How a Power Post Hole Digger Works

Purpose

Powered post hole diggers are used in farms, ranches and large garden communes to dig holes for fence posts. They also aid in digging holes for trees and to aid in aerating the soil of new gardens. Land surveyors and water quality technicians use them to check the depth of the water table on lands in preparation for construction or to find the best place to sink a new well.

Diesel Models

Diesel post hole diggers are larger than the man-portable designs and install on most tractors or earth-moving machinery. The screw can be up to 8 feet in length and a foot in diameter. The back end of it sits against a flywheel which connects by means of a cv joint to a splitter in the undercarriage of the tractor. The splitter runs to both the drive axle of the tractor and to the transmission. The transmission connects to the tractor's diesel engine. Depending on the model tractor, the number of pistons and firing chambers in the engine may be as many as 12 or as few as four. In any case, they all operate in the same four-part series. The intake valve on the first firing chamber opens, allowing air in and letting the piston fall to the bottom of the chamber. While the intake process repeats in the second firing chamber, the piston in the first firing chamber moves back toward the top, compressing the air. While the intake process repeats in the third firing chamber and the compression process repeats in the second firing chamber, the fuel injector sprays diesel fuel into the first firing chamber, which ignites because of the high air pressure. It pushes the piston down, turning the crankshaft. While the intake process repeats in the fourth firing chamber, the compression process repeats in the third firing chamber, the ignition process repeats in the second firing chamber, the piston in the first firing chamber is pushed back up to force the burnt gasses out of the exhaust process. In this way, all firing chambers work in concert to constantly turn the crankshaft, which causes the screw of the post hole digger to spin.

Two-Stroke Models

The screw--- typically 3 or 4 feet in length and 6 inches in diameter--- is made of cast iron or steel and connected to a crankshaft at the top. Atop this sits a two-stroke engine similar to what you would find on a lawnmower or industrial weed-eater. The operator holds the digger between two horizontally protruding handles and engages the throttle. The throttle is connected to a carburetor, which pulls in air and combines it with a fine spray of fuel. This combination is pulled in through the crank case of the engine, where a single piston sits in a combustion chamber. At the top of the combustion chamber sits the fuel/air mix intake, the exhaust port and a spark plug. As the fuel enters the combustion chamber, the piston rises, compressing the fuel. At the appropriate moment, the spark plug sparks, igniting the mix and forcing the piston back down the crank case. The pressure created by the explosion forces the exhaust out of the firing chamber and the vacuum created by the exiting exhaust pulls in more fuel. The momentum of the turning piston pushes it back up to the combustion chamber and the process repeats. Each time the piston is pushed down it turns a crankshaft. The turning motion is transferred from the crankshaft to the screw, making it revolve so that the operator can drill a hole.

References

• Earth Auger Function
• How a 2-stroke engine works