Exchange of One Member During Gearbox Service

Certain industries that apply large gears in earthmoving and mining equipment request the possibility to exchange a broken, damaged, or excessively worn gear member without replacing its mating member. At first, this appears to be contrary to good mechanical practices. However, these gears in question might have diameters of 500 mm to 2,000 mm and weigh, in some cases, several tons.

The weight and the waste of expensive material, including the value added on the way from a soft steel blank to the finished gear, are compelling arguments not to replace a perfectly good gear. Also, the additional repair hours and the required equipment can be very costly. If the mating member of the damaged gear shows no damage, such as cracks or excessive wear, then the exchange of a single member should be considered.

For Which Kind of Gears Can One Member Be Exchanged?
Couplings and clutches will allow a single member exchange if the mating member does not show any fretting or cracks.

Straight bevel gears can also be exchanged if they are, for example, standard Coniflex gears where the lead function is a straight line and the tooth contact is established by the pressure angle and a standard length and profile crowning.

Spiral bevel gears are more delicate to replace. Here, the cutter diameter, the spiral angle, and the manufacturing method have to match the original components. Face hobbing and face milling are not interchangeable with each other. Generated gears cannot be replaced with nongenerated (Formate) gears. In the case of face hobbing, next to the cutter radius, the number of cutter starts is identical to the original manufacturing method. In critical cases, the damaged gear must be measured with a coordinate measurement machine. The surface measurement results can be used to reconstruct the correct surface form by applying a reverse engineering approach. If the damaged member is too degraded to perform a good measurement, then the mating member must be disassembled and undergo an inspection measurement. Also, in this case, it is possible to create the damaged member with sophisticated computer software. However, in most cases, the latter is unrealistic.

Hypoid gears are, in general, not used for these large applications. It would be considerably more difficult to create a replacement pinion or gear for a hypoid design.

In face gear sets, it is easy to replace the cylindrical pinion. However, if the basic parameters of the cylindrical pinion are known, it is also possible to design and machine a replacement face gear.