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They are used in the same applications as fender washers. A spherical washer is part of a self-aligning nut; it is a washer with one radiused surface, which is designed to be used in conjunction with a mating nut in order to correct for up to several degrees of misalignment between parts.
Specially designed Spherical Washers, two installed in proper order under each nut, corrects for the lack of parallelism and equalizes the load on the tightened nuts. For specific instructions on clamp installation and correct installation of washers, see https://www.woodcousa.com/clamp-install.htm .