Issue link: http://read.dmtmag.com/i/449927
I DA U N I V E R S A L J a n u a r y - Fe b r u a r y 2 0 1 5 43 round bearing pressure is always an issue with heavy construction machinery. For wheeled machines, stabilizers and spreader plates can spread the load, but if you want to move across so ground, crawler tracks are the obvious answer. Even regular crawler tracks are not always enough, however. ere is a groundworks company in the Highlands of Scotland called John Mackay Ltd that does a lot of work in protected forest land. When it drives its excavators across the peat-riddled wetlands of the Flow Country, it is important that the machines not only work eff ectively, but also cause as little damage to the terrain as possible. For this reason, it has come to specialize in adapting its excavators to reduce the ground bearing pressure by adding wider tracks. Working with Caterpillar dealer Finning UK & Ireland, John Mackay Ltd has produced what is said to be the world's fi rst low ground pressure (LGP) Cat 312 hydraulic excavator. e new machine has been modifi ed with reconditioned undercarriage, including old track motors that Finning has reconditioned and 1.4-metre wide non-off set track pads originally belonging to a 2005 Cat 312C LGP model. e complete Cat C4 Acert engine was li ed by crane and placed on the special LGP undercarriage. John Mackay, owner of John Mackay Ltd, says that the adapted machine "gives the local conserva- tion authorities peace of mind that we are not only doing a good job, but protecting this wonderful Scottish cultural heritage." Another Scottish contractor that has to cope with boggy ground is Malcolm Construction, which has a Reducing Ground Bearing Pressure JCB JS145 excavator with a longer track frame and 1500mm-wide track pads. e widest standard JCB track pads are 900mm, but JCB supplied a custom-made model for Malcolm to meet its needs. When thinking about ground bearing pressure, I am reminded of a machine that I fi rst saw in 1999 at the Las Vegas Conexpo of that year – the Manitowoc 21000 lattice boom crawler crane. Although it was 15 years ago, I still remember it well. I am not sure that anything like it had been seen before, nor have I seen it since. e Manitowoc 21000 has a li ing capacity of 750 tonnes in standard confi guration, but exerts less ground bearing pressure than a conventional 70 tonne crane. is was achieved by what Manitowoc called its Octa-trac system – instead of two big crawler tracks beneath the main body of the crane, there were two pair of smaller tracks at each corner, mounted at the ends of a connecting beam, making eight tracks in total. Each track was less than 5-metres long and just 1.2-metres wide, with each pair forming a 3.5m wide module. All eight treads are powered hydraulically, with the four right-side treads operating in synch, and the four le -side treads in synch. ( e photo is taken from the website of U.S. li ing specialist All Erection & Crane Rental.) Manitowoc's motivation for Octa-trac was not just to reduce ground bearing pressure. Elimi- nating the need for huge tracks also made it easier to move the components of the crane from site to site. e 21000 remains in Manito- woc's product line-up, but the Octa-trac idea has not been repeated on any other models. e more recent 31000 model, rated at 2,300- tonnes capacity, has four tracks. ● http://blog.machineryzone.com by Steve Rhine, nov 2014 G