Excavators where the digging action is a downward arch motion are known as backhoes or hoes and even back shovels. Thus they are used to excavate below the ground surface or below the machine track level.
Picture of a crawler track backhoe
Back hoes beside being mounted on crawler tracks can also be mounted on a wheel base. Wheel mounted excavators are not specifically for bulk excavation but designed for mobility and general purpose works. The John Deere excavators and JCB excavators are the most well known examples of the wheeled based types. A typical modern back hoe and it's key components are depicted in the diagram below.
A Backhoe Diagram
Uses of back hoes
These machines are suitable for excavating trenches, pits for basement and smaller machines can handle general grading work. It is a versatile machine in that it can perform both excavation and lifting works. Example in drainage works or utility works , the back hoe can perform the trench excavation and handle the pipes or culverts. Thus this makes the need for a second lifting machine unnecessary.
During excavation the penetration force in to the material being excavated is achieved by the stick cylinder and the bucket cylinder. The buckets can be selected depending on the type of material excavated. For easily excavated material wide buckets are used. When excavating rocky material or blasted rocks, a narrow bucket is used. In utility works, the width of the required trench is the deciding factor in selecting the bucket.