Valve stem seals are sprung-lip seals pressfit over valve guides in internal combustion engines. The molded, radiused seal lip touches the valve stem, which is reciprocating within the valve guide. The magnitude of the lip radius meters (controls) the lubrication of the valve stem such that the stem receives neither too little nor too much oil. A stamping flange on the seal O.D. provides a seat for the valve spring to prevent damage to the surface of the valve seat area.
Select figure to enlarge.

Topics of discussion:

• Isolating power transmission
• Automotive applications
• Sealing shock absorbers

In our last installment, we discussed how a shaft seal is just one of a three-part system — a system that also includes the moving shaft itself, as well as the housing into which the seal is installed. Now we discuss mechanical assemblies containing fluids that are less easily isolated from the assembly. Seals incorporated into such designs prevent leakage at the points where different assembly parts meet — blocking clearance gaps so that nothing passes through it.

Automotive applications

Shaft seals are common in automotive applications because of the many ways in which they may be configured. Diesel engine rear crankshaft applications (shown in Fig. 7) 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 speed that results in high lip temperatures. The seal lip is also poorly lubricated because the area is splash lubricated. This causes an even higher underlip temperature. Large, random shaft deflections caused by piston slap can make it very difficult for the sealing lip to follow the shaft surface properly. Stickslip or lip chatter can also occur, further increasing temperature. Oil degradation and coking can cause sludge to accumulate on the sealing lip. What's more, diesel oils also contain additives that can degrade elastomers and hasten leakage.

Many power steering applications use high-pressure hydraulic systems. Seals for power steering applications include an input shaft seal (also known as a stub shaft seal) and a pinion seal. These seals are for shafts with slow oscillating rotation, and they typically have operating pressures that are 10 to 20 psi. The outer rack seal and inner rack seal are reciprocating applications and can see pressures up to 1,500 psi. Typical rack seals have plastic or PTFE backup rings to prevent seal lip blowout.
Select figure to enlarge.

Because of their high temperatures, early diesel engine crankshaft seals were made from either silicone or fluoroelastomers. However, hydrocarbons and oil additives softened silicone seals, which then disintegrated. FKM seals better resisted chemical attack, but the lubricating properties of newer diesel oils didn't measure up to those of older oils. Poor lubrication in turn lead to lip chatter and stick-slip. Higher temperatures were generated, and seal damage was common. High temperatures also caused oil burn and sludge buildup on the sealing lip, resulting in leakage. Blisters would also often form on the airside of the seal lip.

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 PTFE blended with fillers. The inherent slickness of PTFE compensates for poor lubrication and eliminates stick-slip, which in turn helps to keep underlip temperature down. PTFE is also resistant to chemical attack, making degradation of the lip by oil additives unlikely.

Because PTFE is stiffer than traditional elastomers, it cannot develop the microasperities vital to inpumping 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 unidirectional and can be used only if the shaft always rotates in one direction.

An alternative design features a PTFE lip bonded to a rubber substrate, which is, in turn, bonded to a metal case. An example of this is shown in Fig. 8. Notice that the PTFE lip features a dual coined spiral pattern. The spiral on the airside pumps oil back to the oil side unidirectionally. 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.

Though gasoline engines incorporate crankshaft and camshaft seals like diesel engines, gasoline engine seals are different. Gasoline engines have become smaller and more powerful, so the need for increasingly heat and additive-resistant rear crankshaft seals has caused designers to turn away from silicone (for years the typical engine seal material) to fluoroelastomers. FKM seals are now the norm for rear and front crankshaft, camshaft, and auxiliary shaft seals.

A rear crankshaft seal with a halfrubber, half-metal O.D. has advantages over either a full metal O.D. or a full rubber O.D. It may be more forgiving than a full-rubber O.D. seal during installation. The metal portion of the half-and-half O.D. has a chamfer that helps guide the seal into the bore; the metal edge also prevents the rubber from shearing off the leading edge of the O.D. during installation — which sometimes happens with full-rubber O.D. designs. Finally, with less rubber on the O.D. and a metal-to-metal pressfit, springback is reduced. (Springback is a phenomenon in which a seal unseats itself after installation due to shearing stresses between the O.D. and the bore.) One caveat: Half-andhalf seals may require more force to install than a full-rubber O.D. shaft seal.

Continue to next page