Wednesday, 29 May 2019

Concentricity in GD&T

Concentricity in GD&T


Concentricity:- Concentricity is indicated by two concentric circles. Concentricity also known as coaxially. It controls tolerance of central axis of the referenced feature, to a datum axis.
*Concentricity describes a condition in which two or more feature, in any combination, have a common axis.

The need for this symbol has been challenged, but it is still used to specify concentricity. Care should be taken to avoid the use of this symbol if the needed control can be position or runout tolerances.


Concentricity is a 3-Dimensional cylindrical tolerance zone that is defined by a datum axis where all the derived median points of a referenced circular feature must fall into. the median points of the reference surface cross sections form the theoretical axis that must be in this tolerance zone.

When Used:



Due to its complex nature, Concentricity is usually reserved for parts that require a high degree of precision to function properly. Transmission gears, which need to always be coaxial to avoid oscillations and wear, may require concentricity to ensure all the axes line up correctly. Equal mass or inertial concerns are one of the leading causes for the concentricity callout. Any application where the median points of a feature need to be controlled relative to a datum would require cylindricity. However in many cases, the use of runout or true position can replace the need for concentricity and be much easier to measure for.

Example:

An intermediate shaft in a transmission is composed of two different diameter sections which are coaxial. Datum A is the drive side and relatively fixed with bearings to the housing, The referenced surface B is desired to be concentric with Datum A to avoid oscillations at high speed.



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