Accounting for Plating Thickness in Assembly Design
The main consideration when plating threaded components would be to ensure that the plated threads properly fit into their mated counterparts. As shown in Figure 1-1, the dimensions to consider here would be the diameter and pitch of the threads. This translates into the tolerance of the plated coating being accounted for within the assembly because the plated coating will slightly change the geometry of the threaded component. The thickness of the plating should be accounted for when the part is designed. The most ideal location to account for the plated coating would be within the specifications of the mating part, including diameter and pitch. Of course, certain calculations are required to determine the slight change in the diameter or some other dimension of the mated part which will not impact the overall strength of the assembly. But, this is a calculation that should be performed before arbitrarily designating a plating thickness.
Figure 1-1 (image credit: Shighley’s Mechanical Engineering Design Ninth Edition)
The increase in thickness can best be visualized using Figure 1-2, which gives a representation of the M or MJ type threads. But, some studies performed by reputable companies have resulted in some tested and proven recommendations for the plating thickness. In the detailed research study performed by (cite Lockheed Martin paper) the target thickness of 300 μ inches performed the best, as far as torque requirements and lubricity were concerned, for several different plated metals.
|Figure 1-2 (image credit: Shighley’s Mechanical Engineering Design Ninth Edition)|
In addition, it is important to note that because electroplating is not simply a coating process-the thickness of the plated metal across a complex part may have some variation. This is because electroplating is a process that uses the transfer of electrons to create a bond between the substrate and the plated metal. The distribution of the electric field of charge around a part can be used to model the areas of high and low current density (see figure 1-3).
|Figure 1-3 (image credit: Shighley’s Mechanical Engineering Design Ninth Edition)|
So, at the locations of sharp edges on the threads, or the crest of the pitch, may build up with plating faster than the troughs of the threads. This will vary depending on the vessel the parts are plated with. But, this is simply the nature of electroplating, a highly complex process.
Torque-Diameter-Coefficient of Friction Relationship
It is important to understand the relationship between the plated material’s coefficient of friction, the torque needed to raise or lower the threaded component, and the plated diameter of the component. For an M type thread, the relationship is the following, as diagramed in Figure 1-4:
|Figure 1-4 (image credit: http://www.wtools.com.tw/Relation-Formula.shtml)|
So, even though the diameter of the threads increases with plating thickness, the coefficient of friction generally decreases. But, this is something that should be computed before seeking a plating process out, unless guidelines have already been established.
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