Surface Finishing Methods.

Paper polishing by hand using emery cloth can generate lead if the paper is not held perpendicular to the axis of the shaft. Automated polishing can ensure the paper is perpendicular to the shaft, resulting in an acceptable, lead-free finish. An example of what a paper-polished surface would look like if viewed through a high-power microscope is shown in Figure 116.

As shown in close-up in Figure 117, honing can be problematic because it generates lead in the form of a crosshatched surface. This crosshatching creates channels through which fluid can escape.

Machine turning is unsuitable for shaft finishing because it always creates lead. If used, machine turning should always be coupled with an additional finishing operation that will eliminate lead. A close-up example of a machine-turned surface is shown in Figure 118.

Unlike the preceding finishing methods, glass bead blasting doesn’t generate shaft lead. Unfortunately, it doesn’t remove it, either. Dimpling of the metal masks lead caused by machine turning without eliminating it. A close-up example of a glass bead blasted surface is shown in Figure 119.

Similarly, metal peening also camouflages shaft lead caused by machine turning with dimples. Unlike with glass bead blasting, however, these dimples have tiny notches on one side. As shown in close up in Figure 120, these notched dimples mimic lead.

Stone tumbling results in a uniform (rather than dimpled) shaft surface finish. However, stone tumbling still does not remove lead. A close-up example of a metal surface that has undergone stone tumbling can be seen in Figure 121.

Roller burnishing doesn’t generate lead, but it also doesn’t remove it. Roller burnishing compresses – rather than eliminates – lead grooves. A close-up example of a roller-burnished surface is shown in Figure 122.

As illustrated in Figure 123, through-feed centerless grinding (also known as transverse grinding) can create shaft lead if the feeding process is too fast. Through-feed centerless grinding can produce lobing (out-of-round shafts).

Machine lapping involves the finishing of a shaft by rotating it between two rollers of varying speeds. One of the rollers utilizes an abrasive medium to wear the metal surface. The grit size of this medium can be chosen to give the proper surface texture, but machine lead can be created if the rollers are not properly aligned. In addition, machine lapping may not remove enough material to eliminate lead caused by turning. A close-up example of machine lapping is shown in Figure 124.

Grit blasting is a process wherein abrasive particles (such as sand) are shot against metal to compress the surface and to leave behind tiny indentations capable of holding lubrication. Unlike glass bead blasting (which can only be used on unhardened shafts), grit blasting is possible with hardened surfaces. Applied correctly, grit blasting can eliminate mild to moderate (though not major) machine lead. A close-up view of a grit-blasted metal surface is illustrated in Figure 125.

Sintering of compressed metal particles in a mold is sometimes used to produce porous shafts capable of holding lubrication. Though this lubrication-holding ability is helpful to the seal’s lip, it comes at the expense of increased seepage and metal impurity, as well as reduced shaft strength. A close-up view of a sintered metal surface is shown in Figure 126.

A drawn stamping involves use of a stamping die to form a metal sleeve. Many wear sleeves are produced in this way. Out-of-roundness is possible, and draw or work lines (such as are shown in Figure 127) on the sleeve surfaces can cause leakage. It is recommended that the surface on which the seal rides be plunge ground or paper finished to form the proper surface texture without machine lead.

Plunge grinding has proven to be the most reliable finishing method for removing machine lead on rotating shafts. This is because plunge grinding eliminates any axial movement of the grinding wheel relative to the surface of the shaft. A mixed number (rather than whole number) RPM ratio (for example, 9.5 to 1) between the grinding wheel and the shaft (which should be rotating in opposite directions) is suggested to help prevent the introduction of spirals onto the shaft surface. Plunge grinding using mixed number ratios also greatly reduces the time required to achieve sparkout, the point where sparks are visible during the grinding operation. You must leave enough material on the shaft so that you can grind it to remove all traces of lead. If all of these recommendations are followed, then the shaft surface should be free of lead. A grinding wheel with an 80-grit size will provide a surface finish of 8 to 17 µin Ra per RMA recommendations. An example of a plunge-ground surface is shown in Figure 128.

As noted in Table 34, there are two other interrelated concepts relating to shaft finishing. These are out-of-roundness and chatter. Out-of-roundness (OOR) pertains to the oval geometry of a lobed shaft. OOR can make it difficult for the sealing lip of a shaft seal to maintain proper contact with the shaft, particularly at elevated shaft speeds. OOR has two causes: deformation during assembly, and machining inconsistencies. When OOR becomes excessive (greater than 45 cycles or lobes as defined by the RMA) it is considered grinding chatter (also known as waviness). RMA specifications are that OOR be less than 0.0050 mm (0.0002 in) at a maximum of 2 lobes, and less than 0.0025 mm (0.0001 in) at a maximum of 7 lobes. Figure 129 shows an example of a lobed, out-of-round shaft.



“Shaft seal design is never complete without giving due consideration to the shaft on which the seal will be asked to function.”


Figure 116

Figure 117

Figure 118

Figure 119

Figure 120

Figure 121

Figure 122

Figure 123

Figure 124

Figure 125

Figure 126

Figure 127

Figure 128

Table 34

Figure 129