The CICA technical committee discusses the impact of changing ground conditions have on the operation of articulated pick and carry cranes.
The Pick and Carry crane is a type of mobile crane that is designed to lift and often carry a load on wheels, to its destination. Different from slewing mobile cranes, pick and carry cranes normally do not have outriggers; they often have a permanent articulation joint in their construction, allowing the crane to manoeuvre and place loads.
Pick and Carry cranes are popular in Australia. There are over 5000 Pick and Carry cranes in operation which make up approximately 50%= per cent of the total number of mobile cranes in Australia. Nearly every crane company has at least one Pick and Carry crane and many companies have fleets of these cranes. Pick and Carry cranes have displaced the work usually completed by smaller hydraulic truck cranes as the set-up time is significantly shorter and the load can be picked up and carried by the crane to the destination.
During the lifting operation, slewing mobile cranes usually lift and place the load from and to a fixed location, whereas pick and carry cranes often travel on sites with their load. When operating the crane to lift and carry a load, the crane operator should follow the manufacturer’s instructions. Adverse operating conditions should be considered, including the supporting surface, pendulum action of the load, jerking or sudden stops of the crane that creates load swing and other factors affecting stability. Crane operators should take into consideration changing ground conditions that can occur when travelling whilst carrying load, as changing ground conditions (uneven road surface, potholes, etc.) will cause a reduction in crane rated capacity. These conditions effectively create a side slope condition and the rated capacity of the crane reduces to ensure the crane remains stable.
When lifting a load on a side slope, it induces a side load as the load swings down the slope and this affects the tipping line of the crane. Crane operators should follow the deration percentage specified on the deration load chart provided by the crane manufacturer to calculate the appropriate crane rated capacity for their lifting job.
Figure A below is an example of how to use the crane capacity deration chart. The deration load charts are provided by the crane manufacturers to guide the operator on how to calculate the rated capacity reduction when a side slope is encountered.
For example, a crane with a deration load chart below has a rated capacity (on level ground) of 4000kg (operating at 7m working radius with a boom angle of 34°). When it’s operating on a side slope up to 5°, according to the chart, the crane capacity should reduce 40 per cent to:
4000kg – 4000kg x40% = 2400 kg (Example PT)
But Why?
Crane stability can change during crane articulation. The position of crane weight gets closer to the axis of rotation which causes the moment from the crane weight to be reduced (Figure B1)
When operating on a side slope, if the load offset goes beyond the wheel, crane tipping line changed to the side wheel line. In this case, distance between crane weight and tipping line is greatly reduced.
When operating on a side slope in articulated configuration, the crane tipping line changed to the side wheel line. In this case, distance between crane weight and tipping line greatly reduced and distance between load and tipping line increased (Figure B2)
Unlike slewing mobile cranes that can perform lifting operations with a slewing angle of 360°, the articulated cranes’ operation is only permitted through the crane articulation range, typically up to 40°. Most articulated cranes have a reduced rated capacity above 10° articulation. During crane articulation, the crane’s centre of gravity gets closer to the axis of rotation which causes the moment from the crane counterweight to be reduced. A reduced rated capacity chart has to be used if entering the articulation range of 10° to 40° during the operation. Because the crane often articulates during carrying operations while travelling on site, planning the lift is key to ensuring crane capacity is maintained throughout the lift operation.
Refer to the articulated crane load chart example below. In the chart, crane rated capacities when the crane articulation angle less than 10° (in green) and crane articulation angles greater than 10° (in yellow) are listed.
When crane is operating with a 9-metre boom at 4 metre radius,
Rated capacity of the crane is 8350kg when the crane is articulated less than 10°, or
Rated capacity of the crane is 7250kg when the crane is articulated more than 10°.
As discussed above, there are several key set-up and operation differences between a slewing mobile crane and a pick and carry mobile crane. Careful consideration needs to be given to the safe use of pick and carry cranes.
To improve safety for our industry, CICA has developed several useful tools for the industry on the topic of safe use of pick and carry cranes:
Stability of pick and carry cranes video. This is a training video for pick and carry crane operators, the video explained the different situations where the crane could lose its stability (side slope, articulation), and the reasons behind these situations in a simple, easy to understand way. The video can be accessed on YouTube:
www.youtube.com/watch?v=d9cDQ2rLkwE.
CICA-PA-009-B CICA Position Paper Articulated Crane Operator Requirements.
CICA training pack “Safe use of Articulated Pick & Carry Cranes”
CICA Pick and Carry crane lift planning tool.
For more details on the materials listed above, please contact CICA Technical Project Engineer Alice Edwards (projeng@cica.com.au).