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Program: Arrow Gear’s Advanced Design and Development Assistance Capabilities

The Understanding Gearbox Displacement

There is another critical issue to consider when assessing how the contact pattern will perform in an operating gearbox. This issue is gear displacement.

In the operation of many gearboxes, the gears and their shafts do not remain in a fixed orientation. Due to thermal forces and stress from being under load, the components of the gearbox will actually move from their axes.

There are typically four different types of movement which can take place – and these movements are described as Offset, Pinion in and out of Mesh, Gear in and out of Mesh, and Shaft Angle. It is this movement that is referred to as gear displacement and it can occur in any combination of the four types.

In aerospace gearboxes, where keeping weight to a minimum is a high priority, the mass of the gearing used is usually lighter, and these displacements can be significant.

On the other hand, in commercial applications where the gearbox components are typically more rigid, there is not the same degree of displacement.

Conventional Methods for Contact Pattern Development

The size and position of the contact pattern has always been a primary design consideration for gears. And for many years, achieving a good contact pattern was performed through the same methods that the vast majority of gear producers still use today.

The conventional method of achieving an ideal contact pattern is performed in this way.

First an engineer will make an educated guess at how the gear should be made so that the contact pattern will be correct. Next the part is fabricated and the gear teeth are machined.

When the gear and its mating part are finished, they are run together in a tester. More often than not, the contact pattern will not be correct in this first attempt and this requires going back and changing the settings on the gear tooth cutter and grinder, then producing a new gear and pinion.

The parts are checked again. This trial and error process can continue through many cycles until the best educated guess for contact pattern location is achieved.

But how will the gear perform under load in a gearbox and what will the contact pattern look like then? This leads to more steps in the trial and error process.

First the gears must be mounted in the gearbox and run at operating speed and load. Then the gears are visually inspected to check the contact pattern – which is indicated by the wear pattern on the mating tooth surfaces. If the pattern is not correct, which is commonly the case, its back to the tooth cutting and grinding machines, where settings changes must be made, new gears must be produced and the cycle continues.

For a new gear design – this process can take up to 6 months or more to complete. And while this is a time consuming and costly process, it was just the way it had to be done – or it was until recently.

To see how Arrow has eliminated the need for this costly trial and error method of contact pattern development – please proceed to the next chapter.