Your chain is made up of a simple string of components, flexible enough to wrap itself around your sprockets but strong enough to pull your back wheel around when you stamp hard on the pedals.
Each link is made up of a small selection of components – two sets of 8-shaped plates, a rivet that allows each set of plates to rotate relative to its neighbour and a doughnutshaped roller encircling the middle section of the rivet. Each link of the chain sits in turn in the valley between two of the teeth on your chainring, and so gets pulled forwards as the chainring rotates. The chain meshes in turn with the teeth on your sprocket, each link sitting in a valley and pulling against the back face of a sprocket tooth to drag your wheel forward.
The mating surface between tooth and chain link is very small, under high pressures and often dirty – a classic recipe for wear. But careful design means that chains last much longer than you’d expect them to in the circumstances. This longevity is down to a critical component – the roller shrouding each rivet. These can rotate freely around the rivet – give one a wiggle on your chain to see what I mean. This means that as each roller settles into the valley between two teeth, it can rotate into place, even under pressure, minimizing the kind of metal-on-metal sliding that promotes rapid wear.
As a result, the external surfaces of the chain require very little lubrication. This is a good thing, as oil is sticky and makes dirt stick to your transmission, which causes wear. However, there is an area where there is metal-on-metal contact – where the inside surface of the roller rotates about the rivet. This would wear if not lubricated. Luckily, because the roller is trapped between the chain plates, dirt finds it pretty difficult to work its way into this surface.
Outer and inner plates, roller, rivets and link
So, for minimum wear, the hidden, internal surfaces of your chain, which stay relatively clean, need lubrication. The external surfaces, that you can see, need only enough lubrication to limit corrosion – not a thick sticky layer that will pick up dirt. That’s why the most effective maintenance routine is to clean your chain, oil it with good-quality oil, leave enough time for this to soak into the gaps between the plates and inside the roller, then wipe excess oil off the outside of the chain.
Bike chains all have the same pitch, namely 1⁄2 in. However, they have different widths, determined according to the number of sprockets on the cassette at the back. Sprockets on, for example, nine-speed cassettes are narrower (both the sprocket plate and the gap) than eight-speed cassettes so that the extra sprocket can be packed into the same space. The narrower chains are also more flexible, so they change more slickly. By the same token, they need to be kept clean and well-lubricated or they will wear more quickly. A chain that is too narrow for the block will stick, whereas a chain that is too wide won’t shift properly. Both will wear down the sprockets very quickly.
Compatibility between parts is an important issue. You can be as careful as you like with adjustments for your front and rear gears, but if the components aren’t compatible, the adjustment won’t make any difference. Shimano is the undisputed giant in the transmission department; other manufacturers make their components compatible with Shimano parts. The most sensitive combinations are chain, shifters and cassettes. For example, nine-speed chains are considerably narrower than eight-speed ones, and have matching narrow sprockets, so there is no compatibility between the two systems.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
See also bike maintenance tips, tricks and techniques “How gears work”