The assembly argument
Every loose washer is a part that can be dropped, misaligned or forgotten. In automated assembly that is a stoppage; in manual assembly it is a defect that reaches the customer. A captive washer removes the part from the operator's hands entirely, which is why SEMS and combination screws dominate appliance and automotive joints.
The preload argument
A washer spreads the clamp load over a larger area, which matters on plastic and thin sheet. With a spring washer the joint also keeps preload as the materials creep — the screw does not lose tension, so it does not come loose. That is the mechanism behind 'anti-vibration', not a coating.
Specifying the finish
On appliances the finish has to survive steam, detergent and food contact, which attack coatings differently. Specify the salt spray hours to red rust, not the coating name: 'zinc' can mean 24 hours or 240 depending on thickness and passivation.
What goes wrong
Washer retention that is too loose lets the washer fall off in the feeder; too tight and it will not rotate, so it galling against the part. The undercut depth is the whole specification, and it is set by the die, not by assembly.
Which combination for which joint
| Joint | Washer type | Why |
|---|---|---|
| Plastic housing | Flat + spring | Spreads load, keeps preload as plastic creeps |
| Thin sheet metal | Flat | Prevents pull-through at the hole |
| High-vibration motor mount | Spring or serrated | Maintains tension under cyclic load |
| Food-contact surface | Sealed or coated | No trapped residue, coating to spec |
If the joint comes loose in the field, the fix is usually the washer and the preload, not a threadlocking compound.