Seals represent another important source of energy loss due to friction. In a 100-car freight train
traveling at 60 mph, an estimated 50% of the energy consumed arises from overcoming friction
in seals, of which there are 16 per car. In an automobile, there are dozens of seals in the engine,
transmission, driveline, suspension, steering system, air conditioner, and fuel tank.
The design of a seal (see Figure 7) must consider the entire system, including the fluid and the
surface of the rotating shaft. An incompatible fluid could chemically attack the elastomers,
resulting in reversion, hardening, or softening and swelling. Other potential problems include oil
carbonization from excessively high temperatures, high friction from excessive viscosity at low
temperatures, or leakage from low-viscosity synthetic oils.
Figure 7. Elements of a Rotary Seal
The optimal finish of the shaft will depend on whether it undergoes high-speed rotation or lower-
speed oscillation, and on the pressure of the fluid. In addition, any wear-resistant coating on the
shaft will affect the seal design.
Future challenges faced by seal manufacturers include: