Bottom (support) rollers
Bulldozer bottom rollers are classified into single flanged and double flanged. Bottom rollers lean against inner part of the chain. With their help happens excavator, bulldozer movement. They withstand main loads.
Top (carry) rollers
Top rollers carry track chain, so that it does not fall on frame. How many carrying rollers will be mounted depends on what is the distance between excavator (bulldozer) sprocket and idler.