Spring-loaded valve assembly

The purpose of this inspection is to determine whether a spring-loaded valve assembly has been properly assembled. Two wire-springs are inserted into a small diameter valve port to control the motion of a plastic valve. If the wire springs do not seat properly (to within about 10 µm) the assembly may not function properly. The challenge was to be able to determine the difference between a good and defective part by non contact optical inspection.

This inspection is currently being performed by human inspectors. The inspectors quickly inspect a part visually for obvious defects and then drop a plastic test part though the assembly. If the test part can move through the assembly the assembly passes. If the assembly fails, it is not clear precisely what caused the failure. The ID of the assembly was 11.35 mm and the OD of the test part was 11.33 mm. A photo of the assembly is shown in Figure 1a and a photo of the test part is shown in Figure 1b.

              

        Figures 1. a) Valve assembly                                  b) Test part

The image obtained from the IOMS probe for a good assembly is shown in Figure 2. The image for a defective part is shown in Figure 3.

Figure 2. Image of an acceptable assembly


Figure 3. Image of a defective assembly

The main difference between these images is that for a properly seated spring the turns of the spring in its slot are clearly distinguishable and not touching each other. In the defective assembly the wires are touching or crossing. Figure 3 shows that only one of the springs are improperly seated in the defective assembly.