Unit 20: Overview of Fits Between Mating Parts

Description of Fits

  • Fits describe the range of tightness or looseness resulting from the application of specific combinations of allowances and tolerances in the design of mating parts.
  • Three general types of fits:
      - Clearance Fits
      - Interference Fits
      - Transition Fits

Types of Fits

Clearance Fits

  • Definition: Clearance fits have limits of size prescribed so that assembly results in a clearance when mating parts are fitted together.
  • Purpose: Intended for accurate assembly of parts and bearings.

Interference Fits

  • Definition: Interference fits have limits of size prescribed such that an interference must result when parts are assembled together.
  • Assembly Method: Typically, parts are pressed together using an arbor press.

Transition Fits

  • Definition: Transition fits have limits of size prescribed such that either clearance or interference may result when parts are assembled together.
  • Nature: They act as a compromise between clearance and interference fits.

Description of Specific Fits

Running and Sliding Fits

  • Characterization: Special types of clearance fits.
  • Intended Performance: These are designed to provide similar running performance with suitable lubrication across a range of sizes.
  • Reference: See Table 17 of the Appendix for tolerances and clearances.
Locational Fits
  • Definition: Intended to determine the location of mating parts.
  • Features: May provide either rigid or accurate location (interference fits) or some freedom of location (clearance fits).
  • Types of Locational Fits:
      - Clearance fits
      - Transition fits
      - Interference fits
Drive and Force Fits
  • Definition: A special type of interference fit, characterized by maintaining constant bore pressures across various sizes.

  

Standard Inch Fits

  • Designation: Standard fits are designated for design purposes in specifications and sketches by letter symbols:
      - RC - Running and sliding fit
      - LC - Locational clearance fit
      - LT - Locational transition fit
      - LN - Locational interference fit
      - FN - Force or shrink fit
  • Additional Information: Following these letter symbols are numbers that indicate the class of fit. See Appendix Tables 17–21 for details.

Details on Fitting Classes

Running and Sliding Fits

  • RC1: Precision Sliding Fit - Accurate location without perceptible play; used for high-precision work like gauges.
  • RC2: Sliding Fit - Designed for accurate location but has greater maximum clearance than RC1; parts can move and turn easily but are not intended to run freely.
  • RC3: Precision Running Fit - Closest fit expected to run freely; suitable for oil-lubricated bearings at slow speeds and light pressures.
  • RC4: Close Running Fit - Designed for grease- or oil-lubricated bearings in accurate machinery with moderate surface speeds.
  • RC5 and RC6: Medium Running Fits - Suitable for higher running speeds and temperature variations.
  • RC7: Free Running Fit - Where accuracy is not essential; accommodating large temperature variations.
  • RC8 and RC9: Loose Running Fits - For parts made to commercial tolerances (e.g., cold-rolled shafting).

Locational Clearance Fits

  • Purpose: These fits are designed for stationary parts that can be freely assembled or disassembled.
  • Classes:
      - LC1 to LC4: Minimum zero clearance in theory but usually have actual clearance.
      - LC5 and LC6: Small minimum clearance for close location fits in non-running components.
      - LC7 to LC11: Larger clearances and tolerances for assembly of bolts and similar parts.

Locational Transition Fits

  • Definition: Compromise between clearance and interference fits; used where accurate location is necessary, allowing slight clearance or interference.
  • Classes:
      - LT1 and LT2: Averaging slight clearance for light push fits.
      - LT3 and LT4: Little to no average clearance, tolerating some interference.
      - LT5 and LT6: Slight interference on average, requiring appreciable assembly force.

Locational Interference Fits

  • Application: Used for parts requiring precise alignment with no specific bore pressure requirements.
  • **Classes:
      - *LN1 and LN2:* Light press fits with small interference, suitable for dowel pins.
      - LN3: Heavy press fit for steel and brass, or light in elastic materials.
      - LN4 to LN6: Primarily press fits for softer materials, LN4 can be permanent assembly of steel.

Force or Shrink Fits

  • Definition: Special type of interference fits; the interference directly correlates with diameter.
  • Classes:
      - FN1: Light Drive Fit - Requires light assembly pressure, producing permanent assemblies.
      - FN2: Medium Drive Fit - Suitable for heavier steel parts or shrink fitting on light sections.
      - FN3: Heavy Drive Fit - Appropriate for heavier steel parts or medium sections as shrink fits.
      - FN4 and FN5: Force Fits - Suitable for high-stress components.

Unit 21: Metric Fits

  • Overview: The ISO (Metric) system of limits and fits is used for general applications in Canada and the United States.
  • Designations: Tolerance grades used to define specific dimensional limits on drawings. An “International Tolerance Grade” defines the tolerance zone's magnitude or dimension variation allowed.
  • Tolerance grades range from 1 to 16, with lower numbers indicating smaller tolerance zones.
Precision Grades
  • Grades 1-4: Highly precise, for gauge-making.
  • Grades 5-16: Suitable for cutting operations and manufacturing processes.
      - Grade 5: Most precise from fine grinding.
      - Grade 16: Coarsest grade for rough operations.

International Tolerance Grades

  • Application Guide: Displays IT grade numbers and general applications for measuring tools, materials, and fits performance.

Tolerancing Grades and Machining Processes

  • Machining Processes: Different processes such as lapping, grinding, turning, etc., are associated with specific tolerance grades.

Metric Tolerance Symbols

  • Structure: Tolerance symbols are established by combining IT grade numbers with position letters. Capital letters for internal dimensions and lowercase for external dimensions.
  • Basis Fits:
      - Hole Basis Fits: Fundamental deviation is “H”.
      - Shaft Basis Fits: Fundamental deviation is “h”.

Fit Symbol Usage

  • A fit is specified by the basic size common to both parts followed by a symbol for each component in the order of internal (hole) to external (shaft) parts.

Hole Basis Fits System

  • Basic size is the minimum size of the hole; limitations are provided in the Appendix Tables.
Example:
  • For a hole fit: ${25.000 ext{ to } 25.033}$ with a shaft fit from ${24.959 ext{ to } 24.980}$.
      - Minimum clearance: 0.0200.020
      - Maximum clearance: 0.0740.074

Shaft Basis Fits System

  • Recommended when multiple fits are needed on the same shaft, with identical tolerances as the hole system but with maximum shaft size as the baseline.
Example:
  • For a shaft fit: ${16.095 ext{ to } 16.205}$ with a hole fit from ${15.890 ext{ to } 16.000}$.
      - Minimum clearance: 0.0950.095
      - Maximum clearance: 0.3150.315

Types and Examples of Millimeter Fits

  • Underlaying diagrams illustrate various maxima for shaft diameters, tolerances, clearances, and fit examples.
  • Example of clearance fit: Max shaft diameter of ${19.980}$ with clearance limits between ${19.959}$ and ${20.033}$.
  • Example of transition fit: Max shaft diameter would allow certain interference for accurate assembly.

Conclusion

  • The comprehensive understanding of fits is essential in mechanical engineering, impacting assembly, durability, and performance of machine parts.

Note: For practical applications and examples, tables and figures referenced should be consulted in the appendix, as they provide critical data and dimensional tolerances relevant to specific fits discussed.