The old saying "the devil is in the details" certainly applies to the industry's quest for improved fuel efficiency. Many opportunities still exist to make the internal-combustion engine and other fundamental mechanical systems more efficient by eliminating parasitic losses. One opportunity is the reduction of friction on rotating surfaces.
"Seals are typically a significant energy loss for any type of machinery," noted Dr. Ted Duclos, Vice President, Operations and Technology, at Freudenberg-NOK. The sealing-systems supplier recently introduced a new radial shaft sealing technology, called ESS (for energy-saving seals), that is aimed at improving systems and vehicle fuel efficiency and emissions.
According to Duclos, Freudenberg-NOK's new seal designs can be used in a variety of automotive applications, including engine, transmission, and driveline.
Current "conventional" designs for such applications feature a knife-edge main sealing lip combined with a garter spring to press the seal against the rotating shaft. A secondary dust or dirt lip is also often a feature. The new ESS technology combines a proprietary elastomer with a new optimized lip loading design that in many cases eliminates the need for the garter spring.
The result is a seal that works with lower radial pressure and thus reduced friction—60-70% less than most conventional designs, claimed Duclos.
There are other benefits in the ESS technology. Reduced friction also leads to lower seal lip temperatures, which in turn reduce the potential for coked oil buildup and hardening of the rubber, according to Duclos. He explained that the Freudenberg-NOK design optimizes shaft misalignment, if desired, ensuring the sealing lip remains in contact with the shaft.
“What our new elastomer provides is high resistance to compression set,” explained Duclos. Maintaining the elasticity of the rubber maintains the efficiency of the seal—a key enabler in the wider yet lower friction design of the sealing lip.
Alternatively, polytetrafluoroethylene (PTFE, also known by the Teflon trade name) shaft seals were designed for similar reasons. These also eliminate garter springs and reduce radial pressure compared to conventional springs. The Freudenberg ESS design offers greater efficiency improvements vs. PTFE, according to Duclos.
In a test with a 93-mm (3.66-in) diameter shaft turning at 2500 rpm, the measured parasitic loss for an ESS seal was 35 watts, compared to 110 watts for a PTFE seal.
“We are working to develop an expanded range of ESS solutions within diverse markets,” remarked Duclos. One such example is an ESS design for heavy truck applications called MileMaker. Though this design requires a garter spring, the company notes its durability, claiming over 8 million miles (approximately 13 million km) of fleet testing with zero failures.
“What we want to stress is that engineers who chose this design will not have to sacrifice durability for improved fuel economy,” stated Duclos.