A tripod consists of three legs lashed or secured at the top. The advantage of the tripod over other rigging installations is its stability, and it requires no guy lines to hold it in place. The disadvantage of a tripod is that the load can be moved only up and down. The load capacity of a tripod is approximately 11/2 times that of shears made of the same-size material.
Figure 6-51.-Erecting a gin pole.
Figure 6-52.-Hoisting with a gin pole.
1. Rigging. There are two methods of lashing a tripod, either of which is suitable provided the lashing material is strong enough. The material used for lashing can be fiber rope, wire rope, or chain. Metal rings joined with short chain sections and large enough to slip over the top of the tripod legs also can be used. The method described below is for fiber rope 1 inch in diameter or smaller. Since the strength of the tripod is affected directly by the strength of the rope and the lashing used, more turns than described below should be used for extra heavy loads and fewer turns can be used for light loads.
a. Select three masts of approximately equal size and place a mark near the top of each mast to indicate the center of the lashing.
b. Lay two of the masts parallel with their tops resting on a skid or block and a third mast between the first two, with the butt in the opposite direction and the lashing marks on all three in line. The spacing between masts should be about one half or the diameter of the spars. Leave the space between the spars so that the lashing will not be drawn too tight when the tripod is erected.
c. With a 1-inch rope, make a clove hitch around one of the outside masts about 4 inches above the lashing mark, and take eight turns of the line around the three masts (fig. 6-53). Be sure to maintain the space between the masts while making the turns.
d. Finish the lashing by taking two close frapping turns around the lashing between each pair of masts. Secure the end of the rope with a clove hitch on the center mast just above the lashing. Frapping turns should not be drawn too tight.
a. An alternate procedure (fig. 6-54) can be used when slender poles not more than 20 feet long are being used or when some means other than hand power is available for erection.
b. Lay the three masts parallel to each other with an interval between them slightly greater than
Figure 6-53.-Lashing for a tripod.
Figure 6-54.-Alternate lashing for a tripod.
twice the diameter of the rope to be used. Rest the tops of the poles on a skid so that the ends project over the skid approximately 2 feet and the butts of the three masts are in line.
c. Put a clove hitch on one outside leg at the bottom of the position the lashing will occupy, which should be approximately 2 feet from the end. Weave the line over the middle leg, under and around the outer leg, under the middle leg, over and around the first leg, and continue this weaving for eight turns. Finish with a clove hitch on the outer leg.
2. Erecting. The legs of a tripod in its final position should be spread so that each leg is equidistant (fig. 6-55) from the others. This spread should not be less than one half nor more than two thirds of the length of the legs. Chain, rope, or boards should be used to hold the legs in this position. A leading block for the fall line of the tackle can be lashed to one of the legs. The procedure is as follows:
a. Raise the tops of the masts about 4 feet, keeping the base of the legs on the ground.
b. Cross the two outer legs. The third or center leg then rests on top of the cross. With the legs in this position, pass a sling over the cross so that it passes over the top or center leg and around the other two.
c. Hook the upper block of a tackle to the sling and mouse the hook.
d. Continue raising the tripod by pushing in on the legs as they are lifted at the center. Eight men should be able to raise an ordinary tripod into position.
e. When the tripod legs are in their final position, place a rope or chain lashing between the legs to hold them from shifting.
3. Erecting Large Tripods. For larger tripod installations it maybe necessary to erect a small gin pole to raise the tripod into position. Tripods, lashed with the
Figure 6-55.-1Yipod 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.