Whether it is a $2.6 million dollar crane, or an $11.00 eye bolt, how you use a piece of machinery, or a device impacts its capacity.  

Let’s start with the machine itself

You would be surprised at the number of individuals who operate a machine only occasionally, say a forklift, or something like a low-capacity carry deck crane, who don’t know that the machines “capacity” is not a static number, but in fact can vary with how the machine is used.

I mean, a 15 ton carry deck should be able to pick up 15 tons, right?

Not necessarily.

The farther away the pick is from the machine, the less the machine can pick, all other things being equal.  Experienced mobile, articulating and tower crane operators, as well as forklift and Excavator operators all pretty much know that.  

Radius

“Radius” . . . the horizontal difference from the center of rotation of the crane (center of the swing circle) to the center of gravity of the suspended load . . . dramatically affects capacity. A 15-ton capacity industrial crane at 6’ has less than 1 ton capacity at 50 feet. 

Hold a 5-pound weight at your chest. Now stick your arm straight out parallel to the ground.  What “feels” heavier?  That’s leverage working. Boom cranes operate on the principle of leverage.  If you hold the weight above your head with your arm straight it will not feel heavier. It’s not about your arm being straight, it’s about how far away, horizontally, the weight is from your shoulder. “Radius.”

The other thing that you may feel as you extend your arm horizontally is that your weight shifts more to your toes.  If you were on outriggers (or tires or two tracks), the pressure would increase on the lift side. While this does not technically lower the capacity of the equipment, it impacts the safety of the lift. The greater the pressure, the more firmly compacted the ground must be and / or the more blocking must be used.

Wind

Wind can decrease the capacity of some machines as the crane manufacturer requires the gross capacities of some cranes to be de-rated based on wind speed. Many manufacturers also place a max wind speed for conducting operations. Some employers or organizations will specify a wind speed cap. The Army Corps of Engineers is 20 mph, for example.       

Side Loading

Also, side loading the boom can reduce machine capacity. Now understand, generally it is not permissible to side load the boom because of slides, side pulls, swing drift etc., some manufacturers will permit a small amount of side loading with suspended loads when during “duty cycle” operations. 

During clam shell, drag line, concrete operations and other duty cycle operations some machine manufacturers require their cranes to be derated by 20%. Another operation that requires de-rating of the crane include multi crane lifts which require that each crane in the operation be derated to 75% of its capacity as configured. When hoisting personnel both the crane as configured and the load line is derated by 50%.

Rigging

Now let’s talk rigging. No matter what kind of rigging you are dealing with the Working Load Limit (WLL) and the size should be on it (OSHA) as well as the name or trademark of the manufacturer (ASME).  If this information is not on the rigging, or if it simply says “China”, throw it away. 

For slings, the WLL is usually based on a straight vertical (90 degree) pull. This would be a 100% lift.  As most of you know, as the sling angle to the horizon decreases, the capacity of the sling decreases because tension on the sling leg increases. This is called load angle factor.

At an angle of 60 degrees, the sling will lose 14% of its capacity. So, a sling with a 10 Ton capacity at a vertical, will have a capacity of 8.6 Tons or 17,200 pounds. At 45 degrees from horizontal a sling will lose 30% of its capacity so our 10 Ton (at 90 degrees) sling would have a 7 Ton capacity. At 30% a sling will lose 50% of its capacity so our sling’s capacity at 30 degrees would be 5 Ton. Using slings below 30% from horizontal is generally prohibited. 

The slings you use probably came with paperwork that tells you a lot of valuable information on how to use the sling properly, to include more detailed load angle factor information. If you lost this information (or more likely, threw it away), it can probably be found on the manufacturer’s web site.  

The hitch you use will also impact the capacity of the sling. As stated above, most sling WLLs are based on a vertical sling, so a vertical hitch is a 100% hitch. A basket hitch will actually increase the WLL of the sling to 200% of the vertical WLL, as there are two legs supporting the load. This is assuming 90-degree legs. If the legs are not vertical, then the load angle factor comes into play and the lower the angle, the lower the capacity until you get down to 30 degrees, the capacity of the basket will be back to 100% of the sling WLL.

A choke is nominally a 75% hitch, so the 10 Ton (at 90 degrees) sling would have a capacity in a choke of 7.5 Tons.  However, this varies from sling to sling so it is best (as it always is) to seek out the manufacturers information for your particular device.  Also, the angle of the choked leg impacts the hitches capacity with a straighter angle yielding a higher capacity and a more acute angle yielding a lower capacity, so again, make sure to consult the manufacturers information.

Some devices may only be used for in-line pulls (Eye Nuts, Non-Shouldered Eyebolts and Turnbuckles come immediately to mind), in which case the angle of the pull is not an issue because they are not to be used for angular pulls.  Some devices, like eye bolts which are shouldered to the load, may be used in angular pulls (in the case of an eye bolt, only in the plane of the eye), but as the angle increases from 0 degrees (straight in line pull), the capacity decreases as per manufacturers specifications.  Now, swivel hoist rings do no lose capacity at an angular pull BUT, be careful that the load angle factor doesn’t increase tension on the sling so much that it exceeds the WLL of the swivel hoist ring.  

D/d Ratio

And finally (for the purpose of this blog), is D/d Ratio. As a wire rope with a diameter of “d” is wound around a drum, sheave or other object with a diameter of “D”, it can lose capacity. The more tightly the wire rope is bent, the more capacity it loses.  A boom tip sheave might need to be 16 times the diameter of the rope (a “D/d Ratio” of 16/1), while a snatch block may only need a sheave 6 times the diameter of the wire rope (a “D/d Ratio” of 6/1).  In the interest of keeping the math to a minimum, just do what the manufacturer tells use. Use the replacement tip sheave and the wire rope the manufacturer tells you to. A snatch block will invariably list the size of wire rope permitted to be used with it. Do what they say. 

In fact, the entire purpose of this blog is to encourage you to always follow the manufacturer’s (or a qualified person’s) guidance on using any equipment or device. This blog only contains general information.  It provides no answers, only questions. These questions can be answered by the manufacturer of whatever equipment you are using. So use machines and rigging only as authorized by the manufacturer (or a qualified person) and remember, slow is smooth and smooth is fast.  

How can you become Qualified or Certified?

Crawford Custom Consulting offers certified and qualified training. Classes are available that can help candidates pass their National Commission for the Certification of Crane Operators (NCCCO) exams.  These classes are held several times per year (please check the schedule on the CCC website for exact dates) and will assist candidates with the written exams and provide practice time on the appropriate crane(s) to prepare the candidate for the practical exam(s).  

Both the NCCCO written and practical exams can be completed at that location and tests are scheduled conveniently for the day after the classroom preparation is completed.  Additionally, if you have a number of candidates who require certification, we may be able to come to your location and conduct the preparation, and testing.

Tony earned a BS in Geology from Juniata College and an MS in Educational Leadership from Miami University.  During his more than two-decade career in higher education Tony was also a reserve component Infantry Officer and his military schooling includes the Air Assault School, 101st Airborne Division (Air Assault), Commandant’s List Graduate of The Infantry Officer Basic Course, and The Distinguished Honor Graduate of The Nuclear, Biological and Chemical Warfare Officer’s Defense Course.  The positions that he held include Platoon Leader, Company Executive Officer, Rifle Company Commander and he served as the Logistics Officer (S4) for the 463D Engineer Combat Battalion (Heavy) that included a yearlong combat deployment to the Sunni Triangle in Iraq.