Читать книгу Inventors at Work, with Chapters on Discovery - George Iles - Страница 20

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In lofty structures, the box girder is frequently employed as a column or a beam because it has even greater rigidity than the I-beam; usually it has four sides, but it may have eight, sixteen, or more, and thus step by step we come to a hollow cylindrical column which has, indeed, the best form that can be bestowed on supporting material. Chinese builders learned its economy on the distant day when they adopted the bamboo for their walls and roofs. Comparison with a solid stick of timber of like weight and substance will show that an equal length of bamboo is decidedly preferable. The inner half of a round solid stick does comparatively little in holding up a burden; to remove that half is therefore as gainful as to strip from a joist the timber surrounding its working skeleton. At first the journals or axles of engines and large machines, as well as the axles of railroad cars and the shafts of steamships were solid; to-day, in a proportion which steadily increases, they are hollow. The advantage of this form comes out when we take two cylinders of rubber, alike in length and weight, one solid, the other hollow. Supporting both at their ends, the hollow form sags less than the solid form, proving itself to be the more rigid of the two.

Solid rubber cylinder sags much.

Hollow rubber cylinder sags less.

Handle-bar of bicycle in steel-tubing.

A sulky in steel tubing.

A pneumatic hammer, steel tubing.

Fishing-rod in steel tubing.

Bridge of steel pipe.

With like advantage seamless tubing is adopted for a broad variety of purposes. It builds bicycles and sulkies which far out-speed vehicles of solid frames; it is worked up into elevator cages, mangle rolls, pneumatic tools, fishing-rods, magazine-rifle tubes, inking rollers, farm machinery, poles, masts and much else where strength and lightness are to be united. Steel tubing is readily bent into any needed contour, even when of considerable diameter. Mr. Egbert P. Watson has pointed out its availability for highway bridges of about forty feet span, no professional bridge-builders being needed for their construction. Near Saxonville, Massachusetts, a pipe-arch bridge, eighty feet long, provides a roadway across the Sudbury River, while carrying within its pipe a stream which forms part of the Boston water system. A bridge of similar form, 200 feet long, spans Rock Creek in the City of Washington. The Eads bridge crossing the Mississippi, at St. Louis, employs for each span eight steel tubes of nine inches exterior diameter. Tubes large and small have been strengthened by adopting the model of an old-fashioned fire-lighter, or spill, a bit of paper rolled spirally as a hollow tube. Blow sharply into it and you but tighten its joints. In like manner tubes and pipes of metal are all the tighter when their seams are spiral instead of longitudinal. An eager quest for combined strength and lightness in the bicycle has ended in the choice of tubes spirally welded.

Arch bridge of steel pipe,

Sudbury River, near Saxondale, Mass.

Spiral fire-lighter.

Spiral weld steel tube.