Center Distance Variations Influence Noise in Compound Gearboxes

Although gearboxes are a well-understood technology, they don’t always operate as designed. For example, many different manufacturing and assembly factors, vibrations from the gears as well as various running conditions can produce noise and transmission errors. These errors typically occur because the gears cannot operate at a standard centered distance within their predetermined tolerances. In order to provide a framework for creating a more optimal gearbox design, we recently simulated how center distance variation influences noise.

This figure shows the layout and cut section of the gearbox used in the simulation. Here’s what we observed:

•	The displacements eventually cause the shaft misalignments and impact the dynamic behavior and noise characteristics via transmission error (TE) and acoustic noise.
•	The third gear mesh exhibits a higher TE as compared to the other two gear meshes.

Our analysis focused on the third gear mesh pair in the layout and explained the significance of TE harmonics and its impact on acoustic behavior.

•	Misalignment variation. The simulation used displacements of the bearings’ supporting shafts. The front and rear bearings were displaced in the X and Y directions and the shaft misalignment in skew position were used. No-load condition has lower TE when the front bearing is displaced in the X and Y directions versus the rear bearing. We allowed displacement tolerances of 30, 60 and 80 microns. When we analyzed the skew positions, the peak-to-peak TE for X-80 and Y+80 microns was higher in comparison to skew positions of X-30, Y+30 microns and X-60, Y+60 microns.
•	Acoustic Analysis. We used a shrink-wrap mesh to link the structural vibration domain and the acoustic domain. The mesh is watertight, and it completely and closely encloses the structural mesh of the component being analyzed. Here’s what we observed: ○ With a skew of X-30, Y+30 microns and a maximum noise value of 50 dBA: As compared to nominal conditions, the displaced condition resulted in higher deflection in the shaft and the housing due to higher TE and noise levels. The maximum noise amplitude was observed at 3,200 rpm and the dBA level was 50. ○ With a skew of X-60, Y+60 microns and a noise value of +50 dBA: : Similar results were found at 60 microns as were found at 30 microns, which includes no load and no misalignment where there are nominal conditions. ○ With a skew of X-80, Y+80 microns and a noise value of +52 dBA: Similar results were found at 80 microns as were found at 30 and 60 microns.
•	System level deflection. When 0.5 Nm torque is applied with a displacement of X-30 and Y+30 microns for the rear bearing, the maximum deflection is approximately 68 microns — somewhat high.

These demonstrations of how center distance variation influences noise can be used as a guide to reduce TE, address noise at its source and to keep noise levels lower to create optimal compound gearbox designs.

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