Though more expensive to buy than hammer drills, rotary hammers (which can also be used for driving in ground rods) are also more efficient to operate. A rotary hammer operates at a significantly lower rpm than a hammer drill, but offers a much faster drilling rate because of the design of the mechanism used to provide the impact force.
Rotary hammers have two designs. Smaller rotary hammers suitable for electricians setting anchors feature a hollow piston design, while larger ones use a crank-type piston.
In the hollow piston design, the ram and the beat piece both travel within the cylinder, or barrel, that is inside the front of the tool. The beat piece is what actually contacts the bit, hitting the SDS bit into the material. The beat piece moves in very small increments when hit by the ram, which is traveling back and forth within the hollow piston, where the compression is created. After the bit is hit, the rebound energy coming back from the bit pops the beat piece back and the ram is rebounded backward toward the rear of the piston, never touching the back of the piston. Air compression is built up and the piston is cranked forward, slamming the ram forward again toward the beat piece. The compressing air pressure when the ram moves backward absorbs the backward forces.