Motion System Design
Applying shaft seals
By Rick Hudson Dr. Les Horve RL Hudson, Tulsa, Okla., | June 1, 2007
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.
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. Stick-slip 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.
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 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 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 half-rubber, 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-and-half seals may require more force to install than a full-rubber O.D. shaft seal.
With their complex combination of mechanical, hydraulic, electrical, and computerized systems, automatic transmissions pose considerable challenges to seal designers. Generally speaking, there are two transmission designs: Rear wheel and front wheel drive. Rear drive designs typically employ an input seal (also known as the front seal) to prevent leakage of transmission fluid at the interface between the torque converter and the transmission case. Rear drive designs also use an output seal (otherwise known as the rear seal) to prevent leakage past the output shaft — where the transmission connects to the driveshaft.
In front wheel drive designs, an input seal prevents leakage between the torque converter and the transaxle. They employ two output seals, one at both of the opposing interfaces between the transaxle and the front drive axles. For example, an input seal might consist of a non-unitized design — or a dual lip designed to facilitate fluid separation. (Flanges in both can make removal easier.) Output seal designs can be non-unitized designs with shields, or unitized designs featuring axial dirt lips.
Wheel axle seals
Shaft seals on automotive, truck, and off-road vehicle wheel assemblies contain grease and exclude dirt, mud, and other contaminants. The specific seal design typically depends on how and where the seal must function. The most effective designs include an axial dirt lip that contacts a vertical surface (for example, a unitized wear sleeve) to exclude contaminants. The driven axles in light-duty applications are generally sealed with standard spring-loaded seals. Some wheel bearings are sealed and greased for life; these are called hub unit bearings. Fig. 10 shows a hub unit application for a driven axle. The spring-loaded seal prevents axle oil from entering the greased hub unit bearing.
Heavy-duty wheel end seals (also called oil bath seals) have become the focus of increasing demands for longer warranty periods, higher temperature resistance, and increased contamination exclusion. Both non-driven and driven axles in heavy-duty truck applications often feature oil-lubricated bearings that must be sealed. The hub rotates while the spindle is stationary, and unitized seals are commonly used.
The external seal design features a rotating seal sleeve in conjunction with a stationary sealing element. Because the O.D. of this seal is metal, tools are required for proper installation. Some internal seal designs feature a stationary sleeve and a rotating sealing element. With rubber on both the seal I.D. and O.D., these can be installed by hand, without tools, if necessary. Note that this internal design also features both an axial dirt lip and a flinger to maximize exclusion of contaminants.
Heavy-duty wheel end applications lubricated with hypoid grease can also utilize spring-loaded, unitized designs featuring both a metal I.D. and O.D.
Heavy-duty pinion seals
Like so many other seals, pinion seals — found in rear wheel drive transmissions where the driveshaft yoke enters the differential housing and so named because they're near the differential's pinion gear — are under increasingly stringent demands. Pinion seals (also known as input seals) must also resist being broken down by increasingly aggressive additives in gear oils. High shaft speeds are common, and so are temperature extremes. The placement of pinion seals means they are constantly inundated by dust, water, and mud; axial endplay is also typical. In particularly dirty environments, a flinger may also be installed on the yoke to help deflect contaminants.
One common pinion seal configuration is a fluoroelastomer seal with a spring-loaded primary lip and dual dirt lips. The space between the primary lip and the dirt lips is packed with grease as a further barrier to contaminants. The seal is placed in the differential housing, and the yoke connecting the drive shaft to the pinion gear provides the sealing surface.
Contamination reaching the sealing lips can cause wear on both the lips and the yoke surface, so in some cases a unitized pinion seal design is better. As shown in Fig. 11, this design features two axial dirt lips, maximizing contaminant exclusion. A thrust bumper is molded on the seal to accurately locate the axial dirt lips. Sometimes a PTFE thrust bumper is utilized to minimize friction between the seal and the flange. This bumper further impedes contamination, as does the flange, which acts as a flinger. The wear sleeve provides the running surface for the seal, thus protecting the yoke.
This article excerpted from theShaft Seal Design and Materials Guide of RL Hudson and Co. The book can be read in its entirety and purchased at http://www.rlhudson.com. To access the MSD seals library, search seals at http://www.motionsystemdesign.com.