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THE Top Tip – Open Circuits and Pivot Points (1)

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One of the basic issues/pitfalls involving self propelled modular transporters (SPMT) is unintended rotation around pivot points.

This usually occurs when there is a narrow support point above a tower of packing. When combined with an open hydraulic circuit this can creates a potential incident.

What is an open circuit?
An open hydraulic circuit is the term used to describe the free flow of oil between hydraulic suspension rams. This occurs when the rams on each side of a 2-file trailer are linked together in one circuit, as occurs in the point circuit of a 3-point system. Ordinarily this will not be an issue as there is tipping or twisting resistance provided by the side circuits and the chassis strength.

But when there are no side circuits, such as in an overall 4-point setup, or multi trailer arrangement, where single trailers are piped into a much larger hydraulic group, but are not physically connected to each other, then there exists the potential for a trailer to “roll” around its pivot point.

This roll can be prevented in the short term by ensuring the ram tops are covered by additional supports as a minimum.

This “roll” can also occur in the longitudinal direction, when a trailer is on a single support in the centre. Any minor imbalance in hydraulic pressure centre, transporter cog (perhaps caused by the PPU) and the position of the load point on the transporter can quickly develop into a transporter rotating underneath a piece.