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Program: The Installation of Bevel Gears


Backlash

Another critical parameter for a bevel gearset to perform properly is Backlash.  Backlash in bevel gears is defined as outer normal backlash, or transverse backlash at the tightest point of mesh. Backlash can be larger at other points of the mesh but not smaller.

Unless otherwise specified, normal backlash is measured normal or perpendicular to the tooth surface. In other words, backlash should not be measured in the plane of rotation. In most cases, the manufacturer marks the normal backlash on the gear. For the purpose of illustration, we’ll show how the backlash is checked in a tester. However, checking backlash in a gearbox is theoretically performed in the same way.

The mounting distance of the gears is first set and the mating teeth are aligned. Then an indicator is placed on the gear tooth surface. Again, the indicator must be located normal to the tooth surface as is shown here. First, the pinion member is locked so it will not rotate. Then the gear member is rocked lightly back and forth.

The maximum value on the indicator is considered to be the backlash reading.

It’s important to keep in mind that the orientation of the tooth to the indicator is essential for obtaining an accurate backlash reading. Here we see the proper orientation.

For critical applications where backlash is small, it is a good idea to check the backlash in several areas. Runout from the gearbox’s bearings and shafts can change the location of the tightest point of mesh from the matched teeth to another location.

An important note here - sometimes, it is not possible to locate the indicator normal to the surface and the backlash measurement must be made in the plane of rotation. Measuring backlash in the plane of rotation will provide a reading that is called transverse backlash.

However, this value can be as much as 40% larger than “normal backlash”. Please contact Arrow Gear’s Design Engineering Department if you need to use this method.

When mounting distances have been marked on one or both gears, position the gears with these distances first.

Then use the normal backlash value to verify proper assembly. It’s important to note – before checking the backlash, at least one of the gears - preferably the pinion - must be installed and positioned at its marked mounting distance.
However, it is best to do both members whenever possible.

The reason is this. Because bevel gears are conical in shape, they can be assembled in an almost infinite number of positions - most of which cause poor performance - while still having the desired backlash value.

On gear ratios greater than one, the pinion position controls the contact pattern more than the gear. The gear controls the backlash more than the pinion. As the ratio increases, the effect becomes more dramatic. One to one ratio gears control pattern and backlash equally.

Frequently, an assembly technician will assemble the bevel gears and obtain the desired amount of backlash without regard to the mounting distance.

In this example, we see a set of gears that are dramatically out of position from their proper mounting distance. It is actually possible to obtain a proper backlash reading in this position, but overlooking the mounting distance can have a very negative result.

This is sometimes done with low quality or lightly loaded bevel gears. Although overlooking mounting distance will occasionally work, it is a risky approach when the gears will be loaded to their maximum capacity.

This will most often result in shorter life and poor performance. So it’s important to remember to start with the proper mounting distance, then obtain the designated amount of backlash.