Surface Finishes

Surface Finishes

The mating surface properties and quality play critical role to the function, reliability, and service life of the dynamic seal components and systems. 

 

 

Proper definition and control are required in order to achieve desired performance and functional reliability. Generally, the matting surface quality is defined by the surface finish quality.

The surface finish of the mating surface is a result of the utilized manufacturing/machining processes. All machining processes provide typical “surface topographies” (surface roughness, lay and waviness) including the irregularities.

The irregularity of a machined surface is the result of the actual machining process and its machining parameters, such as turning speed, cutting feed and feet rate, selected tool and its geometry.  This irregularity consists of high and low spots, which machined into a surface by the tool(s) and create typical surface texture. The peaks (high spots) and valleys (low spots) can be measured and used to describe the surface condition.

Surface roughness is a measure of the irregularities (peaks and valleys) produced on a mating surface according to the manufacturing process used to create the surface. The standard method of measuring roughness is by an average value of the profile variation from a centre line over a reference length (L).

The parameters for specifying a surface finish are defined in ISO 4287, 4288 and DIN 4762 standards. The parameters are measured and/or calculated from the roughness mean line. The most commonly used value is Ra – arithmetic average roughness, which is the arithmetic mean deviation of the surface profile from the Center Line/ (Figure 1).

The other important characteristics frequently used to describe the surface roughness are Rz –  which is a peak-to-valley height and Rmax – which is maximum peak-to-valley height (Figure 2).

Calculation of Rz: Rz= (RZ1 + RZ2 + RZ3 + RZ4 + RZ5)/5

Figure 2. – Rz surface value and calculation of Rmax

The Ra, Rz, and Rmax values alone do not quantify the proper, required sealing surface. Additional parameter is needed. The importance of the additional requirement related to surface quality is demonstrated by Figure 3., where we have two different surface profiles with the same Ra, Rz and Rmax values. The closed surface profile will a better sealing surface compared to the open profile, even if they have the same roughness values.

Figure 3. – Closed and open surface profiles

The new parameter what helps to get better understanding of the surface profile is call  material contact area (or sometimes called material profile bearing length ratio, Rmr provides more information about the surface profile characteristics (Figure 4).

Figure 4. – Material contact area and its curve

The closed profile has higher material contact area value versus open one. The closed profile is called “seal-friendly” profile, which will help to provide optimum sealing surface conditions to the dynamic lips of the seals. The material contact area Rmr should be in the range of 45-90% depending on the seal material (based on a cut depth c=0.25 x Rz and relative to a ref. level (zero line) of 5%.)

Figure 5. – Open and closed profiles with their material ratio curves  

An optimal surface texture will have ideal pocket depths that retain lubrication in small enough volumes to provide a lubrication film between seal and mating surface, reducing friction and seal wear. If the surface is too rough, it will abrade the seal surface by plowing grooves in it and create a leak path. Alternatively, a surface that is too smooth will increase friction and wear because it does not have the ability to retain enough lubrication to provide a boundary lubrication film.

Surface roughness value recommendations are distinguished between static and dynamic surfaces. Static surfaces are easier to seal, that is why requirements for static surface roughness are less rigorous.

Surface finish recommendations will vary depending upon the utilized seal material (Chart 1).

Chart 1.

As it was discussed earlier both the roughness and texture of the surface finish produced can vary widely by different machining processes.

Chart 2. gives typical values likely achieved with different processes

Chart 2.