Text 6. Winch drum

A winding drum transmits power to the wire rope, and it also serves as a reservoir for storing rope that is not needed at the moment. Each turn of the rope around the full circumference of the drum is called a wrap. Rope is helically wrapped around the drum, starting at one flange and progressing toward the opposite flange. A series of wraps extending from flange to flange is called layer. If spooling continues after the completion of a layer, the wraps proceed back toward the starting flange in a second layer.

The end of a rope is attached to the drum barrel by a socket or clamp. When rope is spooled out, at least three full wraps must remain on the drum for safety. To make first-layer wraps closely placed and uniform, drum barrels are often grooved. A good first layer is a necessity if succeeding layers are to wrap properly; the rope itself provides a groove effect for subsequent layers. Grooves are cut to suit a particular rope diameter, and grooved drums can be properly used only for that diameter of rope. A helical pattern can be used, but better performance results from grooves cast parallel to the rope direction in an arrangement known as Lebus grooving.

To help proper spooling and to prevent excessive wear on the rope and on drum grooves, the maximum angle at which the rope leads onto the drum, called the fleet angle, must be kept within controlled limits. Figure below illustrates the fleet angle, which is the angle whose trigonometric tangent is one-half of the drum-barrel width divided by the distance between the shaft centerlines of the drum and lead sheave. The lead sheave is a fixed-position deflector sheave aligned with the center of the drum. Fleet angles should be no less than 1⁄ 2° and no more than 11⁄ 2° for smooth drums or 2° for grooved drums when the lead sheave is centered on the drum.

Wire ropes are manufactured with oversized diameters, but then they reduce in diameter because of wear and tear. Also the tightness of the wraps can vary from one spooling to the next. These factors affect the length of rope that spools onto a drum and make spooling capacity calculations difficult. Good values can be determined, nonetheless, using the equation

L = (D + E)EBs

L is given in feet when dimensions D, E and B are in inches. The spooling factor is calculated using the following equations.

s = 0.26218 / d² in U.S. customary units

s = 0.002285 / d² in SI units

Hoist drums are rated by line pull (the tension that the drum can withstand) and by line speed. Ratings are generally specified at the first layer but may be given for the top layer as well. When rope spools onto the drum, line pull decreases and line speed increases, but available torque remains constant. If the rated line pull on the first layer equals to P_r, then the rated torque T is given by

T = P_r (D + d) / 2

for a system with nominal rope diameter d. The line pull P_u at any other drum layer is found from

P_u = 2T / (A - 2C - d)

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