Limits, Tolerances, Fits, and Terminology
Limits and tolerances refer to the allowable variations in size and geometry of a part or assembly. They are used to ensure that parts fit together correctly and function as intended.
Fits refer to the specific relationship between the size of one part and the size of the mating part it is designed to connect to. There are several types of fits, including clearance fit, interference fit, and transition fit.
Clearance fit: A clearance fit occurs when the hole is larger than the shaft. This type of fit allows for easy assembly and disassembly and provides room for expansion and contraction due to temperature changes.
Interference fit: An interference fit occurs when the shaft is larger than the hole. This type of fit is used to create a tight connection that will not come apart easily.
Transition fit: A transition fit is a combination of clearance and interference fit. This type of fit is used when the hole size is between the maximum and minimum size of the shaft.
Terminology
The terminology used in fits and tolerances is shown in Fig. 1.3. The important terms are:
Basic size: It is the exact theoretical size arrived at by design. It is also called nominal size.
Actual size: The size of a part as may be found by measurement.
Maximum limit of size: The greater of the two limits of size.
Minimum limit of size: The smaller of the two limits of size.
Allowance: It is an intentional difference between maximum material limits of mating parts. It is a minimum clearance or maximum interference between mating parts.
Deviation: The algebraic difference between a size (actual, maximum, etc.) and the corresponding basic size.
Actual deviation: The algebraic difference between the actual size and the corresponding basic size.
Upper deviation: The algebraic difference between the maximum limit of size and the corresponding basic size.
Lower deviation: The algebraic difference between the minimum limit of size and the corresponding basic size.
Zero line: It is the line of zero deviation and represents the basic size.
Tolerance zone: It is the zone bounded by the two limits of size of the parts and defined by its magnitude, i.e., tolerance, and by its position in relation to the zero line.
Fundamental deviation: That one of the two deviations which is conveniently chosen to define the position of the tolerance zone in relation to the zero line, as shown in fig. 1.4.
Basic shaft: A shaft whose upper deviation is zero.
Basic hole: A hole whose lower deviation is zero.
Clearance: It is the positive difference between the hole size and the shaft size.
Maximum clearance: The positive difference between the maximum size of a hole and the minimum size of a shaft.
Minimum clearance: The positive difference between the minimum size of a hole and the maximum size of a shaft.
Hole Basis and Shaft Basis for Fits:
1. Hole basis system: In this system, the different clearances and interferences are obtained in associating various shafts with a single hole, whose lower deviation is zero.
2. Shaft basis system: In this system, the different clearances and interferences are obtained in associating various holes with a single shaft, whose upper deviation is zero.
No-Go Gauge and Go Gauge
A No-Go gauge and a Go gauge are both types of inspection tools used to ensure that a manufactured part meets the required specifications.
A No-Go gauge is used to check if a part is too large or too out of tolerance in a particular dimension. It is a gauge with a maximum size limit, and if the part being inspected is larger than the gauge, it will not fit, indicating that the part is out of tolerance and should be rejected.
A Go gauge, on the other hand, is used to check if a part is within tolerance for a particular dimension. It is a gauge with a minimum size limit, and if the part being inspected fits within the gauge, it indicates that the part is within tolerance and can be accepted.
Both No-Go and Go gauges are used together to ensure that a part is within the required tolerance for a particular dimension. The No-Go gauge is used to check for parts that are too large, and the Go gauge is used to check for parts that are too small. Together, they ensure that parts are within the acceptable tolerance range and are fit for use.
In summary, No-Go gauges are used to check if a part is too large or too out of tolerance in a particular dimension, while Go gauges are used to check if a part is within tolerance for a particular dimension. They are used together to ensure that parts are within the acceptable tolerance range and are fit for use.
