Upper deviation
What is an upper deviation?
The upper deviation is a concept used in measurement and manufacturing technology, which specifies the difference between the maximum permitted size of a workpiece (the upper limit size) and the basic size (nominal size). This measurement is a key part of the tolerance system and helps ensure that the manufactured workpiece dimensions remain within the defined limits. The upper deviation (denoted as ES for holes and es for shafts/pins) defines the difference between the upper limit size and the nominal size.
Tolerances and basic concepts
- Nominal size: The size of a perfectly shaped element defined by a drawing, used for setting limit sizes.
- Actual size: The real measured size of an element.
- Deviations (upper and lower deviation): Indicate how much the actual size is allowed to differ from the nominal size. Upper deviation (ES, es): The difference between the upper limit size and the corresponding basic size.
- Lower deviation (EI, ei): The difference between the lower limit size and the corresponding basic size.
- Upper limit size: The maximum permitted size of an element.
- Lower limit size: The minimum permitted size of an element.
- Tolerance: The difference between the upper and lower limit sizes.
- Tolerance range: Defines the different values a size can take between the tolerance limits, including the limits themselves.
- Fundamental deviation: In the ISO system, the deviation that determines the position of the tolerance range with respect to the nominal size.
- Tolerance grade: In the ISO system, a group characterizing precision for various nominal sizes (e.g., IT7).
ISO tolerance system
For manufacturing parts measured in the metric system, ISO 286:1988 tolerance standards are commonly used. These tolerances are used to describe the fits and compatibility between parts. Tolerance groups are defined from A to ZC, and precision grades (IT) range from 1 to 18. In these standards, uppercase letters are used for holes and lowercase letters for shafts. The letter indicates whether the hole or shaft needs to be larger or smaller than the nominal value, and the number represents the permissible range of variation. For example, an H7 tolerance for a 3 mm hole means the diameter must be 3 mm +0.010/-0 mm, while an h7 tolerance for a shaft means its diameter must be 3 mm -0.010/+0 mm.
Fits
- Fit: A property determined by the difference in dimensions of two elements to be joined (e.g., a shaft and a hole) before assembly.
- Interference fit: An interference where the shaft diameter is greater than the hole diameter after assembly.
- Clearance fit: A clearance created when the shaft diameter is smaller than the hole diameter after assembly.
- Transition fit: A situation where, depending on the actual sizes of the hole and shaft, the fit may result in either clearance or interference.
Practical example of upper deviation
For example, if a workpiece's nominal size is 50 mm and the upper deviation is +0.020 mm, the upper limit size is 50.020 mm. This means the component must not be larger than 50.020 mm to meet quality standards and be compatible with other parts.
Summary
The upper deviation is critical in component manufacturing because it specifies the maximum allowed size of a workpiece relative to its nominal size. This concept helps control the accuracy of components and ensures compatibility. The upper deviation, together with other tolerance concepts such as nominal size, actual size, upper and lower deviation, and fits, forms the foundation for a precise and reliable manufacturing process across various industries.
