An excavator arm consists primarily of a boom, stick, and attachments. Hydraulic cylinders power the movements of the parts, pins bind parts together, and linkage attaches parts to one another in ways that allow for parts to pivot to perform actions. While not part of the arm, the entire hydraulic system is controlled by the main hydraulic pump and main hydraulic valve, located in the house of the machine, and connected to hydraulic cylinders through hydraulic lines throughout the arm and the entire machine.

Boom

The excavator’s boom is the largest of the main components of the arm. It is pinned directly to the house and pivots at that connection. This pivoting action allows the arm to rise and lower relative to the digging surface and also extend the reach of the arm or draw closer as it pivots.

Stick

The excavator’s stick sits between the boom and the attachment and pivots at the connection to the boom. This pivoting action further allows the arm to position the attachment further or closer to the machine, as well as control the height of the bucket relative to the digging surface. By changing the angle of the stick the operator can position the bucket deeper than the tracks of the machine to dig or at a point high off the ground to load a machine or pile materials.

Attachment/Bucket

The attachment on an excavator is the tool at the far end of the arm. Increasingly, a wide range of attachments are used with excavators from attachments for clearing ground to attachments for hammering and breaking large rocks. The most common attachment is the bucket, but even excavator buckets can be as diverse as the materials they handle from teeth and side cutters for digging to thumbs to grab materials.

Boom Cylinders

Most commonly, two cylinders on each side of the machine will be attached at one end to the house and at the other to the boom. Extending and contracting the cylinders will rise and lower the boom and subsequently the entire arm.

Stick Cylinder

A stick cylinder attached to the boom and the stick and mounted at the top of the boom will rotate the stick as it pivots around its connection to the boom.

Bucket Cylinder

Unlike the other cylinders on the excavator arm, the bucket cylinder is connected to the bucket through linkage. By extending the bucket cylinder, the attachment will perform specific actions. In the case of a bucket, the bucket cylinder will control the loading and dumping of the bucket. Since the angle of the bucket can also be changed as the stick and boom are moved, holding a load with efficiency can require changing the angle of the bucket even after a digging motion has been completed.

Linkage

The linkage on an excavator arm are heavy-duty connectors that help define and control the movements of the attachments. These parts link together the stick and attachment while also allowing the attachment to move in very specific ways.