Wire Rope Installation
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UNREELING AND UNCOILING
Wire rope is shipped in cut lengths, either in coils or on reels. Great care should be taken when the rope is removed from the shipping package since it can be permanently damaged by improper unreeling or uncoiling. Looping the rope over the head of the reel or pulling the rope off a coil while it is lying on the ground, will create loops in the line. Pulling on a loop will, at the very least, produce imbalance in the rope and may result in open or closed kinks (Fig. 18). Once a rope is kinked, the damage is not repairable. The kink must be cut out or the rope is unfit for service.
Figure 18. Improper handling can create open (a) or,qlosed kinks b). The open kink will open the rope lay: the closed kink will close it. . Srarring loop (c): Do not allow the rope to form a loop. If. however, a loop does form and is removed at the stage shown. a kink can be avoided. Kink (d): In this case. the looped rope was put under tension, the kink was formed. the rope is permanently damaged.
Unwinding wire rope from its reel also requires careful and proper procedure.
There are three methods to perform this step correctly:
- The reel is mounted on a shaft supported by two jacks or a roller payoff (Fig.19). Since the reel is free to rotate, the rope is pulled from the reel by a workman holding the rope end, and walking away from the reel as it unwinds. A braking device should be employed so that the rope is kept taut and the reel is restrained from over-running the rope. This is necessary particularly with powered de-reeling equipment.
- Another method involves mounting the reel on an unreeling stand (Fig. 20). It is then unwound in the same manner as described above (1). In this case, however, greater care must be exercised to keep the rope under tension sufficient to prevent the accumulation of slack. Slack can allow the rope to drop below the lower reel head and be damaged or loose wraps on the reel to fall
over the rope coming off the reel and become tangled.
- In another accepted method, the end of the rope is held while the reel itself is rolled along the ground. With this procedure, the rope will payoff properly however, the end being held will travel in the direction the reel is being rolled. As the difference between the diameter of the reel head and the diameter of the wound rope increases, the speed of travel will increase.
Figure 19. The wire rope reel is mounted on a shaft supported by jacks. This permits the reel to rotate freely. and the rope can be unwound either manually or by a powered mechanism.
When re-reeling wire rope from a horizontally supported reel to a drum it is preferable for the rope to travel from the top of the reel to the top of the drum; or, from the bottom of the reel to the bottom of the drum (Fig. 21). Re-reeling in this manner will avoid putting a reverse bend into the rope during installation. If a rope is installed so that a reverse bend is induced, it may cause the rope to become "twisty" and, consequently, harder to handle. When unwinding wire rope from a coil, there are two suggested methods for carrying out this procedure in a proper manner:
1) One method involves placing the coil on a vertical unreeling stand. The stand consists of a base with a fixed vertical shaft. On this shaft there is a "swift," consisting of a plate with inclined pins positioned so that the coil may be placed over them. The whole swift and coil then rotate as the rope is pulled off. This method is particularly effective when the rope is to be wound on a drum.
2) The most common as well as the easiest uncoiling method is merely to hold one end of the rope while rolling the coil along the ground like a hoop (Fig. 22). Figures 23 and 24 show unreeling and uncoiling methods that are most likely to cause kinks. Such improper procedures must be avoided in order to prevent the occurrence of loops. These loops, when pulled taut, will inevitably result in kinks. No matter how a kink develops, it will damage strands and wires, and the kinked section must be cut out. Proper and careful handling will keep the wire rope free from kinks.
DRUMS - GROOVED
Drums are the means y which power is transmitted to the rope and then to the object to be moved. For the wire rope to pick up this power efficiently and to transmit it properly to the working end, installation must be carefully controlled. If the drum is grooved, the winding conditions should be closely supervised to assure adherence to the following recommended procedures:
I) The end of the rope must be secured to the drum by such means as will give the end termination at least as much strength as is specified by the equipment manufacturer.
2) Adequate tension must be maintained on the rope while it is being wound sothat the winding proceeds under continuous tension. Back tension applied to the rope during installation' should be from 2 to 5% of the minimum breaking force of the rope being installed.
3) The rope must follow the groove.
4) It is preferable to have at least three dead wraps remaining on the drum when the rope is unwound during normal operation. Two dead wraps are a mandatory requirement in many codes and standards. If the wire rope is carelessly wound and, as a result, jumps the grooves, it will be crushed and cut where it crosses from one groove to the other. Another, almost unavoidable problem is created at the drum flange; as the rope climbs to a second layer there is further crushing and the wires receive excessive abrasion.
Riser and filler strips may help remedy this condition. Another factor that must be given serious consideration is the pitch of the
drum grooves relative to the actual rope diameter. Wire rope is normally manufactured to a plus tolerance. (See Table 3.) The oversize tolerance of the rope must be taken into account or the rope will be damaged by poor spooling caused
by a groove pitch that is either too small or too large. As an example, a grooved drum made for 1/4-inch rope may have a pitch of .250
inches. Yet, by Federal standards, a 1/4-inch rope may have a diameter as large as .265 inches. If a rope of this size were to be operated on a drum with a .250 inch pitch, crowding would occur and the rope would be forced out of the groove.
Figure 32. By holding the right or left hand with index finger extended, palm up or palm down, the proper procedure for applying left-and right-lay rope on a smooth drum can be easily determined.
<p >The proper direction of winding the first layer on a smooth drum can be determined by standing behind the drum and looking along the path the rope travels, and then following one of the procedures illustrated in Figure 32. The diagrams show: the correct relationship that should be maintained between the direction of lay of the rope (right or left), the direction of rotation of the drum (overwind or underwind) and winding from left to right or right to left.
Many installations are designed with requirements for winding more than one layer of wire rope on a drum. Winding multiple layers presents some further problems.The first layer should wind in a smooth, tight helix which, if the drum is grooved,
is already established. The grooves allow the operator to work off the face of the drum, and permit the minimum number of dead wraps. A smooth drum present' an additional problem, initially, as the wire rope must be wound in such a manner that the first layer will be smooth and uniform and will provide a firm foundation for the layers of rope that will be wound over it. The first layer of rope on the smooth drum should be wound with tension (2 to 5% of the minimum breaking force of the rope) sufficient to assure a close helix - each wrap being wound as close as possible to the preceding wrap. The first layer then acts as a groove which will guide the successive layers. Unlike wire ropes operating on grooved drums, the first layer should not be unwound from a smooth-faced drum with multiple layers. After the rope has wound completely across the face of the drum (either smooth or grooved), it is forced up to a second layer at the flange. The rope then winds back across the drum in the opposite direction, lying in the valleys between the wraps of the rope on the first layer. Advancing across the drum on the second layer, the rope, following the "grooves" formed by the rope on the first layer, actually winds back one wrap in each revolution of the drum. The rope must then cross one or two rope "grooves" (depending upon the type of grooving - single or double cross-over) in order to advance across the drum for each turn. The point at which this occurs is known as the cross-over. Cross-over is unavoidable on the second, and all succeeding layers. Figure 33 illustrates the winding of a rope on the second layer from left to right, and from right to left-the direction is shown by the arrows.
At these cross-over points, the rope is subjected to severe abrasion and crushing as it is pushed over the "grooves" and rides across the crown of the first rope layer. The scrubbing of the rope, as this is happening, can easily be heard.
There are, however, special drum groovings available that will greatly minimize the damage that can occur at cross-over points - e.g. Counterbalance Drum Grooving* with a double cross-over.
Helical grooving does not employ a built in cross-over and does not work as well for multiple layer spooling as a counterbalanced drum because it does not have the cross-over and does not consistently put the rope in the proper position at the
flanges to rise from one layer to the next layer.
Counterbalance grooving with two cross-overs is made so that each wrap of rope winds parallel to the drum flange for a distance less than half the circumference around the drum, then follows a short cross-over to complete half the drum circumference. The cross-over is at an angle with the drum flange and displaces the rope laterally by half the pitch of grooving.
Around the other half of the drum circumference each wrap again winds parallel to the flange for a distance, and then follows another short cross-over to a point one full circumference from the start. At this point the lateral displacement is equal to the full pitch of grooving.
The grooving for this type of winding is similar to the parallel grooving except that half the drum circumference is laterally displaced from the other half by half the pitch of grooving, and between these two halves the grooves make short cross-overs to guide the rope properly. The two cross-over areas are on opposite sides of the drum, or 1800 apart.
Since the lateral displacement of each cross-over is one half the pitch of grooving, or one half the displacement of the cross-overs encountered with other types of winding, "throw" of the rope is reduced, decreasing the whipping action. However, if the interval between these displacements happens to match the rope's vibration cycle, whipping can still become severe because this action is cumulative.
Since the cross-over areas are spaced opposite each other, or 1800 apart, raised portions of the winding caused by vertical displacement at the cross-overs also occur opposite each other. These raised sections become quite pronounced where many layers are involved and the balancing effect of keeping them opposite gave name to the method.
With counterbalance winding, the change of layers can be controlled better than with other systems and is preferred when a rope must wind in many layers on the drum.