1. Tolerancing II Engr 135

2. Tolerancing Terminology
Basic Size
The theoretical size used as a starting point the application of tolerances.
Tolerance
The acceptable dimensional variation that is permitted on a part.
Limits
Maximum and minimum size shown by the toleranced value
1.00 ±0.05
Upper limit = 1.05
Lower limit = 0.95
Allowance
The minimum clearance or maximum interference between two parts
Smallest hole – Largest Shaft

3. Tolerancing Terminology
Maximum Material Condition (MMC)
The condition when the part contains the most amount of material
For a shaft (external feature) this would be the upper limit
For a hole (internal feature) this would be the lower limit
Least Material Conditions (LMC)
The condition when the part contains the least amount of material
For a shaft (external feature) this would be the lower limit
For a hole (internal feature) this would be the upper limit

4. What is the allowance between the two parts?

5. Tolerancing Terminology
Fits between parts
Clearance Fit (running fit)
A condition in which an internal part is smaller than the external part it fits into.
Hole is always larger than the shaft
Interference Fit (force fit)
A condition in which an internal part is larger than the external part it fits into.
Hole is always smaller than the shaft
Transitional Fit (locational fit)
The limits of size applied to a part can either result in a clearance fit or a transitional fit.

6. Fit Systems Selective Assembly Basic Hole System Basic Shaft System
An assembly method where the parts are measured and grouped into small, medium and large parts, all within tolerance. The groups are then assembled with similar matching parts of small, medium and large part for a better than random fit.
Basic Hole System
The basic hole system applies the basic size to the smallest hole.
Most used as there are limited standard tools for making holes
The allowance is applied to the shaft which can typically be made any size
Basic Shaft System
Used in the textile industry where many different parts fit on long shafts
Shaft is made nominal and holes are machined to fit to achieve the desired clearance or interference fit

7. Basic Hole System Preferred Precision Fits Fit Classes
English unit tolerance relationships
ANSI Standard B4.1
Specifies a series of standard fits between cylindrical parts
Fit Classes
Running and Sliding Fits (RC)
Clearance Locational Fits (LC)
Transitional Locational Fits (LT)
Interference Locational Fits (LN)
Force and Shrink Fits (FN)

8. Running and Sliding Clearance Fits
There are nine classes of running and sliding clearance fits.
Running and sliding clearance fits always provides a clearance between the shaft and the hole.
Smaller classes of fit (RC1 or RC2) provide less clearance than larger classes of fit (RC8 or RC9).
The amount of tolerance permitted by an RC fit is proportional to the size of the part.

11. Lecture Example
Determine the tolerances for a 3/4 in. shaft and hole using with RC3 class fit using the Basic Hole System.
Determine the nominal size of the hole and shaft.
Determine class of fit.
Locate the Clearance and Standard Tolerance Limits on the appropriate Tolerance Table.
Determine the maximum and minimum hole size by adding the tolerances to the nominal size of the hole.
Determine the maximum and minimum shaft size by adding the tolerances to the nominal size of the shaft.

12. Lecture Example n 0.7500 Nominal Size RC3 Class Fit
0.71 – 1.19 Nominal Size Range in Tolerance Table
Minimum Clearance =
Maximum Clearance =
Tolerance on hole
+.0008
-.0000
Tolerance on the shaft
-.0008
-.0013

13. Lecture Example (cont’d)
Maximum Hole Size = ( ) = .7508
Minimum Hole Size = ( ) = .7500
Maximum Shaft Size = ( ) = .7492
Minimum Shaft Size = ( ) = .7487