Understanding Limits, Fits, and Tolerances in Engineering: A Comprehensive Guide, Schemes and Mind Maps of Engineering

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Limits & Fits

Explain the necessity for tolerancing.

Identify the symbols used for fundamental deviation.

Identify the numbers used for grades of tolerances.

Select fits for mating components.

Translate limits and fits symbols to engineering drawings usingBS4500A.

At the end of this presentation, the

students should be able to :

Lower limit =



27. 8

Upper limit =



28. 2

Shaft

2. Limits (extreme sizes of a part)

Tolerancezone

Tolerance = UL – LL= 28.2 – 27.8= 0.4Or



0.

Ø Zero line (Basic)

Limits of Size

Unilateral Limits

occurs when both maximum limit and minimum limit are

either above or below the basic size. Upper

its mtii LimerL Low

Basic Size

e.g. Ø

+0.18 +0.

Basic Size

= 25.00 mm

Upper Limit = 25.18 mmLower Limit = 25.10 mmTolerancee.g. Ø

-0.10 -0.

Basic Size

=

0.08 mm

Basic Size

Upper Limits

Lower Limit

= 25.00 mm

Upper Limit = 24.90 mmLower Limit = 24.80 mmTolerance

=

0.10 mm

3. Fits(assembly condition between “Hole” & “Shaft”)

Hole – Shaft –

A feature engulfing a componentA feature being engulfed by a component

Hole

Shaft

Min C

Max C

Clearance Fits

Tolerance zones

never meet

Max. C = UL of hole - LL of shaftMin. C = LL of hole - UL of shaft

Shaft

Max I

Hole

Max C

Transition Fits

Tolerance zones

always overlap

Max. C = UL of hole - LL of shaftMax. I = LL of hole - UL of shaft

Terminology Related to Limits and Fits

Main Menu

4a. Fundamental Deviation

is chosen to locate the tolerance zone w.r.t. the zero line

Holes are designated by capital letter:

Letters A to G - oversized holesLetters P to ZC - undersized holes Shafts are designated by small

letter:

Letters m to zc - oversized shaftsLetters a to g - undersized shafts

H is used for holes and h is used for shafts

whose fundamental deviation is zero

4b. Grades of Tolerances

Grades of Tolerances

Cost

• Grade is a measure of the

magnitude of the tolerance.

• The lower the grade the finer the

tolerance.

• There are total of

18 grades

which are allocated the numbersIT01, IT0, IT1, IT2 ..... IT16.

• Fine grades are referred to by the

first few numbers.

• As the numbers get larger, so the

tolerance zone becomesprogressively wider.

• Selection of grade should depends

on the circumstances.

• As the grades get finer, the cost of

production increases at a sharperrate.

Basis of Fits - Shaft Basis

Basic Size

I

HoleShaftTolerance

C - ClearanceT - Transition I- Interference

•Here the hole size is varied toproduce the required class of fit with abasic-size shaft.• A series of drills and reamers is

required for this system,therefore it tends to be costly.

• It may, however, be necessary to

use it where different fits arerequired along a long shaft. Forexample, in the case of drivingshafts where a single shaft mayhave to accommodate to a varietyof accessories such as couplings,bearings, collars, etc., it ispreferable to maintain a constantdiameter for the permanentmember, which is the shaft, andvary the bore of the accessories.

T

Shaft Basis Fits

Legends:

Selected ISO Fits- Hole Basis {Table 1.24(a) on Pg 56/57}

The ISO system provides many holes and shaft tolerances soas to cater for a wide range of conditions.

• The following selected hole and shaft tolerances have been

found to be commonly applied:

Selected hole tolerances:

H7, H8, H9, H

Selected shaft tolerances: c11, d10, e9, f7, g6, h6, k6, n6, p6, s6 • Data sheet 4500A shows a range of fits derived from these

selected hole and shaft tolerances.

• It covers fits from loose clearance to heavy interference and are

suitable for most general engineering applications.

• This data sheet covers all basic sizes up to 500 mm.

Identify fitting conditions fromFundamental deviation. (e.g 30 H7/ g6)

Convert from F.D to limits of tolerancefor hole and shaft. (e.g 100



0.5 )

Calculate max. & min. limit of size ofhole and shaft.

Max./ Min. Clearance or Interference

(for Transition, we have max. clearance and max interference)



You must know….

6. Application of Tolerances to Dimensions - Tolerances should be specified in the case where a dimension

is critical to the proper functioning or interchangeability of acomponent.

• A tolerance can also be supplied to a dimension which can

have an unusually large variation in size.

• General tolerances are generally specified as a note at the

bottom of the drawing.