LIMIT , FIT AND TOLERANCE

LIMIT , FIT AND TOLERANCE

LIMIT :

WHEN WE HAVE TO ASSEMBLE TWO MACHINE PARTS  ( FOR EXAM . NUT AND BOLT ) . THAN ACCURACY IN THE DIMENSIONS OF EACH PART IS NACESSARY . OTHERWISE IT WOULD BE VERY DIFFICULT ( SOMETIMES IMPOSSIBLE T0 ASSEMBLE THEM ).

                THE EXACT DIMENSION IS KNOWN AS BASIC OR NORMAL  SIZE.

UNFORTUNATELY , DUE  TO INACCURACY OF MANUFACTURING  METHODS IT IS ALMOST IMPOSSIBLE TO MAKE A PART WITH EXACT DIMENSIONS. HENSE IT IS NECESSARY FOR US TO ALLOW SOME DEVIATION IN THE DIMENSIONS OF MACHINE COMPONENTS.

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TYPE OF LIMIT ( OR DEVIATION ) :

1. MAXIMUM LIMIT  ( OR UPPER DEVIATON ) 
2. MINIMUM LIMIT  ( OR LOWER DEVIATION )

THE  MAXIMUM ALLOWED SIZE IS  KNOWN AS UPPER LIMIT AND MIINIMUM ALLOWED SIZE IS KNOWN AS LOWER LIIMITS.

TOLERANCE :

TOLERANCE IS THE DIFFERENCE BETWEEN MAXIMUM AND MINIMUM  

ALLOWBLE SIZE OF THE MANUFACTURED PARTS.

FOR EXAMPLE :

                                 A SHAFT OF EXACT 10.00 MM DIAMETER IS  DIFFICULT TO PRODUCE BY MACHINING PROCESS .BUT  IF YOU PROVIDE TOLERANCE , i.e. THE AMOUNT OF VARIATION PERMITTED IN THE SIZE , AND THEN SUCH PARTS CAN BE EASILY PRODUCED.

A dimension means a shaft may be produced between 10.05 and 9.95. These two figures represent limit and the difference, (10.05 – 9.95) = 0.10 is called tolerence.

TYPE OF  TOLERANCE :

1. UNILATERAL TOLERANCE
2. BILATERAL TOLERANCE

1. UNILATERAL TOLERANCE :  UNILATERAL TOLERANCE IS A TYPE OF TOLERANCE IN WHICH EITHER ONLY LOWER OR UPPER DEVIATION . FROM THE NORMAL SIZE IS ALLOWED .

                           +0.05                    +0.00

                           200                        300            

                           +0.00                    +0.02

   2.  BILATERAL TOLERANCE :   BILATERAL TOLERANCE IS A  TYPE OF 

        TOLERANCE IN WHICH BOTH UPPER ANDLOWER DEVIATION FROM THE 

         BASIC SIZE IS ALLOWED.

                    +0.05                          +0.08

        200                           300

                    -0.03                            -0.02

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ZERO LINE :

THE IMAGINARY STRAIGHT LINE  CORRESPONDING TO THE BASIC SIZE OF THE SHAFT OR HOLE   ,  WHICH IS USED TO MEASURE THE DEVIATION .

MANUALY TWO SYSTEM USED IN MANUFACTURE PARTS.

1. HOLE BASIS SYSTEM : HOLE  SIZE IS FIXED ( OR CONSTANT ) AND SHAFT SIZE IS VERY .
2. SHAFT BASIS SYSTEM : SHAFT SIZE IS FIXED ( OR CONSTANT ) AND HOLE SIZE IS VERY.

    FIT :  THE RELATIONSHIP BETWEEN THE ASSEMBLED PARTS DUE TO THE DIFFERANCE IN THEIR SIZE BEFORE ASSEMBLY ,IS KNOWN AS A FIT.

TYPE OF FITS :

1. CLEARANCE FIT
2. INTERFERENCE FIT
3. TRANSITION FIT

1. CLEARANCE FIT  : IN CLEARANCE FIT , UPPER LIMIT OF SHAFT DIMENSION IS ALWAYS LESS THAN LOWER LIMIT OF THE HOLE DIMENSION .IT ALWAYS PROVIDES A POSITIVE CLEARANCE BETWEEN THE HOLE AND SHAFT , OVER THE ENTIRE RANGE OF TOLERANCE. 
   
2. INTERFERENCE FIT :IN INTERFERENCE FIT  , LOWER LIMIT OF SHAFT DIMENSION IS ALWAYS MORE THAN UPPER LIMIT OF HALE DIMENSION . IT ALWAYS PROVIDES INTERFERENCE OVER THE ENTIRE RANGE OF TOLERANCE .
   
3. TRANSITION FIT : TRANSITION FIT CAN EITHER BE A CLEARANCE FIT OR INTERFERENCE FIT. IT DEPENDS ON ACTUAL VALUES OF THE DIMENSION .
   

CLASSIFICATION  OF FIT :

1. CLEARANCE FIT : THE HOLE SIZE IS  ALWAYS GREATER THAN SHAFT SIZE .  WHEN VARIATION IN HOLE SIZE AND SHAFT SIZE. MINIMUM SIZE OF HOLE IS ALWAYS GREATER THAN MAXIMUM SIZE OF SHAFT. WHEN  ASSEMBLED TWO PART HOLE AND SHAFT .IT IS ASSEMBLE SLIDELY WITHOUT ANY FORCE.
2. INTERFERANCE FIT :  HOLE TOLERANCE ZONE AND SHAFT TOLERANCE ZONE ARE BOTH OVERLAP.                                                                                                                                                           1. WHEN HOLE SIZE IS MAXIMUM  AND SHAFT SIZE IS MAXIMUM . THEN SHAFT SIZE IS LESS THAN HOLE SIZE . SHAFT IS ASSEMBLED EASLY.                                                                                                                          Y2.  WHEN HOLE SIZE IS MINIMUM AND SHAFT SIZE IS MAXIMUM . THEN SHAFT  SIZE IS GREATER THAN HOLE . SO THE FORCE IS REQUIRED FOR ASSEMBLED SHAFT SHAFT IN HOLE.                                                                                                  
3. TRANSITION FIT :  TRANSITION  FIT IS NEITHER LOOSE NOR TIGHT AS LIKE CLEARANCE FIT AND INTERFERANCE FIT . THE TOLERANCE ZONE OF THE SHAFT AND THE HOLE WLL BE OVERLAPED BETWWEN THE INTERFERANCE AND CLEARANCE FITS.