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05 – Fits and Tolerances

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The intent of this presentation is to present enough information to provide the reader with a fundamental knowledge of the different type fits and tolerances used within Michelin and to better understand basic system and equipment operations.

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**Types of Tolerances and Fits**

05 – Fits and Tolerances Types of Tolerances and Fits Introduction to Tolerances and Fits Definition of Tolerance A tolerance is the acceptable difference between the maximum and minimum size of a mechanical part as a basis for determining the accuracy of its fit with another part. When to use tolerances Theoretically, each time a dimension is put down on a drawing, a tolerance should be also. In practice, only a few critical dimensions have particular tolerances and the others are subject to general tolerances.

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**05 – Fits and Tolerances Why tolerance? **

Since it is impossible to realize an absolute dimension, it is necessary to determine acceptable limits for this dimension outside of which the part cannot be used. Since, in industry, a large number of identical parts are used, it is necessary to be able to interchange these parts and maintain the desired fit without additional machining. Tolerance also allows us to determine the degree of precision needed in different applications. This selection of appropriate tolerance avoids unneeded precision of non-precise parts, therefore reducing machining costs. How to use tolerances Tolerances are used by putting the tolerance of each dimension on the drawing and by taking into account the function of each part, considering the way in which the part is to be manufactured and used.

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**05 – Fits and Tolerances 4” Types of tolerances **

The following are the types of size tolerances most often used: Particular (Special) This type of tolerance is typically seen on mechanical drawings when specific sizes of a part are needed. Examples of particular tolerances: 1.001” 1.0005” +0.01 −0.00 3" +/ 4” Particular tolerance is also in the ANSI and ISO tolerance systems. This will be discussed in more detail in the following pages.

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**05 – Fits and Tolerances General Workshop Tolerance **

General workshop tolerances are usually found in the title block of a detail drawing. These tolerances set the acceptable limits when the fabricator or machinist has no other tolerances given on the drawings. ANSI (American National Standards Institute) This type of tolerance system is seen mainly in the United States only. It is associated with the English measuring system. Examples of ANSI tolerances: RC1 LC3 FN4 The letters and numbers in these tolerances represent specific tolerances. Further detailed explanations will follow.

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**05 – Fits and Tolerances I.S.O. (International Standards Organization) **

This type of tolerancing system is seen on most all of the European drawings along with many other countries. It is associated with the metric system. Many U.S. manufacturers are moving towards this type of tolerancing. Examples of I.S.O. tolerances; 27 H p N9 The letters and numbers in these tolerances represent specific tolerances. Further detailed explanations will follow. Terminology associated with tolerances Nominal Dimension - The dimension that the tolerances are applied to. Upper Limit - The maximum allowable size of the part based on the tolerance given. Lower Limit - The minimum allowable size of the part based on the tolerance Interval of Tolerance - The upper limit minus the lower limit, also known as the range of tolerance.

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**05 – Fits and Tolerances Fits **

When assembling two parts together such as a key in a shaft, a bearing on a shaft, a seal into an end-cap, or an end-cap into a housing, it is termed as the fitting of male and female parts. The compared size of the male and female parts at the point of contact determines the type of fit the assembly will form. A fit is defined as the condition of clearance or interference between two parts Terminology associated with fits Allowance - The amount of clearance or interference that exists between two parts. Maximum Allowance - The greatest amount of clearance or interference that can exist between mating parts. Equal to the largest female size (upper limit) minus the smallest male size (lower limit). Minimum Allowance - The least amount of clearance or interference that can exist between mating parts. Equal to the smallest female size (lower limit) minus the largest male size (upper limit).

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**05 – Fits and Tolerances The three types of fits **

Clearance - There will always be some amount of space between mating parts. No force should be required to put these parts together. When the calculation is performed the allowance values will always be positive. Interference (Press) - There will always be some amount of contact (interference) between mating parts. Usually force will be required to put these parts together. Sometimes a great amount of force and sometimes a firm hand push. When the calculation is performed the allowance values will always be negative. Uncertain (Transition) - There could exist between mating parts sometimes a condition of clearance or interference. This would depend on how the parts were tolerated and exactly where within the tolerance range each part was made. When the calculation is performed the allowance values will be positive and negative.

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**05 – Fits and Tolerances Example 1: Clearance fit Note: All dimensions**

Clearance fit Note: All dimensions in inches Female Male Equals Max. dia. = 4.003" - Min. dia. = " (+) 0.006" Pos. clearance Min. dia. = " - Max. dia. = 3.999" (+) 0.001" Pos. clearance

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**05 – Fits and Tolerances Example 2: Interference fit**

Interference fit Female Male Equals Max. dia. = 6.001" - Min. dia. = " (-) 0.001" Neg. clearance Min. dia. = " - Max. dia. = 6.003" (-) 0.003" Neg. clearance

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**05 – Fits and Tolerances Example 3: Uncertain fit Female Male Equals**

Uncertain fit Female Male Equals Max. dia. = 8.005" Min. dia. = 7.999" (+) 0.006" Pos. clearance Min. dia. = 7.998" Max. dia. = 8.003" (-) 0.005" Neg. clearance

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**05 – Fits and Tolerances Systems of Fits **

ANSI (American National Standard Institute) and ISO (International Standard Institute) specifications are systems of fit. The fit specifications are based on application of the part and the tolerances selected to limit the size of two mating parts to achieve an allowable type of fit. The type of fit is determined based on the function of the part within the design of the equipment. The fit dictates the part tolerance(s) and establishes the mating specification needed for two parts to correctly fit together. Theoretically, an infinite number of fits could be chosen based on the specifications. Standard fits used to cover most applications are shown in a series of ANSI and ISO tables found in engineering textbooks, machinist manuals, and quick reference guides for millwrights. The tables that are displayed in this book are condensed versions provided to give an understanding of the system itself. Full tables have many more nominal size ranges. Having knowledge of tolerances will help to understand ANSI and ISO specifications.

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05 – Fits and Tolerances American National Standards Institute Fits (ANSI) Designation of Standard Fits Standard fits are designated by means of the following symbols. The symbols are not intended to be shown on manufacturing drawings; instead, sizes should be specified on drawings. The letter symbols used are as follows: RC Running or Sliding Clearance Fit LC Locational Clearance Fit LT Transition Clearance or Interference Fit LN Locational Interference Fit FN Force or Shrink Fit These letter symbols are used in conjunction with numbers representing the class of fit; thus FN4 represents a Class 4, force fit. With clearance fits, the higher the number-class the greater the clearance between mating parts. Consequently, with interference fits, the higher the number-class the greater the interference between mating parts. 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.

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**05 – Fits and Tolerances Description of Fits **

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. 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). 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 size. 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.

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05 – Fits and Tolerances International Standards Organization (ISO) tolerances and fits The ISO system of limits and fits for mating parts is approved and adopted for general use in the United States. It establishes the designation symbols used to define specific dimensional limits on drawings. The general terms "hole" and "shaft" can also be taken as referring to the space containing or contained by two parallel faces of any part, such as the width of a slot, or the thickness of a key. International Tolerance Grades An "International Tolerance Grade" establishes the magnitude of the tolerance zone or the amount of part size variation allowed for internal and external dimensions alike. The smaller the grade number the smaller the tolerance zone. Grades 1 to 4 are very precise grades intended primarily for gage making and similar precision work, although grade 4 can also be used for very precise production work. Grades 5 to 16 represent a progressive series suitable for cutting operations, such as turning, boring, grinding, milling, and sawing. Grade 5 is the most precise grade, obtainable by the fine grinding and lapping, while 16 is the coarsest grade for rough sawing and machining. Grades 12 to 16 are intended for manufacturing operations such as cold heading, pressing, rolling, and other forming operations.

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**05 – Fits and Tolerances Table 6**

As a guide to the selection of tolerance grades, on the following page, Table 7 has been prepared to show grades which may be expected to be held by various manufacturing processes for work in metals. For work in other materials, such as plastics, it may be necessary to use coarser tolerance grades for the same process.

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**05 – Fits and Tolerances Tolerance Position **

A fundamental deviation establishes the position of the tolerance zone with respect to the basic size. A tolerance may be above, below, or astride the nominal dimension. Fundamental deviations are expressed by "tolerance position letters". Capital letters are used for internal dimensions (bore), and lower case letters for external dimensions (shaft). There are 28 possible positions. They are A, B, C, CD, D, E, EF, F, FG, G, H, JS, J, K, M, N, P, R, S, T, U, V, X, Y, Z, ZA, ZB, ZC; for internal dimensions. There are also 28 possible positions for external dimensions. They are a, b, c, cd, d, e, ef, f, fg, g, h, js, j, k, m, n, p, r, s, t, u, v, x, y, z, za, zb, zc. By combining the IT grade number and the tolerance position letter, the tolerance symbol is established which identifies the actual maximum and minimum limits of the part. The toleranced sizes are thus defined by the basic size of the part followed by the symbol composed of a letter and a number.

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05 – Fits and Tolerances

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**05 – Fits and Tolerances Fits **

A fit is indicated by the basic size common to both components, followed by a symbol corresponding to each component, with the internal part symbol preceding the external part system. Hole Basis Fits System In the hole basis fits system, the basic size (nominal dimension) will be the size of the bore. For example, for a 25 H7/f7 fit, which is a preferred hole basis clearance fit, the limits for the hole and shaft will be as follows: Hole limits = Shaft limits = Minimum clearance = 0.020 Maximum clearance = 0.062 If a 25 H7/p6 preferred hole basis interference fit is required, the limits for the hole and shaft will be as follows: Shaft limits = Minimum interference = (-)0.001 Maximum interference = (-)0.035

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05 – Fits and Tolerances Normally a bore of H7 is used. The variations in clearance and interference which are necessary in order to realize the assemblies, are obtained by the choice of the shaft tolerance position.

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Recommended fits

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**Application and Calculation**

05 – Fits and Tolerances Application and Calculation Use of Standard Fit Tables Example 1 - A Class RC1 fit is to be used in assembling a mating hole and shaft of 1.5 inch diameter. This class of fit was selected because the application required accurate location of the parts with no perceptible play. From the data in Table 1, establish the limits of size and clearance of the hole and shaft. Maximum hole = = ; minimum hole = inches Maximum shaft = = ; minimum shaft = = inches The resulting clearance possibilities are between and inches. Tolerance limits given in the tables are added to or subtracted from basic size ( as indicated by + or - sign ) to obtain maximum and minimum sizes of mating parts. NOTE: All values shown on an ANSI tolerance chart are in thousandths of an inch. Example: = +1.2 =

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05 – Fits and Tolerances TABLE 1

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05 – Fits and Tolerances TABLE 2

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05 – Fits and Tolerances TABLE 3

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05 – Fits and Tolerances TABLE 4

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TABLE 5

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05 – Fits and Tolerances * Class FN3 for these size ranges does not exist. Applications in this range would use either a FN2 or FN4. NOTE: There are only five different classes of fits for force and shrink applications. This normally suffices for typical applications. For unusual circumstances, a designer would assign specific tolerances to suit the need.

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ISO Tolerance Chart

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**05 – Fits and Tolerances Exercises **

Find maximum and minimum size for the following tolerances. a) 30h6 b) 62p6 c) 48H7 d) 105g6 e) 16f7 f) 55N7 g) 4" +/ h) " -0.001 Answer Key

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05 – Fits and Tolerances Determine the shaft and hole sizes for the following fits. Also, determine the maximum and minimum clearances for these fits. a) 1.2" RC6 b) 0.5" RC2 c) 1.1" LT6 d) FN2 e) 35 H7/g6 f) 72 H11/c11 g) 23 H8/f7 h) 55 H7/p6 Answer Key Answer Key Answer Key Answer Key

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**05 – Fits and Tolerances Fit calculation blocks Example: 10H7/g6 +15 F**

F 10H7 10.015 10.000 0.015 + -5 -14 m 10g6 9.995 9.986 0.009 0.024 +0.029 +0.005 0.024 -

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**Form and Position Tolerance**

05 – Fits and Tolerances Form and Position Tolerance Geometric Tolerances Of Form And Position When? When the function of a part in a mechanism calls for surfaces whose form and position requires a tolerance. Why? Because it is impossible to realize an ideal surface (the theoretical form prescribed by the drawing) which occupies an ideal position in relation to a reference. Therefore, for the form and position of this surface, it is necessary to fix acceptable limits beyond which the part can no longer be used. How? By indicating on the functional dimension drawings, the tolerances of form and position, but only if these tolerances correspond to a functional necessity; this is done in order to avoid overloading and cluttering up the drawing, which automatically entails needless expenditure.

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**05 – Fits and Tolerances Form and Position Symbols **

Form tolerance of an individual element

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**05 – Fits and Tolerances Form tolerance of associated elements**

Position Tolerances

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**05 – Fits and Tolerances The Drawing Tolerance Block**

All boxes are drawn in the horizontal position. Arrows go out from 3 sides of the symbol box (centered), as straight lines with as few bends as possible. Boxes are located to give the shortest arrows (leaders) as possible. When the solid reference triangle is used, the reference surface box is omitted. Only one reference triangle can be used with each tolerance. Arrows and the reference triangle will touch the surface, or an extension line from the surface. Do not use center lines or dimension lines

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**05 – Fits and Tolerances Definitions: **

Upper plane: (of a surface indicated by being flat) Plane which is parallel to the theoretical plane of the surface in question and which touches the surface without cutting it. Lower plane: This plane is parallel to the upper plane and is situated at a maximum distance indicated by the tolerance Median plane: (of two surfaces indicated as being flat and parallel) This ideal plane is situated midway between the upper and lower plane and is parallel to both of them. Whatever the tolerance of form and position or indicated, we shall always apply the same basic principle, be it a question of a straight line, a cylinder or a circle, depending upon the type of tolerance sought after.

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05 – Fits and Tolerances

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05 – Fits and Tolerances Geometric Tolerances

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05 – Fits and Tolerances

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**Surfaces must be concentric within 0.03mm to the reference surface**

Surface must be symetrical within 0.05mm to the reference surface A

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**Surface must be concentric within 0.1mm to reference surface A**

Surfaces must be perpendicular within 0.01mm Surface must be flat within 0.03mm Surfaces must be parallel within 0.01mm

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**Surface must be perpendicular within 0.1mm to reference surface A**

Surface must be concentric within 0.02mm to reference surface Surface must be parallel within 0.1mm to reference surface A

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**05 – Fits and Tolerances Surface Finish What is surface finish?**

What is surface finish? Regardless of how smooth a surface looks or feels, it contains some roughness. Surface finish is a measure of how smooth or rough a surface is.

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**05 – Fits and Tolerances Why is surface finish important?**

Why is surface finish important? The surface finish will be the determining factor in how well mating parts: Wear: if one slides against the other if one turns in the other Seal: mating surfaces must be flat

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**05 – Fits and Tolerances Some examples of these situations would be:**

A shaft turning in a seal A cylinder head on a car A ball in a ball bearing The back and forth action of the ram on the shaper How is surface finish shown on the drawing? The surface finish for a part is indicated on the detail drawing of the part. One of the two symbols shown below will be used to indicate the finish of a surface

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05 – Fits and Tolerances The numbers in the finish symbol are micro-meters (microns) in the ISO system and micro-inches in the U.S. system……..…millionth of a meter or inch. Practical Applications of Surface Finish: Three important surface finishes to remember are: 1. Bearing seat (housing or shaft) Ra 1.6 (63 in U.S.) 2. Place on shaft where oil seal rides Ra 0.8 (32 in U.S.) 3. “O” ring in a housing or on a shaft Ra 0.4 (16 in U.S.) Checking Surface finish: Surface finish is checked by sight and feel. This is done by comparing the surface finish on the object we are checking with a surface having a known roughness value. The device used for this is a surface finish comparator.

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05 – Fits and Tolerances

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05 – Fits and Tolerances

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Ra1.6 Ra0.8

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05 – Fits and Tolerances End of Chapter Five Exit

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**05 – Fits and Tolerances Return**

1) Find maximum and minimum size for the following tolerances. Size Tolerance Maximum Minimum a) 30 h6 0.00 30.0 29.987 b) 62 p6 62.051 62.032 c) 48 H7 48.025 48.0 d) 105 g6 e) 16 f7 15.984 15.966 f) 55 N7 54.991 54.961 g) 4” ” ” ” ” h) 3.025” ” ” 3.03” 3.024” I) ” ” ” ” ”

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**2) Determine the shaft and hole sizes for the following fits**

2) Determine the shaft and hole sizes for the following fits. Also, determine the maximum and minimum clearances for these fits. a) Size Tolerance Maximum Clear./Inter. Minimum Interval of Tol. (Range) F 1.2” RC6 ” 0.000 1.2025” 1.2” 0.0025” M ” ” 1.198” 1.1964” 0.0016” Fit = Clearance ” ” 0.0041” Return b) Size Tolerance Maximum Clear./Inter. Minimum Interval of Tol. (Range) F 0.5” LC2 ” 0.000 0.5007” 0.5 0.0007” M ” 0.5” 0.4996” 0.0004” Fit = Uncertain ” 0.0” 0.0011”

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**c) Clear./Inter. Interval of Tol. (Range) F 1.1” LC9 + 0.005 0.00**

Size Tolerance Maximum Clear./Inter. Minimum Interval of Tol. (Range) F 1.1” LC9 0.00 1.105” 1.1” 0.005” M ” ” 1.0955” 1.092” 0.0035” Fit = Clearance ” 0.0085” Return Size Tolerance Maximum Clear./Inter. Minimum Interval of Tol. (Range) F 0.75” FN2 ” 0.000 0.7508” 0.75” 0.0008” M ” ” 0.7519” 0.7514” 0.0005” Fit = Interference ” ” 0.0013” d)

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**e) Clear./Inter. Interval of Tol. (Range) F 35 H7 + 0.025 0.0 35.025**

Size Tolerance Maximum Clear./Inter. Minimum Interval of Tol. (Range) F 35 H7 0.0 35.025 35.00 0.025 M 35 g6 34.991 34.975 0.016 Fit = Clearance + 0.05 0.041 Return f) Size Tolerance Maximum Clear./Inter. Minimum Interval of Tol. (Range) F 72 H11 + 0.19 0.0 72.19 72.0 0.19 M 72 c11 - 0.15 - 0.34 71.85 71.66 Fit = Clearance + 0.53 + 0.15 0.38

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**g) Clear./Inter. Interval of Tol. (Range) F 23 H8 + 0.033 0.0 23.033**

Size Tolerance Maximum Clear./Inter. Minimum Interval of Tol. (Range) F 23 H8 0.0 23.033 23.0 0.033 M 23 f7 - 0.02 22.98 22.959 0.021 Fit = Clearance + 0.02 0.054 Return h) Size Tolerance Maximum Clear./Inter. Minimum Interval of Tol. (Range) F 55 H7 + 0.03 0.0 55.03 55.00 0.03 M 55 p6 55.051 55.032 0.019 Fit = Interference 0.049

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05 – Fits and Tolerances

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