A baler is a piece of farm machinery used to compress a cut and raked crop (such as hay, straw, or silage) into compact bales that are easy to handle, transport and store. Several different types of balers are commonly used, each producing a different type of bales – rectangular or cylindrical, of various sizes, bound with twine, netting, or wire. Industrial balers are also used in material recycling facilities, primarily for baling metal, plastic, or paper for transport.

SMALL RECTANGULAR BALER:

A type of baler which is less common today in some places but which is still prevalent in many countries such as New Zealand and Australia to the exclusion of large bales produces small rectangular (often called "square") bales. Each bale is about 15 in x 18 in x 40 in (40 x 45 x 100 cm). The bales are wrapped with two, three, or sometimes four strands of knotted twine. The bales are light enough for one person to handle, about 45 to 60 inches (1.1 to 1.5 m).

To form the bale, the material to be baled, (which is often hay or straw) in the windrow is lifted by tines in the baler's pickup. This material is then dragged or augered into a chamber that runs the length of one side of the baler. A combination plunger and knife moves back and forth in the front end of this chamber. The knife, positioned just ahead of the plunger, cuts off the material at the spot where it enters the chamber from the pickup.

The plunger rams the material rearwards, compressing it into the bales. A measuring device measures the amount of material that is being compressed and, at the appropriate length it triggers the mechanism (the knotter) that wraps the twine around the bale and ties it off. As the next bale is formed the tied one is driven out of the rear of the baling chamber onto the ground or onto a special wagon hooked to the rear of the baler. This process continues as long as there is material to be baled, and twine to tie it with.

http://en.wikipedia.org/wiki/Baler