This seemingly insignificant small part often causes controversy in fastener connections. Some on-site experts insist they must be installed, while some standards and specifications clearly state that they are "generally not recommended for use". So, what's the deal with this little washer?

01 The Big Confusion About a Small Part

In practical engineering, one often encounters this contradictory scenario: bolts on equipment flanges and pipeline flanges are installed without flat washers, and experts point out that this does not meet requirements. However, consulting the relevant standards reveals that the situation is not so absolute.

The Technical Specification for Piping Material Design of Chemical Plant clearly states: "Washers are generally not recommended for use. They are only used when metal bolts (or studs) and nuts are used to fasten non-metallic or non-ferrous metal connectors."

The Pressure Piping Code - Industrial Piping similarly takes a conservative stance on washer use, merely mentioning that "when washers are required, not more than one shall be used per bolt". These regulations indicate that washers are not a mandatory component for fastened connections, but rather an optional item that needs to be judged based on the specific situation.

02 Why Not Add Washers Casually?

In the field of pressure vessels, professionals generally do not recommend installing washers on flange fasteners, especially spring washers. This is because pressure vessel flange bolts are subjected to alternating loads, requiring very high pre-tightening force.

When the pre-tightening force is sufficiently large, the anti-loosening force provided by a spring washer becomes negligible, and it may even fail due to over-compression. The Technical Conditions for High-Strength Fasteners for Construction Machinery and Equipment clearly stipulates: "Spring washers are generally not allowed for anti-loosening when using Grade 8.8 or 8.9 bolts."

For high-strength bolted connections, spring washers provide almost no anti-loosening effect; instead, they can lead to rapid decay of the bolt's clamping force. This is the main reason why spring washers are being phased out for high-strength connections abroad.

03 Application Scenarios for Flat Washers

Protecting the surface of soft materials is a primary task of flat washers. When bolts require frequent disassembly, and the connected parts are made of relatively soft material, flat washers can effectively prevent the surface of the connected parts from being scratched.

In high-strength pre-stressed bolted connections, the role of flat washers is even more critical. This type of connection usually requires precise control of the friction coefficient. The bolt, nut, and washer need to be provided as a set by the same manufacturer to ensure performance matching.

When the strength of the bolt is much higher than the bearing capacity of the connected part, the flat washer reduces the surface compressive stress by increasing the bearing area, preventing deformation or damage to the connected part.

Furthermore, in situations where bolt holes are machined into slotted holes, thickened flat washers or special washers are essential, as they can compensate for installation position deviations and ensure connection stability.

04 The Limits of Using Spring Washers

The main function of spring washers is anti-loosening, but their applicable range is relatively limited. They are typically used in situations with low requirements for bolt pre-tightening force and relatively minor dynamic loads.

When the bolt clamping length is short, bolts are prone to stress relaxation. Spring washers can compensate for this relaxation to a certain extent, maintaining the tightness of the connection.

It is important to note that in critical high-strength connections, the anti-loosening effect of spring washers is not ideal. For such key connections, using other anti-loosening methods like double nuts is more reliable.

05 Special Circumstances Require Special Consideration

For large-diameter fasteners, industry standards have special requirements. For example, HG/T 20583-2020 stipulates that nuts with a diameter of M48 and above should be equipped with hardened flat washers and require the use of special devices capable of controlling the tightening load.

This regulation considers the huge pre-tightening force that may be generated when tightening large-diameter bolts. The hardened flat washer here not only helps distribute pressure but also ensures the stability of the tightening process.

Under conditions with vibration and significant temperature changes, the selection of washers requires even more caution. Ordinary spring washers may not be able to cope with severe operating condition changes, necessitating consideration of specialized anti-loosening solutions.

There are no one-size-fits-all rules in the field of engineering. The selection of washers is essentially a comprehensive consideration of the connection system—from material hardness to load characteristics, from installation conditions to maintenance needs. Understanding the mechanical principles behind each connection is more important than mechanically following rules.

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