TECHNICAL INSIGHT PRODUCT AND APPLICATION ENGINEERING INFORMATION A PUBLICATION OF NSK AMERICAS INTERNAL CLEARANCE - TYPES AND NORMS Internl clernce is the distnce which the two rings of non-instlled bering cn move when they re pushed in opposite directions. A distinction is mde between rdil nd xil clernce. Rdil clernce is mesured perpendiculr to bering s centrl xis, while xil clernce is mesured long the centrl xis. The distnce between one finl position nd the other is mesured. roller berings, where the internl clernce defined prior to instlltion nd the theoreticl clernce re the sme. Plese refer to the min NSK bering ctlog for tbles showing the internl clernce for specific types of berings. Converting rdil clernce into xil clernce for tpered roller berings Rdil clernce Axil clernce Internl clernce hs mjor impct on the operting performnce of berings. It ffects fctors such s their service life, vibrtion, noise level nd het genertion. Axil clernce D = D r cot α @ D r α e : Rdil clernce : Contct ngle : Constnt 1.5 e D r MEASURED AND GEOMETRIC INTERNAL CLEARANCE To obtin ctul vlues, the theoreticl clernce is mesured by pplying defined mesuring lod to the bering. This mesured vlue is lwys slightly higher thn the theoreticl clernce lso known s the geometric clernce. The difference between the two figures corresponds to the elstic deformtion cused by the mesuring lod. The theoreticl clernce cn be clculted by subtrcting the mount of elstic deformtion from the mesured clernce. This elstic deformtion is miniml in the cse of FACTORS AFFECTING INTERNAL CLEARANCE Reduction in Rdil Clernce due to the Fit If the inner ring or outer ring is mounted on the shft or in the housing with n interference fit, the rdil clernce is reduced when the inner ring expnds or the outer ring contrcts. This reduction vries for different bering types nd sizes, nd depends on the shft or housing design. The reduction is pproximtely 70 to 90% of the interference. After subtrcting these reductions (d fe nd d fi ) from the theoreticl clernce ( 0 ), the ply is referred to s residul clernce ( f ).
Reduction in Rdil Ply due to Differences in Temperture between the Inner nd Outer Rings The frictionl het generted during opertion is conducted long the shft nd through the housing until it reches the outside. As housings re better t conducting het thn shfts due to their mss, the temperture of the inner ring nd the rolling elements is normlly 5 to 10 C higher thn tht of the outer ring. If the shft hets up or the housing is cooled down, the resulting difference in temperture between the inner nd outer rings cuses the rdil ply to decrese. Subtrcting d t from the residul clernce ( f ) gives us the effective internl clernce ( ). Chnges in the Rdil Clernce : Effective Clernce Outer ring : Clernce reduction due to fit between the outer ring nd the housing bore Internl clernce (theoreticl) Roller : Residul clernce Clernce reduction due to fit between the inner ring nd the shft : Clernce reduction due to temperture differences between the inner nd outer rings INTERNAL CLEARANCE 2
The following equtions cn be used to clculte the extent of this reduction: t = α t D e t : Rdil clernce reduction due to temperture differences between the inner nd outer rings (mm) α : Liner therml expnsion coefficient for bering steel = 12.5 10 6 (1/ C) t : Difference in temperture between the inner nd outer rings ( C) D e : Outer ring rcewy dimeter (mm) For bll berings For roller berings D e @ 1 (4D+d) D e @ 1 (3D+d) 5 4 A bering with n effective clernce of 0 or low positive number should be chosen. If single-row ngulr-contct bll berings or tpered roller berings re mounted s duplex set, the effective clernce should be miniml (unless prelod is needed). If two cylindricl roller berings with shoulder on one side re used opposite one nother, level of xil clernce must be selected which enbles free expnsion of the shft. Internl Clernce in Specific Applictions Operting Conditions Exmples Internl Clernce Severe shft deflection Semi-floting wheel berings in crs C5 or similr Stem flow through hollow shfts or pressure brs exposed to het Dry end of pper mchines Conveyor rollers in rolling mills C3, C4 C3 High impct lods nd vibrtions, or both inner nd outer rings with n interference fit Trction motors for trins Vibrting screens Hydrulic clutches Gerboxes for trctors C4 C3, C4 C4 C4 Loose-fitting inner nd outer rings Roll tenons for rolling mills C2 or similr Very quiet, vibrtion-free running Smll motors with specil specifictions C1, C2, CM Set ply to void shft deflection etc. Min spindle of lthes CC9, CC1 INTERNAL CLEARANCE 3
PRELOAD A SPECIAL TYPE OF NEGATIVE CLEARANCE Normlly, berings retin certin mount of internl ply when they re operting. However, in some cses it is dvntgeous to set negtive clernce to increse the bering rrngement s rigidity. This is clled preloding. Prelods re designed for use with berings whose ply cn be set during instlltion, e.g. ngulr-contct bll berings nd tpered roller berings. Generlly, two berings re mounted in fce-to-fce or bck-to-bck configurtion to crete duplex set with prelod. In prctice, three methods re normlly used to obtin position prelod: Instlling duplex bering set with previously djusted stnd-out dimensions nd xil clernce. Using spcer or shim of the correct size to obtin the required spcing nd prelod. Utilizing screws or nuts to set the xil prelod. In this cse, the strting torque should be mesured to verify the proper prelod. The purpose of prelod in typicl pplictions: Min shfts of mchine tools nd precision instruments: Holding the berings in their exct position both rdilly nd xilly s well s mintining the shft s rottionl ccurcy nd incresing rigidity. Min shfts of mchine tools, pinion shfts in cr gerboxes: Incresing bering rigidity nd optimizing ger meshing. Smll electric motors: Reducing noise cused by xil vibrtion nd resonnce. High-speed or high-ccelertion pplictions with ngulrcontct or xil bll berings: Preventing slippge between the rolling elements nd the rcewys cused by gyrtory moments Position Prelod 1b) Position Prelod nd Rigidity When the inner rings of berings A nd B re fixed xilly, the spce mesuring 2 d 0 is eliminted. A prelod of F 0 is imposed on ech bering. Axil bll berings nd self-ligning sphericl roller thrust berings mounted on horizontl shft: Keeping the rolling elements in the correct position with the bering rings. Bering Bering A Bering Bering B B TYPES OF PRELOAD F F 0 F 0 1) Position Prelod A position prelod is chieved by fixing two xilly opposed berings in such wy tht prelod is imposed on them. Once they hve been fixed in plce, their position remins unchnged during opertion. Bck-to-bck Duplex Bering Prelod INTERNAL CLEARANCE 4
Bering rigidity i.e. the reltionship between lod nd xil displcement with given xil lod F imposed on duplex set. Bering B Axil Lod Bering A 2b) Constnt-pressure Prelod nd Rigidity The digrm shows the rigidity of duplex berings under constnt-pressure prelod. The deflection curve of the spring is nerly prllel to the horizontl xis becuse the rigidity of the springs is lower thn tht of the bering. As result, the rigidity under constnt-pressure prelod is pproximtely equl to tht for single bering with prelod of F 0 pplied to it. F F 0 A 0 0 B F B F A Axil Displcement 5 Axil Lod F Bering A F : Externl xil lod F A : Axil lod on bering A F B : Axil lod on bering B A B : Displcement of the duplex set : Displcement of bering A : Displcement of bering B F 0 Axil Displcement with Position Prelod 2) Constnt-pressure Prelod A constnt-pressure prelod is chieved using coil or lef spring. Even if the reltive position of the berings chnges during opertion, the size of the prelod remins reltively constnt. 0 Axil Displcement with Constnt-pressure Prelod A Axil Displcement COMPARISON OF BEARING RIGIDITY AND PRELOADING METHODS Position prelod nd constnt-pressure prelod cn be compred s follows: (1) When both prelods re equl, the position prelod provides greter bering rigidity. In other words, the deflection due to externl lods is less for berings with position prelod. Constnt-pressure Prelod (2) Constnt-pressure prelods re more suitble for highspeed pplictions, to prevent xil vibrtion nd for use with thrust berings on horizontl shfts. INTERNAL CLEARANCE 5
(3) When position prelod is used, the prelod vries depending on the following fctors: Vrition in the xil expnsion due to temperture differences between the shft nd the housing Vrition in rdil expnsion due to temperture differences between the inner nd outer rings Deflection due to lod (4) When using constnt-pressure prelod, vrition is miniml becuse the effects of shft expnsion nd contrction re negligible. Axil lod Bering with position prelod Bering with constnt-pressure prelod F Bering without prelod Amount of Prelod If the prelod is lrger thn necessry, it cn result in unwnted het genertion, incresed torque nd shorter service life. The mount of prelod should be crefully clculted, tking the operting conditions nd the purpose of the prelod into ccount. In extreme cses, the bering my lst for just few hours. (1) Preloding duplex ngulr-contct bll berings As generl rule, n extr light or light prelod should be selected for grinding spindles nd the min shfts of mchining centers. A medium prelod should be dopted for the min shfts of lthes requiring rigidity. When speeds result in vlue of D pw n (d m n vlue) higher thn 500,000 the prelod should be very crefully considered nd selected. Axil Displcement Comprison of Rigidity nd Preloding Methods C 0 n F min = 100 C 0 F min = 1000 N mx F min : Minimum xil lod (N), {kgf} n : Speed (min -1 ) C 0 : Bsic sttic lod rting (N), {kgf} N mx : Limiting speed (oil lubriction) (min -1 ) 2 (2) Preloding xil bll berings When the blls in xil bll berings rotte t reltively high speeds, slippge my occur due to gyrtory moments cting on the blls. The lrger of the two vlues obtined from the equtions should be dopted s the minimum xil lod in order to prevent this kind of slippge. (3) Preloding self-ligning sphericl roller thrust berings When self-ligning sphericl roller thrust berings re used, dmge such s broken rings my occur due to slippge between the rollers nd the outer-ring rcewy. The minimum xil lod necessry to prevent such slippge (F min ) cn be clculted using the following eqution: F min = C 0 1000 www.nskmerics.com INTERNAL CLEARANCE - REV 2 / ATI / 17