THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

378 14. Machinery Noise ControlOther Aspects of Gear Noise ControlGear production methods involve die casting, milling, drawing, extruding, stamping,and production from sintered metal. These methods tend to produce gearswith significant tooth error to a more or lesser degree. Most gears produced bymilling cutters also tend to have their tooth forms only approximate the correctconfiguration. In order to produce a precise tooth form, each milling cutter shouldbe dedicated to only one diametral pitch or module and for a specific tooth number.In practice, a different milling cutter is used for each diametral pitch, but eachcutter is used for a range of tooth numbers. Inaccuracies in cast, drawn or extrudedgears result from shrinkage and other dimensional changes.Precision methods of generating gear teeth usually result in cutting down noisedue to tooth error. These include computer-controlled generating rack cutter, thegenerating gear shaper cutter, and the generating hob. Precision gears are usuallyfinished by grinding and shaving, and other finishing methods include burnishing,honing, and lapping.Large gears benefit by damping to absorb vibration energy. Constrained layerdamping, which sandwiches a layer of damping material between the gear web anda rigid steel plate, can reduce gear noise. If loads are low and the temperatures arenot excessive for the material, fiber, plastic, fiberglass reinforced, and compositegears are used in such applications. These materials have a low modulus of elasticityand high internal damping compared with steel. Shock loads from tooth meshingand tooth error are absorbed, thereby reducing noise.Gear enclosures or housing can and should be designed to control noise. Theenclosure should be isolated so as not to transmit vibration to adjacent structures;and provisions for adequate lubrication of the gears should be included. Resonancesof the enclosure should not correspond to the tooth meshing or any other excitationfrequency; otherwise the housing will radiate a large amount of noise energy. Thiscan be avoided by stiffening the enclosure structure to “tune out” its resonance toa higher frequency. The stiffeners themselves should be designed so that they havea low-radiation efficiency. Advantage should be taken of the directivity patterns ofnoisy gear trains in the orientation of machinery, so that personnel noise exposureis minimized.14.9 Journal BearingsA journal bearing is the simplest type of bearing. It consists of a portion of a shaftrotating inside a circular cylinder with a layer of lubrication separating the shaftand bearing surfaces. Hydrodynamic rotation depends on shaft rotation to pump afilm of lubricant between the shaft and the bearing. If the lubricant viscosity andshaft rotational speed are adequate for the load on the project bearing area, thenthick-film hydrodynamic lubrication will prevail. This ensures stable operationwithout metal-to-metal contact, as well as quiet operation. Starting, stopping, and