I created those skates for a company I used to work for. They are meant to connect to the corner castings of a container and while the other end is lifted by a forklift or a electric pallet jack. That makes it possible to move a LOADED container on hard concrete or, with use of steel plates on asphalt. We moved containers heavier than 25 ton using four of those but at that weight it gets into heroics department. Low tech but will get you out of a bind if the floor of the building is reasonable. The rollers are steel for low friction and load capacity. Call or email to this posting. NO TEXT. If the ad is up the skates are available and no need to ask.