The lifting moment of a crane is the load multiplied by its working radius, the horizontal distance from the slewing centre to the hook. It is written in tonne-metres (t·m) or kilonewton-metres (kN·m). A crane holding 8,000 kg at 2 m works at 16 t·m, and the same 16 t·m could also be 1,600 kg at 10 m. Every truck crane has a maximum lifting moment printed on its drawing, and a load moment limiter watches that figure during each lift.
BOB-LIFT has built truck mounted cranes in Xuzhou since 2003, and every model has a dimension drawing and a load chart. This guide uses those drawings to show how to calculate lifting moment, why the rated moment is not the same at every radius, and what a load moment indicator or limiter (LMI) does on a truck crane.
Key takeaways
- Lifting moment = load × working radius. Using tonnes and metres produces t·m, and multiplying by 9.81 converts the result into kN·m.
- The maximum lifting moment on a technical drawing represents a peak value: our SQ8ZA5 is rated at 16.7 t·m, and its own load chart reaches that figure only at approximately 4.5 m.
- Never calculate capacity backwards from the moment alone, because at 12.63 m the SQ8ZA5 chart permits 1,160 kg, which corresponds to only 14.65 t·m.
- Telescopic cranes drop further at long reach: the SQ5SA4 is rated at 12.5 t·m, but its longest boom allows 600 kg at 11.8 m, about 7 t·m.
- A load moment limiter continuously senses load and radius, warns the operator, and automatically stops movements that would increase the moment.
- OSHA requires articulating cranes built after 8 November 2011 to have an overload prevention device or a load moment system.
What is the lifting moment of a crane?
Lifting moment is the rotational effect that a suspended load creates around the crane’s slewing centre and its outrigger supports. The load pulls down at the hook, and because the hook sits some distance away from the slewing centre, that downward pull tries to tip the crane forward. Consequently, the further out the hook goes, the larger the turning effect becomes, even though the load itself has not changed at all.
OSHA describes this quantity in its definition of a load moment indicator: a system that senses “the overturning moment on the equipment, i.e., load multiplied by radius” (29 CFR 1926.1401). In other words, load is the total weight hanging from the hook, while radius is the horizontal distance from the centre of rotation to that hook.
Truck crane manufacturers commonly use the maximum lifting moment as a size classification. That is why you will often see a crane described as a “16 t·m” or a “21 t·m” crane, rather than only as an 8 t or a 10 t crane. The tonnage tells you the heaviest load close to the column, whereas the moment tells you considerably more about how the crane performs once it reaches out over a truck bed or a wall.
How to calculate lifting moment
The formula itself is short, and it works for knuckle boom, telescopic and marine cranes alike:
Lifting moment (t·m) = load (t) × working radius (m)
The load is not only the object you are lifting. OSHA’s definition of load covers “the load block, ropes, slings, shackles, and any other ancillary attachment” as well (29 CFR 1926.1401). Therefore, add the hook block, the slings and any grab or basket to the weight of the object before you multiply.
Worked example. You need to lift a 1,600 kg pallet of blocks with 50 kg of slings and shackles, and the hook will be 8.5 m from the slewing centre. The load is 1.65 t. Multiply 1.65 t by 8.5 m and you get 14.0 t·m. Next, check the chart. Our SQ8ZA5 knuckle boom crane allows 1,820 kg at 8.51 m, so this lift is inside its rating with some margin.
Converting t·m and kN·m
Some drawings give the moment in kilonewton-metres instead. Since one tonne-metre is approximately 9.81 kN·m, divide a kN·m figure by 9.81 to convert it into t·m. For example, our BLZ90K4TJ marine crane drawing shows 90 kN·m, which is about 9.2 t·m. Older documents may also use “ton.m” or “tm”; these mean the same thing as t·m.
Why the rated moment is not the same at every radius
The maximum lifting moment on a drawing is a peak. It does not mean the crane can hold that moment at every point of its reach. To see this, multiply each load on a real chart by its radius. The SQ8ZA5 chart below is taken from our technical drawing.
| Radius on the SQ8ZA5 chart | Rated load | Load × radius |
|---|---|---|
| 2.00 m | 8,000 kg | 16.0 t·m |
| 4.51 m | 3,700 kg | 16.7 t·m |
| 6.49 m | 2,470 kg | 16.0 t·m |
| 8.51 m | 1,820 kg | 15.5 t·m |
| 10.56 m | 1,420 kg | 15.0 t·m |
| 12.63 m | 1,160 kg | 14.7 t·m |
As the table shows, the crane only reaches its full 16.7 t·m at about 4.5 m. Close to the column, the 8,000 kg maximum load is the governing limit, so the moment is lower. Further out, however, the extension booms and the dead weight of the boom itself use up part of the capacity, and as a result the moment falls to about 14.7 t·m at full reach.
Dividing 16.7 t·m by 12.63 m would suggest approximately 1,320 kg at full reach, yet the official chart permits only 1,160 kg. This gap is precisely why you should never work out capacity from the moment alone: the load chart is the actual rating, and the moment is only a convenient summary of it.
Knuckle boom and telescopic cranes behave differently
Telescopic booms show a considerably bigger drop at long reach. The SQ5SA4 telescopic crane is rated at 12.5 t·m. Its load chart gives 5,000 kg at 2.5 m, which is that peak. With its longest 12 m boom, however, the chart permits 600 kg at 11.8 m, which is approximately 7.1 t·m, or a little more than half of the rated figure.
The main reason is the boom itself. In particular, a long, slim telescopic boom has to carry its own weight far from the column, and its rating at full length is set by the strength of the boom, not just by the overturning moment. A knuckle boom, by comparison, distributes its weight across articulated sections, so it retains considerably more of its moment at reach. Our comparison of knuckle boom vs telescopic boom truck cranes covers the other differences.
Lifting moment of BOB-LIFT truck cranes by size
The following table lists the maximum lifting moment printed on several of our technical drawings, together with the rated load at the final point of each chart. Use it to compare capacity classes, and then request the complete chart for the particular model you are considering.
| Model | Boom type | Max lifting capacity | Max lifting moment | Load at the last point on the chart |
|---|---|---|---|---|
| SQ5ZA4 | Knuckle | 5,000 kg | 10.5 t·m | 700 kg at 9.9 m |
| SQ5SA4 | Telescopic | 5,000 kg | 12.5 t·m | 600 kg at 11.8 m |
| SQ8ZA4 | Knuckle | 8,000 kg | 16.7 t·m | 1,420 kg at 10.5 m |
| SQ8ZA5 | Knuckle | 8,000 kg | 16.7 t·m | 1,160 kg at 12.6 m |
| SQ10ZA6 | Knuckle | 10,000 kg | 21 t·m | 1,150 kg at 14.7 m |
| SQ10SA4 | Telescopic | 10,000 kg | 25 t·m | 1,050 kg at 15.5 m |
| SQ12ZA4 | Knuckle | 12,000 kg | 27 t·m | 2,100 kg at 10.2 m |
In addition, two cranes with the same moment can still suit quite different jobs. The SQ8ZA4 and SQ8ZA5 are both rated at 16.7 t·m, but the SQ8ZA5 reaches about 2 m further with a lighter load at the end. For more on choosing a size, see our guide to knuckle boom crane lifting capacity and the 8 ton knuckle boom truck crane page.
What does a load moment limiter (LMI) do?
A load moment limiter is the safety device that watches the lifting moment for the operator. OSHA defines it as a system that senses the overturning moment and compares it with the rated capacity. When the rated capacity is reached, it “shuts off power to those equipment functions which can increase the severity of loading”, such as hoisting, telescoping out or luffing out. Movements that reduce the load, such as lowering or telescoping in, normally stay available (29 CFR 1926.1401).
On a hydraulic truck crane, the system typically reads the pressure in the lifting cylinder, and on a telescopic crane it also reads the boom length and angle. From these it works out the load and the radius, compares them with the chart, and shows the result as a percentage. Our product pages list a load moment limiter on every crane, and our telescopic models additionally have a live display.
The screen in the photograph displays practically every input that the limiter uses. You can see the boom angle, the boom length, the working radius, the rated load at this position and the actual load. The horizontal bar at the top converts the comparison into a percentage, so the operator does not need to perform any calculation during the lift.
What the rules say about load moment systems
Under OSHA’s construction crane rules, articulating cranes (knuckle boom cranes) “manufactured after November 8, 2011 must have at least one of the following: automatic overload prevention device, load weighing device, load moment (or rated capacity) indicator, or load moment (rated capacity) limiter”. Other cranes over 6,000 pounds built after 29 March 2003 must have a load weighing device, an indicator or a limiter (29 CFR 1926.1416(e)(4)).
The same section treats these devices as Category II operational aids. If one stops working properly, it must be repaired within 30 calendar days, and in the meantime the weight of every load must be determined from a recognised source and communicated to the operator before lifting. In Great Britain, the HSE code of practice for LOLER says that where there is “a significant risk of overturning and/or overloading”, rated capacity indicators and limiters should be provided, and it notes that these were once called “moment load indicators” (HSE L113).
Load moment indicator vs limiter vs rated capacity limiter
These names often cause confusion between buyers, operators and suppliers, so here is how the official texts actually use them:
- Load moment indicator (LMI) or rated capacity indicator: shows the operator the percentage of capacity in use, often with lights or a buzzer as the load gets close to the limit. It warns but does not stop the crane.
- Load moment limiter or rated capacity limiter: does the same sensing, and also cuts the functions that would increase the moment once the rating is reached.
- Automatic overload prevention device: the term OSHA uses for articulating cranes. It stops an overload without necessarily showing a percentage.
OSHA’s definitions page links the terms directly: “Rated capacity indicator: See load moment indicator” and “Rated capacity limiter: See load moment limiter”. In daily use, “LMI” is often used for both the indicator and the limiter, so check what the device on your crane actually does.
What changes the usable lifting moment on site
A load chart assumes ideal conditions, which are rarely found on a real site. For example, the following factors all reduce the moment you can safely use:
- Outriggers: the rated moment needs the outriggers set as the chart states. Short-jacked or uneven outriggers reduce stability. Our guide to crane outriggers and ground bearing pressure explains the checks.
- Level ground: a tilted crane moves the load further out than the indicated radius, which increases the actual moment.
- Rigging and attachments: slings, a hook block, a hydraulic grab or a personnel basket all count as part of the load.
- Boom angle and length: on a telescopic crane the radius comes from both. Our article on crane boom angle shows how they connect.
- Movement and wind: swinging a load or stopping it sharply adds dynamic force. Wind on a large flat load can push it out past the planned radius.
How to check a lift before you make it
- Determine the total load, meaning the object together with the hook block, slings and any attachment.
- Measure the radius at both the pick-up position and the set-down position, and use whichever is larger.
- Look up the rated load at that radius on the crane’s own chart, for the boom length and outrigger setting you will use. Our guide on how to read a crane load chart walks through this step.
- Confirm that the total load is below the rated load; multiplying load by radius provides a useful cross-check, although the chart remains decisive.
- Check that the load moment limiter is working before the shift. Our truck mounted crane inspection checklist includes the safety devices.
Example scenario (an illustration, not a customer record): a builder wants to place 2,000 kg roof trusses, with 60 kg of slings, 7 m behind the truck. The total is 2.06 t, and 2.06 × 7 gives 14.4 t·m, which is below the SQ8ZA5 rating. However, the chart shows 2,470 kg at 6.49 m and 1,820 kg at 8.51 m. At 7 m the safe value is the 1,820 kg of the next point out, unless the chart lists a closer row, so this lift should be re-planned with a shorter radius or a larger crane.
The hydraulic system matters as well, since a limiter can only protect the crane effectively when the relief valves and holding valves work as designed, which our guide to the hydraulic crane for truck explains.
Frequently asked questions
What is the formula for lifting moment?
Lifting moment equals the load multiplied by the working radius, so a 2 t load held at 5 m produces a moment of 10 t·m. Include the hook block, slings and attachments in the load, and multiply t·m by 9.81 if you need kN·m.
What does t·m mean on a truck crane?
It is the maximum lifting moment expressed in tonne-metres; our SQ10ZA6, for example, is a 10 t crane with a rated lifting moment of 21 t·m. It tells you the peak load-times-radius on the chart, not the load at every radius.
What is an LMI on a crane?
LMI stands for load moment indicator, a device that senses load and radius, compares them with the crane’s rated capacity, and displays the percentage currently in use. A load moment limiter goes one step further and stops movements that would increase the moment.
Can I calculate capacity from the lifting moment?
Only as a rough plausibility check, because manufacturers derate capacity at long reach. The SQ8ZA5 is rated at 16.7 t·m, but at 12.63 m its chart allows 1,160 kg, less than the 1,320 kg the moment would suggest. Always use the load chart.
Do knuckle boom cranes need a load moment limiter?
In the United States, articulating cranes built after 8 November 2011 must have an overload prevention device, a load weighing device, or a load moment indicator or limiter under 29 CFR 1926.1416. Every BOB-LIFT crane is listed with a load moment limiter.
How we compiled this guide
The capacities, radii and lifting moments come from the BOB-LIFT technical drawings and load charts of the SQ5ZA4, SQ5SA4, SQ8ZA4, SQ8ZA5, SQ10ZA6, SQ10SA4, SQ12ZA4 and BLZ90K4TJ, and we compiled the load × radius figures from those charts. The regulations are quoted from 29 CFR 1926.1401 and 1926.1416 on eCFR and from HSE L113 (second edition, 2014), as they stood in November 2026. The photos show real BOB-LIFT cranes. The guide was reviewed by Achord Chan, Product Operations Director, in November 2026. It replaces an earlier, shorter version of this page and does not replace your crane’s load chart, operating manual or local regulations.
Get the load chart for your lift
Tell us the load, the radius and the lifting height you need, and we will suggest a model and send its dimension drawing and load chart. Every crane carries a 12-month warranty covering the complete machine, common spare parts are dispatched within 48 hours, and our service department replies within 12 hours. Browse our knuckle boom truck cranes and telescopic boom truck cranes, or contact us to start.
