MOBILE CRANE STABILITY AND LOAD RATINGS

This section deals with mobile crane capacities and load ratings. Mobile crane load ratings are governed by one of two modes of failure:

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The rated load, or chart value, at a particular radius is based on a percentage of the load that will cause failure of the crane by either tipping or structural failure. The percentage, or factor of safety, is stipulated by ASME B-30.5 when tipping is the mode of failure. The ASME tipping factors of safety are 75 percent for crawler cranes on crawlers and 85 percent for cranes on outriggers. Because of complexity, other codes (design codes) govern the factor of safety based on structural failure. Structural failure modes are not discussed here.

Thus, to determine a load rating (chart value), one must first determine the two load values that cause failure, one from tipping and one from structural failure. Then apply the appropriate factor of safety to each of the failure loads and choose the lower value. This will be the rated load for that radius.

3.7.1 Tipping Load and Conditions

The method of determining the load that will cause tipping is outlined in SAE J76S and basically consists of loading weight onto the hook so that the test crane begins to tip. There are stringent conditions under which the tipping test must be carried out. First the crane must be set up on a firm, level (within 1 percent) foundation. Second, the test load condition must be static. Dynamic effects from hoisting, lowering, or swinging must be completely eliminated or minimized. Similarly, the effects of wind must be eliminated by conducting the tests on calm days.

These conditions are the laboratory test conditions and, as such, do not truly represent conditions in the field. The laboratory conditions signify a consistent, repeatable, controlled, tipping value. The factor of safety is added to this baseline tipping value to yield the chart value or load rating for use in the field.

3.7.2 Tipping Load

The predominant factor controlling load ratings for cranes is stability against tipping. In the United States, crane load ratings are established when a crane load stability test is performed per SAE J76S under controlled conditions to determine the tipping load.

The tipping load is the hook load at a specified radius about a line called the tipping fulcrum, which causes the crane to tip. The crane rating is based on taking a percentage of the tipping load. In the United States and Canada, the ASME crane load rating is 75 percent for crawlers and 85 percent for truck cranes of the tipping load. In other industrial countries the crane load

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rating is 66.67 percent and 75 percent, respectively. A crane will tip when the overturning moment (moment of the load and boom about the tipping fulcrum) becomes close to equal to the crane resisting moment (moment of the machine weight about the tipping fulcrum). A stability test is conducted for truck and hydraulic cranes when the machine is in a state of balance about its tipping fulcrum. At this condition, the entire weight of the machine and load is being supported on two outriggers.

Cranes are designed based on full structural rated loads with a 20 mph side wind and a side loading at the boom tip for 2 percent of the rated load. For the normal operating conditions, the above design parameters provide some allowance for the dynamic effects due to the boom swing and luffing.

3.7.3 Tipping Fulcrum Location for Crawler Cranes

Crawler cranes do not have a leveling device and normally operate on timber mats or on firm, level ground. The crawler’s tracks are loose cast steel and their purpose is to provide runways for the track rollers and distribute the machine weight and load to the supporting surfaces. The track rollers define the position of the side fulcrum. The track opposite the tipping fulcrum is not effective in resisting the tipping. When operating over the front, the tipping fulcrum is located below the centerline of the idler or drive sprocket. See Figure 3.7-1.

The weight and center gravity location for various crane components can be calculated. From these data, the stability-based ratings for the crane can be calculated. The accuracy of the calculated stability ratings can be determined by actual testing, which is performed in accordance with SAE J76S.

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3.7.4 Tipping Fulcrum for Truck and Hydraulic Cranes on Outriggers It is essential to raise the crane off of the tires and place it on fully extended outriggers to increase the crane’s stability against tipping. By extending the outriggers, the side tipping lines are extended, thus providing a higher resisting moment against tipping. See Figure 3.7-2.

Figure 3.7-2 Tipping Fulcrum for Truck Cranes

At the construction site, occasionally, cranes are operated improperly with outriggers not fully extended. This could be due to lifting light weights or due to the side condition restraining outriggers for full extension. The following example illustrates the significance of extending the outriggers in regard to the stability.

The above exercise shows clearly why outriggers must be fully extended for lift stability. Truck and hydraulic crane manufacturer lift crane capacity charts clearly state: