Tolerances and Fits

Tolerance and Fit

Tolerance: Permissible deviation of a dimension from the specified basic size
Fit: The relative looseness or tightness (clearance and interference) of mating parts

  • Clearance fit: A fit that allows free motion b/w two parts to be mated (loose, running or sliding fit)
  • Transition fit: A fit requiring light force to (dis)assemble
  • Interference fit: A fit that requires two parts to be pressed together and prevents any relative movement during operation (tight fit)

Blanket Tolerance

If no tolerance is given for a dimension, then the blanket tolerance applies. This is blanketed based on the amount of significant digits given by the dimension

We also have unidirectional or bi-directional tolerances which specify the direction in which a tolerance applies…

International Tolerance Grade

  • A system of standardized tolerances divided into two categories: hole and shaft
  • Labelled with a letter (upper case for hole, lower case for shaft) and a number which is the grade number

Fundamental Deviation

  • For an example: p6…
  • p is the starting deviation from nominal
  • If in our box plot, we are above 0, then our shaft is oversized
  • The 6 tells us the IT band or how much we can deviate in total from the nominal size
  • From A (hole) to H (shaft) (going down left to right), “+”ve means housing “hole” is larger with more clearance

General Rules for Bearing Fits

Note

The load is applied to the shaft

Case 1

  • Inner ring rotating w/ stationary load on inner ring
  • Load circles around inner ring, but is concentrated at a single point for the outer ring

Case 2

Hint

This is the case for our project

  • Outer ring rotating w/ rotating load on outer ring
  • Load is concentrated at a single point for the outer ring, but circles around inner ring
  • Shaft is not rotating

Case 3

  • Shaft is stationary but the housing is rotating
  • The housing is rotating so the outer ring is rotating and sees the load as rotating
  • The inner ring is stationary and sees the load as stationary
  • Load circles around outer ring, but is concentrated at a single point for the inner ring

Case 4

  • Inner ring rotating w/ rotating load on inner ring
  • Load is concentrated at a single point for the inner ring, but circles around outer ring

Summary on Bearing Fitting

  • The inner ring always moves with the shaft and the outer ring always moves with the housing regardless of their fits
  • In usual (nominal) situations, either inner ring or outer ring has tight-fit while the other has loose-fit
  • The one that gets a dynamic load circling around its circumference is to be tight-fitted
  • Once the fit is determined, the specific tolerance can be found following the guidelines provided in the bearing catalogue (usually specified in IT grade)

Examples

  • Gears, chain sprockets, belt pulleys are usually assembled with modest clearance fit with respect to the shaft for easy assembly
  • Small turbines usually needs interference fit with respect to shaft to eliminate any looseness that can cause vibrations at high speeds
  • Bearings usually, either inner ring (w/ shaft) or outer ring (w/ housing) is interference fit (with the other ring clearance fit)