236. Chains And Cables

Every link of a chain has to transmit the whole strain to which the chain is subject, so that one faulty link renders the chain useless.

To have a perfect chain, its strength should be equal to that of two rods of iron from which the links are made; that is to say, a chain made of iron one inch in diameter should bear in tension a load equal to that which two one-inch rods would be capable of sustaining. To resist shearing, wrought iron is safe at 7500 pounds per square inch of sectional area.

Chains should be tested, as one imperfect link or pin will injure the whole.

Fig. 1 in the accompanying plate shows elevation and plan of a portion of a flat-link chain; it consists of a series of short bars fastened together by pins or rivets.

It will be observed that the links are arranged alternately in twos and threes; hence, to keep the strength uniform throughout, those links which occur in twos should be thicker than those where there are three together. Thus we might have alternately two links 3/4 of an inch thick and three links 1/2 inch thick.

The requisite size of the pin is determined by considering the strain on the chain and the number of places where the pins must be sheared, together with the mode in which such strain is distributed through the links.

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Where the three links occur, it may be assumed that the load is equally distributed throughout, or that there is one third of the load on each of the three links; then it is evident that in the case of the outside link the pin requires to be sheared in one place only for rupture to ensue; therefore we have the following simple Rule. To find the greatest strain on one section of a pin in a flat-link chain, divide the total load on the chain by the greatest number of links placed together.

It is common practice to test chains up to two thirds the breaking weight; but a lower limit should be set, as in many cases failure is due to overstraining the material in the process of testing.

An ordinary chain will give way at the weld. To ascertain that the welds are perfect in every link, the chain is passed through a smith's fire and heated to redness, after which cold water is poured on each link: if the shut be imperfect it will open and thus exhibit the defect.

In Fig. 1 the links are in one piece and punched from steel or iron plates, A being the length and B the width of each link. C is the pin.

Fig. 2 is punched in a similar manner, as shown at A, and linked together as shown in elevation B and plan C. There being no pins, the tensile strain of the iron or steel is alone considered.

237. Wirework

Of the various ways in which wirework is employed in building construction, the following are particularly notable:

(a) For the protection of glass over skylights, and where glass is to be protected from being broken by falling objects.

1" diam. mesh No. 13 wire, round frame (Fig. 2), 3/4 lb. per sq. ft. 1" " " " " " grooved " 1 " " " "

(b) For window guards, bank and office railings and elevator enclosures.

1 1/4" mesh, No. 10 and 11 wire, round frame, 1 1/4 lbs. per sq. ft. 1 1/4" " " " " " " grooved " 1 1/2 " " " "

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Fig. 1.

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Fig. 2.

The following sizes are also used to enclose well-holes of elevators, guard windows, etc. They make the cheapest guard that can be produced consistent with strength.

1 1/2" mesh, No. 9 or 10 wire, round frame, 1 lb. per sq. ft. 1 1/2" " " " " " " grooved " 1 1/2 " " " "

(c) For guards requiring rough usage, such as window guards for factories or warehouses.

1 1/2" mesh, No. 8 wire, 3/4" grooved frame, 1 3/4 lbs. per sq. ft.

(d) Where light is not considered, but glass is to be protected.

1 1/2" mesh, No. 7 wire, 7/8" grooved frame, 2 lbs. per sq. ft.

(e) For railings, partitions and guards requiring strength.

2" mesh, No. 7 wire, 7/8" grooved frame, 2 lbs. per sq. foot. 2 1/2" " No. 5 or 6 wire, 1" " " 2 1/2 " " " "

(f) To screen pipes and machinery when situated in recesses.

1" square mesh (Fig. 1), flat wire 5/16" wide, 2 1/4 lbs. per sq. ft.

The flat wire can also be applied as railings, partitions and enclosures. Then use the following sizes:

1 1/4" and 1 1/2" mesh, 3/4" grooved frame, 1 1/4 lbs. per sq. ft.

(g) In fire-proof buildings wire is extensively used in place of wooden lath.

238. Cast-Iron Boiler Flues

Cast-Iron Boiler Flues are cast in sections from 5 to 8 feet in length and 1/2 inch thick, enclosed in brickwork, with an air-space between the flue and the brick enclosure. Expansion and contraction are to be considered when joining the sections. The maximum distance that the flue will expand or contract nearest the boiler is 1/8 inch per foot; therefore for a section 8 feet long 1 inch should be allowed in the overlapped joint, and proportionately less at a greater distance from boiler.

239. Wrought-Iron Boiler Flues

Wrought-Iron Boiler Flues are made of 1/4-inch plate, constructed similarly to cast-iron flues, with wrought-iron angles for the overlapped joints.