Although they perform a simple, almost primal feat of pounding and breaking, the inner workings of jackhammers are complex. Their exact construction varies from model to model, but there are some similarities in how they function.
Electric jackhammers are usually lighter weight models best suited for small jobs or for handymen. Gas versions are designed for job sites where there's simply no power or compressed air.
Pneumatic jackhammers were (and still are) among the most popular types of jackhammers. They became the standard in large part because of the specific needs of underground mining, in which any sparks from combustion engines threatened to ignite explosive gases. Not only was compressed air nonflammable, it didn't lose its power through long delivery hoses, even when the compressor was located many feet from the pneumatic tool at the other end.
A pneumatic tool is a device that uses highly compressed air as its power source. Typically, a diesel engine drives the air compressor, which then forces the air through a hose to the end tool -- in this case a jackhammer.
The body of a jackhammer is a vertical cylinder, which acts as a pressure chamber. Compressed air enters the chamber and activates a trigger valve, which rapidly opens and closes. When opened, the piston allows pressurized air into a piston chamber. Pressure rises in the piston chamber, causing the piston to move, striking the bit. The bit (or chisel) strikes the surface below, causing rock to fracture.
After the piston slams into the bit, the trigger valve closes and redirects the compressed air to the cylinder below the piston. The energy of the air, along with a spring, forces the bit back into its original position. Then the process begins anew. Typically, there's at least one exhaust valve on the cylinder that relieves pressure during the up-and-down piston cycle.
Hydraulic jackhammers work in the same manner but use fluid instead of compressed gas. These are typically larger, heavier and more powerful models that are mounted to big construction rigs and, thus, are used for jobs too big for a handheld jackhammer.