Wire Ropes and Accessory Fittings
Wire ropes used in oceanographic work must have the following properties:
They must be of strong material so that rope of relatively small diameter can be used, thus reducing the bulk of the winch.
They must be flexible and not liable to kink or unravel.
They must be made of a metal that is resistant to corrosion, and thus long-lived, and free from materials that will contaminate water samples and plankton catches.
Such requirements are best answered by multistrand ropes of stainless steel, but this alloy is expensive, and in practice tinned or galvanized steel ropes are satisfactory. Phosphor-bronze and aluminum-bronze ropes are also used, as they are noncorroding, but they are only about one half as strong as steel and their life is limited because they crystallize with use and lose their strength. Many wire ropes have hemp cores, but these are not so satisfactory as ropes with wire cores, because the hemp may shrink and break when submerged, and it is also liable to rot unless specially treated. High-grade manila rope is about one tenth as strong as steel rope of comparable diameter.
The strength of steel depends upon its composition and treatment and varies from about 50,000 to 400,000 lb/in2. Steels employed in wire ropes are usually of relatively high tensile strength, the tensile strength of the rope increasing with decreasing diameter of the individual wires. The greater strength of a rope made up of smaller but more numerous wires is offset, however, by the greater surface offered for corrosion and by the greater likelihood of the individual wires breaking after a certain amount of wear.
Single-strand wire of the type known as piano or music wire is used for deep-sea sounding and for running taut-wire traverses (p. 342). This wire is of extremely high tensile strength, but is stiff and liable to break if kinked. If no bottom sample is required when sounding at great depths and in taut-wire traverses, the wire is usually cut away, as it is not worth the time required to reel it in again. Piano wire of
The wire ropes used on the hydrographic and heavy winches are generally of either the 7 × 7 or the 7 × 19 types. A 7 × 7 wire rope consists of six strands, each composed of seven individual wires that are wound around a central core strand which itself contains seven wires. A 7 × 19 wire rope is similarly constructed, but each of the seven strands contains nineteen individual wires. The 7 × 19 ropes are slightly heavier than the 7 × 7 type, and the ropes of smaller diameter are considerably stronger, but the small size of the individual wires is a disadvantage. Ropes of the 7 × 7 type with diameters between about one eighth inch and one quarter inch are sufficiently flexible for oceanographic use, but, to obtain the desired flexibility in larger ropes, it is necessary to employ 7 × 19 rope on the heavy winch. In table 58 are given the characteristics of 7 × 7 galvanized steel ropes of the type known as aircraft cord.
For most purposes it is considered that the working load of wire rope should not be more than one fifth of the breaking strength—that is, a safety factor of five. In marine investigations it is sometimes impossible to maintain such a high factor of safety, but, if the anticipated strain is known, the diameter of the wire should be such that the maximum load is never more than half the breaking strength. There is not only the danger of losing valuable equipment, but also the hazard to those on deck if the wire should break near the water surface. Steel has a greater elasticity than bronze, and the safety factor must be somewhat greater when bronze ropes are employed.
When great lengths of wire rope are paid out, the weight of the rope in the water may approach the breaking strength and exceed the safety factor of five, even when there is no gear suspended from the rope. Ropes of the type listed in table 58 exceed the safety factor of five when more than 4000 m of wire are suspended in the water. When taking water samples and temperatures or other observations with light gear where there are no sudden strains upon the rope, the safety factor may be reduced and the work extended to great depths. However, in trawling, dredging, and taking cores of the bottom sediments, and when anchoring in deep water, the equipment must be such that it can withstand a heavy working load in addition to the weight of the wire rope. The increased working strength necessary for observations in deep water is gained by using tapered wire ropes, which are of the smallest diameter
|Diameter||Weight in air per 100 m||Breaking strength|
|Data through Courtesy of John A. RoebIing's Sons Co., Trenton, New Jersey.|
Many preparations are on the market to be used for the preservation of wire ropes. Whether or not any of these are suitable depends upon the manner in which the wire rope is to be used, as those which flake off will contaminate plankton samples and other collections. Relatively frequent application of used crankcase oil is a satisfactory method for preserving steel ropes. The oil is applied before the rope is first placed in the water, and thereafter at intervals, particularly when the rope will not be in service for a long period.
The wire rope used on the hydrographic winch must be smooth and free from kinks or stray broken wires that will prevent the passage of messengers or weights. Where a break or other damage necessitates a join, wire rope may be repaired with a long splice, which does not materially increase the diameter of the wire or decrease the breaking strength. Kinks that form in the wire should never be pulled out, but should be eliminated by untwisting the wire, straightening the strands, and realigning them.
Sheaves are aIways necessary for leading the wire rope outboard. They should be free-running and of such diameter that the wire passing over them will not be cramped or strained. Unless the circumstances
An essential part of the equipment for each winch is a meter wheel (fig. 83) for measuring the amount of rope payed out. A meter wheel is a sheave with a wheel of appropriate circumference fitted with a device that records the number of revolutions. The device is so designed that the difference between two readings of the dials gives directly in meters, fathoms, or feet the amount of wire that is run out or hauled in. For soundings and for obtaining temperatures and water samples the meter wheel must be carefully constructed and checked from time to time for wear. The effective diameter of the wheel is its own diameter plus the diameter of the wire rope. The meter wheel may be mounted on the boom or davit that leads the wire outboard, or it may be built into the spreader on the winch. The latter arrangement is a convenient one because the winch operator can then see the recorder at all times. If the meter wheel itself is in such a location that the dial is not readily seen, recorders operated by a flexible speedometer cord may be mounted in a more convenient place. To avoid slippage of the wire rope over the meter wheel, the angle of contact should be at least 90 degrees.
Meter wheel with dial indicators and stainless steel sheave.
The wire rope is led outboard from the winch through sheaves and finally from a boom or davit extending out over the side of the vessel on the windward side. The lead from the heavy winch must be approximately amidships because, when dredging, the vessel must be maneuvered under sail or power, which is possible only if the wire rope is suspended from a sheave near the middle of the vessel. To avoid striking the side of the vessel the outboard leads should be arranged so that the wire, when hanging vertical, is several feet away from the hull, and to facilitate the handling of gear it is usually necessary to have a working platform extending out from the hull and large enough to accommodate two men. If the wire rope on the heavy winch is used for deep-sea anchoring,
Sudden strains on the wire rope that may be caused by the rolling of the vessel or by fouling equipment on the bottom are a hazard to both the apparatus and the wire. These strains may be equalized somewhat by the use of accumulators, which are usually coil springs mounted with one end secure and the other end attached to the sheave through which the wire rope passes. Accumulators can usually be calibrated so that the extension or compression of the spring may be used to measure the strain on the wire rope. Some types are secured to the outer end of the boom or davit with the meter wheel or a plain sheave attached to the free end, or they may be an integral part of the davit or winch. The strain on the accumulator may be used in deep soundings to determine when the weight strikes the bottom, and in dredging and trawling it should be watched so that sudden strains may be eased by slacking off on the winch. Special devices known as dynamometers may be used to measure the strain on the wire rope.