Mechanical face seal

• eliminates seal squeal
• maintains longevity

As manufacturers of major appliances, such as dishwashers and washing machines, continue to improve machines to run quieter, every part of the assembly is now considered a potential for noise. One potential source of noise is the face seal.

Face seals provide long life sealing solutions (over 10 years) but can have a tendency to squeal. The appliance owner may think a bearing is failing, thereby increasing replacement warranty costs and loss of customer loyalty. While there are alternative seal designs that do not squeal, none of them can replicate the long life of the mechanical face seal.

Seal squeal starts to occur when the mating seal faces run dry. The seal materials are usually stainless steel or ceramic seats running against a softer graphitic non-rotating seal head. The mating surfaces should have a very thin, full film between each other. Once a seal runs dry during pump-out cycles, the full film is gone and the boundary layer lubrication causes a phenomenon known as “slip-stick.” The seal seat and seal head no longer run smoothly, but rather temporarily stick together and then slip apart many times. With the seal rotating at 3450 rpm, it is not hard to imagine the effects of these minute sticks and slips. This causes vibrations that sound like a squeal. (An example of how slip-stick causes the squeal can be demonstrated at home, by rubbing a partially wet finger around the rim of a wine glass. The glass will not ring if the finger is too wet or too dry. If the finger is partially wet and dry (boundary layer lubrication), the finger actually slips and sticks to the glass many times causing vibrations to produce the ring.)

To retain the long life characteristics and reduce the face seal squeal, Freudenberg-NOK developed and patented the Zero Lash Positive Drive (ZLPD) mechanical face seal. The ZLPD eliminates the slip-stick by removing the vibrations. A molded-in metal insert of the seal boot has finger spring tabs that straddle the drive lugs on the neck of the seal. Molded around these tabs is a rubber tube that conforms to the neck and drive lugs. The design relies on circumferential beads protruding from the tube interior to hug the neck and the sides of the drive lugs. This configuration eliminates radial and rotational clearances (lash) between the moveable and fixed portions of the seal head and absorbs all vibrations. The ZLPD seal is designed to run at 30 psig, 3450 rpm, capable of containing one of the most difficult fluids to seal — water.

Broader applications include residential and commercial water circulation as well as intake and discharge pumps.