Digging for riches Mining and construction operations kick up a lot of dust, particulate that can clog engines and cause them to grind to a halt. Donaldson Company makes filtration systems that prevent engines like those from Caterpillar ( NYS: CAT ) from seizing up. It counts the heavy equipment manufacturer as one of its top customers, generating just under 10% of total revenues, so it's easy to see the immediate future for Donaldson as pretty bright.