ansi b4.1 fit chart

8/3/2019 Ansi B4.1 Fit Chart

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ANSI Fits Practices(Based on ASME B4.1 and B4.2)

Bruce A. Harding

Mechanical Engineering Technology


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Factors affecting fit selection Length of engagement

Bearing load

Lubrication

Materials

Temperature

Humidity

Modifications to ANSI fits may be required to satisfyextreme conditions. Subsequent adjustments may alsobe required as a result of particular application

experience to suit critical functional requirements or topermit optimum manufacturing economy.


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ANSI Preferred Tolerances & Allowances(ANS B4.1-1967)

Thousandths of an inch


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ANSI Standard Tolerances (ANSI B4.1-1967)

Thousandths of an inch. Above line to ABC agreement


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Relationship of Machining Tolerances and

Tolerance GradesThis chart may beused as a generalguide todetermine themac n ngprocesses thatwill under normalcircumstancesproduce work

within thetolerance gradesshown.


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ANSI Standard Fits (Inch)

In selecting limits of size for anyapplication, the type of fit is determined

,from the equipment being designed.

Then the limits of size of the mating parts

are established, to insure that the desiredfit will be produced.


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ANSI Standard Fits (Inch) RC = Running or Sliding Clearance Fit

LC = Locational Clearance Fit

LT = Transition Clearance or Interference Fit

FN = Force or Shrink Fit

The symbols are not intended to be shown on drawings.Instead, sizes (dimensions) based on the fits should bespecified on the drawings. The letter symbols are thenused in conjunction with class of fit numbers, thus FN 4represents a Class 4, force fit.


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Running & Sliding Fits (RC 1-4)

Values represent limits for hole & shaft and min. & max. clearances.


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Description of Fits (RC 1-2)Running and Sliding Fits (RC) are intended to provide a similar

running performance, with suitable lubrication allowance, throughoutthe range of sizes. The clearances for the first two classes, usedchiefly as slide fits, increase more slowly with the diameter than forthe other classes, so that accurate location is maintained even at

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

RC 2 Sliding fits intended for accurate location, but with

greater maximum clearance than class RC 1. Partsmade to this fit move and turn easily but are notintended to run freely, and in the larger sizes may seizewith small temperature changes.

.


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Graphical Presentation (RC)

Disposition of hole & shaft tolerances in thousandths of aninch. Basic size 0.


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Description of Fits (RC 3-4)

Running and Sliding Fits (RC) are intended to provide a similar


RC 3 Precision running fits are about the closest fits that can beexpected to run freely, and are intended for precision work at slowspeeds 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 journalpressures, where accurate location and minimum play are desired.

t e expense o ree re at ve mot on.


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Running & Sliding Fits (RC 5-9)



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Description of Fits (RC 5-9)Running and Sliding Fits (RC) are intended to provide a similar


RC 5 and RC 6 Medium running fits are intended for higher runningspeeds, 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 beencountered, or under both these conditions.

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

the expense of free relative motion.


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Description of Fits (LC 1-11)Locational Fits (LC, LT, and LN) are intended to determine only the

location of the mating parts. They may provide rigid or accuratelocation, as with interference fits, or provide some freedom oflocation, as with clearance fits. Accordingly, they are divided intothree groups: clearance fits (LC), transition fits (LT), and

LC Locational clearance fits are intended for parts whichare normally stationary, but that can be freelyassembled or disassembled. They range from snug fits

for parts requiring accuracy of location, through themedium clearance fits for parts such as spigots, to thelooser fastener fits where freedom of assembly is ofprime importance.

n er erence s .


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Locational Clearance Fits (LC 1-5)



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Graphical Presentation (LC)



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Locational Clearance Fits (LC 6-11)



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Graphical Presentation (LC)



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Description of Fits (LT 1-6)

Locational Fits (LC, LT, and LN) are intended to determine only the

location of the mating parts. They may provide rigid or accuratelocation, as with interference fits, or provide some freedom oflocation, as with clearance fits. Accordingly, they are divided intothree groups: clearance fits (LC), transition fits (LT), and

LT Locational transition fits are a compromise betweenclearance and interference fits, for applications where

accuracy of location is important, but either a smallamount of clearance or interference is permissible.

interference fits (LN).


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Locational Transitional Fits (LT 1-6)



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Graphical Presentation (LT)



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Description of Fits (LN 1-3)

Locational Fits (LC, LT, and LN) are intended to determine only thelocation of the mating parts. They may provide rigid or accuratelocation, as with interference fits, or provide some freedom oflocation, as with clearance fits. Accordingly, they are divided into

LN Locational interference fits are used where accuracyof location is of prime importance, and for parts

requiring rigidity and alignment with no specialrequirements for bore pressure. Such fits are notintended for parts designed to transmit frictional loadsfrom one part to another by virtue of the tightness of fit.These conditions are covered by force fits.

, ,

interference fits (LN).


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LocationalInterference Fits

(LN 1-3)

Values represent limits for hole& shaft and min. & max.clearances.


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Graphical Presentation (LN)



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Description of Fits (FN 1-2)

Force Fits: (FN): Force or shrink fits constitute a special type of

interference fit, normally characterized by maintenance of constantbore pressures throughout the range of sizes. The interferencetherefore varies almost directly with diameter, and the differencebetween its minimum and maximum value is small, to maintain the

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

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

resulting pressures within reasonable limits.


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Description of Fits (FN 3-5)

Force Fits: (FN): Force or shrink fits constitute a special type of

interference fit, normally characterized by maintenance of constantbore pressures throughout the range of sizes. The interferencetherefore varies almost directly with diameter, and the differencebetween its minimum and maximum value is small to maintain the

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

FN 4 and FN 5 Force fits are suitable for parts that canbe highly stressed, or for shrink fits where the heavypressing forces required are impractical.

resulting pressures within reasonable limits.


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Force & Shrink Fits (FN 1-5)



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Force & Shrink Fits (FN 1-5) Cont.



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Graphical Presentation (FN)



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Acknowledgments

The author wishes to acknowledge the support from the Society forManufacturing Engineers - Education Foundation, SME-EF Grant #5004for Curriculum Modules in Product Lifecycle Management.

ansi b4.1 fit chart

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