Over the limit

Know you limits to make safe lifts

While lifting and rigging requires a consistent application of good old common sense, it is easy to exceed crane and sling capacity limits without workers even realizing it, says Mike Parnell, president of Wire Rope and Rigging Consultants in Woodland, Washington.

Parnell teaches riggers and trainers. With more than 22 years of experience, he shares common areas overlooked by contractors that can make lifts inefficient and, in some cases, dangerous.

How do I know if slings are bad?
Sling damage is common on construction sites. Construction crews often use rigging equipment that’s shared by several workers, perhaps who are not even direct employees of the general contractor or building owner. It’s possible no one has the responsibility for sling maintenance, inspection and replacement. That sets the stage for sling damage. The most common cause: using slings around loads without protecting them from sharp edges.

Other causes of damage: improper storage, abrasion from dragging the sling across the ground or surfaces while under load; or sudden jerks during lifts.

Sling damage is almost always visible; you just need to know what to look for, and that should be done before every lift. If a sling has a nick, cut, abrasion or fray, it no longer meets its rated capacity and should be taken out of service.

Web slings often have contrasting threads in the web. If those colors show up against the normal web color, it should be taken out of service. If the protective cover on any roundsling is damaged, consider it unusable.

On chain slings, look for damaged links. Also, measure link lengths from time to time. If any link is longer than the length noted on the sling tag, replace the sling.

How can I prevent sling damage?
The best way is to select and use slings properly. Make sure the load does not exceed the sling’s capacity and that it is attached to the load in a way that doesn’t reduce the sling’s carrying capacity.

When lifting loads, do not jerk or make sudden load swings.

Slings can be protected with guards made from discarded web sling material. Take two or more pieces of web sling, have them sewed together and put Velcro or other loop fastener material on the left and right edges. Sandwich the sling with these protectors at every sharp corner the sling makes around the load.

I don’t always know the load’s weight. 
How can I safely approximate weight?
Multiply the load’s width, height and  length to approximate its volume.

Multiply the volume by 500 pounds. Then, envision the load if it were melted down and estimate how much space the load would take up as a percentage of its original size (known as its density). Multiply the total weight by the density factor, arriving at a best estimate for final load weight.

The final number is a safe approximation of the load’s weight. For example, an electric motor’s density is about 30 percent. If it measures 2'x2'x3', then it has 12 cu. ft. in volume x 500 = 6,000 lbs. x a density factor of 30 percent = 1,800 lbs. Based on that, select your rigging.

What reduces a sling’s load-carrying capacity?
A sling’s capacity is most often reduced by rigging the load so the sling’s angle is too acute (close to horizontal) to the load. The most efficient sling angle forms an equilateral triangle from the load, sling and hitch point (two slings and their span are all the same length).

As the load’s angle to the hitch becomes less than 30 degrees, the tension on the sling dramatically increases and carrying capacity decreases. Refer to manufacturers’ charts to estimate load derations vs. sling angle.

How the sling connects to the load can also affect its capacity. If welded attachment points are not available, the next best way to secure a load is to use a double-wrap basket sling. Here, the sling wraps twice around the load, which seats and compresses the load. It takes a longer sling, but is one of the safest ways to lift a load because it offers up to 200 percent greater rated sling capacity based on a single vertical hitch.

However, using a choker hitch reduces sling capacity by up to 25 percent from the single vertical hitch because the choke causes vertical and horizontal stresses on the sling.

Where can I get lifting and rigging training?
Start with your lifting equipment distributor but rigging professionals are also a good source.

Other resources: The Association of Crane and Rigging Professionals (www.acrp.net).

For reference cards, books, field guides, videos and posters check out the Training and Inspection Resource Center (www.tirc.net).

Published in the May/June 2001 issue of Contractor Tools and Supplies magazine.

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