7. Limiting and indicating devices
7.1 Reliability of devices
7.2 Rated capacity limiters
7.3 Motion limiting devices
7.4 Working radius indicator
7.5 Load indicators
Limiting and indicating devices must be fitted to mobile cranes as required by AS 1418.5 Cranes, hoists and winches - Mobile cranes (non-Queensland Government link), unless otherwise required by this code. The purpose of limiting devices is to stop a specific crane motion before the crane moves out of its limits into an unsafe situation. Indicating devices are used to visually or audibly warn the crane operator that the crane may be approaching its set limits or an unsafe situation. These devices may be used individually, or together, for specific crane motions.
7.1 Reliability of devices
Limiting and indicating devices are intended as an aid to crane operators. The devices should not be relied upon to replace the use of the crane’s load chart and operating instructions under any circumstances. Sole reliance on these devices in place of good operating practices may cause an accident.
Where limiting and indicating devices are to be installed on a mobile crane, the safety circuits of these devices should generally meet either:
- a reliability level of Category 4 under AS 4024 : Safeguarding of machinery (non-Queensland Government link); or
- a safety integrity level (SIL) of 3 under AS 61508 : Functional safety of electrical/electronic/programmable electronic safety-related systems (non-Queensland Government link).
These categories of reliability level and SIL are related to the concept of 'fail-safe'11.
7.2 Rated capacity limiters
A rated capacity limiter prevents overloading of the crane by stopping all relevant crane functions when an overload is detected. Rated capacity means the maximum load that may be attached and handled by the crane, and may not include the weight of the hook block, falls of rope, slings and rigging hardware. The load to be raised must include the weight of all lifting appliances that are not permanently attached to the crane. The crane’s load chart will provide guidance on any deductions that may need to be made.
Rated capacity limiters must be provided on all mobile cranes manufactured since 2002 with a maximum safe working load of more than three tonnes. The limiter should prevent:
- hoisting of a load, within the tolerance of 100 to 110% of the maximum rated capacity; and
- the radius being increased when the load exceeds 100 to 110% at the particular radius.
7.3 Motion limiting devices
Motion limiting devices are used to prevent physical damage to the crane or part of the crane due to movement of the crane or part of the crane past its designed range of motion.
Motion limiting devices must be fitted to a mobile crane to prevent motion out of its service limits. These devices cause braking, including deceleration where appropriate and stopping, when the following extreme permissible positions have been reached:
- the highest position of the hook (this is generally known as ‘anti-two block’);
- the extreme permissible operating positions of the jib (luff limiter); and
- the end positions of horizontally telescoping or movable jibs.
7.4 Working radius indicator
A radius indicator displays the radius of the suspended load generally measured from the centre of the slew ring. A radius indicator should be fitted on all mobile cranes that were originally designed with this feature. The indicator should be displayed in metres and be accurate to +10% and -3% of the actual radius.
7.5 Load indicators
Load indicators should be fitted to all mobile cranes with a maximum rated capacity of more than three tonnes. Load indicators measure and display the mass of the load being lifted. This indicator assists the crane operator to stay within the load chart and safe working limit of the crane. The load indicator should be capable of displaying the mass of the suspended load at all times.
11. See appendix 1 for definitions.
Last updated 21 August 2008