Thrust Bearing End Play

Thrust Bearing End Play (Axial Clearance) Basics

Scope:

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Purpose of End Play (EP) Typical Values and Tolerance

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Setting End Play

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Changes in End Play

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The purpose of thrust bearing end play p lay is to provide axial clearance between the thrust collar and the thrust bearing asseblies! The end play allows roo for the foration of an oil fil" isalignent" and theral expansion of the bearing co ponents! End play is the total distance the shaft can ove between the two thrust bearings and is soeties called float" thrust bearing clearance or axial clearance! Typical Typical values of end play can be calculated by the following forula: EP (ils) # $!% & 'abbitt utside ia! (in!) * + Exaple: a ,$!-. thrust bearing would re/uire !$,-. of end play! The noral tolerance is $!$$-." so the EP range would be $!$,0. 1 $!$,2.! 3hile these values are typical" larger or saller values are often used! 4n general" higher shaft speeds re/uire ore end play then lower speed applications! 5s end play becoes larger" the unloaded (slac6) bearing will see a larger gap! 4n soe cases" slac6 side pad flutter can occur if the gap" speed" and oil viscosity are at precisely the right values to excite it! n any achines"  pad flutter never occurs regardless of the end play! play! 5t 5t very large gaps" the oil fil is not fored fored at all and so there are no dynaic effects on the slac6 side! Soe gear applications use up to $!-$. of end play! 4n cases such as these" the shoes should be retained so they do not fall towards the collar! The end play ust not be so large to allow rotating eleents to contact stationary coponents! 5nother proble that can occur with lightly loaded thrust bearings is shaft shuttling! 4n this case" the whole shaft bounces bac6 and forth between the loaded and slac6 thrust bearing! 3ith 3ith saller end play values" a strong oil fil will develop on both side of o f the bearing which increases the stiffness and daping of the bearing! Values Values of half the typical aount of end en d play have been used with great success to reduce shuttling and pad flutter! The challenge with reduced end play is the tolerance! The idea is to reduce the end play enough to solve the proble without getting it too tight which can raise oil fil teperatures due to increased forces! 4n these cases" the tolerance should be reduced! Values Values less than 7-8 the typical end en d play should be avoided! The setting and chec6ing of end play is a critical step in the setup of a achine! There are several ethods utili9ed in the process of setting end p lay! ne ethod is to achine the thrust bearing and cavity to a very accurate value so that the end play is designed in! The ost coon ethod however" is to use shis or filler plates behind the thrust bearing that can b e used to adust the

 bearings overall height (stac6ed height!) This ethod also allows for the positioning of the rotor within the achine by shiing one bearing ore or less than the other! 4nitially setting end play re/uires easuring the available space be tween the bearing housing and the thrust collar and the bearing;s overall height! nder this condition the bearing babbitt surface will not wear" however there are conditions that can cause a reduction in babbitt thic6ness! The rate of reduction would vary depending on the cause and severity! Several factors that can contribute to such a situation include erosion" cavitations" cheical attac6" electrostatic discharge" and stray electric currents! These conditions will eventually lead to a bearing wipe and failure if not addressed! 'abbitt wear can also occur in achines that start and stop under load? however even with any start@stop cycles" the wear will be very inial with clean oil! 'abbitt loss can be deterined by easureent of the shoe height and surface profile! 5nother ethod to deterine babbitt loss is by routine oil ana lysis! 5n analysis can identify trace eleents of babbitt (tin) in the lubricating oil however it does not indicate the aount or location of loss! Changes in onitored shaft position or end play ay also occur as a result of elastic and plastic deforation of the bearing coponents! Aine and point contacts in the bearing coponents create high contact stresses which can cause sall peranent indentations! Bost of this deforation occurs early in the operation of the syste and then levels off (see attachent!) The end play an d  position can be adusted at the next aintenance cycle bac6 to the specified end play! 4f the  bearing is disassebled" it is iportant to put the coponents bac6 in their original locations! 5fter this first adustent" the end play should change very little! 5nother reason for increasing end play is axial shaft vibration! The coon source is collar wobble where the leveling plates are constantly wor6ing to e/uali9e the rotating high p oint of the collar! E/uali9ing thrust bearings are not designed to ta6e high dynaic loads or swashplate loads! These loads cause the contact points of the leveling plates to wear" reducing the bearing;s

height and leading to increased end play! 4t is iportant to chec6 the runout of the assebled collar and shaft prior to installation! The following is a /uote fro a ,%++ ingsbury catalog: .Buch tie ay be saved if it is reali9ed that" for ost installations" the aount of end play is not an exacting atter! >sually the noinal aount" plus or inus a few thousandths" is /uite satisfactory!. 3hile this ay still hold true today" 4 would suggest 6eeping the end play on the saller side rather than on the larger side!