Pipeline & Civil were looking to certify an arm extension for one of their excavators to provide greater reach. The problem required updated lifting charts which is based on both the strength of the extension and the machine capacity. The extension additionally needed to meet any relevant PECPR regulations and required CPEng certification.

A linear static FEA (Finite Element Analysis) was conducted to determine the lifting capacity of the extension itself.

The machine capacity is driven by its overturning limits, but also the boom, arm, and bucket cylinder limits and hydraulic capacities. Having the original lifting capacity charts allowed an accurate baseline to be developed as the mass distribution for a standard excavator is generally not available from the manufacturer.

From the boom pivot there are three degrees of freedom (based on the movement of the three hydraulic cylinders). To keep things simple, the extension was assumed to be in-line with the arm, reducing the problem to a single-solution two-degree-of-freedom system.

From here a calculation set was developed and a macro written to iterate through every possible reach and height combination required for the lifting capacity chart (see Figure 5). It was interesting to see at which points the different limiting factors over-ride the others: in this case the arm cylinder limits at maximum extensions.

Motovated was able to provide input and results in a number of key areas including a strength assessment using FEA, stability calculations and generation of lifting charts, advice on how the extension fits into the PECPR regulations and CPEng certification.
Putting everything together allowed Pipeline & Civil to continue work at a site where access was challenged and an extra 2.5m of reach made all the difference.

Figure 5: Lifting Capacity Chart