Tolerance and Fits

Tolerance and Fits - Tolerance and Fits Chapter 13 Material taken from Mott 2003 Machine Elements in Mechanical Design Tolerances and Fits The term

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1 Tolerance and Fits Chapter 13 Material taken from Mott, 2003, Machine Elements in Mechanical Design . Tolerances and Fits The term tolerances refers to the permissible deviation of a dimension from the specified basic size. The proper performance of a machine can depend on the tolerances specified for its parts, particularly those that must fit together for location or for suitable relative motion. Tolerances and Fits The term 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. Such fits are usually called running or sliding fits.
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2 Tolerances and Fits Fit can also refer to the amount of interference that exists when the inside part should be larger than the outside part. Interference fits are used to ensure that mating parts do not move relative to each other. Factors Affecting Tolerances and Fits Consider the plain surface bearings. A critical part of the design is the specification of the diametrical clearance between the journal and the bearing. The typical value is just a few thousandths of an inch. But some variation must be allowed on both the journal outside diameter and the bearing inside diameter for reasons of economy of manufacture. Factors Affecting Tolerances and Fits con’t There will be variation of the actual clearance in production devices, depending on where the individual mating components fall within their own tolerance bands. Too small a clearance could cause seizing. Conversely, too large a clearance would reduce the precision of the machine and adversely affect the lubrication.
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3 Tolerances, Production Processes, and Costs A unilateral tolerance deviates in only one direction from the basic size. A bilateral tolerance deviates both above and below the basic size. The total tolerance is the difference between the maximum and minimum permissible dimensions. Tolerances, Production Processes, and Costs con’t The term allowance refers to an intentional difference between the maximum material limits of mating parts. For example, a positive allowance for a hole/shaft pair would define the minimum clearance between mating parts from the largest shaft mating with the smallest hole. A negative allowance would result in the shaft being larger than the hole (interference). Tolerances, Production Processes, and Costs con’t The term fit refers to the relative looseness (clearance fit) or tightness (interference fit) of mating parts, especially as it affects the motion of the parts or the force between them after assembly. Specifying the degree of clearance or interference is one of the tasks of the designer.
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4 Tolerances, Production Processes, and Costs con’t It is costly to produce components with very small tolerances on dimensions. It is the designer’s responsibility to set
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Tolerance and Fits - Tolerance and Fits Chapter 13 Material taken from Mott 2003 Machine Elements in Mechanical Design Tolerances and Fits The term

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