Washers are often treated as “just spacers,” but selecting the right washer can improve joint reliability by:
- Distributing load and protecting surfaces,
- Compensating for embedding/settlement and thermal effects,
- Increasing friction in light-duty anti-rotation cases,
- Improving electrical bonding/grounding on sheet metal.
This post breaks down three common families: flat, spring (split and conical), and serrated/toothed washers.
1) Flat washers — primarily for load distribution
What they do
- Increase the bearing area under the bolt head or nut → reduces local stress.
- Protects surfaces from gouging, especially on softer materials.
- Helps when holes are slightly oversized or slotted.
When to use
- Soft or thin materials (aluminum, plastics, sheet metal).
- Slotted holes where you want to “bridge” the slot.
- Painted or plated surfaces where you want to reduce damage from tightening.
What they don’t do
- A flat washer is not a “strong vibration lock” by itself. Anti-loosening still depends mainly on preload and the locking method.
2) Spring washers — split vs conical (Belleville)
In practice, “spring washer” can mean different hardware. Two common types behave differently:
2.1) Split lock washers (helical spring)
Practical effect
- Provides some spring action at low loads and can lightly “bite” the surface.
- Can be acceptable for light-duty assemblies where requirements are modest.
Limitations
- Under proper clamp loads, split lock washers can flatten, reducing their spring effect.
- For high vibration or critical joints, they’re usually not sufficient alone—use proven locking strategies per your design.
2.2) Conical/Belleville washers (disc springs)
Why they’re useful
- They act as a real spring element and help maintain clamp force when:
- Surfaces settle (embedding),
- Soft layers/gaskets creep,
- Temperature changes cause expansion/contraction.
When to use
- Assemblies with softer materials or compressible layers.
- Joints where maintaining preload over time is important.
3) Serrated/toothed washers — friction and grounding
What they do
- Teeth increase friction and can cut through oxide/paint → improves:
- Light-duty anti-rotation resistance,
- Electrical bonding/grounding on sheet metal.
When to use
- Sheet-metal assemblies, especially at grounding points.
- Under heads/nuts when surface marking is acceptable and friction is desired.
When to be cautious
- Painted/plated surfaces: serrations can damage coatings.
- Soft materials (e.g., aluminum): teeth may gouge deeply and change friction/tension behavior.
4) Quick selection table
| Situation | Prefer | Why |
|---|---|---|
| Soft/thin material, avoid dents | Flat washer | Distributes load, protects surface |
| Oversized/slotted holes | Flat washer | Bridges hole/slot, reduces pull-through |
| Settlement/creep or thermal cycling | Conical (Belleville) washer | Maintains preload with spring action |
| Grounding on sheet metal | Serrated washer | Bites through oxide/paint for contact |
| High vibration, critical joint | (Don’t rely on washer alone) | Correct preload + engineered locking method |
5) Common washer mistakes
- Stacking multiple split lock washers: often increases variability more than performance.
- Using serrated washers where corrosion protection or appearance matters.
- Choosing a flat washer that’s too small for the hole/slot, leading to edge pull-through.
6) One important note about tightening
Washers change friction under the head/nut, which affects the relationship between torque and bolt tension (preload). For important joints:
- Follow the specified tightening method and avoid mixing washer types without re-validating the process.
If you want a refresher on metric sizing and thread pitch notation, see: How to Read Metric Bolt Sizes (M6/M8/M10) and Thread Pitch (P).
