3.2.1 Influence factors
There are many factors which experience has shown to influence the amount of sound generated by operating gears in a power transmission system.
Table 3--1 lists items which are often considered when quiet operation is desired.
Table 3--1 -- Considerations for noise control -- Type of gearing
-- Tooth profile and pitch -- Pressure angle
-- Geometry modifications -- Total gear contact ratio -- Design load versus operation -- Quality (accuracy and finish) -- Unbalance and alignment -- Tooth ratios
-- Type of bearing support -- Gear blank design -- Pitch line velocity
Each item in table 3--1 can be argued as to its relative importance; however, they all must be considered with the thought that the main object is to achieve smooth operation under certain performance condi-tions. To transfer a specific power with minimum change in the angular velocity of the meshing gears is the most desirable characteristic. One can see that there is great difficulty in describing a “simple” model of a gear driven system. Therefore, to say the generation of gear noise can be simply described or analyzed may be an impossibility. However, one
should reserve thinking about the many degrees of freedom (lateral, longitudinal and torsional) and their modes that may develop into gear noise, and concentrate on eliminating the system vibrations.
3.2.1.1 Manufacturing quality
One major internal source of gear noise can be related to differences in gear geometry during engagement (meshing) of teeth. If a “perfect” gear set could be designed and manufactured, there would not be any sudden accelerations or decelera-tions of the rotors during the transfer of power, and noise or vibration, due to operation, would be minimized. Therefore, the imperfect geometry, profile, and spacing of gear teeth must be controlled to minimize the noise. The items in table 3--1 pertaining to gear geometry must be discussed in terms of reducing the sudden accelerations and decelerations during tooth mesh for a particular application. One prime consideration is that no matter how good the design, it is the result of the realistic quality of manufacturing that determines the sounds generated.
3.2.1.2 Types of gearing
The type of gearing for quiet operation must be selected for the speed and power transferred. The various types of gears such as spur, helical, herring-bone, worm, bevel, etc., all have individual charac-teristics lending themselves to specific applications, speeds, and loads. The tooth profile, pitch, pressure angle, overlap, quality, and ratios are all items that can be discussed in terms of quiet operation for one or more of the following reasons:
-- enables a manufacturer to reduce inaccuracy;
-- averages out individual tooth errors over a wide number of teeth;
-- reduces abrupt changes in tooth contact action;
-- places generated noise frequencies outside the sensitive audible range or at a frequency that can be more easily controlled.
3.2.1.3 Geometry modifications
Other items such as geometry modifications for designed operating load, misalignments, unbalance and types of bearings can also be controlled to minimize the changes in angular velocity while transferring loads. Because of the number of items to be considered and their dependence on load deflections of teeth, it becomes difficult to produce a
gear unit that will be its quietest at all required speeds and loads. Therefore, the user should not expect a gear unit to be quiet under all conditions of operation.
3.2.2 Total gear contact ratio
The variation of meshing stiffness is one of the major influences on noise generation. The total contact ratio as a sum of the helical overlap ratio and the transverse contact ratio significantly influences the stiffness variation. By careful selection of both ratios, the influence of transmission error and noise can be reduced significantly. In general, increasing both ratios will lead to a lower noise level, but experience and investigation have shown the best results are at values for both factors which are more or less a whole number. For example, selecting a transverse contact ratio of 2 can result in a sum of the total contact ratio as a whole number. Therefore, simply increasing the ratios is not enough; rather finding an optimum value is required. A very high total contact ratio in itself may not result in an improved noise level.
The helical overlap ratio can be increased by a higher helix angle, increased face width, or use of a finer pitch. But its benefit is limited because the transverse contact ratio will in turn be diminished.
The transverse contact ratio can be increased by using a lower pressure angle and profile modifica-tion, see AGMA 913--A98.
A possible difference in action between spur and helical gears of similar geometry is shown by comparing figures 3--1 and 3--2 and is further defined in AGMA design standards.
Knowledgeable design of helical gears is an implied requirement as shown figure 3--3. The more constant the contact length and the more uniform the contact conditions, the lower the dynamic forces which cause noise. The same advantages lie in spiral bevel gears when compared to straight bevel gears.
3.2.3 Optimum tooth geometry
The selection of optimum tooth geometry to reduce noise is complicated by the fact that compromises are necessary. If a tooth of increased height is used to get a greater transverse overlap, both the strength and scoring resistance might be reduced. If the tooth pitch is reduced or the helix angle increased too greatly, reductions in strength are also possible. It might appear simple to increase the size of the gears
to increase strength capacity, but a larger gear runs at a higher pitchline velocity normally producing increased noise.
3.2.3.1 Pressure angle selection
Some noise control is related to the selection of
pressure angle. A lower pressure angle reduces the effect of radial runout and in a minor way decreases tooth stiffness to reduce meshing impulse.
Figure 3--1 -- Contact of helical gears
Figure 3--2 -- Contact of spur gears
Facewidth, inches
Figure 3--3 -- Variation of length of contact lines/face ratio with face width
3.2.3.2 Profile modifications
Meshing impulses or dynamic forces can be reduced to control noise by profile modifications. As an example, tip relief can “ease” an incoming tooth into contact to compensate for the elastic deflection of the teeth already in contact as illustrated in figure 3--4.
3.2.3.3 Lead modifications
Besides profile modifications, crowning (or barrel-ing) across the face and tapering of the lead may be done to compensate for misalignment and deflec-tions under a given load. Crowning and tapering can also affect the durability and strength capacity of the gears. Excessive profile and lead modifications can increase rather than decrease gear contact noise.
3.2.4 Pitchline velocity
Normally gear contact velocities will affect the generation of noise. Lower pitchline velocities and sliding generally result in lower noise levels. Lower velocities may be achieved by changing types of gears, materials, hardness, ratios, size, etc.
3.2.5 Practical design changes
It is important to realize that, generally, the present accepted practices for gear design in accordance with AGMA standards are such that reductions of only 2 to 4 dBA are practically obtainable by changing items listed in table 3--1.
The gear manufacturer must be given the design flexibility to balance load capacity with the items that may affect the sound generation.