Load Capacity

The rated capacities of mobile cranes are based on both strength and stability. Manufacturers of cranes will normally denote on the load charts a shaded area or a bold line across the chart dividing the lifting capacities based on strength or stability of the crane. It is extremely important to know the difference for, in one case, one of the structural components of the crane will break and, in the other case, the crane will tip over. Additionally, the following factors must be recognized and the capacity adjusted accordingly:

1. Do not use stability to determine lifting capacity. Use the load chart installed by the crane manufacturer. The load chart is securely attached in the operator's cab.

2. The number of parts of line on the hoist and the size and type of wire rope for various crane loads.

3. Length of boom.

4. Boom angle.

5. Boom pendant angle (when the telescopic/ folding gantry is down, the angle decreases and the stress increases).

Figure 12-46.-Leveling procedures using a builder's level.

6. Gantry and/or live mast in the highest position.

7. Quadrant of operation (that is, over the side, over the rear capacities).

Load Rating Chart

Atypical load rating chart is shown in figure 12-47. To determine the capacity of the crane by using the load chart, the operator must know the length of boom, the load radius, the boom angle, and if the lift is to be performed over the side or over the rear.

When performing lifts using the boom angle indicator that indicates an angle not noted on the load chart, use the next lower boom angle noted on the load chart for determining the capacity of the crane. For example, using the load charts in figure 12-47, the crane is rigged with 60 feet of boom, and the boom angle indicator indicates a boom angle of 57 degrees. A 57-degree boom angle load capacity is not noted on the load chart, so you must use the next lower noted boom angle of 53 degrees for determining the capacity of the crane.

NOTE: Do not rely on the boom angle indicator for radius accuracy when lifts exceed 75 percent of the rated capacity. Measure the radius to avoid the possibility of error.

When using a radius measurement not noted on the load charts, use the next longer radius measurement noted on the load chart for determining the capacity of the crane. For example, using the load charts in figure 12-47, the crane is rigged with 50 feet of boom, and the radius measurement is 32 feet. A 32-foot radius measurement is not noted on the load charts, so you must use the next longer radius measurement of 35 feet noted on the load chart for determining the capacity of the crane.

The number of part lines reeved on the main hoist block can affect the capacity of the crane. If the crane is capable of being reeved with an eight-part line and the reeving is changed to a six-part line, the capacity of the crane changes. On newer models of cranes, the capacity for different parts of line configurations is noted on the load charts. On older models, you must refer to the manufacturer's manual.

The load chart provides the capacity of the crane with outriggers set and without outriggers. "Outriggers set" means the outriggers are fully extended and the weight of the crane is off of the suspension system or the tires are off the ground. If a situation arises where the outriggers cannot be fully extended, you must use the without outriggers load capacity ratings.

NOTE: Load capacities change when swinging from each quadrant of operation, such as from over the rear to over the side.