched in a water or oil bath.
The resulting surface and internal hardness are comparable to induction hardening, but flame hardening generally results in shallower and less consistent hardness penetration depth. For most applications, we recommend induction hardening over flame hardening, as induction hardening offers tighter tolerance control.
Carburized and Ground Gears
Carburized gears have an incredibly hard outer surface while maintaining a soft internal structure. In the carburizing process, a low-carbon steel gear is placed in a furnace and a gas with high carbon content is pumped into the furnace, baking into the metal over time.
This process creates a carbon-rich surface layer on the gear tooth with a hardness of 58-64 HRC, surrounding a softer core. The enhanced surface hardness resists higher contact stresses than the previously mentioned heat treatment methods.
Due to materials and processing time, this process typically costs more than the others. The special steels required are slightly more expensive than those used in through hardening or induction hardening. Plus, the preparation time for parts is much greater than that of the other methods. The extended heat exposure of carburizing can distort gear teeth, requiring gears to be ground back into tolerance after treatment. This step adds to lead time and cost.
However, the added hardness improves lifespan, which can make up for the added costs. This process is necessary for certain applications in process duty cranes.
If your processes have changed, or your gearing is not lasting in service, heat treatment is an available and effective solution to improving your crane’s operating capabilities. I’m happy to review your crane processes with you to determine the best heat treatment for your application.
Product Manager, Gearing
Component and Assembly Modernizations
The following table summarizes information shared above:
|Through Hardened||Induction Hardened||Carburized and Ground|
|Key Benefits||Cost-effective, good bending fatigue||Cost-effective, good bending fatigue, good surface hardness||High surface hardness, good bending fatigue|
|Key drawbacks||Surface wears quickly||Lead time for heat treat||Lead time for heat treat and grinding|