Question

Explain the motions of each mechanism in an excavator arm

Answer 1

Excavator consists of three important components namely backhoe, tractor and loader. The backhoe is where the excavator arm is and it consists of numerous parts such as bucket, bucket cylinder, arm cylinder, boom cylinder, swing link, swing cylinders, boom link, arm link and fixed link.

Backhoe Mechanism:

Backhoe excavator attachment is four degrees of freedom system, because each of the four links (swing link, boom link, arm link and bucket link) are allowed to be rotated with their respective joint axes only. Backhoe consists of four different mechanisms each of which can be controlled independently.

A) The first mechanism is for the swing motion of the swing link relative to the fixed or base link, and can be actuated by swing cylinders.

B) The second mechanism is for the rotation of the boom, which is actuated by boom cylinder thus forming an inverted slider-crank mechanism relative to the frame.

C) The third mechanism is for the rotational motion of an arm, which is actuated by arm cylinder, and is also an inverted slider-crank mechanism.

D) The fourth mechanism is for the rotational motion of the bucket. Since a large bucket oscillation is required, the mechanism used is a series combination of a four bar mechanism, and an inverted slider-crank mechanism, which forms a six link mechanism relative to the arm.

Apart from this, the boom assists only in positioning the bucket and the arm for the digging operation; it does not directly contribute in digging operation. On the other hand, the arm and the bucket directly contribute in the digging operation by generating the required digging forces with the help of the hydraulic actuators. The bucket cylinder generates the bucket curl force, and the arm cylinder generates the arm crowd force to excavate the ground. Maximum crowd force is developed when the arm cylinder operates perpendicular to the arm. The ability to break the material is better at the bottom of the arc because of the geometry of the boom, arm, and bucket and the fact that at that point, the hydraulic cylinders exert the maximum force drawing the arm in and curling the bucket.