Rotary Seal Design Guide - Seals Unlimited

Catalog EPS 5350/USAEngineeringHow Do They Work?Rotary shaft seals work by squeezing andmaintaining the lubricant in a thin layer betweenthe lip and shaft. Sealing is further aided by thehydrodynamic action caused by the rotating shaft,which creates a slight pumping action.Rotary shaft seals provide protection byperforming two critical functions. In mostapplications the primary function of the seal is toretain the bearing or system lubricant. There arethousands of different types of lubricantsavailable today, but in general bearings are eitheroil or grease lubricated.The second function of the seal is to excludeoutboard material that can contaminate thesystem lubricant or directly damage the bearing.The type of contamination the seal will need toexclude is dependent on the application. The morecommon types are moisture and water, and drymaterials including dust, sand, dirt or particulatessuch as those generated by manufacturingprocesses.ContaminantsSeal LipHydrodynamic Pumping ActionFigure 2-1. Rotary Shaft Seal at WorkLubricantThe seal’s ability to retain the system lubricantand exclude contaminants plays a key role in theservice life of equipment components such asbearings, gears and any other component thatrelies on the system lubricant. The seal can have adramatic impact on the service life of the systemlubricant by retaining the optimal level, reducingexposure to excessive frictional heat andexcluding foreign matter.Typical petroleum oil has a useful life of thirtyyears at 86 °F (30 °C) if it is not contaminated withwater or particulate matter, but the same oil has alife of only a month at 212 °F (100 °C). As little as0.002% water in oil lubrication can reduce ballbearing life by 50%, primarily through hydrogenembrittlement. Solid particles cause more rapiddamage to the bearing race through high-localizedstresses and increased frictional heat.The sliding contact between the seal lip andthe shaft will generate friction, increasing thecontact temperature beyond the temperaturecaused by the bearings and other sources. Heataccelerates the breakdown of the oil and startsforming a varnish on the hot spots. Over time,the varnish changes to carbon and builds inthickness as the surrounding oil loses itslubricity. How quickly this happens is dependenton temperature. The deposit can lift and abradethe lip, causing leakage. The time to reach eachstage is cut in half for each 18 °F (10 °C)increase in temperature. The heat alsoaccelerates the cure of the rubber, especially atthe contact surface between the seal lip and therotating shaft. Eventually the lip surface hardens,small cracks form and the surrounding rubberstiffens. The cracks get larger and the lipstiffer,until it can no longer follow the movementof the shaft or seal. In order to maximize seal life,it is critical to minimize the amount of frictionalheat of the application.The amount of frictional heat that is generatedis a combination of many operating parameters.Shaft surface, internal pressure, operating speed,lubricant type, lubricant level, lip geometry and lipmaterial are just a few of the conditions that needto be considered. It is important to note that theseconditions are very interactive. For example, anincrease in shaft speed will increase the sumptemperature. If not vented, the temperature risewill increase the pressure inside the housing. Theinternal pressure will push on the seal lip andcreate additional force between the seal lip and theshaft. In turn, the operating temperature under theseal lip will see a significant rise in temperatureand can cause premature seal failure within hours.2203/28/062-3 Parker Hannifin CorporationEPS DivisionToll Free: (800) 233-3900www.parkerseals.com