Difference Between Hole Basis System And Shaft Basis System

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Engineering fits are generally used as part of geometric dimensioning and tolerancing when a part or assembly is designed. In engineering terms, the ‘’fit’’ is the clearance between two mating parts and the size of this clearance determines whether the parts can, at one end of the spectrum, move or rotate independently from each other or at the other end, are temporarily or permanently joined together. Engineering fits are generally classified as hole basis system and shaft basis system.

Hole Basis System

In the hole basis system, the size of the hole is taken as basic hole i.e the nominal size and the limits on the hole are maintained constant and the shaft limits are varied to obtain the required fit. Mostly, the hole basis system is used because standard tools such as drills and reamers are used for producing the hole whose size is generally not adjustable.

In hole basis system, the hole is kept as the constant and the shaft upper and lower deviation values determine the type of fit. In hole basis system the lower deviation of the hole will be zero.

What You Need To Know About Hole Basis System

• In this system, the hole/bearing size is kept constant and the size changes to achieve the desired fit.
• The basic size of the hole is taken as low limit.
• In this system, Hole gets the letter H and the shaft gets letter O which decides the position of tolerance.
• The high limit of the size of the hole and the two limits of size of the shaft are selected to give the desired fit.
• The actual size of the hole is within the tolerance limit.
• The hole basis system is preferred to in mass production because it is convenient and less costly to make a hole of correct size due to availability of stand grills.
• The hole basis system is commonly used because it is easier to maintain a standard size of hole with the use of drills and reamers while the shaft size can easily be changed during the turning and grinding process.

Shaft Basis System

In shaft basis system, the shaft is kept as the constant and hole upper and lower deviation values determine the type of fit. In shaft basis system, the upper deviation of the shaft will be zero. This system is generally used in industries where shaft is important e.g propeller shafts and stepped shafts.

Generally hole basis system is preferred over shaft basis system because from a manufacturing perspective, it is easy to adjust the shaft dimension by using grinding or turning operation whereas holes are produced with drilling operations and it becomes complicated to do any further turning or grinding operations in holes.

What You Need To Know About Shaft Basis System

• In this system, Shaft size is kept constant and the hole/bearing size changes to achieve the desired fit.
• The basic size of the shaft is taken as one of the limits (maximum) of size of shaft.
• In this system, shaft gets the letter h and the hole gets a different letter o which decides the position of the tolerance zone to obtain desired fit.
• The limit of size of the shaft and two limits of hole are then selected to give the desired fit.
• The actual size of a hole that is within the tolerance limits is always less than the basic size.
• Shaft system is not suitable for mass production because it is inconvenient and time-consuming and costly to have a shaft of the correct size.
• It requires large capital, storage space because a large number of tools are required to produce holes of different size.