Open Cut » Overburden Removal
p>After draglines, rope shovels are the most important part of coal uncovery and yet up until now, the process of excavating with a shovel has been largely based on experience and intuition. The work done during this project has advanced understanding of dippers and processes greatly. This project involved a range of site visits, modelling, (qualitative observations and quantitative testing) new dipper design and finite element analysis. In excess of 2,500 tests were undertaken on GBI's model shovel with five different model dippers. The key findings of this work provide a series of key dipper design principles, (which are subject to a provisional patent held by GBI Consulting Pty Ltd). These design principles have been developed from qualitative observations on the dipper filling and quantitative measurement of various factors.
Operational principles have also been identified.
DIPPER DESIGN PRINCIPLES
- When looking into the mouth of the dipper, the height / width ratio will depend on what is being dug.
- The dipper needs to be designed in such a way so as to minimise the void volume as a percentage of apparent spoil volume.
- The back of the dipper needs to be designed in such a way as to minimise the interaction between the spoil flowing into the dipper and the top, rear of the dipper.
- Rounded (high radius) corners should be utilised wherever possible. The shape of the back needs to be rounded to account for the shape of spoil flowing into the dipper.
- This work leads to the recommendation that the use of the term "tooth attack angle" should be replaced with "dipper angle".
- The angle between the centre line of the teeth and the floor of the dipper is described as "tooth angle".
- The hoist rope connection needs to be as far back as possible; at least in line with the rear of the dipper.
- Spoil which is well fragmented and has a range of particle sizes, rather than all large or all small, provides the highest payloads and subsequently best productivity. Differences between average payloads are up to 25%.
- Payload and productivity increase in line with available power. Mines should maximise the power available for digging.
- The application of excess crowd force during filling can reduce potential productivity by up to 10%.
- It is inappropriate to disengage the dipper as soon as it is visually full.
- The optimum productivity occurs at a bench height of about 50% of the boom point height.
- The decision about when to move the shovel is dependent on the distance from the face and is difficult for the operator to optimise.