Limits and Fits
It is impossible to produce a component to an absolute or 100% dimensional accuracy. The variations in size of the components are caused by the following factors.
1. Type of manufacture.
2. Method of machining.
3. Condition of machine.
4. Skill of machinist.
5. Time taken.
6. Geometry of the product.
7. Condition of measuring tools.
8. Condition of measuring.
Therefore depending upon the functional requirements of a work, some variation is permitted on it. This permissible variation is called tolerance. Though functional requirement is the primary consideration there are other factors like standardization, methodisation, manufacturing needs etc., which influence the choice of tolerance.
1 Limit system
It is a system of standard tolerances and deviations.
2 Interchangeability
In earlier times the majority of components in assembly were matched together, their dimensions being adjusted until the required type of fit is obtained. These methods demanded craftsmanship of high order. Today manufacturing techniques are changed. In mass production the process is broken in to several smaller activities and as a result various components will come from several shops. Under such conditions it becomes absolutely essential to have a strict control over the dimension of parts, which have to match with other parts. Any part selected at random should assemble correctly with any other matching component that too selected at random. When a system of this kind is ensured, it is called interchangeable system.
2.1 Interchangeable parts
Interchangeable parts are those which ensure the possibility of assembling a unit or machine or replacing a worn out component without doing any extra machining or fitting operations.
Elements of limit system
3.1 Nominal size: The nominal size of a dimension is the size specified in the drawing. It is usually given in the drawing as rounded of whole millimeters.
3.2 Basic size: The basic size of dimension is the size in relation to which all limits of variations
are determined.
3.3 Actual size: the actual size of a dimension is its measured size.
Limits of size: limits are the two extreme permissible sizes for that dimension, there being an upper limit and a lower limit. The upper limit is the largest and lower limit is the smallest permissible sizes.
Tolerance: Tolerance is the difference between upper and lower limit of a size. If the tolerance is allowed on both sides of the basic size it is called bilateral tolerance.
Eg: 45± 0.02 , 30 ± 0.04
If the tolerance is allowed on one side of the basic size it is called unilateral tolerance.
Eg: 45+0.03 , 30 +0.04
Fit
The relationship existing between two parts, shaft and hole, which are to be assembled with
respect to their difference in the sizes before assembly is called fit.
Hole: The term used by convention to designate all the internal features of a part
including those, which are not cylindrical.
Shaft: The term used by convention to designate all external features of a part including those, which are not cylindrical.
4.3 Types of fit:
Depending upon the actual limit of the hole or shaft the fit in Indian Standard shall be divided
into three main classes,
4.3.1: Clearance fit The fit, which always provides the clearance is called clearance fit. Here,
the tolerance zone of the hole is entirely above that of shaft.
4.3.2 : Interference fit The fit, which always provides an interference is called interference fit.
Here, the tolerance zone of the hole is entirely below that of shaft.
4.3.3 : Transition fit : The fit, which provides either a clearance or an interference is called
transition fit. Here the tolerance zones of the hole and shaft overlap.
5 Indian standard ISO system of limits and fits ( IS 919 , ISO 286 ).
This Indian standard which is identical with ISO 286-1: ISO system of limits and fits: Bases of tolerances, deviations and fits was adopted by the Bureau of Indian Standards on the recommendations of the Engineering Standards Sectional Committee (LM 01) and approval of the Light Mechanical Engineering Division Council.
In this system fundamental deviations are indicated by letter symbols for both holes and shafts. [Capital letters A to ZC for holes and a to zc for shafts]. Letter symbols used to indicate fundamental deviations are A B C CD D E EF F FG G H J JS K M N P R S T U V X Y Z ZA Z B ZC. And 20 tolerance grades are indicated by number symbols from IT 01, IT 0, IT 1 ……………IT 18.
Terminology:
5.1 Zero line: In a graphical representation of limits and fits the straight line to which deviations are referred is called zero line. It represents the basic size. When the zero line is drawn horizontally positive deviations are shown above and negative deviations below it.
5.2 Deviations : It is the algebraic difference between a size and corresponding basic size.
Upper deviation : It is the algebraic difference between the maximum limit of size and the corresponding basic size. It is designated by ‘ES’ for holes and ‘es’ for shafts.
Lower deviation : It is the algebraic difference between the minimum limit of size and the corresponding basic size. It is designated by ‘EI’ for holes and ‘ei’ for shafts.
Tolerance: It is equal to the algebraic difference between the upper and lower deviation.
5.4 Tolerance zone: In a graphical representation of tolerance, the zone bounded by two limits of size of the part and defined by its magnitude and by its position in relation to zero line.
5.5 Fundamental deviation: According to convention, it is the one defining the nearest limit to the zero line.
6 Symbol for tolerances, deviations and fits
The tolerance is designated by a number symbol called grade. The position of tolerance zone is indicated by a letter symbol. [Capital letter for hole and small for shaft.]
The tolerance size is just defined by basic value followed by a letter and numeral. Eg. 50H7, 35g6. A fit is indicated by the basic size common to both components followed by symbols corresponding to each component, the hole being written first.
The following symbols are used to denote upper and lower deviations.
Upper deviation of hole - ES
Lower deviation of hole - EI
Upper deviation of shaft- es
Lower deviation of shaft- ei
ES = EI+IT
es = ei+IT
6.1 Go and not go limit :
Go limit refers to upper limit of shaft and lower limit of hole. It corresponds to maximum material condition.
Not Go limit refers to lower limit of shaft and upper limit of hole. It corresponds to least material condition.
6.2 Hole basis and shaft basis system
A limit system is said to be on a hole basis, when the hole is held a constant member and different fits are obtained by varying the sizes of the shafts. In this system a single hole whose lower deviation is zero( H ) is used.
A limit system is said to be on a shaft basis, when the shaft is a constant member and different fits are obtained by varying the sizes of the holes. In this system a single shaft whose upper deviation is zero (h) is used.
All modern limit system employed the hole basis system. The chief reason is that it is easier to vary the size of shaft than that of hole. In majority of drawings in engineering work are produced with drill, reamer or some similar tools and vary the size of the hole would necessitate the use of very large number of tools of varying sizes. However in some instants shaft basis system goes to more advantages to use than that of an hole basis system.
6.3 Guideline for selection of fit
In the hole basis system various grades of holes used are,
H5: This grade can be obtained by precision boring, honing and fine internal grinding.
H6: This can be obtained by fine hand reaming, honing and precision boring.
H7: This grade can be obtained by Internal grinding, broaching, or careful reaming.
H8: This can be obtained by machine reaming or boring.
H9: This can be obtained by boring and reaming. It is mainly used for non circular dimensions.
H10: This grade is used for milled widths, drill holes and unimportant parts.
H11: This grade being very coarse is never used for fits. Eg. coarse drilled and punched holes.
6.4 Clearance fit
Shafts a, b and c gives large clearance and therefore not widely used.
Shaft d is used for loose running fit. Shaft e is used for large high-speed heavily loaded bearing.
Shaft f is used for normal grease lubricated or oil lubricated bearings.
Shaft g is used in precision equipments. Shaft h is used for normal location and spigot
fits and in the finer grades is used as precision sliding fit.
6.5 Transition fit
Shaft j is used for location fits where a slight interference is permissible. Also used for spigot fits.
Shaft k is best suited for location fits. Shaft m gives location fits. Eg. Dowell hole, dowel pin.
Shaft n gives clearance only on extreme sides. It is recommended for generally ‘tight’ assembly fits.
6.6 Interference fit
Shaft p gives a true interference. It is a standard press fit used for steel and cast iron. An example of this fit is fixing of bush on to a gear.
Shaft r gives a medium drive fit on ferrous parts, and on non-ferrous parts a light drive fit which can be easily dismantled when required.
Shaft s is used for permanent and semi permanent assemblies.
Shaft t, u and v give more interference. Shaft x , y , z , za , zb and zc give a very large interference and therefore these shafts are not recommended for fits.
6.7 Problems
1) Calculate the maximum & minimum clearance for the following fits. Take the value of deviations from tolerance chart .
20 h7/g6
20h7
ES = +21
EI = 0
Maximum hole size = 20.021
Minimum hole size = 20.000
20g6 es = -7
ei = -20
Maximum shaft size = 19.993
Minimum shaft size = 19.980
Minimum clearance : Minimum clearance exists when the shaft is made to its maximum size and
hole to its minimum size. i.e when shaft size 19.993 and hole size is 20.00 mm
Minimum clearance =20.000-19.993
= 0.007mm
Maximum clearance : Maximum clearance exists when the shaft is made to its minimum size and
hole its maximum size. i.e when shaft size is19.980mm and hole size is 20.021mm
Maximum clearance = 20.021-19.980
= 0.041mm
2) Calculate the maximum and minimum interference in the fit 20H7/p6
20H7 - ES = +.021
EI = 0
Maximum hole size =20.021mm
Minimum hole size =20.000mm
20p6 - es = +35
ei = +22
Maximum shaft size = 20.035mm
Minimum shaft size =20.022mm
Maximum interference = maximum shaft size - minimum hole size
= 20.035 -20.000 = 0.035mm
minimum interference = minimun shaft size - maximum hole size
= 20.022 - 20.021 = 0.001mm
3) Calculate the maximum and minimum interference and clearance in the fit 30H7/j6
30H7 - ES =+21
EI = 0
Maximum hole size =30.021mm
Minimum hole size = 30.000mm
30j6 es = +9
ei = -4
Maximum shaft size=30.009mm
Minimum shaft size = 29.996mm
Maximum interference = maximum shaft size- minimum hole size
= 30.009-30.000
= 0.009mm
Maximum clearance = maximum hole size - minimum shaft size
= 30.021- 29.996
= 0.025mm
7 Selective Assembly
Selective assembly is adopted to reduce the production cost of a job without sacrificing its quality. In selective assembly the components produced by a machine are classified into several groups according to size. This is done both for hole and shaft and then corresponding groups are matched.
Eg: If some part [shaft and hole] to be assembled are manufactured to normal tolerance of 0.01mm [and both are within curve of normal distribution] an automatic gauge can segregate them into ten different groups with in a 0.001mm limit for selective assembly of the individual parts.
Thus part with tolerance of 0.001mm is obtained and both the conditions of high quality and low cost can be served by selective assembly techniques.
A practical example of selective assembly is found in the production of ball bearings. The balls are sorted into groups according to their size, to facilitate the assembly of any bearing with balls of uniform size.
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