Diesel engine rear crankshaft applications (such as is shown in Figure 200) can be particularly challenging for shaft seal designers, and for a number of reasons. High crankshaft speeds and large shaft diameters are common. This combination produces high shaft surface speeds that results in high lip temperatures. The seal lip is also poorly lubricated because the area is splash lubricated. This causes the underlip temperature to be even higher. Large, random shaft deflections caused by piston slap can make it very difficult for the sealing lip to follow the shaft surface properly. Stick-slip or lip chatter can also occur, further increasing temperature. Oil degradation and coking can cause sludge to accumulate on the sealing lip. And as if all this weren’t enough, diesel oils also contain additives that can degrade elastomers and hasten leakage.

Because of the high temperatures, early diesel engine crankshaft seals were made from either silicone or fluoroelastomers (FKM). Because of incompatibility with hydrocarbons and oil additives, silicone seals often became soft and disintegrated. FKM seals resisted chemical attack, but the lubricating properties of newer diesel oils were less than with older oils. Poor lubrication resulted and, in turn, led to lip chatter and stick-slip. Higher temperatures were generated, and seal damage was common. High temperatures also caused the oil to burn and sludge to build up on the sealing lip, resulting in leakage. Blisters would also often form on the air side of the seal lip. All things considered, diesel engine rear crankshaft applications are very tough.

Seal designers attempting to address these issues have found that the most effective seals for diesel engine crankshafts are those that feature a sealing lip made of a blend of PTFE (Teflon®) and fillers. The inherent slickness of the PTFE compensates for poor lubrication and eliminates stick-slip, which in turn helps to keep underlip temperature down. PTFE is also very resistant to chemical attack, making degradation of the lip by oil additives unlikely.

Conventional PTFE shaft seals are made by crimping one or more PTFE wafers into a steel case along with steel spacers and gaskets (which prevent leakage between the inside of the case and the PTFE lips). An example of this type of design is shown in Figure 201. The seal shown in this illustration has a radial dirt lip to exclude contamination and a rubber gasket between the inside of the metal case and the dirt lip to prevent internal leakage. Because PTFE is stiffer than traditional elastomers, it is not able to develop the microasperities vital to in-pumping of oil. To compensate for this, a spiral groove must be machined or coined into the surface of the primary sealing lip; this groove screws oil back into the sump. The seal is uni-directional and can be used only if the shaft always rotates in the same direction.

R.L. Hudson & Company is proud to offer an alternative design featuring a PTFE lip bonded to a rubber substrate, which is, in turn, bonded to a metal case. An example of this is shown in Figure 202. Notice that the PTFE lip features a dual coined spiral pattern. The spiral on the air side pumps oil back to the oil side (uni-directionally). The coined spiral ridges on the oil side of the lip improve lip flexibility. Notice also that this design features rubber ribs molded on the seal O.D. to improve sealing between the housing bore and the seal O.D.

Diesel engine applications also include front crankshaft and camshaft seals. An auxiliary drive seal is common as well. These are all typically smaller than the rear crankshaft seal, so the speed and temperature difficulties inherent in rear crankshaft seals are not as pronounced. And though these seals also tend to be better lubricated than rear crankshaft seals, PTFE is still the lip material of choice for these diesel applications.

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