Last post, we covered how easy it is to assemble an A-frame gantry. As one of the most used pieces of portable rigging equipment, it also holds that it would be one of the most misused pieces of equipment. Below are seven horrible mistakes made using aluminum gantries.
Gantries are meant to spread a load evenly across the top beam and spreading the load between the two legs. With that said, the load also needs to utilize rigging hardware to safely spread the load along the beam, whether that means using one or multiple trolleys or beam clamps. Occasionally, someone may get lazy, desperate to save a buck or just not realize the damage they are doing and will wrap a sling around the beam, causing point-loading and that can cause the beam to fold in on itself.
Bull rigging or using two gantries to lift a load improperly can easily cause a gantry to tip over or to collapse upon itself. Using two gantries with a monorail beam is the correct way to arrange this system to spread the load evenly and keep the forces pulling downward, rather than on an angle.
Once a gantry is under load, to move the load, both legs should be moved at the same time. This keeps the load static rather than introducing dynamic loads into the equation. A swinging load can cause too much force to be applied to the beam and legs, creating an unstable base.
Even a slight angle can send the entire load rolling away once it is raised up or worse. In addition to the load traveling when it is not supposed to, the force can be focused on one portion of the leg and, in rare occasions if overloaded, can cause one of the legs to buckle.
Everything about a gantry relies on stability and controlling the force of a load. Before making a pick, calculating the location of the center of gravity allows the rigging to be adjusted to capture the CG and keep the load from bouncing or shifting upon being picked, thereby, avoiding failure of the system.
Only as strong as its weakest part. A gantry is a series of parts that make up the whole. The wheels, legs, inserts, top plates and beam all add up to make the whole. If you take a q-ton gantry leg and pair it up with a 3-ton beam, you still only have 1-ton capacity. Likewise, if you attempt to use a gantry with suspect components, whether it be a weak top plate or an undersized beam, the whole is only as strong as the sum of its parts.
Where the main header beam and vertical posts meet, the connecting hardware not designed to withstand any dynamic forces. You would not want the gantry crane to literally fall to pieces with this unsafe practice.