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Tolerances and fits This book includes tables and calculations for easy option of fits of machine parts and determination of their dimensional tolerances and deviations. Using this tool the following tasks can be solved:. Selection of suitable fits of machine parts according to the international standard ISO 286. Determination of dimensional tolerances and deviations of machine parts according to the international standard ISO 286. Selection of preferred fits of machine parts and determination of their dimensional tolerances and deviations according to ANSI B4.1. Determination of non-prescribed limit deviations of linear and angular dimensions according to ISO 2768. Automatic design of a fit for the given clearance or fit interference respectively.
The data, procedures, algorithms and specialized literature and standards ANSI, ISO, DIN and others were used in the calculations. List of standards: ANSI B4.1, ANSI B4.2, ISO 286, ISO 1829, ISO 2768, EN 20286, JIS B 0401. Information on the syntax and control of the calculation can be found in the document '.
It is necessary that the dimensions, shape and mutual position of surfaces of individual parts of mechanical engineering products are kept within a certain accuracy to achieve their correct and reliable functioning. Routine production processes do not allow maintenance (or measurement) of the given geometrical properties with absolute accuracy. Actual surfaces of the produced parts therefore differ from ideal surfaces prescribed in drawings. Deviations of actual surfaces are divided into four groups to enable assessment, prescription and checking of the permitted inaccuracy during production:.
Dimensional deviations. Shape deviations. Position deviations. Surface roughness deviations This toll includes the first group and can therefore be used to determine dimensional tolerances and deviations of machine parts.
As mentioned above, it is principally impossible to produce machine parts with absolute dimensional accuracy. In fact, it is not necessary or useful. It is quite sufficient that the actual dimension of the part is found between two limit dimensions and a permissible deviation is kept with production to ensure correct functioning of engineering products. The required level of accuracy of production of the given part is then given by the dimensional tolerance which is prescribed in the drawing.
The production accuracy is prescribed with regards to the functionality of the product and to the economy of production as well. A coupling of two parts creates a fit whose functional character is determined by differences of their dimensions before their coupling. Basic size Dmax, Dmin. Limits of size for the hole dmax, dmin. Limits of size for the shaft ES. Hole upper deviation EI. Hole lower deviation es.
Preferred Fits (Iso)
Shaft upper deviation ei. Shaft lower deviation Depending on the mutual position of tolerance zones of the coupled parts, 3 types of fit can be distinguished:. Clearance fit. Transition fit.
Interference fit 1 This paragraph can be used to choose a fit and determine tolerances and deviations of machine parts according to the standard ISO 286:1988. This standard is identical with the European standard EN and defines an internationally recognized system of tolerances, deviations and fits.
The standard ISO 286 is used as an international standard for linear dimension tolerances and has been accepted in most industrially developed countries in identical or modified wording as a national standard (JIS B 0401, DIN ISO 286, BS EN 20286, CSN EN 20286, etc.). The system of tolerances and fits ISO can be applied in tolerances and deviations of smooth parts and for fits created by their coupling. It is used particularly for cylindrical parts with round sections. Tolerances and deviations in this standard can also be applied in smooth parts of other sections. Similarly, the system can be used for coupling (fits) of cylindrical parts and for fits with parts having two parallel surfaces (e.g. Fits of keys in grooves). The term 'shaft', used in this standard has a wide meaning and serves for specification of all outer elements of the part, including those elements which do not have cylindrical shapes.
Also, the term 'hole' can be used for specification of all inner elements regardless of their shape. Note: All numerical values of tolerances and deviations mentioned in this paragraph are given in the metric system and relate to parts with dimensions specified at 20 °C. 1.1 Basic size.
It is the size whose limit dimensions are specified using the upper and lower deviations. In case of a fit, the basic size of both connected elements must be the same. Attention: The standard ISO 286 defines the system of tolerances, deviations and fits only for basic sizes up to 3150 mm. 1.2 Tolerance of a basic size for specific tolerance grade. The tolerance of a size is defined as the difference between the upper and lower limit dimensions of the part. In order to meet the requirements of various production branches for accuracy of the product, the system ISO implements 20 grades of accuracy. Each of the tolerances of this system is marked 'IT' with attached grade of accuracy (IT01, IT0, IT1.
Field of use of individual tolerances of the system ISO: IT01 to IT6 For production of gauges and measuring instruments IT5 to IT12 For fits in precision and general engineering IT11 to IT16 For production of semi-products IT16 to IT18 For structures IT11 to IT18 For specification of limit deviations of non-tolerated dimensions Note: When choosing a suitable dimension it is necessary to also take into account the used method of machining of the part in the production process. The dependency between the tolerance and modification of the surface can be found in the table in paragraph 5. 1.3 Hole tolerance zones.
The tolerance zone is defined as a spherical zone limited by the upper and lower limit dimensions of the part. The tolerance zone is therefore determined by the amount of the tolerance and its position related to the basic size. The position of the tolerance zone, related to the basic size (zero line), is determined in the ISO system by a so-called basic deviation. The system ISO defines 28 classes of basic deviations for holes.
These classes are marked by capital letters (A, B, C. The tolerance zone for the specified dimensions is prescribed in the drawing by a tolerance mark, which consists of a letter marking of the basic deviation and a numerical marking of the tolerance grade (e.g.
H7, H8, D5, etc.). This paragraph includes graphic illustrations of all tolerance zones of a hole which are applicable for the specified basic size 1.1 and the tolerance grade IT chosen from the pop-up list. Though the general sets of basic deviations (A. ZC) and tolerance grades (IT1. IT18) can be used for prescriptions of hole tolerance zones by their mutual combinations, in practice only a limited range of tolerance zones is used.
An overview of tolerance zones for general use can be found in the following table. The tolerance zones not included in this table are considered special zones and their use is recommended only in technically well-grounded cases.
ANSI Standard Limits and Fits (ANSI B4.1-1967,R1974) ANSI, This American Standard for preferred limits and fits for cylindrical parts presents definitions of terms applying to fits between nonthreaded cylindrical and makes some recommendations on preferred sizes, fits, tolerances, and allowances for use where they are applicable. The ANSI B4.1 charts data are provided in thousandths (.001) of an inch. ANSI can be defined ANSI metric or ANSI Inch system. ANSI decimal inch system is based on ANSI Standard (B4.1 - 1967(1979). ANSI Standard (B4.2 - 1978)metric system is based on ISO millimeter system.
ANSI - American National Standards Institute. ISO is International Organization for Standardization. ANSI standard is in accord with the recommendations of American-British-Canadian (ABC) conferences up to diameter of 20 inches.The ABC proposal comprises:(a)A grade system of fundamental tolerances for diameters 0.04 inch to approximately 20 inches, the tolerance values in each grade being to the diameter by a given law of formula and each grade of tolerance being approximately related to the others in the system in a preferred number ratio (R5 series, with 60 percent increments). (a) A series of unilateral hole limits derived from the fundamental tolerances, for use with a hole basis system.
(c) A series of shaft, one of these limits being determined by the allowance ( according to some technical requirement), the other being derived from it by addition or subtraction of the fundamental tolerance.(d) A selection of fits made by recommended association of certain of the holes and shaft, the selection being adequate to cover most engineering requirements. What can be effective for selection of the fits? Many factor can be affective for selecting proper fits such as bearing loads, speed, materials, length of engagement, lubrication, humidity, and temperature etc. This all affect must be taken into consideration in the selection of the fits for proper applications.
ANSI B4.1 - 1967(1979), R1974 Standard Tolerances ANSI B4.1 standard implements 10 tolerance grades to meet the requirements of various production branches for accuracy of products. ANSI B4.1 standard defines a system of preferred fits only for basic sizes up to 19.69 inch. ANSI B4.1 system of tolerances is prescribed by the standard for basic sizes up to 200 in. ANSI B4.1 Tolerance Table provides a suitable range from which appropriate tolerance for holes and shaft can be selected. This enables the use of standard gages.
Table shown below. NOMINAL( BASIC) SIZES (INCHES) INTERNATIONAL TOLERANCE GRADES OVER UP TO INCL.
PREFERRED FITS AND TOLERANCES CHARTS (ISO & ANSI METRIC STANDARDS) Preferred metric fits and tolerances for hole and shaft basis systems which are given in ISO 286-1 (2010) and ANSI B4.2-1978 standards are summarized in the following tables. The usage of these tolerances is advised for economic reasons. Supplement: Preferred fits advised by ISO and ANSI standards ISO Symbol Description Hole Basis Shaft Basis Clearance Fits H11/c11 C11/h11 Loose running fit for wide commercial tolerances or allowances on external members. H9/d9 D9/h9 Free running fit not for use where accuracy is essential, but good for large temperature variations, high running speeds, or heavy journal pressures.
H8/f7 F8/h7 Close running fit for running on accurate machines and for accurate location at moderate speeds and journal pressures. H7/g6 G7/h6 Sliding fit not intended to run freely, but to move and turn freely and locate accurately.
H7/h6 H7/h6 Locational clearance fit provides snug fit for locating stationary parts; but can be freely assembled and disassembled. Transition Fits H7/k6 K7/h6 Locational transition fit for accurate location, a compromise between clearance and interference. Andrew wiles fermat's last theorem solution proof.
H7/n6 N7/h6 Locational transition fit for more accurate location where greater interference is permissible. Interference Fits H7/p6 P7/h6 Locational interference fit for parts requiring rigidity and alignment with prime accuracy of location but without special bore pressure requirements. H7/s6 S7/h6 Medium drive fit for ordinary steel parts or shrink fits on light sections, the tightest fit usable with cast iron.
H7/u6 U7/h6 Force fit suitable for parts which can be highly stressed or for shrink fits where the heavy pressing forces required are impractical. Oberg, E., Jones,F.D., Horton H.L., Ryffel H.H., (2016). 30th edition. Industrial Press Inc.
Oberg, E., Jones,F.D., Horton H.L., Ryffel H.H., (2012). 29th edition. Industrial Press Inc. IS0 286-1 (2010) Geometrical product specifications (GPS) - ISO code system for tolerances on linear sizes - Part 1: Basis of tolerances, deviations and fits. IS0 286-2 (2010) Geometrical product specifications (GPS) - ISO code system for tolerances on linear sizes - Part 2: Tables of standard tolerance classes and limit deviations for holes and shafts. ANSI/ASME B4.2 (1978) Preferred Metric Limits and Fits.
Grade NOMINAL HOLE SIZES (mm) over 3 6 10 18 30 40 50 65 80 100 120 140 160 180 200 225 250 280 315 355 inc.
Grade NOMINAL SHAFT SIZES (mm) Over 3 6 10 18 30 40 50 65 80 100 120 140 160 180 200 225 250 280 315 355 Up to and incl.