3.11.
Dynamic Oil Seals
Dynamic oil seals are used between surfaces having relative motion such as a shaft and a
housing. The seals keep liquid and gases in and keep contaminants out. Oil seals must not press
very hard against the moving parts to minimize surface wear and must provide minimum drag
or friction. Seal selection is made considering the rubbing speed, fluid pressure, operating
temperature, shaft surface requirements, and space available. Some dynamic seals, such as
piston rings, are designed to withstand a lot of pressure, while others such as front and rear
crankshaft oil seals, seal against little pressure. Seals around rotating shafts in an engine are
called as radial positive contact seals. Three most common types of seals are,
(a) the radial-packing seal
(b) the radial-lip seal, and
(c) the spiral-thread clearance seal.
Dynamic seals used in automotive engines are most commonly made from a rope packing
or from synthetic rubber. Rope packing is the least expensive type of dynamic seal and has very
low friction and wear characteristics. Lip-type dynamic seals used in automobile engines are
made from synthetic rubber. They can stand more shaft eccentricity and run-out than rope type
seals. They can operate at higher shaft speeds, but they require a finer shaft finish, for longer
sealing life. Lip seals place more load on the shaft than the rope type seal, and therefore, they
seal better. Lip seals are usually held in a steel case or are supported by bonding on to a steel
support member. The seals must run with a very thin flow of lubrication otherwise it would
wear the shaft very quickly.
3.11.1.
Front and Rear Crankshaft Oil-seals
Crankshaft Pulley-end Oil-seals.
The radial-packing and radial-lip seals are generally used in the timing-chain or oil-pump
housing surrounding the crankshafts extended nose. The graphitized-asbestos packing or
synthetic-rubber lip rubs against a sleeve and forms part of the pulley hub or the inner oil-pump
member (Figs. 3.69 and 3.70). Only in a very few cases, spiral-thread clearance seals are used.
During operation, most of the oil is squeezed out of the front main-end journal and is flung
radially outwards as the oil spreads on the rotating oil-flinger washer. However, some oil creeps
along the shaft and meets oil-seal packing or lip end which prevents oil going beyond the casing,
so it is then drained back to the sump. In the case of the spiral-thread seal, the rotating
movement of the shaft tends to continuously screw back any excess oil, which is then returned
to the sump by gravity.
Crankshaft Flywheel-end Oil-seals.
The radial-packing, radial-lip, or spiral-thread clearance seals are used at the flywheel end
of the crankshaft (Fig. 3.67). The axial movement of the escaping oil between the journal and
the bearing is initially diverted by an oil-flinger or the side of the flywheel flange, which deflects
it radially instead of flowing in an axial outward direction. The small quantity of oil which still
continues to move axially is blocked off by the sealing action, and the excess oil is allowed to
return to the sump.
