Limits, Tolerances And Fits

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IPE 301Limits, Tolerances and Fits

Basics

IPE-301 (Measurement)

Generally in engineering, any component manufactured is required to fit or match with some other component.

The correct and prolonged functioning of the two components in match depends upon the correct size relationships between the two, i.e., the parts must fit with each other in a desired way.

If a shaft is to rotate in a hole, there must be enough clearance between the shaft and hole to allow the oil film to be maintained for lubrication.

If the clearance is too small, excessive force would be required in rotation of shaft.

If clearance is too wide, there would be vibrations and rapid wear and ultimate failure

Basics


Impossible to achieve dimensional perfection due to temperature, tool wear, vibrations, human error.

Lot of time and cost of manufacturing will be high, to maintain very close degree of accuracy.

Accuracy

Cost

Terminology


Shaft: Not only the diameter of the circular shaft but also to any external dimension on a component.

Hole: Not only the diameter of the circular hole but also to any internal dimension on a component.

Basic Size or Basic dimension: It is the theoretical size from which limits of size are derived by the application of allowances and tolerances.

Actual Size: The measured size of the finished part.

Limit of size: Maximum and minimum permissible sizes of part.

Terminology


Terminology


Maximum Limit: Maximum limit or high limit is the maximum size permitted for the part.

Minimum Limit: Minimum limit or low limit is the minimum size permitted for the part.

Tolerance: Difference between maximum and minimum limit of size.

Zero line: Line of zero deviation and represents the basic size.

Allowance: It is the intentional difference between the Maximum Material Limit(MML) of mating parts. For shaft, the maximum material limit will be its high limit and the for the hole the maximum material limit will be its low limit

Terminology


Terminology


Deviation: It is the algebraic difference between a size and corresponding basic size

Upper Deviation: The algebraic difference between the maximum limit of size and the corresponding basic size. Positive quantity when the maximum limit of size is greater than basic size and negative for vice versa.

Lower Deviation: The algebraic difference between the minimum limit of size and the corresponding basic size. Positive quantity when the minimum limit of size is greater than basic size and negative for vice versa.

Fundamental Deviation: It is equal to either upper or lower deviation which is closer to the zero line.

Terminology


Terminology


Terminology


Unilateral limit: both the limits of the size are on the same side of the zero line. One of the limits may be basic size.

Bilateral limit: One of the limits is one side and the other at the other side of zero line.

Unilateral system is more satisfactory and realistically applied where it is common knowledge that dimensions will most likely to deviate in one direction. Ex: Drilling a standard size hole, the drill most likely to produce oversize hole, than undersize hole.

Terminology


Fits


Fit is the general term used to signify the range of tightness or looseness that may result from the application of a specific combination of allowances and tolerances in mating parts.

Three types of fit

Clearance Fit

Interference Fit

Transition Fit

Movable Joint (Clearance): Shaft running in a bush

Fixed Joint (Interference): Pulley mounted on a shaft

Fits


In clearance fit shaft is always smaller than hole. Positive allowance between largest possible shaft and smallest possible hole.

Minimum clearance: Difference between maximum shaft and minimum hole.

Maximum clearance: Difference between minimum shaft and maximum hole.

Types of Clearance Fit:

Slide Fit

Easy Slide Fit

Running Fit

Slack Running Fit

Loose Running Fit

Fits


In Interference fit shaft always larger than the hole. Tolerance zone of shaft entirely above the hole.

Minimum interference: difference between maximum hole and minimum of shaft.

Maximum interference: difference between minimum hole and maximum of shaft

In Transition fit tolerance varies between clearance and interference fits. Tolerance zone of shaft and hole overlap.

Types of Transition Fit:

Force Fit

Tight Fit

Wringing Fit

Push Fit

Interference fit

Transition fit

Hole and Shaft System


SHAFT BASED SYSTEM

Size of shaft is kept constant,

hole size is varied

to get different fits.

Basic size = High limit of shaft

Shaft h

HOLE BASED SYSTEM

Size of hole is kept constant,

shaft size is varied

to get different fits.

Basic size = Low limit of hole

Hole H

Tolerance


Tolerance can be defined as the magnitude of permissible variation of a dimension or other measured or control criterion from the specified value.

The primary purpose of providing tolerances is to permit variation in dimensions of shaft or hole without degradationof the performance beyond the limits established by the specification of the design.

For Example a dimension given as 1.625 .002 means that the manufactured part may be 1.627 or 1.623, or anywhere between these limit dimensions.

Tolerance


The amount of tolerance depends on:

The function of the product (the allowance desired in the fit)

Manufacturing process available

Cost of production and assembly.

Cost of production or assembly decrease with increase of tolerance:

Need for some sophisticated machinery

Higher skill required

Increased attention to inspection and handling

Tolerance assignment balanced between cost and quality.

.

Tolerance


Tolerance can be based on the two methods of analysis:

Complete interchangeability

Statistical assembly

Complete interchangeability means that all parts will assemble even though the parts are at their extreme limits. There should be no rejected assembly for improper tolerance.

Statistical assembly methods considers the probability that the dimensions will not occur so frequently at or near the extreme limits as between or near the centre of these limits.

Tolerance


Complete Interchangeability:

It is necessary to chose one direction as positive and the other as negative. The sum of distance is zero.

In order to determine the tolerances of the individual components, it is necessary to consider the tolerance of entire assembly.

Tolerance


Statistical Assembly:

A well controlled machinery operation will not produce many parts with exactly extreme limits dimension.

Frequency of occurrence follow the normal distribution (Mean and Standard Deviation)

Symmetrical about its vertical axis

About 99.73% values fall under the curve and only 0.27% are expected to fall beyond these limits.

Tolerance


A hole and mating shaft have a nominal assembly size of 38mm. The assembly is to have a maximum total clearance of 0.15m and a minimum total clearance of 0.05mm. Determine the specifications of the parts (a) for 100% interchangeability and (b) for statistical average interchangeability.

Mean Value of Clearance

X clearance = X bearing - X shafts

Standard Deviation of the Variation Of Clearance

Tolerance


Tolerance


100% interchangeability

BD + DE = BEBD = BE DE

DE = Half of minimum clearance = 0.05/2 = 0.025mm

BE= Half of maximum clearance = 0.15/2 = 0.075mm

BD = Tolerance of shaft = 0.075 0.025 = 0.05 mm

Maximum limit of shaft = 38 0.025 = 37.975mm

Minimum limit of shaft = 37.975 0.05 = 37.925mm

Similarly, FH = HE EF = Tolerance of bearing = 0.075 0.025 = 0.05 mm

Minimum limit of bearing = 38 + 0.025 = 38.025mm

Maximum limit of bearing = 38.025 + 0.05 = 38.075mm

Tolerance


Statistical interchangeability:

6clearance = Max total clearance Min total clearance = 0.15 0.05 = 0.1mm

clearance = 0.0167mm

bearing = shaft = P = 0.0118mm

tolerance for bearing or shaft = 6P = 0.0707mm

Maximum limit of shaft = 38 0.025 = 37.975mm

Minimum limit of shaft = 37.975 0.0707 = 37.904mm

Minimum limit of bearing = 38 + 0.025 = 38.025mm

Maximum limit of bearing = 38.025 + 0.0707 = 38.0957mm

Almost 41.42% more tolerance than 100% interchangeability and only 3out of 1000 can be expect not in tolerance zone. So manufacturing cost will be less, but not suitable for large quantities.

Tolerance


ISO (International Organization for Standardization):

ISO systems cover holes and shafts from the smallest size up to 3150 mm.

For different fits, series of tolerance grades available for different range.

Hole and shaft basis system can be used.

28 different holes and shafts for any basic sizes

28 Holes: A, B, C, D,, Z, Za, Zb, Zc

28 Shafts: a, b, c, d,.., za, zb, zc

18 grades of tolerance IT0, IT01, IT2,.IT15, IT16 tolerance grades decides the accuracy for manufacture.

Tolerance


H : lower deviation of hole is zero

h : upper deviation of shaft is zero

E.S. upper deviation

E.I. lower deviation

The selection of letter freezes one limit of hole / shaft

(how much away from Basic size)

45 E8/e7

One can have different possible combinations; eg. 45H6g7, 45H8r6, 45E5p7

Calculation of Tolerance Grades


Tolerance grades is multiplier of standard tolerance unit i ,

Where, D = (Geometric mean diameter steps in mm)

i = 0.004 D + 2.1 (m)

Up to 500 mm

Above 500 mm





Calculate limits of tolerance and allowance for 25H8/d9.

Steps: 1-3, 3-6, 6-10, 10-18, 18-30, 30-50, 50-80, 80-120.

Limits, Tolerances And Fits

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