hi all can you please explain to me what a rubber tyre gantry is & how it works
Well, a gantry is a 2-D or 3-D positioning device - a usually rectangular arch goes across the workspace and runs on linear bearings of some sort so that it can be moved across the workspace in the direction perpendicular to the direction of the arch. (It's an arch if the bearings are on the ground or floor of the workspace, a ceiling-mounted version is basically a pair of rails or similar bearings attached to the ceiling.) Another linear bearing or pair of bearings runs along the top of the rectangular arch. A hoist or some other device for vertical positioning is mounted on those bearings. So the arch itself can be moved in one direction the hoist can be moved in the perpendicular direction and whatever is hanging from the hoist can be moved vertically. The combination of the 3 movements allows positioning whatever is hanging from the hoist in 3-D. In a rubber tyre gantry the 1st and often the 2nd dimension bearings are rubber tyres, usually on some sort of rail-like support such as the inner bottom surface of an I-beam with the I-beam web (cross piece) vertical.
Gantries are used in cranes, especially for loading ships, but also often indoors and built into the structure of factories. The general setup is also widely used in machine tools for positioning a router or other cutting device. See wikipedia http://en.wikipedia.org/wiki/Gantry_crane" [Broken].
Here is a gantry-type CNC machine: http://www.legacycncwoodworking.com/artisan-ii-cnc/
(Better mechanisms for machine tools that don't use the cartesian x-y-z movements are possible - see Don Lancaster's http://www.tinaja.com/glib/flutwump.pdf )
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