In the mechanical fastening system, bolted connections are the most widely used detachable connection method due to their simple structure, convenient assembly, and high reliability. In practical engineering applications, washers are critical matching components that effectively improve connection stability, anti-loosening performance, and service life. Based on GB/T, DIN, ISO standards and measured engineering data, this article systematically explains the functions of flat washers and spring washers, as well as applicable scenarios where washers are unnecessary.

 一,Flat washers strengthen bolt connections

1.Protect the surface of connected parts

With a surface roughness of Ra ≤ 0.8 μm, flat washers isolate rough bolt heads/nuts from workpieces, reducing surface scratches and indentation by over 90%, especially suitable for aluminum alloy, plastic, and coated plates.

2.Disperse contact pressure and prevent collapse

Flat washers increase the pressure-bearing area by 200%–400%, reducing contact pressure by 60%–85% to avoid plastic deformation of soft materials without reducing fastening performance.

3.Compensate for installation hole deviations

Flat washers increase the pressure-bearing area by 200%–400%, reducing contact pressure by 60%–85% to avoid plastic deformation of soft materials without reducing fastening performance

4.Adapt to oblong holes and oversized holes

Provide stable support for adjustable mounting holes; custom thickened or stacked washers meet special assembly requirements.

Standards: GB/T 97.1, DIN 125, ISO 7089

Common sizes: M3–M36, thickness 1.0–6.0 mm

Materials: Carbon steel, stainless steel 304/316, brass, engineering plastic

二,Strengthening Effects of Spring Washers on Bolted Connections

Spring washers are elastic locking elements that maintain preload by compensating for stress relaxation:

1.Timely compensate for stress relaxation

When bolt grip length is L ≤ 3d, spring washers provide a rebound stress of 150–350 MPa to stabilize preload.

2.Suitable for low-strength bolts

Compatible with Grade 4.8/5.6/6.8 bolts; strictly prohibited for Grade 8.8 and above high-strength bolts to avoid stress concentration and reduced reliability.

3.Improve anti-loosening life

Under vibration conditions, the anti-loosening life is 3–8 times longer than connections without washers.

Standards: GB/T 93, DIN 127, ISO 7092

Elastic recovery rate: ≥ 80%

Applications: Static load, low-to-medium vibration equipment, ordinary bolt connections

三,Bolted Connections Without Washers

Washers can be omitted in the following conditions without affecting reliability:

1.Pure shear-loaded bolt connections

Bolts bear transverse shear force with axial preload < 5% of design load; small holes and low contact pressure eliminate the need for washers.

2.Flange bolts / flange nuts

The flange structure provides 1.5–2.5 times the standard bearing area, equivalent to flat washers.

3.High-precision high-strength assembly

When hole tolerance ≤ ±0.1 mm and base material strength ≥ 300 MPa, direct fastening is applicable.

四,Conclusion

Washers are key components to enhance bolted connection performance:

• Flat washers: Disperse pressure, protect surfaces, compensate hole deviations;
• Spring washers: Prevent loosening and compensate stress (only for ordinary bolts);
• Flange fasteners and pure shear connections: Washers are not required.

Reasonable selection based on standards, load type, material strength, and assembly accuracy ensures safety and durability in engineering applications.