In lifting any load, it takes two personnel to ensure a safe lift: an equipment operator and a signalman. In the following paragraphs, we will discuss the importance of the signalman and a few of the safety rules to be observed by all hands engaged in hooking on.
One person, and one person only, should be designated as the official signalman for the operator of a piece of hoisting equipment, and both the signalman and the operator must be thoroughly familiar with the standard hand signals. When possible, the signalman should wear some distinctive article of dress, such as a bright-colored helmet. The signalman must maintain a position from which the load and the crew working on it can be seen, and also where he can be seen by the operator.
Appendix III at the end of this TRAMAN shows the standard hand signals for hoisting equipment. Some of the signals shown apply only to mobile equipment; others, to equipment with a boom that can be raised, lowered, and swung in a circle. The two-arm hoist and lower signals are used when the signalman desires to control the speed of hoisting or lowering. The one-arm hoist or lower signal allows the operator raise or lower the load. To dog off the load and boom means to set the brakes so as to lock both the hoisting mechanism and the boom hoist mechanism. The signal is given when circumstances require that the load be left hanging motionless.
With the exception of the emergency stop signal, which may be given by anyone who sees a necessity for it, and which must be obeyed instantly by the operator, only the official signalman gives the signals. The signalman is responsible for making sure that members of the crew remove their hands from slings, hooks, and loads before giving a signal. The signalman should also make sure that all persons are clear of bights and snatch block lines.
ATTACHING A LOAD
The most common way of attaching a load to a lifting hook is to put a sling around the load and hang the sling on the hook (figure 4-36). A sling can be made of line, wire, or wire rope with an eye in each end (also called a strap) or an endless sling (figure 4-37). When a sling is passed through its own bight or eye, or shackled or hooked to its own standing part, so that it tightens around the load like a lasso when the load is lifted, the sling is said to be choked, or it may be called a choker, as shown in figures 4-36 and 4-37. A two-legged sling that supports the load at two points is called a bridle, as shown in figure 4-38.
The following safety rules must be given to all hands engaged in hooking on. They must be strictly observed.
The shear legs are formed by crossing two timbers, poles, planks, pipes, or steel bars and lashing or bolting them together near the top. A sling is suspended horn the lashed intersection and is used as a means of supporting the load tackle system (figure 4-39). In addition to the name shear legs, this rig often is referred to simply as a "shears". (It has also been called an A-frame.)
The shear legs are used to lift heavy machinery and other bulky objects. They may also be used as end supports of a cableway and highline. The fact that the shears can be quickly assembled and erected is a major reason why they are used in field work.
A shears requires only two guy lines and can be used for working at a forward angle. The forward guy does not have much strain imposed on it during hoisting. This guy is used primarily as an aid in adjusting the drift of the shears and in keeping the top of the rig steady in hoisting or placing a load. The after guy is a very important part of the shears rigging, as it is under considerable strain when hoisting. It should be designed for a strength equal to one-half the load to be lifted. The same principles for thrust on the spars or poles apply; that is, the thrust increases drastically as the shear legs go off the perpendicular.
In rigging the shears, place your two spars on the ground parallel to each other and with their butt ends even. Next, put a large block of wood under the tops of the legs just below the point of lashing, and place a small block of wood between the tops at the same point to facilitate handling of the lashing. Now, separate the poles a distance equal to about one-third the diameter of one pole.
As lashing material, use 18- or 21-thread small stuff. In applying the lashing, first make a clove hitch around one of the legs. Then, take about eight or nine turns around both legs above the hitch, working towards the top of the legs. Remember to wrap the turns tightly so that the finished lashing will be smooth and free of kinks. To apply the frapping (tight lashings), make two or three turns around the lashing between the legs; then, with a clove hitch, secure the end of the line to the other leg just below the lashing (figure 4-39).
Figure 4-39.Shear legs.
Now, cross the legs of the shears at the top, and separate the butt ends of the two legs so that the spread between them is equal to one-half the height of the shears. Dig shallow holes, about 1 foot (30 cm) deep, at the butt end of each leg. The butts of the legs should be placed in these holes in erecting the shears. Placing the legs in the holes will keep them from kicking out in operations where the shears are at an angle other than vertical.
The next step is to form the sling for the hoisting falls. To do this, take a short length of line, pass it a sufficient number of times over the cross at the top of the shears, and tie the ends together. Then, reeve a set of blocks and place the hook of the upper block through the sling, and secure the hook by mousing the open section of the hook with rope yarn to keep it from slipping off the sling. Fasten a snatch block to the lower part of one of the legs, as indicated in figure 4-39.
The guysone forward guy and one after guyare secured next to the top of the shears. Secure the forward guy to the rear leg and the after guy to the front leg using a clove hitch in both instances. If you need to move the load horizontally by moving the head of the shears, you must rig a tackle in the after guy near its anchorage.
A tripod consists of three legs of equal length that are lashed together at the top (figure 4-40). The legs are generally made of timber poles or pipes. Materials used for lashing include fiber line, wire rope, and chain. Metal rings joined with short chain sections are also available for insertion over the top of the tripod legs.
When compared with other hoisting devices, the tripod has a distinct disadvantage: it is limited to hoisting loads only vertically. Its use will be limited primarily to jobs that involve hoisting over wells, mine shafts, or other such excavations. A major advantage of the tripod is its great stability. In addition, it requires no guys or anchorages, and its load capacity is approximately one-third greater than shears made of the same-size timbers. Table 4-1 gives the load-carrying capacities of shear legs and tripods for various pole sizes.
Table 4-1.Load-Carrying Capacities of Shear Legs and Tripods
The strength of a tripod depends largely on the strength of the material used for lashing, as well as the amount of lashing used. The following procedure for lashing applies to a line 3 inches in circumference or smaller. For extra heavy loads, use more turns than specified in the procedure given here. For light loads, use fewer turns than specified here.
As the first step of the procedure, take three spars of equal length and place a mark near the top of each to indicate the center of the lashing, Now, lay two of the spars parallel with their tops resting on a skid (or block). Place the third spar between the two, with the butt end resting on a skid. Position the spars so that the lashing marks on all three are in line. Leave an interval between the spars equal to about one-half the diameter of the spars. This will keep the lashing from being drawn too tightly when the tripod is erected.
With the 3-inch line, make a clove hitch around one of the outside spars; put it about 4 inches above the lashing mark. Then, make eight or nine turns with the line around all three spars. (See view A of figure 4-41.) In making the turns, remember to maintain the proper amount of space between the spars.
Figure 4-41.Lashings for a tripod.
Now, make one or two close frapping turns around the lashing between each pair of spars. Do not draw the turns too tightly. Finally, secure the end of the line with a clove hitch on the center spar just above the lashing, as shown in view A of figure 4-41.
There is another method of lashing a tripod that you may find preferable to the method just given. It may be used in lashing slender poles up to 20 feet in length, or when some means other than hand power is available for erection,
First, place the three spars parallel to each other, leaving an interval between them slightly greater than twice the diameter of the line to be used. Rest the top of each pole on a skid so that the end projects about 2 feet over the skid. Then, line up the butts of the three spars, as indicated in view B of figure 4-4 1.
Next, make a clove hitch on one outside leg at the bottom of the position the lashing will occupy, which is about 2 feet from the end. Now, proceed to weave the line over the middle leg, under and around the other outside leg, under the middle leg, over and around the first leg, and so forth, until completing about eight or nine turns. Finish the lashing by forming a clove hitch on the other outside leg (view B of figure 4-4 1).
In the final position of an erected tripod, it is important that the legs be spread an equal distance apart. The spread between legs must be no more than two-thirds nor less than one-half the length of a leg, Small tripods, or those lashed according to the first procedure given in the preceding section, may be raised by hand. Here are the main steps.
Start by raising the top ends of the three legs about 4 feet, keeping the butt ends of the legs on the ground. Now, cross the tops of the two outer legs, and position the top of the third or center leg so that it rests on top of the cross.
A sling for the hoisting tackle can be attached readily by first passing the sling over the center leg, and then around the two outer legs at the cross. Place the hook of the upper block of a tackle on the sling, and secure the hook by mousing.
The raising operation can now be completed. To raise an ordinary tripod, a crew of about eight maybe required. As the tripod is being lifted, spread the legs so that when it is in the upright position, the legs will be spread the proper distance apart. After getting the tripod in its final position, lash the legs near the bottom with line or chain to keep them from shifting (figure 4-40). Where desirable, a leading block for the hauling part of the tackle can be lashed to one of the tripod legs, as indicated in figure 4-40.
In erecting a large tripod you may need a small gin pole to aid in raising the tripod into position. To erect a tripod lashed according to the first procedure described in the preceding section, you first raise the tops of the legs far enough from the ground to permit spreading them apart. Use guys or tag lines to help hold the legs steady while they are being raised. Now, with the legs clear of the ground, cross the two outer legs and place the center leg so that it rests on top of the cross. Then, attach the sling for the hoisting tackle. Here, as with a small tripod, simply pass the sling over the center leg and then around the two outer legs at the cross.
|David L. Heiserman, Editor||
Copyright © SweetHaven
Revised: June 06, 2015