ENGR 1304- Engineering Graphics - Lecture 8

Inch Tolerances Definitions

  • Maximum Clearance:
    • The maximum amount of space that can exist between the hole and the shaft.
    • Max. Clearance = LMChole – LMCshaft
  • Minimum Clearance (Allowance):
    • The minimum amount of space that can exist between the hole and the shaft.
    • Min. Clearance = MMChole – MMCshaft

Types of Fits

  • Four major types of fits:
    • Clearance Fit
    • Interference Fit
    • Transition Fit
    • Line Fit

Clearance Fit

  • Definition: There is always a space between the shaft and the hole.
  • Condition: Min. Clearance > 0

Interference Fit

  • Definition: There is never a space between the shaft and the hole.
  • Condition: Max. Clearance
    atmost 0

Transition Fit

  • Definition: Depending on the sizes of the shaft and hole, there could be a space or no space.
  • Conditions:
    • Max. Clearance > 0
    • Min. Clearance < 0

Line Fit

  • Definition: There is a space or a contact (hole diameter = shaft diameter).
  • Conditions:
    • Max. Clearance > 0
    • Min. Clearance = 0\n

Examples of Fits

  • Clearance:
    • Lock and Key
    • Door and Door frame
  • Interference:
    • Coin and Coin slot
    • Pin in a bicycle chain
    • Hinge pin
    • Wooden peg and hammer toy

Summary of Fit Types

  • Clearance Fit:
    • Definition: The internal member (shaft) fits into the external member (hole) and always leaves a space or clearance between the parts.
    • When it exists: Min. Clear > 0
  • Interference Fit:
    • Definition: The internal member is larger than the external member such that there is always an actual interference of metal.
    • When it exists: Max. Clear \le 0
  • Transition Fit:
    • Definition: The fit might result in either a clearance or interference fit condition.
    • When it exists:
      • Min. Clear < 0
      • Max. Clear > 0
  • Line Fit:
    • Definition: The limits of size are specified such that a clearance or surface contact may result.
    • When it exists:
      • Min. Clear = 0
      • Max. Clear >0

ANSI Standard Limits and Fits

  • In accordance with ANSI/USAS B4.1-1967 (R2004) standard.
  • Fit Types:
    • Running or Sliding Clearance fit
    • Locational fits
    • Force Fits

RC: Running or Sliding Clearance Fit

  • Intended to provide running performance with suitable lubrication.
    • RC9 (loosest) – RC1 (tightest)
  • Description Table:
    • RC9-RC8: Loose running fit; Used with material such as cold rolled shafting and tubing made to commercial tolerances.
    • RC7: Free running fit; Used where accuracy is not essential, or where large temperature variations occur.
    • RC6-RC5: Medium running fit; Used on accurate machinery with higher surface speeds where accurate location and minimum play is desired.
    • RC4: Close running fit; Used on accurate machinery with moderate surface speeds where accurate location and minimum play is desired.
    • RC3: Precision running fit; This is the closest fit, which can be expected to run freely. Intended for slow speeds. Not suitable for appreciable temperature changes.
    • RC2: Sliding fit; Used for accurate location. Parts will move and turn easily but are not intended to run freely. Parts may seize with small temperature changes.
    • RC1: Close Sliding fit; Used for accurate location of parts that must be assembled without perceptible play.

Locational Fits (LC, LT, LN)

  • Intended to determine only the location of the mating parts.
    • LC = Locational clearance fits
    • LT = Locational transition fits
    • LN = Locational interference fits
  • Description Table:
    • LC: Locational clearance fit; Intended for parts that are normally stationary but which can be freely assembled or disassembled. They run from snug fits (parts requiring accuracy of location), through the medium clearance fits (parts where freedom of assembly is important). The classes are categorized from LC1 being the tightest fit to LC11 being the loosest.
    • LT: Locational transition fit; Used where accuracy of location is important, but a small amount of clearance or interference is permissible. The classes are categorized from LT1 to LT6.
    • LN: Locational interference fit; Used where accuracy of location is of prime importance, and for parts requiring rigidity and alignment with no special requirements for bore pressure. The classes are categorized from LN1 to LN3.

FN: Force Fits

  • Force fits provide a constant bore pressure throughout the range of sizes.
    • FN1 – FN5 (tightest)

Metric Tolerances Definitions

  • Limits, Basic Size, Tolerance, MMC and LMC have the same definition as in the inch tolerance section.
  • Upper deviation:
    • The upper deviation is the difference between the basic size and the permitted maximum size of the part.
    • UD = |basic size – Dmax|
  • Lower deviation:
    • The lower deviation is the difference between the basic size and the minimum permitted size of the part.
    • LD = |basic size – Dmin|
  • Fundamental deviation: The fundamental deviation is the closest deviation to the basic size.
    • The fundamental deviation is the smaller of the UD and the LD.
      Ex: Metric Fit = H11/c11
  • International tolerance grade number (IT#):
    • The IT#’s are a set of tolerances that vary according to the basic size and provide the same relative level of accuracy within a given grade.
    • The number in the fit specification represents the IT#.
    • A smaller number provides a smaller tolerance.
      Ex: Metric Fit = H11/c11
  • Tolerance zone:
    • The fundamental deviation in combination with the IT# defines the tolerance zone.
      • The IT# establishes the magnitude of the tolerance zone or the amount that the dimension can vary.
      • The fundamental deviation establishes the position of the tolerance zone with respect to the basic size.
        Ex: Metric Fit = H11/c11

ANSI Standard Limits and Fits

  • The following fit types are in accordance with the ANSI B4.2-1978 (R2004) standard.

Available Metric Fits

  • Hole Basis
    • H11/c11: Loose running
    • H9/d9: Free running
    • H8/f7: Close running
    • H7/g6: Sliding
    • H7/h6: Locational clearance
    • H7/k6 or H7/n6: Locational transition
    • H7/p6: Locational interference
    • H7/s6: Medium drive
    • H7/u6: Force
  • Shaft Basis
    • C11/h11: Loose running
    • D9/h9: Free running
    • F8/h7: Close running
    • G7/h6: Sliding
    • H7/h6: Locational clearance
    • K7/h6 or N7/h6: Locational transition
    • P7/h6: Locational interference
    • S7/h6: Medium drive
    • U7/h6: Force

Tolerance Designation

  • Fits are specified by using the:
    • fundamental deviation (letter)
    • IT# (International Tolerance Grade #).
  • When specifying the fit:
    • The hole = upper case letter
    • The shaft = lower case letter
      Ex: Metric Fit = H11/c11

Basic Hole / Basic Shaft Systems

  • Metric limits and fits are divided into two different systems; the basic hole system and the basic shaft system.
  • Basic hole system:
    • The basic hole system is used when you want the basic size to be attached to the hole dimension.
    • For example, if you want to tolerance a shaft based on a hole produced by a standard drill, reamer, broach, or another standard tool.
  • Basic shaft system:
    • The basic shaft system is used when you want the basic size to be attached to the shaft dimension.
    • For example, if you want to tolerance a hole based on the size of a purchased standard drill rod.

Selecting Tolerances

  • Tolerances will govern the method of manufacturing.
    • When the tolerances are reduced, the cost of manufacturing rises very rapidly.
    • Specify as generous a tolerance as possible without interfering with the function of the part.
  • Choosing the most appropriate tolerance depends on many factors such as;
    • length of engagement,
    • bearing load,
    • speed,
    • lubrication,
    • temperature,
    • humidity,
    • and material.
  • Experience also plays a significant role.

Tolerance Accumulation

  • The tolerance between two features of a part depends on the number of controlling dimensions.
  • More controlling dimensions = More tolerance accumulation
  • The maximum variation between two features is equal to the sum of the tolerances placed on the controlling dimensions.
  • Remember, even if the dimension does not have a stated tolerance, it has an implied tolerance.
    • For inch tolerances, the default is 2 decimal places.
    • For metric tolerances in mm, the default is 1 decimal place.

Formatting Metric Tolerances

  • Tolerances from standardized fit tables are listed on drawings as;
    • The person reading the print has to have access to the standard fit tables.
  • Unilateral tolerances
    • A single zero without a plus or minus sign.
  • Bilateral tolerances
    • Both the plus and minus values have the same number of decimal places.
  • Limit dimensions
    • Both values should have the same number of decimal places.
  • Using Basic dimensions with the tolerance
    • The number of decimal places in the basic dimension does not have to match the number of decimal places in the tolerance.

Formatting Inch Tolerances

  • Unilateral and Bilateral tolerances
    • The basic dimension and the plus and minus values should have the same number of decimal places.
  • Limit dimensions
    • Both values should have the same number of decimal places.
  • Using Basic dimensions with the tolerance
    • The number of decimal places in the basic dimension should match the number of decimal places in the tolerance.

Formatting Angular Tolerances

  • Angular tolerances
    • Both the angle and the plus and minus values have the same number of decimal places.