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THE Top Tip – Open Circuits and Pivot Points (2)

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Previously we covered the topic of open Hydraulic circuits. One of the basic issues/pitfalls involving self propelled modular trailers (SPMTs) is unintended rotation around pivot points. We looked at the pivot point created at the point of rotation of the axle. Now let us look at the other pivot points that can be created and the compounding effect they could have on the system.

Looking further up from the trailer, we have packing and then the interface with the cargo. Most cargo designers will insist on a single, discrete loading point into the structure, this is for the obvious issues of simplifying the design and analysis of the structure. Unfortunately, as can be seen in figure 7, the single point of contact has inadvertently created a narrow point of contact, which acts as a pivot point. Multiplying this to other points, means that the potential exists to create, in essence, a pin jointed portal frame.

• This is the obvious example. It can also occur, when it would appear that good load-bearing 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.

Solutions to both of these examples are strong shear stops (tubular) and lashings or support props to a different part of the cargo.