Key takeaways
- Grade each part number by criticality before you set the sample size. A position that stops the whole line is inspected harder than one that can be changed within a shift, and one sample size for every part number is waste at one end and risk at the other.
- Six items cover most incoming lots: the three dimensions, hardness, lip and spring, case, markings and lot number, packaging and age.
- Rubber has a shelf life. ISO 2230 groups it by material type, so a lot that sat a long time in the supplier's warehouse is an inspection item, not something to leave to the seller.
- Record the actual measurements, not pass or fail. A column of figures across several lots shows a supplier drifting, while a column of ticks shows nothing.
Incoming inspection of sealing products usually falls into one of two extremes: open the packet, count the pieces and book them into stock, or inspect every piece and block the warehouse. The procedure below sits between the two and can be run on every delivery.
First: grade each part number by criticality
Not every part number deserves the same inspection, and one sample size for every part number is waste at one end and risk at the other.
Three levels are enough:
- Critical. A failure stops the line, or the position takes more than a shift to change. Tighter inspection, larger sample size, first lot inspected in full.
- Standard. Can be changed within a shift, safety stock held. Sample inspection.
- Minor. Several sources available, quick to change. Check markings and quantity.
The level decides the sample size, so it has to be fixed when the part number is set up, not when the goods reach the warehouse door.
The six check items
1. The three dimensions. Inner diameter, outer diameter, thickness. Measure with a caliper, each dimension at two positions at right angles to each other. Compare against the tolerance class declared in the documentation, not against the nominal figure.
2. Hardness. Measure Shore A on the rubber ring, avoiding areas with the steel case underneath. Compare against the figure in the material data sheet. A deviation beyond the declared tolerance is a sign of a change of compound.
3. Lip and spring. The lip must be sharp, with no flash and no cuts. The garter spring must sit in its groove and must not have come unhooked. Inspect visually under adequate light, and for critical part numbers under a magnifier.
4. Case. No dents, no distortion. On seals with a rubber-covered case, the outer edge must be even, with no pitting.
5. Markings and lot number. The part number legible, the lot number on the packaging and matching the certificate of conformity supplied with it. A missing lot number means no basis for lot traceability, and that is reason enough to hold the lot.
6. Packaging and age. Packaging intact, free of stray oil or grease, not left in the sun. Read the date of manufacture.
Shelf life is an inspection item, not something to leave to the seller
Rubber has a shelf life. ISO 2230 gives storage guidance and groups the periods by material type, and a lot that sat a long time in the supplier's warehouse before it reached you has already used up part of that period.
So the date of manufacture is a figure to read on every delivery, not one read only when there is a failure. For sealing products this is one of the few items where goods that pass every measurement can still be unusable.
Your own storage conditions fall under the same guidance: keep away from direct light, keep away from ozone sources such as electric motors, and do not press the parts out of shape when stacking them.
Record figures, not ticks
This is the difference between a form that is useful and a form that is only filed.
Record "pass" and the record answers only one question, whether this lot was accepted. Record the actual measurements and after a few lots you can see whether the supplier is drifting towards one side of the tolerance, and drift is the thing to catch before it becomes scrap.
The minimum form: part number, lot number, date received, sample size, the measurements for each sample, hardness, conclusion, inspector.
When a lot fails
Leave the measured samples as they are and photograph them before the goods go back. State which item failed and the measurements found.
A report that says "poor quality goods" gets nowhere in a negotiation. A report that says the outer diameter measured on three samples falls outside the declared tolerance, with photographs attached, gets somewhere.
Before you inspect, review the documents required to open a supplier code, because most of the items here are checked against that same file.
