Background
CMP Engineers was engaged to design new access systems for the boom, gantry, mast, and hoist and drag drums in accordance with AS 1657, ahead of a major shutdown commencing later in the year.
Challenge
A key concern was weight, because every kilogram added to the boom theoretically reduces the machine’s payload, and the client advised that the dragline was being operated at 115% of its designed RSL (rated suspension load). Compliance with AS 1657 therefore had to be achieved with the smallest possible increase in mass. The design also had to work within several fixed constraints:
- Inspection of the machine was only possible during monthly service days and a single short shutdown
- The standard’s requirements could not always be met on mobile equipment, with one gantry stair requiring 22 rises against the usual maximum of 18
- Drum access had to meet working at heights requirements without interfering with safe rope handling during changeovers, and the platforms had to stow out of the way when not in use.
Solution
CMP began with the client’s parts book and construction drawing discs, then measured the boom, gantry and mast during site visits over a period of two months, presenting the proposed design to site engineering and maintenance personnel in the following month.
The boom chord stairs were designed as flights of no more than 18 rises broken by four landing types, including a barrier landing that forces a change of direction and a stepped landing to suit the 42° chord angle, on C-profile stringers rolled from 3mm steel with Webforge treads selected for lowest weight. An 18-rise flight weighed 220 kg in steel and 190 kg in aluminium, giving a whole-of-boom increase of slightly over 3,000 kg for steel treads or 1,170 kg for alloy, equivalent to a suspended load reduction of around 1% for steel and under 0.5% for alloy.
The gantry and mast design replaced the deflection sheave ladders with stairs, fitted a step-type ladder and cage to the top gantry platform, replaced the tension link walkways with stairs and intermediate landings, and extended the mast top platform, adding 1,200 kg with stairs or 840 kg with a step ladder. A revised gantry option used a 22-rise stair with a 300mm raised bottom landing, retaining clearance to the sheave pin for maintenance.
Drum access combined fixed platforms and stairs at the non-drive end with hinged pivoting platforms lowered by cylinders. This limited any fall to 1200mm. After client review, the fixed platforms were cleat mounted so they could be removed for gearbox work.
We consulted Webforge on tread capacity, galvanic isolation for aluminium treads and fatigue resistant handrail connections, priced the custom rolled C section through local steel suppliers, and issued full detail drawing packages to support fabrication tendering.
Outcomes
- The revised gantry access design was accepted by the site’s dragline superintendent and engineers as meeting their maintenance access needs.
- The client reviewed the boom loading and confirmed the approximately 1.5 tonne weight increase was acceptable for the boom going forward.
- Drawing packages were handed over and fabrication tendering commenced in accordance with the overall shutdown schedule.
CMP delivered a complete detail design covering the boom, gantry, mast and drum systems, with steel and aluminium tread options quantified so the client could assess compliance, cost and payload.
The key takeaway from the project was that on a machine already operating above its rated suspension load, safe and reliable access design is more than simple structural design. Considerations of weight, cost, installation complexity, human factors and lifetime reliability all need to be considered and balanced to achieve a successful outcome.
