Machinery's Handbook, 31st Edition
650 LIMITS AND FITS These letter symbols are used in conjunction with numbers representing the class of fit; thus FN 4 represents a Class 4 force fit. Each of these symbols (two letters and a number) represents a complete fit for which the minimum and maximum clearance or interference and the limits of size for the mating parts are given directly in the tables. Description of Fits.— The classes of fits are arranged in three general groups: running and sliding fits, locational fits, and force fits. Running and Sliding Fits (RC): Running and sliding fits, for which limits of clearance are given in Table 8a, 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. These fits may be described as follows: 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 they 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 intended to determine only the location of the mating parts; they may provide rigid or accurate location, as with interfer ence 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 that 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 impor tance, 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, nor mally characterized by maintenance of constant bore pressures throughout the range of sizes. The interference, therefore, varies almost directly with diameter, and the difference
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