In the constant battle to reduce noise, selection of material can play an important role. To minimize noise the most common approaches design engineers take is either redesigning the gear or specifying tighter dimensional tolerances. However, these two approaches can strain the relationship between the end user and the machine shop and add cost to the finished part. There is a third alternative — using a material that has high noise reducing ability. Cast iron(CI) is one such material.
It is often neglected as an engineering material even though it is well known for being highly machinable. This is because there is a misconception that it is brittle and weak.
While ductile iron is strong it’s a fact that the gray irons are relatively brittle. Graphite in gray iron is in the shape of flakes, but in ductile iron it is in the form of rounded, small nodules.
From gray to ductile all types of CI have inherent sound damping properties and can hence reduce noise. Thanks to the graphite content of CI that gives it the best possible noise decreasing ability. Noise vibration is absorbed by precipitated graphite particles; therefore, ductile iron’s relative damping capacity is twice that of steel.
It is based on the application that materials are selected for gears. For instance, the necessary mechanical properties in the gear tooth and the quantity of damping preferred. In an automobile manufacturer’s four-cylinder engine cast iron was used as the balance shaft. The result was significant noise reduction. Earlier, steel was used for the gear. But when ductile iron was used instead of steel there was significant noise reduction. To obtain the result, no alterations were made to the gear’s dimensional tolerances or design. A significantly lower machining cost was also achieved when they changed the gear material to ductile iron.
In cases where added strength is required converting to ductile iron is not recommended because of fatigue failures. Also, ductile iron is not suitable for gears that are subject to high impact forces. Before making a switch to ductile iron it is important to understand the required safety factors as well as forces acting on the gear.
However, many gear manufacturers are still reluctant to use CI as a suitable material. This is due to several reasons. Manufacturers who set up to machine steel bar stock shy away from using cast iron because of the initial pattern cost. Sand molds are used to make castings, and to produce the shape of the mold cavity patterns are required. To make the part iron is poured into the cavity. To produce patterns manufacturers will have to shell out $20,000 each. In addition, they need bar feeders and machining centers designed to handle bars.
Therefore, they will have to retool the machining center if they want to switch to iron castings. The typical quality problems associated with statically cast iron blanks, such as slag and sand inclusions, shrinkage, hard spots, and porosity also deter manufacturers from switching to cast iron. Due to its graphite content it can also be dirty to the machine.
But whatever the drawback, cast iron, no doubt, is the best choice for reducing gear noise.