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3D S Parameters - Hybrid Parameters


Sds (Summit Density)

Sds, the Summit Density, is the number of summits per unit area making up the surface. Summits are derived from peaks. A peak is defined as any point, above all 8 nearest neighbors. Summits are constrained to be separated by at least 1% of the minimum “X” or “Y” dimension comprising the 3D measurement area. Additionally, summits are only found above a threshold that is 5% of Sz above the mean plane.

(Note that Sds is not strictly defined in the ISO 25178-2 but was established earlier in the research which contributed to ISO 25178-2. Future sections of ISO 25178-2 may address parameters such as Sds as “Feature Parameters”.)

Sds definition

Surface with Sds ~ 2600 summits/mm
Surface with Sds ~ 2600 summits/mm.

Sds Summit Density Diagram
Although a 2D profile is shown here, it is understood that this criteria is applied to the 3D features of the surface.


Sds is a key parameter when considering surfaces used in applications such as bearings, seals and electronic contacts. The manner in which the summits elastically and plastically deform under load is related to the Sds parameter. Depending on the application, a low Sds may result in higher localized contact stresses resulting in possible pitting and debris generation. In applications involving sliding components, a number of summits are needed to prevent optical contacting while maintaining a reasonable load distribution. Summit density may also be related to the cosmetic appearance of a surface once painted.

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