Tight lipped: Understanding lip seals

Tight lipped: Understanding lip seals

January 2, 2020 0 By Ray Bohacz

Anywhere you look on the farm you will find seals of different designs. The purpose of any seal is to keep a fluid or semi-fluid such as grease from leaking out and equally as important, foreign material from entering.

The ability to isolate the component from external influences is just as imperative and possibly in some uses more so, than keeping the fluid inside.

This needs to be understood since most of us only consider a seal failed when there are visible signs of leakage.

A seal employed in a vacuum pump if weakened will introduce dirt and decrease the pump’s operation and useful life without any visible signs.

Many vacuum planters fall prey to a degraded seal that will impact the unit’s performance in the field since dust is highly abrasive.

Though commonly identified as a lip seal, it also goes by the name rotary or shaft seal. On an engine you will find lip seals in use at the front and rear of the crankshaft (one-piece rear main seal), in hydraulic pumps such as for the power steering, in a water pump and other areas.

On a vehicle or machine lip seals are used on the wheel bearings, drive axle bearings, transmission, air conditioner compressor, to name a few.

A lip seal is employed when there is a need to isolate two sides of a component within a rotating member.

Up until around the 1930s leather seals were common but since then lip seals have replaced that style. They have added reliability and improved protection from internal and external leakage.

The lip seal works by having an edge that rides on the rotating part while allowing a film of the fluid to reside between the lip and the shaft.

The confluence of the fluid film and the sealing material (usually some family of rubber or hybrid material such as PTFE) cause a hydrodynamic action.

In addition, a weak but effective pumping action takes place that further enhances the seal’s ability.

As the seal ages not only does the lip that contacts the shaft wears but the material it is made from becomes less pliable, brittle and from thermal excursions, may have the tendency to bow away from the rotating member. When any of this occurs, the seal becomes challenged and can leak both ways.

Important considerations

When changing a lip seal, it is imperative that it is made from the proper material for the fluid that is going to be isolated along with the rpm of the shaft; the higher the speed the greater the heat generated at the contact point.

Also, many lip seals are directional in nature, especially with but not limited to those used in engines. The seal will usually have a small arrow on it to identify the rotation required for the lip to make contact on the shaft. If installed in the wrong rotation the seal will leak immediately.

The cavity the seal resides in needs to be extremely clean and free of any rust or corrosion. If the seal is even minutely cocked it will not function.

The shaft that runs through it must also be clean and smooth and possess no axial grooves. If there are any grooves they have the ability to work like an auger and pull fluid past the lip.

It is important that when the lip seal is being pressed or tapped into place that it is not damaged. Though there are dedicated seal installers that should be used, many improvise and use a socket from a ratchet set. If common sense is employed and the seal and socket can interface properly, it is an acceptable procedure.

If you are careless or the fit is compromised, then the repair may be for naught.

A lip seal installation kit with a variety of proper sizes may be a worthwhile investment.

Always lightly lubricate the cavity the seal will fit into before attempting installation.

The common mistakes made when servicing any lip seal are not lubricating the shaft so that the lip does not run dry on the rotating member before it can pull a film of liquid; reusing even a fairly new lip seal; buying a cheap imported seal that has the proper O.D. but is made from the wrong material.

Most lip seals are designed that once removed not to be put back into service. This is especially true on a component that spins at high speed and is exposed to high temperatures. You may get lucky every now and then, but do not make it a habit.

If you follow these simple steps and invest in the proper name brand lip seals, you will be rewarded with a job well done and an effective and long-lasting repair.