SHEAR LEGS

The SHEAR LEGS is formed by crossing two timbers, poles, planks, pipes, or steel bars and lashing or bolting them together near the top. A sling is suspended from the lashed intersection and is used as a means of supporting the load tackle system. (See fig. 6-9.) In addition to the name shear legs, this rig is often referred to simply as a shears. (It has also been called an A-frame.)

The shear legs is used to lift heavy machinery and other bulky objects. It may also be used as end supports of a cableway and highline. A major reason for using the shears in fieldwork is that it can be quickly assembled and erected.

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 when a load is being hoisted or placed. The after guy is an 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 of the load to be lifted. The same principles for thrust on the spars apply; that is, the thrust increases drastically as the shear legs go off the perpendicular.

Figure 6-9.\Shear legs.

Rigging

In rigging the shears, place your two spars or poles on the ground parallel to each other 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 of the diameter of one pole.

For 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 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 (fig. 6-9).

Now, cross the legs of the shears at the tip and separate the butt ends of the two legs so that the spread between them is equal to one half of the height of the shears. Dig shallow holes about 1 foot (30 centimeters) 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 is 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 shear, and tie the ends together.

Now, reeve a set of blocks and place the hook of the upper block through the sling; then secure the hook by mousing. Fasten a snatch block to the lower part of one of the legs, as shown in figure 6-9.

If you need to move the load horizontally by moving the head of the shears, rig a tackle in the after guy near its anchorage.

The guys\one forward guy and one after guy\are 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. (See fig. 6-9.)

Erecting

Several crew members are needed for safe, efficient erection of the shears, the number being determined largely by the size of the rig. To help ensure good results, the erection crew should lift the top of the frame and walk it up by hand until the after guy tackle system takes over the load. When this point is reached, complete the raising of the shears into final position by hauling in on the tackle.

Remember to secure the forward guy to its anchorage before raising the legs, and maintain a slight tension on the line to control the movement. Also, after the shears has been raised, lash the butt ends with chain, line, or boards to keep them from spreading when a load is applied.

TRIPOD

A tripod consists of three legs of equal length lashed together at the top. (See fig. 6-10.) The fact that the tripod can be used only where hoisting is vertical places it at a distinct disadvantage in comparison with other hoisting devices. Its use will be limited primarily to jobs that involve hoisting over wells, mine shafts, or other 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 1 1/2 times greater than for shears made of the same size timbers.

Figure 6-10.\Tripod.

The legs of a tripod generally are 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.

Rigging

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 line 3 inches (75 millimeters) 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 of the diameter of the spars. This will keep the lashing from being drawn too tight when the tripod is erected.

With the 3-inch (75-millimeter) line, make a clove hitch around one of the outside spars; put it about 4 inches (10 centimeters) above the lashing mark. Then make eight or nine turns with the line around all three spars. (See view A, fig. 6-11.) In making the turns, remember to maintain the proper amount of space between the spars.

Now, make one or two close frapping turns around the lashing between each pair of spars. Do not draw the turns too tight. Finally, secure the end of the line with a clove hitch on the center spar just above the lashing, as shown in view A, figure 6-11.

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 (6 meters) in length, or when some

Figure 6-11.\Lashing for a tripod.

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 tip of each pole on a skid so that the end projects about 2 feet (60 centimeters) over the skid. Then, line up the butts of the three spars, as shown in view B, figure 6-11.

Next, make a clove hitch on one outside leg at the bottom of the position the lashing will occupy, which is about 2 feet (60 centimeters) 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, as shown in view B, figure 6-11.

Erecting

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 not 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 that make up the hand-erection procedure.

Start by raising the top ends of the three legs about 4 feet (1.2 meters), 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 the 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 members may be 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. (See fig. 6-10.)

Where desirable, a leading block for the hauling part of the tackle may be lashed to one of the tripod legs, as shown in figure 6-10.

In erecting a large tripod, you may need a small gin pole to aid in raising the tripod into position. When you are called on to assist in the erection of a tripod lashed according to the first lashing procedure described in the preceding section, the first thing to do is to 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.