Limit Fits and GaugesCh – 4

MEC 263Arijit SenFaculty

Department of Mechanical EngineeringIUBAT

Limits fits and tolerances

• Due to inevitable inaccuracy of manufacturing method it is not possible to make any part precisely to the given dimension and it can be made to lie between two limits maximum and minimum.

• The difference between those two are called permissible tolerance.

• If the difference is in one side then it is called unilateral tolerance and if it is in both positive and negative side then it is called bilateral tolerance.

Limits fits and tolerances

• Generally in Engineering, any component manufactured is required to fit or match with some other component. The correct functioning of the two component in match depends on the correct size relationship between the two.

• if the shaft rotate inside a hole there should be enough clearance.

• If the system required no relative movement between shaft and hole then should be enough interference between shaft and whole so that the force of elastic compression can grip them.

Why Tolerance are specified?

• Variation in the properties of materials being machined introduce error.

• The production machine themselves have some inherent variance and have the limitation to produce accurate parts.

• It is impossible for an operator to make perfect setting.

Different ways of expressing tolerance

• Specifying limiting dimension– Diameter of the hole: 25.00 to 25.02 mm

– Diameter of the shaft: 24.99 to 24.97 mm

• Unilateral or one limiting dimension.– Diameter of the hole: 25.00 +0.02 mm

– Diameter of the shaft: 24.99 – 0.02 mm

• Bilateral or both side tolerance– Diameter of the hole: 25.01 ± 0.01mm

– Diameter of the shaft: 25.00 ±0.01mm

Fits

• Clearance fit: maximum shaft size is smaller than minimum hole size

• Transition fit: maximum shaft is larger than minimum hole size but smaller than maximum hole size

• Interference fit: minimum shaft size is larger than maximum hole size

Selective Assembly

Basic Hole Shaft System

End View

Basic Hole Shaft System

Terminology

Minimum Shaft Diameter

Maximum Shaft Diameter

Minimum Hole Diameter

Maximum Hole Diameter

Tolerance Zone

Zero Line

Ref: R. K. Jain, Page 271

Terminology

• Basic Size

• Actual Size

• Limits of Sizes

• Maximum Limit of Size

• Minimum Limit of Size

• Deviation /Actual Deviation

• Limit Deviation

• Upper deviation

• Lower Deviation

• Fundamental Deviation

• Zero Line

Ref: R. K. Jain, Page 277

Ref: R. K. Jain, Page 279

Different Formulae• Table 4.2 : Formulae for fundamental deviations of

shafts sizes upto 500mm

• Table 4.2 A: Formulae for fundamental deviations of holes sizes upto 500mm

• Table 4.5 : Formulae for fundamental deviations of shaft and hole sizes above 500mm and upto 3150 mm.

• Table 4.1A : Formulae for tolerance grade of shaft and hole sizes upto 500mm

• Table 4.1B: Formulae for fundamental deviations of shaft and hole sizes above 500mm and upto 3150 mm

• Diameter Step: R. K. Jain. Page 276, 2nd paragraph

Problem

• Find out the maximum and minimum diameter for a 25mm shaft and hole pair designated H8d9.

Problem

• Find out the maximum and minimum diameter for a 50mm shaft and hole pair designated C8ef9. Assume D<40.

Plug Gauge and Ring Gauge

Taylor’s Principle

• According to Taylor’s principle ‘Go’ and ‘No Go’ gauges should be designed to check maximum and minimum material limits which are checked below:

• Go Limit: is applied to that limit of the two limits of size which corresponds to the maximum material limit consideration, i.e. upper limit of a shaft and lower limit of a hole.

• No Go Limit: is applied to that limit of the two limits of size which corresponds to the minimum material limit consideration, i.e. lower limit of a shaft and upper limit of a hole.