How Do You Calculate Jib Crane Load Capacity?
Have you ever wondered why holding a brick close to your waist feels lighter than holding it at the end of an extended arm? If you substitute your arm for a crane boom or jib and your waist for a crane’s center of mass, then calculating the crane’s load capacity involves answering this question.
A crane’s lifting capacity is not static but changes according to its configuration. Manufacturers draw up a loading chart for each crane type, in which the maximum gross load is listed for each boom length, angle, and load radius. This chart enables one to calculate the maximum net load.
Is your crane standing on rubber or widened its base with outriggers? Is the wind blowing from the back or the front of the crane? Is the lifting tip at the end of an extended or short boom? Is the boom angle high or low? Is the pick-up point far or near the crane’s center of gravity? Answers to all of these questions may drastically affect a crane’s ability to pick up a given load safely.
Reading A Crane’s Loading Capacity From A Load Rating Chart
A given mass may be safe for a crane to pick up in one configuration but unsafe if the same mass is picked up in a different configuration. Operators should, therefore, not confuse the mass of a picked-up load and the loading stress put onto the crane.
Manufacturers draw up a load rating chart for every type of crane built. The maximum gross load (in pounds) is listed, which a crane can safely handle in a given configuration. A part of the chart for the crane type: Grove RT650E, looks as follows:
The top row numbers in bold (33-60) signify the crane’s boom length in feet(ft). The left column numbers in bold (10-15) signify the lift radius, the horizontal distance between the center of rotation of the crane’s boom, and the suspended load’s center of gravity. Remember that the load is normally suspended from the boom or jib extension tip.
The large number where the column and row numbers meet indicates the maximum gross loading capacity in pounds (lbs.). The smaller number in parenthesis indicates the boom’s extension angle to the horizontal. Due to boom deflection, the correct boom angle can only be measured after the load has been lifted under loading stress.
A wind speed greater than 20mph can significantly impact boom deflection. If a crane is operating in its front quadrant and a strong wind blows the crane from behind, from the back quadrant, it can deflect the boom forward to decrease the boom angle, increase its load radius, and reduce its safe load capacity.
This implies that prior to a 20mph wind blowing, the crane could have been operating within a safe loading capacity. After it blows, the crane could suddenly become unsafe. In theory, the operator could increase the load capacity again by lifting the boom at a higher angle to compensate for the deflection.
For example, see where row figure “33” intersects with column figure “10”. We see the numbers “100 000” and “69.5” at this intersection in parenthesis. These numbers tell you that if the crane is configured so that its boom extends 33ft at an angle of 69.5 degrees to the horizontal, it will pick up or set down a load at a horizontal distance or horizontal working radius of 10ft.
The numbers also tell you that in this configuration, the crane cannot handle more than a total or gross load of 100 000lbs. This does not mean that the crane can lift an object with a mass of 100 000lbs. It means that the mass of the lifted object combined with the mass of all the crane’s lifting accessories should not exceed 100 000lbs at the given configuration.
Many load rating charts further include 4 icons. Each icon specifies information about how the crane must be configured for the figures in the chart to be accurate. One icon indicates the full boom extension range.
The next icon indicates that a specific counter-weight must be installed in the crane; another may specify if outriggers must be deployed and how wide a base the crane must operate from. The last icon indicates whether chart figures are based on lifts where the crane can rotate 360 degrees as opposed to ‘over the rear’ lifts.
When calculating safe loads, operators should keep in mind that the 360-degree sphere of operation around a crane can be divided into ‘weak’ and ‘strong’ quadrants. The quadrant of operation with the most lifting capacity is directly opposite the engine compartment, which acts as a counterweight.
Therefore, if the engine is at the back, the crane will be most structurally stable and resistant when lifting over the front and less stable when lifting over the side or rear. In this case, the crane has its highest loading capacity in its front quadrant. Knowing this, operators should increase their boom angles and reduce their lift radius when operating in a weak quadrant to compensate.
Calculating A Crane’s Maximum Net Load Capacity
A crane’s netload is the mass of the object the crane picks up and offloads during its operations. One calculates the maximum net load a crane can safely pick up after deducting the mass of all lifting accessories from the gross capacity maximum read from the crane’s load rating chart. Therefore:
Maximum Netload = Maximum Gross Load – Deductions
The total mass deducted includes the mass of all lifting accessories and implements suspended below the boom and jib; this includes all blocks, balls, and wire rope reeving. Excess hanging wire rope and jibs unnecessary to make the pick and jibs stowed on the boom are also deducted.
The mass of the boom or jib, which carries the object or netload, is already factored into the gross mass capacity chart figures and thus should not be deducted.
Wire Rope Deductions And Crane Net Load capacity
Concerning all wire rope deductions from the Gross load, to calculate netload:
- Deduct all wire rope mass hanging off the jib, which is not used to carry the load.
- Deduct all wire rope mass used in lifting from boom tip to ground.
Remember that some loads may be attached to multiple lines, so the formula to calculate total wire mass will be as follows:
Total Wire Rope Mass (lbs.) = Number of Lines X Boom Tip To Ground Distance(ft) X Rope Mass/ft
For example: Given the crane’s block and hook with a mass of 5000lbs is reeved with 4 lines, boom tip to ground distance is 150ft and rope mass/ft is 2lbs, and the maximum gross load capacity for that particular crane in that particular boom length/angle, load radius distance configuration was 60 000 lbs, then:
Netload = Gross Load – Deductions (Total Wire Rope Mass + Block&Hook)
= 60 000 – [ (4) (150) (2) + 535000]
= 53 800lbs
Keep in mind that 53 800lbs is the maximum allowed operational object mass the crane can lift safely, given its overall structural strength and stability in that particular configuration. However, a good operator should always remember that the crane’s lifting capacity is always limited by its weakest link.
Therefore, it may be good to check the tensile strength of the wire rope to which the load is attached. Therefore, if each wire rope can carry 10 000lbs and there are 4 of them, the combined line carrying capacity is only 40 000lbs and not 53 800lbs!
Conclusion
When calculating your net allowable operational load, you may pick with your crane; always be mindful of your crane’s specific configuration. When measuring your configuration, always measure after lifting the operational load to allow rubber tire and boom deflection.
Also, remember to compensate for wind and weak vs. strong quadrants of operation and that your weakest link always limits your true net load capacity.
