Risks associated with mobile crane operations Part 3
Wednesday, 30 May 2018
Posted by: Tasveera Singh
Stabilising and overturning moments
The stability factors of load charts allow for variables such as:
a. dynamic factors caused by the crane motion and the load (for example, for boom movement, application of brakes, swaying of the load); and
b. wind effects on the load and boom.
The stability factor of mobile cranes is based on 75% of tipping for stationary mode, and 66.6% for pick-and-carry mode. All mobile cranes should comply with this design requirement and the stability factor should be written on all load charts for the crane. Where second-hand cranes are imported from overseas, the crane should be stability tested to demonstrate it complies with stability requirements.
When the load chart is based on 75% of tipping, the maximum capacity in the stability range of the load chart will be 75% of the suspended load that will cause the crane to overturn. In other words, the actual overturning load will be 33.3% greater than the load being lifted. Therefore, if a crane’s maximum capacity at a given radius in the stability range of the load chart is 10 tonnes, a 13.3-tonne load will cause the crane to overturn. However, it is also possible for a crane to overturn with smaller loads when operating in windy conditions or on sloping ground, or if the crane is not operated smoothly.
The crane counterweight is critical in ensuring crane stability. A counterweight that is too light for a load and boom configuration will cause the crane to overturn in the direction of the suspended load. Additionally, a crane can fall over backwards due to the effect of the counterweight in situations when:
a. the counterweight is too heavy for the boom configuration;
b. the crane is travelling up a slope with the boom luffed up;
c. inadequate timbers are placed under the outrigger pads below the counterweight when the crane is positioned on soft ground; and
d. outriggers are not extended or lowered into position.
On the majority of smaller mobile cranes, the counterweight is fixed and cannot be easily removed. However, on an increasing number of larger cranes, some of the counterweights are designed to be removed for road travel, or when smaller boom and lifting configurations are required. In this situation, it is particularly important to attach the correct type and number of counterweights to the crane for the particular lift to be undertaken.
Counterweights must be secured to the crane in the manner specified by the crane manufacturer. Where counterweights are removable, each counterweight must be clearly and permanently identified with the crane manufacturer’s name or trademark and the mass of the counterweight (preferably in tonnes).
Where the crane is fitted with a rated capacity limiter, the data input into the computer must be correct for the counterweight configuration on the crane, and related to that shown on the appropriate load chart. This also applies to the boom configuration being used on the crane.
In some unusual circumstances, additional counterweights are attached to the crane to increase its capacity. This process requires an engineer.
Strong winds impose additional loads on a crane and affect the crane’s stability. A maximum permissible wind speed of 10 m/s (36 km/h) is specified for mobile crane operation by some crane manufacturers. Crane configurations designed for wind speeds other than 10 m/s should have the design wind speed marked on the rated capacity chart.
Where wind speeds exceed the maximum figure stated by the crane manufacturer for a specific mobile crane, crane operations should cease, and the crane be placed out of service. Crane operators should recognise that dependent on the boom length, the wind speed may be greater at the height of the load compared to the wind speed at the height of the crane’s cabin.
Crane manufacturers will generally only specify a maximum wind speed to operate the crane, ignoring the type of load to be lifted. In some cases, there may not be a maximum wind speed specified for the crane itself. Wind speed may be much greater above the ground level than next to the operator’s cabin. Also, the effect of wind gusts will have a different effect on the crane than a constant wind.
Given these variables, crane operators must base their decision to make a lift on information provided by the crane manufacturer and their experience as a crane operator. If the operator believes it is unsafe to lift the load, written certification should be obtained from the crane manufacturer or an engineer prior to lifting taking place.
Mobile cranes must be operated within their engineered design capacity. To ensure the stability of a mobile crane in windy conditions, the following factors should be addressed:
a. The crane manufacturer should state the maximum wind speed that the crane may be operated in. Generally, the safe operation of a crane becomes difficult to ensure when the wind speed exceeds 36 km/h, irrespective of the size of the load. However, in many situations, this speed may be excessive, particularly where the load and boom have large surface areas
b. Where the crane is lifting close to its rated capacity, the wind will have a greater effect on the crane stability and the potential application of a side load on the crane’s boom
c. Where the lift is a non-standard lift, with a suspended load or large surface area to be undertaken in windy conditions, a competent person should provide written advice on safe lifting conditions
d. Consider attaching wind gauges to mobile cranes or providing another reliable method of measuring wind speed (for example, handheld wind gauge). Where wind gauges are to be attached to the crane, they should be mounted at the top of the main boom, and calibrated at predetermined intervals, to ensure they provide accurate readings. Guidance on this issue should be obtained from the crane manufacturer or supplier. The provision of wind gauges on mobile cranes is strongly recommended where the maximum rated capacity of the crane is 100 tonnes or greater.
Sloping Ground - Pick-and-Carry Cranes
Many crane roll-overs occur when pick-and-carry cranes travel with a load along a side slope. This may also occur to telescopic handlers and other mobile plant when travelling with a suspended load. Working on a slope has the effect of either increasing or decreasing the working radius of the crane, which may in turn affect the stability of the crane, and cause the crane to overturn either forwards, backwards or sideways.
Where the centre of gravity of the mobile crane is high above the ground, a minimal ground slope can be a major factor in causing the crane to overturn. This particularly applies when:
a. the boom has a high luff angle;
b. the boom is telescoped out; or
c. the centre of gravity of the suspended load is high.
A side slope of only 2 or 3˚ can have a drastic effect on the stability of the crane. Soft ground, pneumatic tyres and suspension movement will also tend to increase the side angle of the crane and make the risk of overturning greater.
Most manufacturers of pick-and-carry mobile cranes specify the cranes are to be operated on firm level ground. In practice, it can be very difficult to ensure the supporting surface for a pick-and-carry crane does not exceed a side gradient of 1%. This is particularly the case at a workplace where construction work is being performed where the ground condition and slope may be constantly changing. A pothole in the ground will have the same effect as a gradient if the crane’s wheel enters the hole.
Where possible, avoid working or travelling on sloping ground. If working or travelling on a slope is unavoidable, consider carrying the load on the uphill side of the crane, regardless of the direction of travel. Travel on a slope should be up or down the slope, not across the slope.
Limiting and indicating devices must be fitted to mobile cranes. 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.
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:
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:
a. hoisting of a load, within the tolerance of 100 to 110% of the maximum rated capacity; and
b. the radius being increased when the load exceeds 100 to 110% at the particular radius.
Motion Limited 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:
a. the highest position of the hook (this is generally known as ‘anti-two block’);
b. the extreme permissible operating positions of the jib (luff limiter); and
c. the end positions of horizontally telescoping or movable jibs.
WORKING R 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.
Load indicators should be fitted to all mobile cranes with a maximum rated capacity of more than 3 tons. 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.
Source: Leading equipment Magazine.