HEAVY DUTY DISC SPRINGS: These DIN 6796 Heavy Duty Belleville Washers are well suited to bolted applications. They are designed to help prevent loosening of the bolt/nut which could occur due to setting of a gasket or movement due to expansion of a component.

These Heavy Duty Disc springs are available to purchase online…
https://springfasteners.co.uk/product-category/disc-springs/heavy-duty-disc-springs-din-6796/