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Engineering Tolerance & Fits

As per ASME, Tolerance is the total amount a specific dimension is permitted to vary. e.g. a dimension representing as 25± 0.5 means that this dimension value can be in between 24.5 mm to 25.5 mm.

During mechanical product design, assembly tolerance stackup analysis is done to make sure part are getting assembled even in worst conditions.


Why Tolerance is required?

Interchangeability of manufactured parts is very critical. The production of closely mating parts, with very small tolerances is theoretically possible, but economically it’s unfeasible as this will increase rejection rate and high precise tools and workforce will be required. Increase in rejection rate and requirement of precise tool will increase overall product cost. This is the reason, deviation are provided in part dimensions

Greater accuracy will costs money, so it’s better to not give generous tolerance not the tightest possible tolerance.


Types of Tolerance

Unilateral Tolerance

In this system, the dimension of a part is allowed to vary only on one side of the basic size, i.e. tolerance lies only on one side of the basic size either above or below it

Unilateral system is preferred in interchangeable manufacture, especially when precision fits are required because:

  • with unilateral tolerance its easy and simple to determine deviations
  • With unilateral tolerance „Go‟ Gauge ends can be standardized as the holes of different tolerance grades have the same lower limit and all the shafts have same upper limit
  • Unilateral tolerance is helpful to operator, when machining mating parts. The operator machines to the upper limit of shaft (lower limit for hole) knowing fully well that he still has some margin left for machining before the parts are rejected.

Bilateral Tolerance

In Bilateral Tolerance, dimension of the part is allowed to vary on both the sides of the basic size. The limits of tolerance lie on either side of the basic size, but may not be necessarily equally dispose about it.

In Bilateral Tolerance, it is not possible to retain the same fit when tolerance is varied and the basic size of one or both of the mating parts are to be varied. This system is used in mass production when machine setting is done for the basic size.

 

 


Some general used terms

 

Maximum Material Condition (MMC) : Condition where a feature of a finished part contains the maximum amount of material. That is, the largest shaft or smallest hole.

e.g. MMC (Hole) = 9.5 mm, MMC (Shaft) =4.2 mm

 

Least Material Condition (LMC) : Condition where a feature of a finished part contains the least amount of material. That is, the smallest shaft or the largest hole.

e.g LMC (hole)=  10.5 mm, LMC (shaft) = 3.8 mm

 

Basic/Nominal Size : This is theoretical exact size from which limits of size are defined with the application tolerances.

For the shaft basic size is 4mm

 

Tolerance : Tolerance is allowable variation for any given size by which a given dimension may vary in order to achieve proper function of the product.

 

Max Clearance : The maximum amount of space that can exist between the hole and the shaft.

Max. Clearance = LMC (hole) – LMC (shaft)

 

Allowance : The minimum amount of space that can exist between the hole and the shaft.

Min. Clearance = MMC (hole) – MMC (shaft)


Fits

Fit usually refers to the clearances that are permissible between mating parts in a mechanical device that must assemble easily and that must often move relative to each other during normal operation of the device.


Clearance Fit

A clearance fit results in limits of size that assure clearance between assembled mating parts. A clearance fit has positive allowance.

There is minimum positive clearance between MMC of the shaft and LMC of the hole. For clearance fit size of shaft will always be less than the hole.

 

Interference fit (Force fit / Shrink fit)

Interference fit has limits of size that always result in interference between mating parts. For Interference fit size of shaft will always be larger than the hole. Interference fit has a negative allowance:  interference exists between the LMC of hole and LMC of the shaft.

In interference fit, the tolerance zone of the hole is always below that of the shaft. The shaft is assembled by pressure or heat expansion.

 

Transition fit

Transition fit can be a clearance or interference fit. In this case shaft may be either larger or smaller than the hole in a mating part. Transition fits are a compromise between clearance and interference fits.

Transition Fit are used where accurate location is important but either a small amount of clearance or interference is permissible. Transition fit has overlapping tolerance zones of the hole and shaft.


Systems of Fit

Basic Hole System

In basic hole system size of the hole is kept constant and shaft size is varied to obtain various types of fits.

The minimum hole diameter is considered as the basic diameter (basic size) from which the tolerance and allowance are applied.

Basic hole system is widely used in industry due to standard Drills, reamers being used to produce holes, and standard size plugs used to check hole sizes accurately.On the other hand size of the shaft produced by turning, grinding, etc. can be very easily varied

 

Basic Shaft System

In basic shaft system diameter of the shaft is kept constant and hole size is varied to obtain various types of fits.

In basic shaft system, the largest shaft is assigned the basic size diameter from which the allowance for the mating part is applied.

The basic shaft system is used for shafts that are produced in standard sizes. Tolerances are applied on both sides and away from the assigned allowance. 

 


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