Shafting properties Training

Shafting
Design and Theory
Shaft Properties

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Design and Theory

While on the surface, shafts may appear the same, there are significant performance differences due to the manufacturer’s selected standards and the manufacturing processes used to achieve them.

Roundness

Shaft roundness is vital for linear applications such as spindles and guide rods where accuracy, life or precision is paramount. Roundness ensures uniform distribution of bearing loads for maximized bearing life, longer travel life and improved positional accuracy.

Round shafts can be deceiving to the eye and appear anything but round when properly evaluated using precision tracing techniques.

Two plots of shafts with different roundness tolerances

Surface finish

Surface finish is a key factor affecting travel life, load levels, frictional resistance and smoothness of travel. Industry leading cataloged surface finish is of 8 Ra max. Excellent surface finish and hardness maximize the efficiency and life of linear bearings, shaft riding seals and overall visual appearance.

Exaggerated view of a ball bearing on different surfaces

Straightness

Straightness is the most vital parameter to positioning accuracy for a Linear guide system. The image below shows a typical method for measuring straightness. With the shaft supported at two points 0.2 times the length from the ends, the shaft is rotated and the total deviation indicated is recorded. Beware that even if a shaft is straight when it is made, handling or machining of shafting can cause the material to bend once it leaves the factory. Care to preserve or restore shaft straightness before installation may be necessary.

Checking the straightness using dial indicators

Cylindricity

Cylindricity is a measure of the degree of conformance of the outside surface (diameter along the length of the shaft) to a true cylinder. True conformance (high cylindricity) ensures the benefits of roundness, diameter and straightness are present over the shaft length, or working surface, and not just in a particular location. This ensures uniform distribution of bearing loads, increases load capacity in the working area of the bearing, maximizes bearing life, and increases travel life.

Taper

Taper is a component of cylindricity. In linear bearing applications, taper can cause one portion of the linear bearing to be loaded higher than the other. This can cause dramatic reduction in travel life or load capacity. As taper increases, balls go in and out of preload, causing premature wear and reduction in travel life.