Accuracy of Profile, Lead, and Spacing

Gear noise is affected by the accuracy of the profile, lead, and spacing. Accuracy, in this case, is the conformance to the specifications on the drawing.

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Noise
The primary cause of gear noise is the number of teeth in the gear and the contact ratio of the given set. These are design functions and we will not discuss them here. Control of the three basic elements (profile, lead, and spacing) also affects noise, and they are directly responsible for 2nd, 3rd, and 4th order frequencies of the tooth mesh frequency (Figure 1).

If you could manufacture a perfect set of gears and mount them in an infinitely rigid case, noise would not be a problem. However, we cannot make a perfect gear, and cannot mount it in an infinitely rigid box within the cost parameters established by management.

To maintain a quiet gear set, you must have conjugate motion (i.e., the motion going into the pinion must be the same as the motion transmitted by the output gear times the ratio). The effect errors in the three elements cause the output gear to accelerate and decelerate; this, in turn, causes vibrations that are generally amplified by the transmission system (Figure 2).

In spur gearing, gear profile is the most sensitive feature. In respect to effects on noise, lead has little to no effect. In helical gearing, lead is the most sensitive feature—with profile, it is second. This is due to the way each type of gear meshes (Figure 3). In both cases, the spacing is less critical, but cannot be ignored.

It has been determined from tests and is accepted throughout the industry that a hollow on the center portion of the involute in excess of 0.005mm (0.0002) will result in a noisy spur gear. Figure 4 indicates what that tooth pattern would look like on spur and helical gears respectively. Note the lines of action on the helical flank.

As stated, lead deviation or variation has a greater effect on noise in a helical gear setup (Figure 5).

Spacing has a similar effect as lead on a helical gear because of the angular line of contact. Non-hunting gear tooth and non-hunting “hob-to-gear” combinations generally lead to higher transmission noise. However, this is in the 4th order frequency range, a very high frequency. However, it can affect the overall rating.

Other causes of noise are surface finish. In the higher quality gearing, ground gears and their grinding process can leave minute flats on the surface, resulting in what is known as ghost noise.

Excessive pitch line run out is a problem that shows up as “wow noise” or a “beat”. Control of this problem is directly related to one or more of the following:
• blank preparation,
• tool and part arbors
• machine alignments
• inaccurate set up