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When two tripods are needed, snatch blocks should be used to suspend the cable from the top of the tripods. The blocks reduce wear on the cable and help prevent the tripods from being pulled over as the winch pulls in cable. The wire rope cable between the tripods must always have some deflection. Post signs to warn people of the dangerous work (figure 6a) and tie flagging on rigging cables (figure 6b).
Tripods can lift heavy objects, but are dangerous when loads pull to the side. Crews should not attempt to pick up or drag heavy loads that are not directly under the overhead cable (figure 7). If the load is off to the side of the cable, the tripods should be moved so that the load is directly beneath the cable. Otherwise, one or both of the tripods may tip over.
Figure 7--One portable tripod can be used by itself when lifting heavy
objects, such as one end of a log bridge. The safety chain around the
bottom prevents the legs from spreading too far apart. Hooks welded
near the bottom of each leg hold the chain in place.
Assembling and Setting Up the Tripods
The part numbers are shown on MTDC Drawing No. 1035 (PDF - 759 k). Before assembling a tripod, gather all the parts. Run the cable through the snatch block and choose a location where the cable can be anchored near each of the tripods.
Attach the eyebolt (No. 11) to the top plate assembly (No. 7).
Attach the three legs (No. 1) to the top plate assembly (No. 7) using the hitch pins and hairpin cotters (No. 14).
Attach a sleeve (No. 2) to the bottom of each leg (No. 1) using a hitch pin and hairpin cotter (No. 14). The sleeve allows the length of the tripod's legs to be adjusted.
Depending on the type of ground under the tripod, attach the foot and plate assembly (No. 4) to each leg (No. 1) using a hitch pin and locking hairpin cotter (No. 14).
Attach the snatch block (with the cable running through it) to the eyebolt.
With the assembled tripod lying on the ground and with plenty of slack in the cable, have one person pull one leg of the tripod forward while two persons push the other two legs up until the tripod is standing.
Adjust the lengths of the legs until the tripod head is relatively level, keeping the legs evenly spaced.
Using a level, make small adjustments to the legs so the tripod head is level, keeping the legs evenly spaced.
Attach the chain to the hook on each of the foot and plate assemblies. The chain prevents the legs from spreading.
After completing the rigging setup (connecting the cable to the anchors, attaching the Griphoist to the cable, attaching the load to the snatchblock, and attaching a tether line to the load) load the cable slowly and gently. If the tripods shift, repeat steps 7 and 8 until the tripods are loaded and level.
MTDC Drawing No. 1035 (PDF - 759 k) shows how to construct the portable tripods.
About the Author
Bob Beckley received a bachelor's degree in political science from the University of Montana in 1982. He began his Forest Service career as a timber technician on the Nez Perce National Forest. Bob was a smokejumper when he joined MTDC in 1990. He works as a project leader, public affairs specialist, and blaster.
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