Positional tolerances in rotating components

Adhering to positional tolerances such as coaxiality is necessary for rotating components to run smoothly. The smallest deviations can already cause abrasion damage and failures during operation. Measuring devices check whether manufacturing tolerances have been adhered to.

Important facts about coaxiality

The technical terms coaxiality and concentricity belong to the group of positional tolerances. Yet they also restrict permissible form deviations. The coaxiality is very important for rotationally symmetric parts such as shafts, gears, crankshafts and camshafts. The coaxiality indicates how several components that are arranged along an axis behave in relation to the reference elements. It checks whether the deviations in form and position still meet the requirements. Tolerances and tolerance zones are defined in DIN EN ISO 1101.

As for example, several cylinders with different diameters are supposed to rotate around an axis. To get a better idea of coaxiality, imagine a narrow cylinder with a diameter of 0.3 mm. This cylinder with its axis matching the reference axis is the tolerance zone for the coaxiality. The axis of the tolerated cylinder has to be within this 0.3 mm tolerance zone. This is also valid, depending on the requirements, for every cylinder of the part. The coaxiality is considered out of tolerance if the axis and the reference axis are more than 0.15 mm apart.

Adhering to the maximum coaxiality is essential for the stability of the coaxial part. The camshaft may break during operation if the rotation axis of just one single part does not exactly match the total rotation axis. The part cannot be replaced if the axis of a bore hole deviates above the maximum value of the defined ideal position. Causes for exceeding the coaxiality are:

  • Faulty clamping of the workpiece
  • Yielding of the tool holder
  • Vibration of the machine

Application and industries

The coaxiality plays an important role in drive shaft manufacturing. Coaxial worm gears are manufactured by drive technology companies. Such gears serve as the drive of transportation systems. Camshafts and crankshafts are parts that are used in car manufacturing. Turbochargers consist of a continuous shaft with a mounting point each for a feed and a drive wheel. They must have an exact coaxiality because the shaft rotates at an extremely high speed. Rotors are components that are installed in machines and aggregates. The coaxiality is also important for their performance.

Coaxiality and manufacturing methods

Companies that manufacture coaxial workpieces with the smallest tolerances often use cylindrical turning on high-precision CNC turning machines. They manufacture radial, radially stepped and irregularly shaped shafts. The manufacturing of gears is a challenge for cylindrical turning. The pinions are milled out of the primary material crosswise to the axis. The smoothness of the drive shaft is affected if the milling depth and therefore the coaxiality is not correct. Adhering to the coaxiality also plays an important role when machining inner cylinders. The machining happens with special drilling and milling machines in which the workpiece is clamped during the process.

Checking coaxiality

Nowadays, the measurement of coaxiality happens almost only digitally. The quality assuring inspection is necessary to sift out the workpieces that require reworking or are declared scrap. The measurement of coaxiality is carried out with the help of measuring devices with optical or tactile sensor technology. In this way, several values can be measured in just one run or at different positions.

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