Manhole cover lifters and ladders are manufactured in Finland by Teräs-Kari OY. The first lifting magnet was invented by the company's founder Kari Toivonen in 20 years ago, which also has the patent Pat FI 121467.
1600 kg is a theoretical lifting force calculated from tests and experience. Theoretical because the conditions and materials must be ideal and the lifting must be done without false movements and in a straight line. Dirt such as sand and stones and unevenness and holes in the metal surface weaken the adhesion, but the strongest possible adhesion is required. To achieve a force of 1600 kg, the material must be thick enough to form such a strong attachment. However, this 1600 kg force allows the lifting magnet to be removed from the hatch, as the product has enough torque on the rod. The metal hatch lifting magnet opens easily due to the optimal length of the rod.
