Top Plate fitting castor
This is the most popular method of fixing a castor to somthing. The plate can be square or rectangular and in gerneral will have a hole in each corner. The castor is fitted using the four holes with either nuts and bolts or studs attached to the item. There are may other ways of attaching the plate castors and this will depend on the application. Some people weld the plate castors on for speed but this can cause problems later if you need to change the castors.
The holes on the plates can be slotted in some way. This is quite common and is designed to ease fitting, giving a bit of leway on the fittings.
A plated castor can either be fixed, Swivel or Swivel with a brake. An alternative to a plate fitting castor is to use a single bolt hole castor.
Click on the picture for some examples of plate castors