Engineering fits are generally used as part of geometric dimensioning and tolerancing when a part or assembly is designed. In engineering terms, the "fit" is the clearance between two mating parts, and the size of this clearance determines whether the parts can, at one end of the spectrum, move or rotate independently from each other or, at the other end, are temporarily or permanently joined. Engineering fits are generally described as a "shaft and hole" pairing, but are not necessarily limited to just round components. ISO is the internationally accepted standard for defining engineering fits, but ANSI is often still used in North America.
ISO and ANSI both group fits into three categories: clearance, location or transition, and interference. Within each category are several codes to define the size limits of the hole or shaft – the combination of which determines the type of fit. A fit is usually selected at the design stage according to whether the mating parts need to be accurately located, free to slide or rotate, separated easily, or resist separation. Cost is also a major factor in selecting a fit, as more accurate fits will be more expensive to produce, and tighter fits will be more expensive to assemble.
Methods of producing work to the required tolerances to achieve a desired fit range from casting, forging and drilling for the widest tolerances through broaching, reaming, milling and turning to lapping and honing at the tightest tolerances.
ISO system of limits and fits
Overview The International Organization for Standardization system splits the three main categories into several individual fits based on the allowable limits for hole and shaft size. Each fit is allocated a code, made up of a number and a letter, which is used on engineering drawings in place of upper & lower size limits to reduce clutter in detailed areas.
Hole and shaft basis A fit is either specified as shaft-basis or hole-basis, depending on which part has its size controlled to determine the fit. In a hole-basis system, the size of the hole remains constant and the diameter of the shaft is varied to determine the fit; conversely, in a shaft-basis system the size of shaft remains constant and the hole diameter is varied to determine the fit.
The ISO system uses an alpha-numeric code to illustrate the tolerance ranges for the fit, with the upper-case representing the hole tolerance and lower-case representing the shaft. For example, in H7/h6 (a commonly used fit) H7 represents the tolerance range of the hole and h6 represents the tolerance range of the shaft. These codes can be used by machinists or engineers to quickly identify the upper and lower size limits for either the hole or shaft. The potential range of clearance or interference can be found by subtracting the smallest shaft diameter from the largest hole, and largest shaft from the smallest hole.
Types of fit
The three types of fit are: Clearance: The hole is larger than the shaft, enabling the two parts to slide and / or rotate when assembled, e.g. piston and valves Location / transition: The hole is fractionally smaller than the shaft and mild force is required to assemble / disassemble, e.g. Shaft key Interference: The hole is smaller than the shaft and high force and / or heat is required to assemble / disassemble, e.g. Bearing bush
Clearance fits Category Description and Usage Hole Basis Shaft BasisLoose runningLarger clearance where accuracy is not essential, e.g., pivots, latches, parts affected by corrosion, heat, or contaminationH11/c11C11/h11Free runningLarge clearance where accuracy is not essential and involves high running speeds, large temperature variations, or heavy journal pressuresH9/d9D9/h9Close runningSmall clearances with moderate requirements for accuracy, e.g., moderate running speeds and journal pressures, shafts, spindles, sliding rodsH8/f7F8/h7SlidingMinimal clearances for high accuracy requirements, which can be easily assembled and will turn & slide freely, e.g., guiding of shafts, sliding gears, crankshaft journalsH7/g6G7/h6LocationVery close clearances for precise accuracy requirements, which can be assembled without force and will turn & slide when lubricated, e.g., precise guiding of shaftsH7/h6H7/h6
For example, using an H8/f7 close-running fit on a 50mm diameter: H8 (hole) tolerance range = +0.000mm to +0.039mm f7 (shaft) tolerance range = −0.050mm to −0.025mm Potential clearance will be between +0.025mm and +0.089mm
Transition fits Category Description and Usage Hole Basis Shaft BasisSimilar fitNegligible clearance or interference fit which can be assembled or disassembled with a rubber mallet, e.g., hubs, gears, pulleys, bushes, bearingsH7/k6K7/h6Fixed fitNegligible clearance or small interference fit which can be assembled or disassembled with light pressing force, e.g., plugs, driven bushes, armatures on shaftsH7/n6N7/h6
For example, using an H7/k6 similar fit on a 50mm diameter: H7 (hole) tolerance range = +0.000mm to +0.025mm k6 (shaft) tolerance range = +0.002mm to +0.018mm Potential clearance / interference will be between +0.023mm and −0.018mm
Interference fits Category Description and Usage Hole Basis Shaft BasisPress fitLight interference which can be assembled or disassembled with cold pressing, e.g., hubs, bearings, bushings, retainersH7/p6P7/h6Driving fitMedium interference which can be assembled with hot pressing or cold pressing with large forces, e.g., permanent mounting of gears, shafts, bushes (tightest possible with cast iron)H7/s6S7/h6Forced fitHigh interference shrink fit requiring large temperature differential of parts to assemble, permanent coupling of gears and shafts that cannot be disassembled without risking destructionH7/u6U7/h6
For example, using an H7/p6 press fit on a 50mm diameter: H7 (hole) tolerance range = +0.000mm to +0.025mm p6 (shaft) tolerance range = +0.042mm to +0.026mm Potential interference will be between −0.001mm and −0.042mm.
Useful tolerances Common tolerances for sizes ranging from 0 to 120 mm +Useful TolerancesNominal Diameter (mm)Toleranceover 0 to 3over 3 to 6over 6 to 10over 10 to 18over 18 to 24over 24 to 30over 30 to 40over 40 to 50over 50 to 65over 65 to 80over 80 to 100over 100 to 120Shaftsc11 −0.060 −0.120 −0.070 −0.145 −0.080 −0.205 −0.095 −0.205 −0.110 −0.240 −0.120 −0.280 −0.130 −0.290 −0.140 −0.330 −0.150 −0.340 −0.170 −0.390 −0.180 −0.400d9 −0.020 −0.045 −0.030 −0.060 −0.040 −0.076 −0.050 −0.093 −0.065 −0.117 −0.080 −0.142 −0.100 −0.174 −0.120 −0.207f7 −0.006 −0.016 −0.010 −0.022 −0.013 −0.028 −0.016 −0.034 −0.020 −0.041 −0.025 −0.050 −0.030 −0.060 −0.036 −0.071g6 −0.002 −0.008 −0.004 −0.012 −0.005 −0.014 −0.006 −0.017 −0.007 −0.020 −0.009 −0.025 −0.01 −0.029 −0.012 −0.034h60.000 −0.0060.000 −0.0080.000 −0.0090.000 −0.0110.000 −0.0130.000 −0.0160.000 −0.0190.000 −0.022h70.000 −0.0100.000 −0.0120.000 −0.0150.000 −0.0180.000 −0.0210.000 −0.0250.000 −0.0300.000 −0.035h90.000 −0.0250.000 −0.0300.000 −0.0360.000 −0.0430.000 −0.0520.000 −0.0620.000 −0.0740.000 −0.087h110.000 −0.0600.000 −0.0750.000 −0.0900.000 −0.1100.000 −0.1300.000 −0.1600.000 −0.1900.000 −0.220k6 +0.006 0.000 +0.009 +0.001 +0.010 +0.001 +0.012 +0.001 +0.015 +0.002 +0.018 +0.002 +0.021 +0.002 +0.025 +0.003n6 +0.010 +0.004 +0.016 +0.008 +0.019 +0.010 +0.023 +0.012 +0.028 +0.015 +0.033 +0.017 +0.039 +0.020 +0.045 +0.023p6 +0.012 +0.006 +0.020 +0.012 +0.024 +0.015 +0.029 +0.018 +0.035 +0.022 +0.042 +0.026 +0.051 +0.032 +0.059 +0.037s6 +0.020 +0.014 +0.027 +0.019 +0.032 +0.023 +0.039 +0.028 +0.048 +0.035 +0.059 +0.043 +0.072 +0.053 +0.078 +0.059 +0.093 +0.071 +0.101 +0.079u6 +0.024 +0.018 +0.031 +0.023 +0.037 +0.028 +0.044 +0.033 +0.054 +0.041 +0.061 +0.048 +0.076 +0.060 +0.086 +0.070 +0.106 +0.087 +0.121 +0.102 +0.146 +0.124 +0.166 +0.144HolesC11 +0.120 +0.060 +0.145 +0.070 +0.170 +0.080 +0.205 +0.095 +0.240 +0.110 +0.280 +0.120 +0.290 +0.130 +0.330 +0.140 +0.340 +0.150 +0.390 +0.170 +0.400 +0.180D9 +0.045 +0.020 +0.060 +0.030 +0.076 +0.040 +0.093 +0.050 +0.117 +0.065 +0.142 +0.080 +0.174 +0.100 +0.207 +0.120F8 +0.020 +0.006 +0.028 +0.010 +0.035 +0.013 +0.043 +0.016 +0.053 +0.020 +0.064 +0.025 +0.076 +0.030 +0.090 +0.036G7 +0.012 +0.002 +0.016 +0.004 +0.020 +0.005 +0.024 +0.006 +0.028 +0.007 +0.034 +0.009 +0.040 +0.010 +0.047 +0.012H7 +0.010 0.000 +0.012 0.000 +0.015 0.000 +0.018 0.000 +0.021 0.000 +0.025 0.000 +0.030 0.000 +0.035 0.000H8 +0.014 +0.000 +0.018 +0.000 +0.022 +0.000 +0.027 +0.000 +0.033 +0.000 +0.039 +0.000 +0.046 +0.000 +0.054 +0.000H9 +0.025 +0.000 +0.030 +0.000 +0.036 +0.000 +0.043 +0.000 +0.052 +0.000 +0.062 +0.000 +0.074 +0.000 +0.087 +0.000H11 +0.060 0.000 +0.075 0.000 +0.090 0.000 +0.110 0.000 +0.130 0.000 +0.160 0.000 +0.190 0.000 +0.220 0.000K70.000 −0.010 +0.003 −0.009 +0.005 −0.010 +0.006 −0.012 +0.006 −0.015 +0.007 −0.018 +0.009 −0.021 +0.010 −0.025N7 −0.004 −0.014 −0.004 −0.016 −0.004 −0.019 −0.005 −0.023 −0.007 −0.028 −0.008 −0.033 −0.009 −0.039 −0.010 −0.045P7 −0.006 −0.016 −0.008 −0.020 −0.009 −0.024 −0.011 −0.029 −0.014 −0.035 −0.017 −0.042 −0.021 −0.051 −0.024 −0.059S7 −0.014 −0.024 −0.015 −0.027 −0.017 −0.032 −0.021 −0.039 −0.027 −0.048 −0.034 −0.059 −0.042 −0.072 −0.048 −0.078 −0.058 −0.093 −0.066 −0.101U7 −0.018 −0.028 −0.019 −0.031 −0.022 −0.037 −0.026 −0.044 −0.033 −0.054 −0.040 −0.061 −0.051 −0.076 −0.061 −0.086 −0.076 −0.106 −0.091 -0.121 −0.111 −0.146 −0.131 −0.166
ANSI fit classes (US only)
Interference fits
Interference fits, also known as press fits or friction fits, are fastenings between two parts in which the inner component is larger than the outer component. Achieving an interference fit requires applying force during assembly. After the parts are joined, the mating surfaces will feel pressure due to friction, and deformation of the completed assembly will be observed.
Force fits
Force fits are designed to maintain a controlled pressure between mating parts, and are used where forces or torques are being transmitted through the joining point. Like interference fits, force fits are achieved by applying a force during component assembly.
FN 1 to FN 5
Shrink fits
Shrink fits serve the same purpose as force fits, but are achieved by heating one member to expand it while the other remains cool. The parts can then be easily put together with little applied force, but after cooling and contraction, the same dimensional interference exists as for a force fit. Like force fits, shrink fits range from FN 1 to FN 5.
Location fits
Location fits are for parts that do not normally move relative to each other.
Location interference fit
LN 1 to LN 3 (or LT 7 to LT 21? )
Location transition fit
LT 1 to LT 6 Location fit is for have comparatively better fit than slide fit.
Location clearance fit
LC 1 to LC 11
RC fits
The smaller RC numbers have smaller clearances for tighter fits, the larger numbers have larger clearances for looser fits.
RC1: close sliding fits
Fits of this kind are intended for the accurate location of parts which must assemble without noticeable play.
RC2: sliding fits
Fits of this kind are intended for the accurate location but with greater maximum clearance than class RC1. Parts made to this fit turn and move easily. This type is not designed for free run. Sliding fits in larger sizes may seize with small temperature changes due to little allowance for thermal expansion or contraction.
RC3: precision running fits
Fits of this kind are about the closest fits which can be expected to run freely. Precision fits are intended for precision work at low speed, low bearing pressures, and light journal pressures. RC3 is not suitable where noticeable temperature differences occur.
RC4: close running fits