Figure 6-55. - Tripod assembled for use.
three legs laid together, must be erected by raising the tops of the legs until the legs clear the ground so they can be spread apart. Guy lines or tag lines should be used to assist in steadying the legs while they are being raised. The outer legs should be crossed so that the center leg is on the top of the cross, and the sling for the hoisting tackle should pass over the center leg and around the two outer legs at the cross.
Shears, made by lashing two legs together with a rope, is well adapted for lifting heavy machinery or other bulky loads. It is formed by two members crossed at their tops with the hoisting tackle suspended from the intersection. The shears must be guyed to hold it in position. The shears is quickly assembled and erected. It requires only two guys and is adapted to working at an inclination from the vertical. The shear legs can be round poles, timbers, heavy planks, or steel bars, depending on the material at hand and the purpose of the shears. For determining the size of the members to be used, the load to be lifted and the ratio of the length and diameter of the legs are the determining factors. For heavy loads the length-diameter (L/ d) ratio should not exceed 60, because of the tendency of the legs to bend, rather than to act as columns. For light work, shears can be improvised from two planks or light poles bolted together and reinforced by a small lashing at the intersection of the legs.
1. Rigging. In erection, the spread of the legs should equal about one half of the height of the shears. The maximum allowable drift (inclination) is 45 degrees. Tackle blocks and guys for shears are essential. The guy ropes can be secured to firm posts or trees with a turn of the rope so that the length of the guys can be adjusted easily. The procedure is as follows:
a. Lay two timbers together on the ground in line with the guys with the butt ends pointing toward the back guy and close to the point of erection.
b. Place a large block under the tops of the legs just below the point of lashing (fig. 6-56), and insert a small spacer block between the tops at the same point. The separation between the legs at this point should be equal to one third of the diameter on one leg to make handling of the lashing easier.
c. With sufficient 1-inch rope for 14 turns around both legs, make a clove hitch around one mast, and take 8 turns around both legs above the clove hitch. Wrap the turns tightly so that the lashings are made smooth and without kinks.
d. Finish the lashing by taking two frapping turns around the lashing between the legs and securing the end of the rope to the other leg just below the lashing. For handling heavy loads the number of lashing turns is increased.
2. Erecting. Holes should be dug at the points where the legs of the shears are to stand. In case of placement on rocky ground, the base for the shears should be level. The legs of the shears should be crossed and the butts placed at the edges of the holes. With a short length of rope, make two turns over the cross at the top of the shears and tie the rope together to form a sling. Be sure to have the sling bearing against the masts and not on the shears lashing entirely. The procedures is as follows:
a. Reeve a set of blocks and place the hook of the upper block through the sling. Secure the sling in the hook by mousing. Fasten the lower block to one of the legs near the butt, so it will be in a convenientContinue Reading