Excavator
Excavators are machines to loosen and load of soil in a continuous or step-by-step way. As long as the extraction takes place in a wet environment, they are called dredging machines. Cable excavators use a cable and winches to use the tool, hydraulic excavators move the tool using a hydraulic transmission.
Excavators are usually moved using a hydraulic drive for driving, moving and working.
Construction
Depending on the way how the tool is moved, excavators can be divided into cable excavators and hydraulic excavators. The construction of the basic unit is quite similar and consists of a underbody with running gear, a chassis with engine, hydraulic pumps, working place and a boom to connect the tools.
The running gear is either constructed with wheels or caterpillars. Heavy machines always have a caterpillar due to the better distribution of the forces into the soil. Light and medium sized machines can also be built with a wheel drive (up to 30 tons, 130 kW and 2 m3 shovel size). This enables the machine to drive fast.
Performance calculation
Many factors influence the performance of an excavator:
- the consitensy of the soil - the shovel layout and dimensions - the available force of the machine - the position, size and ability of movement of the machine - the size ration of the shovel to the truck volume - the construction site's efficiency
The cycle time of an excavator consists of the four parts
- filling the shovel - moving of the full shovel - emptying of the shovel - moving of the empty shovel
The bulking of the soil is an important factor, e.g. rocks have a very high bulking whereas the bulking of sand and gravel is very low.
The fill factor of a shovel depends on the soil. The more cohesive a soil is, the higher is the filling factor.
The performance of an excavator can be calculated with
L (scm^3/h) = V * ff * fa * n * 3600 / t
with
t = cycle time [s] V = volume of the shovel [m^3] ff = filling factor of the shovel [1] fa = bulking factor of the soil [solid cubic meter / loaded cubic meter] n = efficiency factor of the construction site.
Example:
t = 35 s V = 2 m^3 ff = 1.1 (Sand) fa = 0.9 (Sand) n = 0.75 (45 min / h)
leads to L = 152 solid cubic meter per hour