Standard Preferred Tolerance Limits Fits ANSI B4.1

Standard Tolerance Limits and Fits (ANSI B4.1)

This Standard defines preferred tolerances for limits and fits for nonthreaded cylindrical features and defines specific sizes, fits, tolerances, and allowances for use where they are applicable.

The ANSI B4.1 tolerance charts are provided in thousandths (.001) of an inch.

ANSI  B4.1 Standard Fit Designations

The ANSI B4.1 Standard arranged fits classes in three general groups according to the field and type of usage. Standard fits are designated by means of some symbols which facilitate reference to classes of fits to get better understanding. The symbols are not intended to be shown on the manufacturing detail drawings, only sizes should be specified on the detail drawings.

1) Running or Sliding Fits [RC]
2) Locational Fits [LC, LT, LN]. This standard can be explained in three groups.
a) Clearance Locational Fits [LC]
b) Transition Locational Fits [LT]
c) Interference Locational Fits [LN]
3) Force or Shrink Fits [FN]

These letter symbols RC, LC, LT, LN, and FN are used with numbers representing the fit class. e.g.RC4 represent class 4.

Description of Fits. - The classes of fits are arranged in three general groups: running and sliding fits, locational fits, and force fits.

The standard does NOT include location tolerance allowances.

These fits defined as follows:

Running and Sliding Fits (RC): Running and sliding fits, are intended to provide a similar running performance, with suitable lubrication allowance, throughout the range of sizes. The clearances for the first two classes, used chiefly as slide fits, increase more slowly with the diameter than for the other classes, so that accurate location is maintained even at the expense of free relative motion.

RC 1 Close sliding fits are intended for the accurate location of parts that must assemble without perceptible play.

RC 2 Sliding fits are intended for accurate location, but with greater maximum clearance than class RC 1. Parts made to this fit move and turn easily but are not intended to run freely, and in the larger sizes may seize with small temperature changes.

RC 3 Precision running fits are about the closest fits that can be expected to run freely, and are intended for precision work at slow speeds and light journal pressures, but are not suitable where appreciable temperature differences are likely to be encountered.

RC 4 Close running fits are intended chiefly for running fits on accurate machinery with moderate surface speeds and journal pressures, where accurate location and minimum play are desired.

RC 5 and RC 6 Medium running fits are intended for higher running speeds, or heavy journal pressures, or both.

RC 7 Free running fits are intended for use where accuracy is not essential, or where large temperature variations are likely to be encountered, or under both these conditions.

RC 8 and RC 9 Loose running fits are intended for use where wide commercial tolerances may be necessary, together with an allowance, on the external member.

Locational Fits (LC, LT, and LN): Locational fits are fits intended to determine only the location of the mating parts; they may provide rigid or accurate location, as with interference fits, or provide some freedom of location, as with clearance fits. Accordingly, they are divided into three groups: clearance fits (LC), transition fits (LT), and interference fits (LN).

These are described as follows:

LC Locational clearance fits are intended for parts which are normally stationary, but that can be freely assembled or disassembled. They range from snug fits for parts requiring accuracy of location, through the medium clearance fits for parts such as spigots, to the looser fastener fits where freedom of assembly is of prime importance.

LT Locational transition fits are a compromise between clearance and interference fits, for applications where accuracy of location is important, but either a small amount of clearance or interference is permissible.

LN Locational interference fits are used where accuracy of location is of prime importance, and for parts requiring rigidity and alignment with no special requirements for bore pressure. Such fits are not intended for parts designed to transmit frictional loads from one part to another by virtue of the tightness of fit. These conditions are covered by force fits.

Force Fits: (FN): Force or shrink fits constitute a special type of interference fit, normally characterized by maintenance of constant bore pressures throughout the range of sizes. The interference therefore varies almost directly with diameter, and the difference between its minimum and maximum value is small, to maintain the resulting pressures within reasonable limits.

Open Preferred Force Shrink Fits Chart ANSI B4.1 Calculator

These fits are described as follows:

FN 1 Light drive fits are those requiring light assembly pressures, and produce more or less permanent assemblies. They are suitable for thin sections or long fits, or in cast-iron external members.

FN 2 Medium drive fits are suitable for ordinary steel parts, or for shrink fits on light sections. They are about the tightest fits that can be used with high-grade cast-iron external members.

FN 3 Heavy drive fits are suitable for heavier steel parts or for shrink fits in medium sections.

FN 4 and FN 5 Force fits are suitable for parts that can be highly stressed, or for shrink fits where the heavy pressing forces required are impractical.

Use of Standard Fit Tables. - Example 1: A Class RC 5 fit is to be used in assembling a mating hole and shaft of 2-inch nominal diameter. This class of fit was selected because the application required intended for higher running speeds, or heavy journal pressures (see Description of Fits , RC 5 ).

Maximum hole = 2 + 0.0018 = 2.0018; minimum hole = 2 inches

Maximum shaft = 2 − 0.0025 = 1.9975; minimum shaft = 2 − 0.0037 = 1.9963 inches

Minimum clearance = 0.0025; maximum clearance = 0.0055 inch

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