Batteries to power your forklifts or other electric vehicles? To function standby power for utilities or UPS programs? To start up your autos or vans? Each and every of such designs of castor wheels are connected to an item (most often a trolley) applying 4 bolts and nuts (or screws) https://hukums.com/products/elak-2-wheeler-automatic-battery-charger-mx-20-1