Toothed Lock Washers (Star Lock Washers)
In real assembly lines, the failure is rarely “the bolt snapped”—it’s the slow loss of joint integrity: micro-slip under vibration, fretting debris building up at the interface, and electrical bonding drifting out of spec after repainting or zinc flake coatings. Toothed lock washers (star lock washers) are designed for that interface problem: the teeth create localised contact pressure, bite through light coatings/oxides, and increase friction under the fastener head. In sheet-metal enclosures, automotive brackets, and grounding points, this can be the difference between a stable clamp load path and a joint that “walks” over time.
- Bite through coatings, bond
- Prevent rotation under vibration
- Low-profile for tight clearances
- Match DIN 6797/6798 options
- Enable grounding, lower resistance
- Multiple materials, finish choices
Technical Specifications
Product Name
Toothed Lock Washers / Star Lock Washers (Internal & External Tooth)
Standards
DIN 6797 (A/J), DIN 6798 (A/J/V), ASME B18.21.1 (Type A/J variants)
Material
Spring steel (heat-treated), Stainless steel A2 (304) / A4 (316), Phosphor bronze / Silicon bronze (electrical)
Grades
Hardness by standard/type; material-specific
Diameter Range
Metric M2–M30 (typical); imperial on request
Surface Finish
Zinc plated (clear/yellow), mechanically plated (hydrogen-embrittlement sensitive programs), black oxide, passivated stainless, plain/oiled
Certifications
EN 10204 3.1 CoC/CoA on request; RoHS/REACH declarations on request; ISO 9001/IATF program support by project
1: Vibration loosening that starts as micro-slip
What happens: Under transverse vibration, the joint interface experiences micro-motion. That motion polishes contact faces, creates fretting debris, and reduces friction—then clamp load begins to decay.
How toothed lock washers help: The teeth generate high local contact pressure and increase interface friction under the head/nut face. This raises resistance to small-amplitude rotation in early service life—useful in brackets, light assemblies, and access panels where space is limited.
Engineering note: For high-energy vibration joints (powertrain, rail, heavy equipment), a toothed washer is not “positive locking.” Treat it as friction enhancement + bonding aid, and consider wedge-lock washers or prevailing-torque nuts where loss-of-preload risk is critical.
2: Electrical bonding fails after paint, anodising, or oxide build-up
What happens: Painted panels and oxide films raise contact resistance. Even when torque is correct, the electrical path becomes unstable across temperature cycles and humidity.
How star lock washers help: Teeth can pierce thin coatings to create repeatable metal-to-metal contact points, making them common as grounding washers / electrical contact washers in enclosures and harness bonding points.
3: Surface damage vs. “must not slip” requirement
Trade-off: External tooth designs deliver strong bite but can mark mating surfaces; internal tooth versions reduce snagging and visual impact.
Selection logic:
External tooth (DIN 6798 A / DIN 6797 A): maximum bite/anti-rotation on accessible joints.
Internal tooth (DIN 6797 J / DIN 6798 J): cleaner OD, better where you need a smoother perimeter or tight OD clearance.
4: Galvanic corrosion and mixed metals
What happens: A conductive “bite” point can accelerate galvanic activity if you mix stainless, carbon steel, and aluminium in wet environments.
Mitigation: Match materials (A2/A4 for stainless programs; coated carbon steel for general industrial; bronze for electrical + corrosion strategy), and validate with salt-spray or field exposure expectations.
Below are reference dimension examples commonly used for drawing checks and BOM validation. Final acceptance should follow the applicable standard revision and your print requirements. DIN 6798A reference dimensions shown here are based on a published DIN 6798:1971 reference sheet.
DIN 6798A — External Serrated Lock Washer (Type A / External Teeth): example dimensions
| Thread Ø (M) | d1 (ID, mm) | d2 (OD, mm) | s (Thickness, mm) |
|---|---|---|---|
| M3 | 3.2 | 6.0 | 0.4 |
| M4 | 4.3 | 8.0 | 0.5 |
| M5 | 5.3 | 10.0 | 0.6 |
| M6 | 6.4 | 11.0 | 0.7 |
| M8 | 8.4 | 15.0 | 0.8 |
| M10 | 10.5 | 18.0 | 0.9 |
| M12 | 13.0 | 20.5 | 1.0 |
| M16 | 17.0 | 26.0 | 1.2 |
| M20 | 21.0 | 33.0 | 1.4 |
| M24 | 25.0 | 38.0 | 1.5 |
| M30 | 31.0 | 48.0 | 1.6 |
DIN 6797 — Internal Tooth Lock Washers (Type J): example dimensions
(OD and thickness reference values commonly cited for DIN 6797 internal tooth washers.)
| Thread Ø (M) | d2 (OD, mm) | s (Thickness, mm) |
|---|---|---|
| M3 | 6.0 | 0.5 |
| M4 | 7.5 | 0.5 |
| M5 | 9.0 | 0.8 |
| M6 | 11.0 | 0.8 |
| M8 | 14.5 | 1.0 |
| M10 | 18.0 | 1.0 |
| M12 | 20.5 | 1.0 |
| M16 | 26.0 | 1.2 |
| M20 | 33.0 | 1.5 |
| M24 | 38.0 | 1.5 |
1) Define the real objective: locking vs. bonding
If the requirement is anti-rotation only, validate that the joint has sufficient preload and that interface slip is the dominant failure mode.
If the requirement is electrical bonding, measure contact resistance after assembly and after environmental conditioning (humidity/thermal cycling). A toothed washer helps, but coatings and plating choices still dominate long-term stability.
2) Control clamp load: torque is a proxy, not the target
Use torque as an input, but manage Preload as the target. Lubrication changes the torque–tension relationship (K factor scatter can be large), so mixed “dry vs. oiled” builds often create inconsistent clamp load.
Practical method: standardise thread condition (dry/oiled), define tightening method (torque only, torque-angle, or tension control), and audit with periodic joint testing.
3) Tooth engagement depends on surface condition
Teeth need a stable seat. On soft aluminium, thin sheet, or plastics, teeth can embed, relax, and reduce preload (classic preload decay). Consider:
moving the toothed washer to the grounding side only, and using a plain washer to spread load, or
upgrading the locking method (prevailing-torque nut / wedge-lock washers) for vibration-critical joints.
4) Washer orientation and stacking
Internal vs external tooth: choose by clearance and “bite” requirement (external bites harder; internal is cleaner OD).
If stacking with a plain washer, place the toothed washer against the conductive base metal when bonding is required.
5) Hole clearance and alignment (ISO 273)
Excessive hole clearance can reduce effective tooth engagement area and increase micro-motion. Use normal clearance holes per ISO 273 unless your assembly requires close fit; avoid oversize holes on vibration joints where possible.
6) Reuse policy
Treat toothed washers as single-use in most controlled assemblies. If teeth are flattened, plated surfaces are galled, or you see fretting debris, replace—those are indicators that the interface condition has changed.
Related Products
FAQ
What is a toothed lock washer (star lock washer)?
A toothed lock washer is a thin washer with radial teeth that increase friction and create high local contact pressure under a fastener head. It helps resist rotation and can pierce light coatings to improve electrical bonding in grounding applications.
What is the difference between DIN 6797 and DIN 6798 lock washers?
DIN 6797 typically refers to “toothed” (shake-proof) washers, while DIN 6798 refers to “serrated” tooth forms and includes variants such as external (A), internal (J), and countersunk (V). In practice, selection depends on required tooth geometry, clearance, and whether you are pairing with countersunk screws.
Should I choose internal tooth or external tooth star washers?
Choose external tooth washers when you need maximum bite and anti-rotation, and choose internal tooth washers when OD clearance, snag risk, or appearance matters. External teeth generally provide stronger surface engagement; internal teeth keep the outside edge smooth.
Are toothed lock washers enough for high-vibration joints?
Not always—on high-vibration or safety-critical joints, toothed washers should not be treated as a positive locking method. They can improve friction at the interface, but prevailing-torque nuts, wedge-lock washers, or threadlocking strategies are often more reliable for maintaining preload.
Can star lock washers provide grounding through paint or plating?
Yes, star lock washers can improve grounding by biting through thin coatings and creating multiple metal-to-metal contact points. For stable results, control the coating stack-up, verify contact resistance after assembly, and consider corrosion strategy (material pairing and environment).