Page 34 - Gear Technology Solutions

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2 Interchangeability of Gears

2.1 Exchange of One Member During Gearbox Service

Certain industries which 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 di-
ameters of 500mm to 2000mm 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 one single
member should be considered.

2.2 For which kind of gears can one member be exchanged?

Couplings and clutches will allow a single member exchange, if the mating mem-
ber does not show any fretting or cracks.

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

Spiral bevel gears are more delicate to replace. Here, the cutter diameter, the spi-
ral angle and the manufacturing method have to match the original components.
Face hobbing and face milling are not interchangeable between each other. Gen-
erated gears cannot be replaced with non-generated (Formate) gears. In the case
of face hobbing, next to the cutter radius, the number of cutter starts to be identical
to the original manufacturing method. In critical cases, the damaged gear has to
be measured with a coordinate measurement machine. The surface measurement
results can be used to re-construct the correct surface form by applying a reverse
engineering approach. If the damaged member is too destructed to perform a
good measurement, then the mating member has to be disassembled and under-
go an inspection measurement. Also, in this case, it is possible to create the dam-
aged member with sophisticated computer software. However, in most cases, the
latter is unrealistic.

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