Ball bearing butt hinges (also known as ball-race) have capsules between the joints on the knuckle which contain small ball greased bearings which are the ultimate option for heavier doors in particular. Light periodical oiling is recommended to avoid wear. 75mm hinges are ideal for standard internal doors up to 40mm thick, whereas the 100mm size is suited to 44-54mm thick doors such as fire doors and external timber doors. Check the hinge page for suitability for your doors. Ideally, 3 hinges fitted to a door will help avoid a warped door, especially airing cupboards and bathroom doors where heat and humidity can alter the shape of wood.