Adding flexibility to the bolting helps a joint hold load when the gasket relaxes or the joint heats and cools. Conical spring (Belleville) washers are one way to do it. PCC-1-2026 P-4.2.2 covers the benefits and the pitfalls. A printable PDF (PB-TD-220) is below. Values are for reference; confirm against the current edition of the governing standard. Send us your bolting list for a quote or call (281) 581-0176.
Related: Through-Hardened Flange Washers · Flange Live Loading with Belleville Washers (article) · All technical docs
Ways to add bolt flexibility (P-4.2.2)
- Longer bolts with extension collars or spacers.
- Smaller diameter bolts, or reduced-shank (waisted) bolts.
- Conical spring washers made to fit standard flanges, with stiffness matched to common bolt stresses such as 30, 45 or 60 ksi.
Selecting spring washers
- Best practice is to choose a stiffness so the washers are fully compressed at the assembly bolt stress. Follow the washer maker’s literature.
- When fully compressed, effective stiffness is about twice the nominal value, so they are about half as effective as their rating suggests.
- Washer response is not linear.
What can go wrong
- Environmental cracking, fatigue, creep, yielding or loss of spring properties can lead to complete loss of bolt load.
- Installation errors in orientation or stacking.
- Wrong quantity, stiffness or initial compression.
- Creep and relaxation of the washers themselves at high temperature.
- Stacks shifting sideways if the washer ID is loose on the bolt.
- Friction in parallel stacks. Lubricate them.
Related points
A flange bolted into a tapped hole is about twice as stiff and loses about twice the load for each 0.001 in. of gasket relaxation (P-4.2.1). Higher-strength bolting can help where stainless bolts would yield (P-4.3).
References: ASME PCC-1-2026, Guidelines for Pressure Boundary Bolted Flange Joint Assembly. Summaries are paraphrased; clause numbers refer to ASME PCC-1-2026 unless noted. Patriot Bolt manufactures bolting and does not perform bolting, training or joint assembly services.