There are actually two types of links alternating in the bush roller chain. The 1st type is internal links, having two internal plates held with each other by two sleeves or bushings where rotate two rollers. Internal links alternate with the next type, the external links, consisting of two external plates held with each other by pins moving through the bushings of the internal links. The “bushingless” roller chain is similar in procedure though not in structure; instead of separate bushings or sleeves keeping the inner plates jointly, the plate has a tube stamped into it protruding from the hole which serves the same purpose. It has the advantage of removing one part of assembly of the chain.
The roller chain design reduces friction in comparison to simpler designs, resulting in higher efficiency and less wear. The original power transmission chain varieties lacked rollers and bushings, with both the inner and outer plates kept by pins which directly contacted the sprocket the teeth; however this configuration exhibited incredibly rapid wear of both sprocket teeth, and the plates where they pivoted on the pins. This issue was partially solved by the development of 
bushed chains, with the pins holding the outer plates passing through bushings or sleeves connecting the inner plates. This distributed the put on over a larger area; however the teeth of the sprockets still wore more rapidly than is desired, from the sliding friction against the bushings. The addition of rollers encircling the bushing sleeves of the chain and supplied rolling contact with one’s teeth of the sprockets resulting in excellent resistance to put on of both sprockets and chain as well. There is even very low friction, so long as the chain can be sufficiently lubricated. Constant, clean, lubrication of roller chains is of primary importance for efficient operation as well as correct tensioning.