What a Jib or a Boom Extension Does to a Forklift Rating

Why this matters

A jib turns a lift truck's one fixed load center into a variable one that the operator selects with a pin, and it hangs the load from a hook instead of resting it on a fork. Those two changes together mean the truck no longer has a capacity, it has a capacity function, and the machine it has become is a small crane with none of a crane's instrumentation: no load-moment indicator, no anti two-block device, no boom angle indicator, and a hoist that is really a mast. This card is the worksheet that has to be filled in before any of that gets used, with one real set of numbers in it and the two places where the arithmetic you can do yourself stops being on your side.

The worksheet, filled in

Everything below is for one truck, one jib, one hook position and one load. Every line names where its number came from, because a line with no source is the line that fails.

Three things in that table are the whole article.

The jib eats capacity twice before the load arrives

The 9,000 lb-in the jib consumes is not a small correction. It is 7.5 percent of the truck's entire moment budget, spent before anything is hooked. That is the first bite.

The second is larger and it is geometric rather than gravitational. The truck's plate authorized 5,000 lb at 24 inches. The hook at position 3 sits at 60 inches, two and a half times further out. Even with no jib weight at all, the capacity at 60 inches would be 120,000 divided by 60, or 2,000 lb. The jib's own weight then takes that to 1,850 lb. So of the 3,150 lb of capacity that disappeared between the plate and the hook, about 3,000 lb went to the geometry and about 150 lb went to the jib's mass.

That ratio is the reason operators underestimate this. A 300 lb attachment intuitively feels like a 300 lb problem. It is not. The reach is the problem, and moving the hook pin one position further out costs more capacity than the whole attachment weighs.

The shortcut stops being conservative the moment a jib is fitted

A sibling card establishes that the fork-face moment shortcut errs low against an axle-referenced calculation on a bare truck, and that erring low is why it is worth teaching. That protection does not survive a jib, and it is important to say so plainly rather than to carry the habit across.

Here the shortcut produced 1,850 lb and the manufacturer's combination chart says 1,600 lb. The shortcut is 16 percent optimistic, in the direction that drops a load. The reason is that the moment budget only ever priced a static, rigidly supported weight sitting on a fork at a known distance. A jib adds terms it never contained:

• The load swings. A suspended load at the end of a 60 inch hook is a pendulum. Every start, stop, turn and floor joint moves it, and while it swings the effective load center changes continuously and can exceed the pinned hook position. A moment budget computed at a fixed 60 inches is describing a condition that only exists when nothing is moving.
• The truck's stability with a suspended load is not the same problem. The manufacturer's rating for a rigid load and its rating for a hook load are two different numbers, and where the jib manufacturer publishes both, the suspended-load figure governs. The 1,600 lb above is already the suspended rating, so it does not get derated again for the same reason; applying a suspended-load reduction on top of a chart that already contains one is double-charging a correction.
• The jib has its own structure. At the far pin positions the jib's rating can be set by the jib's own steel rather than by the truck's tipping stability, and no truck-side arithmetic reaches that at all.

So the rule is directional and it is easy to remember. On a bare truck, use the shortcut and treat it as a floor. With any attachment fitted, the shortcut is an estimate with no known direction and the manufacturer's combination rating is the only authorization. The estimate still earns its place in the worksheet, because when it comes out far below the combination chart figure that is a signal you have the wrong chart, the wrong hook position or the wrong truck.

The revised plate is the legal instrument, not the chart in the binder

Fitting a jib is a modification affecting capacity. 29 CFR 1910.178(a)(4) requires the manufacturer's prior written approval for it and requires the capacity, operation and maintenance plates, tags and decals to be changed accordingly. 29 CFR 1910.178(a)(5) then requires that a truck carrying front-end attachments other than factory-installed ones be marked to identify the attachments, show the approximate weight of the truck and attachment combination, and state capacity at maximum elevation with the load laterally centred.

Read that last phrase again, because it is the field most often missing from a shop's copy of a chart: at maximum elevation. The 1,600 lb above is a hook-position rating. The capacity with the mast up is lower again, and on a jib job the mast is usually up, because the whole point of a jib is to reach over something.

An attachment fitted without the revised plate leaves the truck carrying a rating that describes a machine that no longer exists, and a sibling card covers how to catch that during a plate read.

What the operator does not have

This is the part that gets people, and it is not on any chart. A mobile crane doing the same 1,150 lb pick has a load-moment indicator warning before it reaches capacity, an anti two-block device, a boom angle indicator, controlled load lowering, and a load chart the operator can read from the seat. A lift truck with a jib has none of them. The operator's only feedback that the truck is approaching its limit is the rear wheels getting light, which is also the last feedback before it tips.

That absence changes how the job is run rather than how it is calculated:

• Nobody is under the load or within its swing arc, at any point. 29 CFR 1910.178(m) prohibits anyone standing or passing under the elevated portion of a truck, loaded or empty, and a jib puts a hook load out where people naturally stand.
• Tag lines are tended from outside the swing arc and never wrapped around a hand, a wrist or a body. A jib load that starts swinging is controlled by stopping the truck, not by grabbing the load.
• Travel with a suspended load only where the jib manufacturer permits it, with the load as low as the surface allows and at walking pace. The load swinging fore and aft during travel is the mechanism that turns a passing calculation into a forward tip.
• No lifting on a grade or a cross-slope. A slope tilts the whole stability triangle and the jib is the longest lever on the machine.
• Operator training is truck-specific under 29 CFR 1910.178(l), and an attachment changes the truck. An operator qualified on the bare truck is not automatically qualified on this configuration, and operator evaluation is required at least every three years under 1910.178(l)(4)(iii) regardless.

How to verify you got this right

Fill the worksheet in the order printed, and refuse to skip a source column. The three lines that fail most often are the hook position (assumed rather than read off the pin), the below-hook rigging (left off, because it feels like part of the load rather than part of the capacity), and the combination rating (taken from a chart for a different truck of the same family).

Then run one check on your own margin. The 405 lb of margin in the example is 25 percent of the governing capacity, and that sounds comfortable. Ask what it buys. It buys the difference between a load weighed on a rated cell and a load someone estimated, and it buys the swing. If the load weight came from a guess rather than a scale, that margin is not margin, it is the size of the guess.

Last, look at the hook position one more time before the pick. Moving from position 3 at 60 inches to position 4 at, say, 72 inches recomputes the estimate to 111,000 divided by 72, or 1,540 lb, and the manufacturer's combination rating at that position will be lower still. One pin, moved for convenience while nobody was watching the chart, is the most common way a jib lift ends up outside its rating with the paperwork all correct.

References

• 29 CFR 1910.178, powered industrial trucks, including 1910.178(a)(4) on modifications affecting capacity, 1910.178(a)(5) on marking front-end attachments and capacity at maximum elevation, 1910.178(l) on training, and 1910.178(m) on operations
• 29 CFR 1926.602, material handling equipment in construction, where the same truck is used on construction work
• The jib manufacturer's data for jib weight, centre of gravity, hook positions and suspended-load ratings, and the truck manufacturer's combination rating and revised plate
• See related: Why Forklift Capacity Falls With Load Center; How to Read a Forklift Data Plate and What Attachments Do to It