SEMS screws (screw and washer assemblies) — spring and plain washer combinations
A screw with the washer fitted before the thread is rolled, so it cannot fall off. Choose spring washer only or spring plus plain washer, and standard or large plain washer.
A SEMS screw is a screw and washer made into one part. The washer is fitted first and the thread is rolled afterwards, so the thread major diameter is larger than the washer hole and the washer cannot come off. The name comes from pre-assembled screws and washers; international standards call them "screw and washer assemblies".
Why use them
- Fewer assembly steps — no fitting washers one by one on high-volume lines.
- No missing washers — the washer cannot be forgotten, so joint quality stays consistent.
- Suits automatic feeders — one part feeds more reliably.
- Simpler stock — no separate screws and washers to buy and manage.
Unit price can be higher than a screw plus loose washer, so for small quantities compare it with assembly cost.
Two washer configurations
| Configuration | Marking | Washers | Our stock | Use where |
|---|---|---|---|---|
| Spring washer only | S/W | Spring (split lock) washer | 116 | Hard mating surface, small hole, anti-loosening |
| Spring + plain washer | S/W+P/W | Spring + plain washer | 349 | Soft mating material (aluminium, plastic) or large hole needing wide bearing |
The split spring washer flattens under load and pushes back, reducing vibration loosening. The plain washer widens the bearing area so the surface is not dented and the head does not pull through the hole.
Large plain washer
Some spring + plain washer items use a large plain washer (34 items) for large or slotted holes where the standard washer does not bear enough. The product name carries "(대)" (large).
Washer dimensions (our drawings)
mm, O.D. / thickness.
| Size | Spring washer | Plain washer (std) | Plain washer (large) |
|---|---|---|---|
| M2.3 | 4.0 / 0.6 | – | – |
| M2.6 | 4.4 / 0.7 | – | – |
| M3 | 5.25 / 0.75 | 5.8 / 0.6 | 8.0 / 0.8 |
| M4 | 6.6 / 1.1 | 8.0 / 0.8 | 10.0 / 0.8 |
| M5 | 8.3 / 1.3 | 10.0 / 0.8 | 11.5 / 1.0 |
| M6 | 11.0 / 1.6 | 13.0 / 1.0 | – |
| M8 | 14.0 / 2.1 | 16.0 / 1.5 | – |
| M10 | 17.0 / 2.6 | 21.0 / 2.0 | – |
| M12 | 20.0 / 3.0 | 24.0 / 1.6 | – |
From the washer specification table on our production drawings. "–" = not stocked. Per-part values are on each product page and data sheet.
Head, material and property class
| Head | Material · finish | Property class | Stock |
|---|---|---|---|
| Pan (cross recess) | Carbon steel · zinc / nickel | 4.8 | 178 |
| Stainless A2 · passivated | A2-50 | 112 | |
| Socket head cap | SCM435 Q&T · nickel | 10.9 | 52 |
| Stainless A2 · passivated | A2-50 | 50 | |
| Hex (with cross recess) | Carbon steel · zinc | 4.8 | 33 |
| Stainless A2 · passivated | A2-50 | 40 |
See steel bolt property classes and A2 stainless property classes.
How to choose
- Does the joint already use a washer? — switching to SEMS removes a part and a step.
- Hard mating surface, small hole? — spring washer only (S/W).
- Soft material or large hole? — spring + plain washer; large plain washer for slotted holes.
- Head and tool — pan or hex (cross recess) for screwdrivers; socket head for hex keys.
- Strength and corrosion — SCM435 10.9 socket head for high clamp load; stainless A2 for corrosive service.
A spring washer flattens with repeated tightening and loses spring force. Where joints are opened often, set a replacement interval.
SEMS in our catalog
465 items, M2.3–M12, each with a 3D model and drawing; washer dimensions are listed by size on the data sheets. SEMS have no KS standard and are made to our in-house standard (JK Standard).
| Type | Stock | Browse |
|---|---|---|
| All SEMS | 465 | SEMS list |
| Pan head SEMS | 290 | Pan head |
| Socket head SEMS | 102 | Socket head |
| Hex SEMS | 73 | Hex |
For head shapes see seven screw head types.
FAQ
What is a SEMS screw?
A screw and washer made into one part. The washer is fitted before the thread is rolled, so it cannot come off. It reduces assembly steps and missing washers.
What is the difference between S/W and S/W+P/W?
S/W has a spring washer only; S/W+P/W has a spring washer and a plain washer. Choose the plain washer version for soft materials or large holes that need a wider bearing area.
What property class are your SEMS screws?
Carbon steel pan and hex SEMS are 4.8, stainless A2 are A2-50, and SCM435 quenched-and-tempered socket head SEMS are 10.9. Each product page shows the class.