In brief
A radial shaft seal is a seal for rotating shafts whose spring-loaded lip runs radially on the shaft and keeps lubricant inside the housing.
Contents
What is Radial shaft seal?
Radial shaft seals seal shaft passages in gearboxes, electric motors, pumps and bearing arrangements. They keep oil or grease inside the housing and prevent dust and splash water from entering. A thin lubricating film under the sealing lip ensures low friction and wear.
A radial shaft seal consists of a metal reinforcing ring, a sealing lip made of elastomer or PTFE and a garter spring. Dimensions and designs are described in DIN 3760 and ISO 6194; common designs have a rubber-covered or metal outer case, with or without an additional dust lip. A radial shaft seal is designated by shaft diameter, bore diameter and width, for example 40 x 62 x 8. Further parameters are material, permissible surface speed, temperature and pressure differential.
Common causes of failure are grooved or damaged shaft surfaces, dry running, lips hardened by overheating, ingress of dirt and overpressure in the housing, for which standard radial shaft seals are not designed. Assembly errors such as pressing the seal in at an angle or damaging it on shaft edges also lead to early leakage.
In practice: maintenance and procurement
In addition to the three dimensions, an enquiry should state the design, material, medium, speed and temperature. If the shaft shows a wear groove, offset installation or a shaft repair sleeve helps; otherwise the new seal will also soon leak.
Synonyms and abbreviations: rotary shaft seal · oil seal · lip seal · shaft seal · RWDR
Frequently asked questions
How do you read the designation of a radial shaft seal?
The numbers indicate the shaft diameter, housing bore and width in millimetres, for example 35 x 52 x 7. These are supplemented by the design, for example with or without a dust lip, and the material of the sealing lip, such as NBR or FKM. Some designations also contain information on the helix direction or on pumping features on the lip.
What is the difference between a radial shaft seal and a mechanical seal?
A radial shaft seal seals with a lip radially on the shaft surface and is suitable for lubricants at low pressure differentials. A mechanical seal seals axially between two precisely lapped rings and is used mainly in pumps and agitators for process media and higher pressures. It is more complex and more expensive than a radial shaft seal.
What requirements does a radial shaft seal place on the shaft?
The running surface must be sufficiently hard, round and smooth and should be machined free of lead so that no oil is pumped out and no dirt is pumped in. A lead-in chamfer on the shaft edge makes assembly easier. If the shaft is grooved at the sealing point, a thin shaft repair sleeve can create a new running surface without replacing the shaft.
Why does a new shaft seal soon start leaking?
Common reasons are a grooved shaft, damage to the lip when pushing it over edges or keyways, installation at an angle, dry running on start-up or the wrong material for the medium and temperature. Increased internal pressure in the housing, for example due to a blocked breather, can also lift the lip.