All but the cheapest chainsets are made up of a spider and separate rings. The spider is the crank, with four or five arms onto which the chainrings bolt. There have been a number of different sizes and shapes of chainrings over the years, each with plausible reasons for existing, resulting in incompatibility between all the different versions. Whatever the science, it makes sense to take your bike or your old chainring to the shop when you buy a new one to make sure you get the right size.
Chainrings are machined to help the chain move across it when you change gear. The most difficult change for the chain to make is from the smallest to the middle ring, so goodquality middle chainrings are shaped to facilitate the change. They often have little ramps riveted onto the sides to lift the chain when it’s halfway across. When the chain moves onto a bigger chainring, it has to lift itself up and over each tooth on the new chainring before it can settle in the valley between two teeth and do some work. Good chainrings are designed to make this shift as easy as possible.
Chainrings under pressure
The chain moves across the chainrings most easily when it’s not under too much pressure, so chainrings are designed to encourage the chain to move across when the pedals are at the top or bottom of your pedal stroke, when you put least pressure on them. On many chainrings, these teeth are shorter than the ones on either side to make it easier for the chain to climb over them and drop into place.
It’s all right for these teeth to be shorter. Because of their position on the chainring they don’t get as much wear as those that are engaged when your pedals are level with the ground. It can be disconcerting though, to buy a new chainset and find that some teeth are shorter than you expect!
Worn teeth mean the chain slips over the chainring
Worn chainrings are a major source of chainsuck – and a sign that you’ve been out riding having fun, so you’re going to have to get used to replacing them. Worn chainrings also stretch your chain quickly, so changing them is a good investment.
You should be able to tell that your chainring is worn by looking at it and comparing it to these pictures. If you wait until a chainring is so badly worn that the chain slips across it, you’re too late! You will usually find that one ring wears before the others, usually the one you use the most. It’s fine to change rings one at a time, they don’t have to be done all at once. When changing your cassette and chain, it’s a good time to also change your most worn chainring.
Changing chainrings can be done with the chainset still on the bike, but the job is easiest if you remove it, saving yourself from bleeding knuckles too. Follow the instruction in the section on removing your chainset.
Once you’ve removed the chainset, turn it over to see what type it is. The chainrings are either bolted to the crank arm using an Allen key or in a few cases are attached with a lockring and a circlip.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
See also bike maintenance tips, tricks and techniques “Chainline”