Limits, fit and tolerance are used to ensure product assembly on assembly line. It is not possible to manufacture 100% accurate parts. Therefore manufacturer always try to approach for true value that are within tolerance limits.
Limits, tolerance and fit in a part provides flexibility to manufacturer .
Following terms and definitions are important to understand Limits, Fit and Tolerances.
This is theoretical exact size from which limits of size are defined. In Example-1 shaft basic size is 4 mm.
It is manufactured part actual measured size. It lies in between upper and lower size limits.
Maximum Material Condition (MMC)
MMC is a condition where a feature of a finished part contains the maximum amount of material. Or largest shaft or smallest hole.
In Example-1 : MMC (Hole) = 9.5 mm and MMC (Shaft) =4.2 mm
Least Material Condition (LMC)
LMC is a Condition where a feature of a finished part contains the least amount of material. Or the smallest shaft or the largest hole.
In Example-1 : LMC (hole) = 10.5 mm and LMC (shaft) = 3.8 mm
Maximum amount of space available between the hole and the shaft.
In Example-1 : maximum clearance is (10.2 – 3.9) mm
Max. Clearance = LMC (hole) – LMC (shaft).
Allowance is the minimum amount of space available between the hole and the shaft. In another words we can say, allowance is minimum clearance.
Allowance (Min. Clearance) = MMC (hole) – MMC (shaft)
Limits Of Size
It is extreme permissible sizes. Actual Dimension of measured part must lie within these limits. In Example-1, shaft basic size is 4 mm and its upper and lower tolerance is +0.2 mm and -0.1 mm respectively. Therefore Shaft Upper limit is 4.2 mm and lower limit is 3.9 mm.
As per ASME, Tolerance is the total amount a specific dimension is permitted to vary. For example, If a dimension represented as 25± 0.5 . It upper and lower tolerance is 0.5 mm.
Tolerance stack-up analysis is used to ensure parts assembly is acceptable even in worst conditions.
Why tolerance is Required?
To ensure product assembly. Manufacturer ensure all parts are manufactured within design limit. Because interchangeability of manufactured parts is very critical.
Production of closely mating parts, without tolerances is economically unfeasible. It results in increased rejection rate. As a result, overall product cost will increase. Therefore, tolerance ensure low rejection rate and smooth product assembly.
Type of Tolerances
Engineering tolerances are further classified in two categories:
- Unilateral Tolerance
- Bilateral Tolerance
In unilateral tolerance Dimension of a part is allowed to vary only on one side of the basic size. Therefore tolerance lies only in one side of the basic dimension. For example, hole basic size is 20 mm and its value can vary from 19.85 mm to 20 mm.
- Unilateral system is used to define precision fits.
- Unilateral tolerance are easy to control in machining parts. Operator machines part to upper limit of shaft or lower limit for hole. As a result operator still has some margin left for further correction.
In bilateral tolerance Dimension of the part is allowed to vary on both sides of the basic size. Therefore limits of tolerance lie on both sides of the basic dimension.
For example hole basic size is 100 mm and it’s value can vary from 19.8 mm to 20.1 mm.
- Bilateral Tolerance system is used in mass production.
- In this system, operator always target to basic size.
Engineering fits are used as part tolerances in an assembly. They defines the clearance between two mating parts. This clearance value determines the type of Fit.
Types of Fits
Engineering fit can be divided in three types.
- Clearance Fit
- Interference Fit
- Transition Fit
In clearance fit, size of hole is always larger than size of shaft. Therefore clearance fit ensures always a clearance between assembled mating parts. Clearance fit has positive allowance.
Examples of clearance fit are door hinges, shaft sliding inside a bearing, Bearing in ceiling fan, electrical switch and board e.t.c
Interference (Force/ Shrink) Fit
In Interference fit, size of shaft is always larger than size of hole. Therefore Interference fit ensures interference between assembled mating parts. Clearance fit has negative allowance.
Examples of interference fit are : push fit inserts in plastic part, Axle and bearing e.t.c
Transition fit is either a clearance or interference fit. Therefore In transition fit, shaft may be either larger or smaller than the hole in a mating part. Transition fits is a compromise between clearance and interference fits.
Transition fit has overlapping tolerance zones of the hole and shaft. Therefore they are used where accurate location is important. But small amount of clearance or interference is permissible.
Systems of Fit
To obtain a required type of fit, Basic hole system and basic shaft system is used. Selection of type of system is done based on availability of standard parts and process used.
Basic Hole System
In basic hole system, size of the hole is kept constant and shaft size is varied to obtain required fit.
e.g Minimum hole diameter is considered as the basic diameter (basic size) from which the tolerance and allowance are calculated.
Basic hole system is widely used because standard Drills, reamers are used to produce holes. On the other hand, size of the shaft produced by turning, grinding can be modified.
Basic Shaft System
In basic shaft system diameter of the shaft is kept constant and hole size is varied to obtain required fit.
e.g largest shaft diameter is considered as basic diameter from which tolerances and allowance are calculated.
The basic shaft system is used for standard size shafts.
To sum up, Engineering tolerance is very important and critical part of product design. Products can not be manufactured without tolerances. You can also use tolerance stackup calculator to define tolerance.
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