Open Circuits and Pivot Points – 2
In a previous tip released last week, we covered the topic of open hydraulic circuits and the issue of unintended rotation around pivot points. This continuation will look at other pivot points that can be created when using a self-propelled modular transporter (SPMT) and the consequent effect on the cargo transportation.
Looking away from the trailer itself, we must consider the packing and then the interface with the cargo. To ensure the load can handle factors such as the route the SPMT travels, the packing of the cargo must be appropriately placed and equally distributed. Most cargo designers will insist in a single and discrete loading point onto the SPMT for the sole purpose of simplifying the design and analysis of the structure. Unfortunately, as can be seen in the diagram, the single point of loading can inadvertently create a narrow point of contact, which acts as a pivot point. Multiplying this to other points means the potential exists to create, in essence, a pin-jointed portal frame.
This diagram may well be an obvious example, however, it can also occur when it would appear that good loadbearing has been achieved:
• If all the load-spreading supports are tied into a single support on the cargo underside, this can create a pivot point.
• If the cargo is a tubular construction and the support is a saddle, there is the possibility that the saddle can rotate around the tubular.
To ensure that the cargo transport is safely executed, solutions to both of these examples are strong shear stops for tubular cargo, as well as lashings or support props to a different part of the cargo.
In order for the SPMT to remain stable, the centre of gravity (CoG) must be identified. Gravity affects the ability to lift, lower, move and stabilise cargo, and therefore has to be considered throughout every stage of the loading and transportation process. The SPMT will either have a three-point or four-point suspension in which the CoG must be located within the triangular (for a three-point) or rectangular (for a four-point) stability zone. Should the CoG move outside of these stability areas, the load will become uncentred and can ultimately result in serious accidents. It is important to note that other elements (such as road conditions, ground slope, and wind) will also alter the CoG during transportation and must be taken into account to avoid the cargo from tipping.
The configuration of cargo loading on a SPMT needs to be accurate to understand not only the CoG, but the dimensions and weight of the item, in addition to the location of the lashings and securing points. To ensure the load can be transported as safely as possible, it is crucial you speak with a professional who can be there for the lifting, moving and setting of the heavy cargo.
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