Vibration Resistant Captive SEMS Machine Screws
What Are Vibration-Resistant Captive SEMS Machine Screws?
SEMS (pronounced “sems”) machine screws arrive from the factory with one or more captive washers pre-assembled under the head. The vibration-resistant style uses a locking washer (commonly split-lock or external-tooth) that helps the joint resist loosening in service. Because the washer is permanently captured on the screw, assembly is faster, parts can’t be misplaced, and clamp force is distributed more evenly.
Why Choose Vibration-Resistant SEMS?
- Built-in locking: pre-assembled lock washer helps resist backing-off in high-vibration environments.
- Faster assembly: no separate washer handling—improves throughput and consistency.
- Repeatable quality: consistent washer position and bearing area under every head.
- Service friendly: fewer loose parts during maintenance; easy tool access with common driver styles.
Head & Washer Options
- Heads: pan (most common), button, hex-washer, and low-profile options depending on size.
- Captive washers: external-tooth or split-lock for anti-vibration; flat or finish washer may be stacked for added bearing area.
- Drives: Phillips, combo, Torx/6-lobe, or hex-socket—choose to match your tooling and torque needs.
Materials & Finishes
- Zinc-plated steel: economical, bright finish for dry interior use.
- 18-8 / 304 stainless: dependable corrosion resistance for most outdoor/indoor environments.
- 316 stainless (select sizes): preferred for marine or chloride exposure.
- Black oxide steel: low-glare appearance for indoor applications.
Sizes, Threads & How to Specify
Order by diameter × thread × length, e.g., #6-32 × 1″, #10-24 × 3/4″, 1/4″-20 × 1-1/2″, or M4-0.7 × 20 mm. Inch series use UNC (coarse) or UNF (fine); metric uses ISO pitches. Length is measured from under the head to the tip (flat/countersunk heads measured overall).
Common Applications
- Electronics, PCBs, power supplies, and rack hardware
- Automotive, motorcycle, and small-engine assemblies
- Industrial machinery, pumps, and HVAC equipment
- Fixtures and enclosures where loose washers are impractical
Installation Tips
- Confirm thread size and pitch match the nut or tapped hole; use proper tap-drill and thread depth.
- Drive straight and use the recommended torque for the screw material and finish; avoid over-tightening thin sheet.
- On painted or soft materials, choose SEMS with a flat + lock stack to protect the surface while adding resistance to loosening.
- For stainless-to-stainless joints, a suitable anti-seize can reduce thread galling.
Related Hardware
- Finished machine screw nuts, keps (external-tooth) nuts, and nylon-insert lock nuts
- Threaded inserts for plastic/soft materials; standoffs and spacers
Vibration-Resistant SEMS FAQs
See the accordion below for sizing, torque, washer types, and material selection.
Disclaimer: This page provides general guidance. Always follow the product datasheet and applicable standards.
A machine screw supplied with one or more captive washers already assembled under the head.
They include a locking washer (split-lock or external-tooth) to help resist loosening from vibration; standard SEMS may use only a flat washer.
Often no. If you need extra bearing area or to protect a finish, choose SEMS with a flat + lock stack or add a finish washer if space allows.
Torx/6-lobe or hex-socket provide higher torque with less cam-out; Phillips/combination maximize tool availability. Match the drive to your tools and torque requirement.
Popular inch sizes include #4-40, #6-32, #8-32, #10-24, #10-32, and 1/4″-20; metric examples include M3, M4, and M5. Availability varies by material/finish.
Yes, ensure the hole is tapped or an insert is installed. For untapped thin sheet or plastics, consider thread-forming/cutting screws instead.
18-8/304 is generally low-magnetic; some magnetism can result from cold-working. 316 is also typically low-magnetic.
Torque depends on diameter, material/grade, finish, lubrication, and joint design. Use the manufacturer’s torque table or your engineering spec.
No, the washer is retained on the screw shank. It spins freely for assembly but is not removable without damaging the fastener.
The lock washer adds mechanical resistance, but some applications still use chemical threadlocker or prevailing-torque nuts for extra security.
Generally yes if threads and the washer are undamaged. Replace fasteners that show wear, bent shanks, or deformed washers.
Zinc-plated steel is economical for dry interiors. Choose stainless for outdoor use or corrosive environments; select 316 for harsher conditions.