|Table of Contents |Section Contents|

LIMITS AND FITS FOR CYLINDRICAL PARTS

(Extracted from American Standard Limits and Fits for Cylindrical Parts, USAS B4.1-1967, with permission of the publishers, The American Society of Mechanical Engineers, United Engineering Center, 345 East 47th Street, New York 10017. Limit dimensions are tabulated in this standard for nominal sizes up to and including 200 in.)

The limits shown in the accompanying tabulations are in thousands of an inch. The size ranges include all sizes over the smallest size in the range, up to and including the largest size in the range. The letter symbols are defined as follows:

• RC: Running and Sliding Fits (Table A15-01) Running and sliding fits are intended to provide a similar running performance, with suitable lubrication allowance, throughout the range of sizes. The Clearance for the first two classes, used chiefly as slide fits, increase more slowly with diameter than the other classes, so that accurate location is maintained even at the expense of free relative motion.
• RC1: Close Sliding Fits (Table A15-01) Close sliding fits are intended for the accurate location of parts which must assemble without perceptable play.
• RC2: Sliding Fits (Table A15-01) Sliding fits are intended for accurate location but with greater maximum clearance than class RC1. 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.
• RC3: Precision Running Fits (Table A15-01) Precision running fits are about the closest fits which can be expected to run freely, and are intended for precison work at slow speeds and light journal pressures, but are not suitable where appreciable tempterature differences are likely to be encountered.
• RC4: Close Running Fits (Table A15-01) 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 is desired.
• RC5-RC6: Medium Running Fits (Table A15-01) Medium running fits are intended for higher running speeds or heavy journal pressures, or both.
• RC7: Free Running Fits (Table A15-01) 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 of these conditions.
• RC8-RC9: Loose Running Fits (Table A15-01) Loose running fits are intended for use where wide commercial tolerances may be necessary, together with an allowance, on the external member.
• L: Locational Fits (Table A15-02 through A15-04) 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, transition fits and interference fits.
• LC: Locational Clearance Fits (Table A15-02) Locational clearance fits are intended for parts which are normally stationary but which can be freely assembled or disassembled. They run from snug fits for parts requiring accuracy of location, through the medium clearance fits for parts such as ball, race, and housing, to the looser fastener fits where freedom of assembly is of prime importance.
• LT: Locational Transition Fits (Table A15-03) Locational Transition fits are a compromise between clearance and interference fits, for application where accuracy of location is important but either a small amount of clearance or interference is permissible.
• LN: Locational Interference Fits (Table A15-04) 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, since these conditions are covered by force fits.
• FN: Force Fits (Table A15-05) Force fits Force and shrink fits constitute a special type of interference fit, normally characterized by maintenance of constant force pressure throughout the range of sizes. The interference therefore varies almost directly with diameter, and the difference between its minimum and maximum value is small so as to maintain the resulting pressure within reasonable limits.
• FN1: Light Drive Fits (Table A15-05) 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.
• FN2: Medium Drive Fits (Table A15-05) 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.
• FN3: Heavy Drive Fits (Table A15-05) Heavy drive fits are suitable for heavier steel parts or for shrink fits in medium sections.
• FN4-FN5: Force Fits (Table A15-05) Force fits are suitable for parts which can be highly stressed or for shrink fits where the heavy pressing forces required are impractical.