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Circumferential measurements improve sealing Surfaces

Leakage from a valve, like flammable gas valves, may have serious safety issues,or affect the efficiency and fuel economy of a fuel injector. Circumferential surface finish helps detect leakage paths.

Circumferential measurements carried out on a Taylor Hobson Talyrond instrument 

Manufacturers of valves for applications involving fluids or gas have to ensure there is no leakage during the function of the components. Leakage from a valve may have safety issues, such as on a gas valve, or may affect efficiency and fuel economy, such as on a fuel injector.  In many circumstances seals may have to combat high pressures, the best example being on a diesel fuel injector where pressures of greater than 1500 bar are common place.

All valves involve seating of two parts, the male and female components of a valve, have to make contact in such a way that no leakage occurs.  This means manufacturers have to control the form of both parts of the seal to ensure correct seating.

If we take the example of a ball valve, the structure of this type of valve is such that the ball is forced against a coned surface, thus creating the seal. If these two seating faces are the wrong form, i.e. the ball component is round but the coned surface is oval, due to poor machining, then the seal will not be made and leakage will occur. Therefore the manufacturer has to control the form of the contacting areas of the seal; this is a fundamental requirement for any valve of this nature.

Not only is roundness an issue, but the concentricity or run out of the valve guide to its seating face is also a critical element. A valve guide and valve body has to have a concentric axis in order for their respective seating faces to sit correctly, thus forming a tight seal.

Other form errors such as parallelism, cylindricity and straightness can also affect the function of the valve, however what about the surface finish of the valve faces?

Surface finish, for the majority of rotationally symmetric components is measured along their axis. This allows the manufacturer to control production and make predictions on tool changes etc.

However is a measurement along the axis of a seal a functional measurement?

In figure 2 (opposite) we can see a sketch of a ball valve. The mating surfaces of this valve require a level of surface finish that ensures the faces make the required seal. In the case shown here we can see scratches on the ball, these scratches are in the axial direction of the seal.

A measurement along the axis of the ball would not find this scratch and as a consequence the inspector may pass a defective seal. The same can be said for the coned area of the valve body, a scratch or deep valley in the longitudinal direction will not be found by measurement in the same direction as the scratch.
It is a bit like measuring the depths of the furrows in a ploughed field by measuring along the furrow, thus revealing a good surface.

Although there are drawing standards for surface finish direction they are very rarely seen on drawings.

In the case of valve seals the most functional form of surface finish measurement is in a circumferential direction, i.e. in the same direction as an inspector would measure roundness; if you think about this it would seem rather logical.

The problem is that roundness measurements are designed to look at the general form of the component; a small scratch although detrimental to the seal will not necessarily be picked up by conventional means. This is because a roundness instrument uses filters to remove high frequency, but more importantly a typical roundness stylus can be anywhere from 0.5 to 2mm diameter; these larger stylus tips will bridge the gap over a valley and disguise the true problem.

In most cases a deep scratch will have an associated peak at either side of the valley caused by the action of scratching the surface (see fig 3 above). When measured with a roundness instrument and conventional stylus, what is actually a deep valley may appear as a small peak on a roundness profile. This peak in turn may not be enough to fail the part.

In fact in a lot of cases, such as on very small valves, it is difficult to separate roundness and surface finish as two individual entities; both contribute towards the function and it may not be practical to see them as individual elements.

Many modern fuel injectors have seating areas of less that 2mm in diameter and lands of tenths of mm.

In these circumstances it is impossible to measure along the axis of the valve using a filter due to the lack of profile i.e. convention states that the length of measurement should include sufficient sampling lengths to represent the surface finish of the component. This is impossible if the valve seat land length is less than the chosen sample length (cut off value). Besides this reducing the filter value or sample length to suit the measurement length can significantly attenuate the peaks and valleys of the surface and hence give a false indication of surface quality.

To overcome this sort of problem the direction of measurement for surface finish should be the same direction as that used for roundness measurement. A circumferential surface finish measurement is a more functional test and will pick up deviations in the surface which may cause leakage such as a scratch. More importantly the surface finish is picked up at the point of contact of the seal.

Its also worth noting that a seal with a small diameter, let’s say 2mm, has a circumference of 2π meaning a circumferential length of 6.248mm.This means even for small valves the correct number of sample lengths can be made for assessment, thus ensuring a more statistical representation of the surface.

To make these measurements the part or the gauge requires rotation. This is simple for a roundness instrument; however there are a number of other requirements and points to note.

First of all the instrument must have a low noise threshold to allow measurement of higher frequency errors around the profile without false peaks and valleys. The gauge used to make these measurements also has to have a high resolution (in the order of nanometres) in order to pick up small changes and imperfections along the surface. In terms of radial or circumferential surface finish, the larger the seal diameter the greater the circumference. Therefore it is often the case that a large amount of radial data is required. This means the roundness system will require a precision spindle encoder to ensure an exact reproduction of the surface with little or no missing data.

The gauge force needs to be light enough to allow measurement using a diamond stylus; this combined with correct spindle speeds ensures the stylus tip tracks the surface correctly without bouncing or missing valley depths.

On a final note, if the measurements are to be made on a roundness instrument, then that instrument needs to support both roundness and roughness software. If you are a manufacturer of seals, remember a “a scratch in the axial direction can only be found with a measurement in the circumferential direction”

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