AGMA 91FTM14-1991
热收缩和膨胀对塑料齿轮几何形状的影响

The Effect of Thermal Shrink and Expansion on Plastic Gear Geometry

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标准号

AGMA 91FTM14-1991

发布

1991年

发布单位

美国齿轮制造商协会

 

 

适用范围

One of the fundamental differences between plastic and metal gears is their differing rates of thermal expansion. An unfilled engineering plastic such as nylon or acetal will have four to five times the thermal expansion coefficient of steel. If the gear mesh is expected to operate at elevated temperatures, the designer must account for this expansion or risk interference at high temperatures or low contact ratio at low temperatures. Historically, this is achieved by altering backlash and root clearance of the mating pair to accommodate expansion. Such an approach is perfectly acceptable for gears with similar expansion rates. However, if a plastic gear with a relatively high thermal expansion is in mesh with a steel gear at an elevated temperature, the method will cause improper meshing action. The higher thermal expansion rate of the plastic gear will cause its basic gear geometry to change much more dramatically than the steel gear. This change in geometry due to thermal expansion is very similar to thermal shrinkage during the cooling cycle in the mold. And the result will be gears operating with dissimilar base pitches. The effect of thermal shrink and expansion on plastic gear geometry must be thoroughly understood before such gears can be properly designed or inspected. This paper will examine the behavior and governing equations for this type of application. In summary, the reader will be able to calculate the actual gear geometry of the mold that produces the finished part as well as determine the change that will occur in the finished part due to operating at elevated temperatures.

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