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. SEMS is the trade's general name for pre-assembled screw and washer products; international standards call them "screw and washer assemblies".
Why use them
- Fewer assembly steps — no fitting washers one by one on high-volume lines.
- Less risk of missing washers — fewer forgotten washers, so joint quality stays consistent.
- Simpler supply and handling — one part is easier to handle; suitability for automatic feeders depends on the equipment.
- 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 | Catalogue items | Use where |
|---|---|---|---|---|
| Spring washer only | S/W | Spring (split lock) washer | 116 | Where the existing drawing specifies a spring washer only; check that bearing area and tightening conditions are adequate without a plain washer |
| Spring + plain washer | S/W+P/W | Spring + plain washer | 349 (incl. 34 large plain washer) | Where the drawing specifies spring + plain washer, or soft mating material (aluminium, plastic) or a large hole needs wider bearing |
The split spring washer deforms elastically as it is tightened. It is traditionally used against loosening, but once fully flattened its effect against vibration loosening can be limited (NASA-STD-5020B, Appendix B.6, notes that free-spinning split lock washers provide minimal, if any, locking and should not be used as secondary locking features). Do not rely on a spring washer alone to prevent loosening; joints with heavy vibration or where loosening matters need preload control and a separate locking design. 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; for slotted holes, check slot width and edge margin to choose an adequate diameter. 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.
※ These washer sizes are our in-house standard for SEMS production and may differ from KS or ISO sizes for loose plain washers; check the drawing of each part for design and substitution. The spring washer thickness is the section thickness, not the free height or the assembled height.
Head, material and property class
| Head | Material · finish | Property class | Catalogue items |
|---|---|---|---|
| Pan (cross recess) | Carbon steel · zinc / nickel | 4.8 | 178 |
| Pan (cross recess) | Stainless A2 · acid cleaned | A2-50 | 112 |
| Socket head cap | SCM435 Q&T · nickel | 10.9 | 52 |
| Socket head cap | Stainless A2 · acid cleaned | A2-50 | 50 |
| Hex (with cross recess) | Carbon steel · zinc | 4.8 | 33 |
| Hex (with cross recess) | Stainless A2 · acid cleaned | A2-50 | 40 |
See steel bolt property classes and A2 stainless property classes.
How to choose
- Does the joint already use a washer? — if the drawing specifies a separate screw and washer, combining them into SEMS removes a part and a step. Check that the washer configuration and dimensions, screw length and tightening conditions suit the existing design.
- What washer does the drawing require? — spring washer only: S/W; spring and plain washer: S/W+P/W.
- Is the bearing area enough? — for soft material or a large hole, S/W+P/W; for slotted holes, a large plain washer after checking width and edge margin.
- 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.
For high-strength SEMS, check not only the screw's property class but also the material, hardness and bearing area of the assembled washer. Class 10.9 describes the screw and does not cover the washer; where high preload is needed, make sure the washer does not deform and the mating surface is not damaged.
Where vibration is heavy or loosening matters, do not rely on the washer type; review preload control and locking design separately.
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. We make their washer sizes and combinations to our in-house standard (JK Standard).
| Type | Catalogue items | 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. Start from the washer the drawing requires, and choose the plain washer version for soft materials or large holes that need a wider bearing area. A spring washer alone does not guarantee against loosening.
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.
Can SEMS screws be reused?
It depends on the service conditions and the condition of the part. Replace it if the thread or washer is damaged, deformed or corroded. Where vibration or loosening matters, do not allow reuse just because a spring washer is attached; decide by the design and the customer's fastening criteria.
References
- KS B 1041 — cross-recessed screw and washer assemblies; washer fitted before thread rolling so it cannot come off
- ISO 10644:2009 Screw and washer assemblies made of steel with plain washers — screw and plain washer assemblies (incl. washer hardness classes 200 and 300 HV); not a statement that all our SEMS with spring washers conform; dimensions and combinations follow our production drawings
- NASA-STD-5020B Requirements for Threaded Fastening Systems in Spaceflight Hardware, Appendix B.6 — split lock washers provide minimal, if any, locking and should not be used as secondary locking features
- ISO 10673:2009 Plain washers for screw and washer assemblies — Small, normal and large series
- KS B 1324 Spring washers / KS B 1326 Plain washers
- ISO 898-1:2013 Fasteners — Mechanical properties of fasteners made of carbon steel and alloy steel — Part 1: Bolts, screws and studs with specified property classes — property classes 4.8 and 10.9
Last revised 2026-10-12