What are ANSI preferred tolerances and fits?

ANSI B4.1 defines preferred limits and fits for cylindrical parts in inch-based systems. It groups fits into classes such as RC running and sliding fits, LC locational clearance fits, LT locational transition fits, LN locational interference fits and FN force fits, each with recommended hole and shaft tolerance combinations.

What is an RC running fit in ANSI B4.1?

RC (running and sliding) fits in ANSI B4.1 are intended for parts that move relative to each other, with controlled clearances. RC1 and RC2 are close sliding fits, while higher classes such as RC3 to RC9 progressively increase clearance for precision running, close running, medium running, free running and loose running applications.

Preferred Tolerances and Fits Charts (ANSI)

Running and Sliding Fits (RC)

Running and sliding fits (RC) are intended to provide 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 diameter than the other classes so that accurate location is maintained, even at the expense of free relative motion.

ANSI B4.1 running and sliding fits (RC) – fit detail and preferred symbols
Fit detail	Description	Symbol
Close sliding fits [RC1]	Intended for the accurate location of parts which must assemble without perceptible play.	H5-g4
Sliding fits [RC2]	Intended for accurate location of parts 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.	H6-g5
Precision running fits [RC3]	The closest fits which can be expected to run freely, intended for precision work at slow speeds and light journal pressures, but not suitable where appreciable temperature differences are likely to be encountered.	H7-f6
Close running fits [RC4]	Intended mainly for running fits on accurate machinery with moderate surface speeds and journal pressures, where accurate location and minimum play are desired.	H8-f7
Medium running fits [RC5]–[RC6]	Intended for higher running speeds or heavy journal pressures.	H8-e7 H9-e8
Free running fits [RC7]	Intended for use where accuracy is not essential, or where large temperature variations are likely to be encountered, or both.	H9-d8
Loose running fits [RC8]–[RC9]	Intended for use where wide commercial tolerances may be necessary, together with an allowance on the external member.	H10-c9 H11-

Note: There is no symbol in the standard for the shaft tolerance of RC9.

Locational Clearance Fits (LC)

Locational clearance fits (LC) are intended for parts which are normally stationary but can be freely assembled or disassembled. They range from snug fits, for parts requiring accuracy of location, through medium clearance fits for parts such as ball races and housings, to looser fastener fits where freedom of assembly is more important than accurate location.

ANSI B4.1 locational clearance fits (LC) – preferred symbols
Fit	LC1	LC2	LC3	LC4	LC5	LC6	LC7	LC8	LC9	LC10	LC11
Symbol	H6-h5	H7-h6	H8-h7	H10-h9	H7-g6	H9-f8	H10-e9	H10-d9	H11-c10	H12-	H13-

Note: There are no symbols in the standard for the shaft tolerances of LC10 and LC11.

Locational Transition Fits (LT)

Locational transition fits (LT) 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.

ANSI B4.1 locational transition fits (LT) – preferred symbols
Fit	LT1	LT2	LT3	LT4	LT5	LT6
Symbol	H7-js6	H8-js7	H7-k6	H8-k7	H7-n6	H7-n7

Locational Interference Fits (LN)

Locational interference fits (LN) 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 the fit; those conditions are covered by force fits.

ANSI B4.1 locational interference fits (LN) – preferred symbols
Fit	LN1	LN2	LN3
Symbol	H6-n5	H7-p6	H7-r6

Force Fits (FN)

Force or shrink fits (FN) constitute a special type of interference fit, normally characterized by maintenance of approximately 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 kept small to maintain the resulting pressures within reasonable limits.

ANSI B4.1 force fits (FN) – fit detail and preferred symbols
Fit detail	Description	Symbol
Light drive fits [FN1]	Fits requiring light assembly pressures and producing more or less permanent assemblies. They are suitable for thin sections or long fits, or in cast-iron external members.	H6-
Medium drive fits [FN2]	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.	H7-s6
Heavy drive fits [FN3]	Suitable for heavier steel parts or for shrink fits in medium sections.	H7-t6
Force fits [FN4]–[FN5]	Suitable for parts which can be highly stressed or for shrink fits where the heavy pressing forces required are impractical.	H7-u6 H8-x7

Note: There is no symbol in the standard for the shaft tolerance of FN1.

Supplements

Reference

Oberg, E., Jones, F.D., Horton, H.L., Ryffel, H.H. (2016). Machinery's Handbook , 30th edition. Industrial Press Inc.
Oberg, E., Jones, F.D., Horton, H.L., Ryffel, H.H. (2012). Machinery's Handbook , 29th edition. Industrial Press Inc.
ANSI/ASME B4.1-1967 (R2004) – Preferred Limits and Fits for Cylindrical Parts.

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ANSI Preferred Tolerances and Fits Charts