A P U B L I C AT I O N O F N S K A M E R I C A S
GettinG A Good fit
SeLeCtinG tHe RiGHt SHAft And HoUSinG toLeRAnCe toLeRAnCe The rst step toward long bearing lie is selecting a bearing o the proper type and size. But that’s not enough, because even a properly selected bearing will ail prematurely i the shat and housing ts are
incorrect. Too Too much or too little intererence between the mating parts can cause problems – or early ailure. This becomes even more critical when bearings are replaced. A ailed bearing can damage the shat and housing, causing them to be out o tolerance. A loose t between the shat and bearing inner ring (or (or the housing and the outer ring) can lead to relative movement, or “creep” between these parts. Creep wears out the mating su races, increasing the clearance between them. Eventually, Eventually, the process can generate abnormal
The shat is worn rom an improper t which creates contamination that could lead to early ailure.
heat, vibration and possible contamination rom accumulated wear particles, as can be seen in the accompanying photograph. At the other end o the spectrum, excessive intererence causes other other
rings. Also, an intererence t can decrease the internal clearance o a
problems that can decrease service lie. Two Two key concerns are racturing racturin g
bearing due to growth o the inner ring or shrinkage in the outer ring.
the inner ring and reducing the bearing’s internal clearance. Too much
When the intererence is too great, internal clearance becomes negative,
intererence builds high stress, which can sometimes racture inner
resulting in excessive heat buildup and premature bearing ailure.
TABLE 1
|
Loading Conditions and Fit
La Applca
Barg opra ir Rg
our Rg
Rotating
Stationary
Stationary
Rotating or Stationary
La Cs
fg our Rg
Rotating Inner Ring Load
Tight Fit
Loose Fit
Electric Motor Pumps Machine Tools Gears
Rotating
Rotating Outer Ring Load
Loose Fit
Tight Fit
Conveyer Rollers Tension Pulleys Auto Wheel Hubs
Rotating or Stationary
Direction o Load Indeterminate
Tight Fit
Tight Fit
Auto Fan Clutch Special Applications
LOAD Stationary
LOAD (stationary)
Direction o load due to variation o direction or unbalanced load
exampls
ir Rg
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A PU BL IC AT IO N OF NS K AM E RI CA S
Staying Fit or Lie
GETTING A GOOD FIT: SELECTING THE RIGHT SHAFT AND HOUSING TOLERANCE
FIGURE 1
Clearly, proper t selection has a narrow margin or error. As a rule o thumb, the rotating part should have the intererence t. To speciy int er ference IN TE RFERENCE
the correct t, you should understand the main actors that infuence t recommendations: Operating Conditions: Which ring rotates, the inner or the outer one? Is the load stationary? These actors tell you which ring should have the intererence t. There are three possible combinations: 1. Tight (or intererence) t on the inner ring and loose on outer – a common approach 2. Tight t on the outer ring and loose on inner – also common
S s E E e e G R r g N O n o B A a b R G r g E N e n I i C c R r N n R A a r R r e E E e n N L l n N i O I o T t
aR rA aN nC ce CcLlEeA E
s S E e G g N n A a R r E e c C n N a A r R e E l L o O t T . . d . D . o O t T f F a A h H s S
Th eThe r elati onshi p bebetween tween Fit desand and relationship Fit Co Codes the the amount of interference for shafts amount o intererence or shats
3. Tight t on both rings – rarely seen tabl 1 shows recommendations or ts (loose or tight) based on typical operating conditions. La: Load decreases the intererence o the inner ring. Thus, heavier
fgur 1 shows the relationship between Fit Codes and the amount o intererence or shats.
loads require more intererence. Husg a Sha Marals: Look at the composition o the housing and shat materials. Varying material strengths and thermal expansion rates make the proper t dierent or each material. For example, aluminum expands more than steel. Thus, an intererence t on an outer ring with an aluminum housing requires more intererence than a steel housing would require. In addition, special conditions should be given to thin wall housings, hollow shats, split housings and high vibration when making t recommendations. And you should avoid split housings when using a tight t on an outer ring – the combination oten leads to an oval-shaped housing and bearing ring. Applications with heavy vibration may need tight ts on both inner and outer rings. Using these guidelines, you should inspect the shat and housing to veriy that they meet the manuacturer’s original specications. This will insure maximum lie or the replacement bearing.
fr mr rma, plas cac nSK a 1.88ThinkNSK (1.888.446.5675) or visit www.nskamericas.com
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