Heavylift Tip – Trailers

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Open Circuits and Pivot Points – 1

A Self-Propelled Modular Transporter (SPMT) – a platform vehicle used for the transportation of heavyweight cargo – is made up of a combination of axles which are operated by a power pack unit (PPU). The PPU is the most important feature of the SPMT and is responsible for the power supply to the hydraulic systems which is used for controlling the axle height, braking and steering. This gives the SPMT the ability to lift and transport virtually every heavy load in the world.

All SPMT wheels have the capability to pivot to allow them to deal with undulations in the ground surface. However, one of the fundamental pitfalls is unintended rotation around these pivot points, which typically occurs when there is also a narrow support point above a tower of packing. When combined with an open hydraulic circuit, the loading on the trailer can become critical, causing it to tip or cargo to slide off.

What is an open circuit?

An open hydraulic circuit is the term used to describe the free flow of oil between hydraulic suspension rams. When the SPMT comes into contact with tilted road surfaces, the hydraulic fluid will flow within each suspension to maintain equal loading. This occurs when the rams on each side of a two-file trailer are linked together in one circuit, similar to that which occurs in the point circuit of a three-point suspension. The load will usually remain balanced – even on irregular or sloping ground – due to the tipping or twisting resistance provided by the side circuits and rigid chassis.

As can be seen in the diagram below, when there are no side circuits and single trailers are piped into a larger hydraulic group without being physically connected to each other (such as in an overall four-point setup or multi-trailer arrangement), there exists the potential for a trailer to “roll” around its pivot point. In these circumstances, this roll can be prevented in the short term by ensuring the ram tops are covered by additional supports.

Additionally, this roll can 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) or the position of the load point on the transporter, can quickly develop into a transporter rotating underneath a piece.

To ensure the cargo has the proper support during its transport and the hydraulic circuits are not overloaded, constant control of the SPMT is required. It is, therefore, imperative to get advice from a professional who understands the affects that uneven terrain, traction power, sharp turns and speed can have on your cargo transportation.

To be continued….

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