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1、FITSSECTION 2.2FitsThe relation betweentwo mating parts is known as a fit.Fits between two mating parts govern the performance of many mechanisms such as reduction gear box.Reduction gear boxTypes of FitDepending upon the actual limits of the hole or shaft sizes,there are three types of fits.A.Clear
2、ance fit:It is a fit that always provides aclearancebetweentheholeandshaftwhenassembled.For example,The lower limit of size of thehole is either larger than or,in the extreme case,equalto the upper limit of size of the shaft.B.Transition fit:It is a fit may provide either a clearanceor an interferen
3、ce between the hole and shaft whenassembled.In a transition fit,tolerance intervals of thehole and shaft overlap either completely or partially.C.Interference fit:It is a fit that always provides ainterferencebetweentheholeandshaftwhenassembled.For example,The upper limit of size of thehole is eithe
4、r smaller than or,in the extreme case,equal to the lower limit of size of the shaft.Illustration of Definitions of FitsRelationship between hole and shaft in fits can be shown in thefollowing figures.Clearance fitTransition fitInterference fitIn order to reduce production costs,the number of possibl
5、e pairs islimited by using two fit systems:Hole-basis fit system and Shaft-basisfit system.Hole-basis Fit SystemHole-base fit system is a system in whichthe size of the hole is keptconstant and shaft sizes are varied to obtain various types of fits.The lettersymbol is H.Fits where the fundamental de
6、viation of the hole is zero.The required clearances or interferences are obtained by combining shafts of various tolerance classes with basic holes.Shaft-basis Fit SystemShaft-basis fit system is a in which the size of the shaft is kept constantand hole sizes are varied to obtain various types of fi
7、ts.The letter symbolis h.Fits where the fundamental deviation of the shaft is zero.The required clearances or interferences are obtained by combining holesof various tolerance classes with basic shafts.Selection of the Fit SystemFrom manufacturing point of view,the“hole-basis fit system”should be ch
8、osen for general use.This choice would avoid anunnecessary multiplicity of tools(e.g.reamers)and gauges.The“shaft-basis fit system”should only be used where it willconvey unquestionable economical advantages(e.g.where it isnecessary to be able to mount several parts with holes havingdifferent deviat
9、ions on a single shaft of drawn steel bar).There are no technical differences regarding the function of theparts.Therefore the choice of the system should be based oneconomic reasons.Commonly Used FitsHole BasisShaft BasisDescriptionMore interferenceMore clearanceClearance fitsH11c11C11h11Loose runn
10、ing fit for wide commercial tolerances or allowances on external members.H8f7F8h7Close running fit for running on accurate machines and for accurate location at moderate speeds and journal pressures.H7g6G7h6Sliding fit not intended to run freely but to move and turn freely and locate accurately.H7h6
11、H7h6Locational clearance fit provides sung fit locating stationary parts but can be freely assembled and disassembled.Transition fitsH7k6K7h6Locational transition fit for accurate location;a compromise between clearance and interference.H7n6N7h6Locational transition fit for more accurate location wh
12、ere greater interference is permissible.Interference fitsH7p6P7h6Locational interference fit for parts requiring rigidity and alignment with prime accuracy of location but without special bore pressure requirements.H7s6S7h6Medium drive fit for ordinary steel parts or shrink fits on light sections;th
13、e tightest fit usable with cast iron.H7u6U7h6Force fit suitable for parts that can be highly stressed or for shrink fits where the heavy pressing forces required are impractical.Example of Fit CalculationQuestion:Calculate the fit:36 H8/f7.Answer:From the table 2 in ISO 286-1:2010(E)Hole:36 H8 resul
14、ts:ES=+0.039 mm,EI=0mmShaft:36 f7results:es=0.025 mm,ei=0.050 mmTherefore:lower limit of size of the hole upper limit of size of the shaft=36.000 35.975=0.025 mm.upper limit of size of the hole lower limit of size of the shaft=36.039 35.950=0.089 mm.Therefore:the fit has a maximum clearance of 0.089 mm and a minimum clearance of0.025 mm.It is a clearance fit.