# Influence of Crushed Rock Properties on the Productivity of a Hydraulic Excavator
## 4 Results and Discussion
### Analysis of the excavator Productivity Determinants  
As we stated several times, all factors that we discussed above (bucket filling, loosening, and swelling) indubitably significantly depend on the granulometric composition of the rock material. Accordingly, the productivity of the excavator must be strongly related to that material characteristic too. Another material property that will surely have a strong influence on excavator productivity is the angle of repose. On that angle will depend the heaped volume of material in the excavator bucket. Except for the material characteristic, organization of the working place, that is the slew angle of the excavator, influences the duration of the working cycle and consequently, the productivity of the excavator. Table 4 shows a comparison of the excavator productivity for a loose state of the material calculated in three ways: Q\({}_{\text{Vm}}\)--productivity calculated by field-measured volume of the material in the bucket and Q\({}_{\text{ISO}}\)--calculated by the ISO procedure of the bucket volume calculation, and Q\({}_{\text{CECE}}\)--calculated by the CECE procedure of the heaped bucket volume calculated by expression (5):  
\[\text{Q}_{\text{ISO,CECE}}=(3600\text{ V k}_{\text{f}}\text{ k}_{\text{l}})/\text{T}_{\text{C  
In the case of an angle of repose of 34.6\({}^{\circ}\), both methods are equally precise in productivity calculation. Above this angle, as can be seen, the ISO method is more suitable since the heaped bucket is calculated assuming an angle of repose 45\({}^{\circ}\).  
Figure 7 shows the influence of the slew angle on excavator productivity, which is calculated by the measured volume of material in the bucket.  
It can be noticed that productivity is inversely proportional to the slew angle. Although a low correlation factor suggests a relatively small dependence between the excavator productivity and slew angle, this is not the case. The total cycle time comprises the time required to load, dump, and maneuver the bucket. A larger angle causes a longer cycle time and thus less productivity.  
However, depending on the task of the excavator, the time for maneuvering the bucket depends on the slew angle, but also on the height of lift and digging depth. If there is a large difference in the height of lifting and lowering the bucket, then the overall cycle can be longer regardless of the relatively small slew angle. An example can be seen from Table 2. In the case when the excavator was used to shift the pile, the total time of the cycle was shorter, although the slew angle was higher than in the case of loading the truck. Litvin & Litvin found that under the same conditions of installations of dump trucks for loading, the excavator cycle time decreased exponentially with the slew angle [44].  
Since it is evident from the performed analysis that there is a considerable influence of several observed properties on excavator productivity, a multiple regression analysis of the simultaneous influence of multiple properties was conducted.  
A linear regression model was assumed in which the excavator productivity for a loose state of material is a dependent variable (6), and all other observed and analyzed properties are independent variables.  
\[\text{Q}_{\text{Vm}}=\text{b}_{1}\,\text{x}_{1}+\text{b}_{2}\,\text{x}_{2}+ \ldots\,+\text{b}_{n}\,\text{x}_{n}+\text{Intercept} \tag{6}\]  
The independent variables are the results of measurements and granulometric analysis from Table 1. The correlation between these variables is shown in Table 5.  
In general, it can be mentioned that \(\text{A}_{\text{r}}\) and \(\text{d}_{80}\) have the most significant degree of correlation with all other variables, while \(\text{d}_{50}\) correlates the least with all other variables.