How to Keep an RT Crane on Its Wheels

By Andrew Baltazar, Associate Editor | September 28, 2010

First introduced in 1959 by Grove, the rough-terrain crane was designed as a multi-purpose construction tool. Equipped with industrial-strength tires, these mobile machines can lift loads on muddy, uneven, or harsh ground, otherwise precarious territory for truck-mounted cranes and other mobile cranes. To a certain extent, they are also able to transport loads on leveled ground. But because rough-terrain cranes mainly operate on treacherous terrain, operators need to conduct their work with utmost caution.

This was not the case in Minturn, Colo., in 2005 when a crane operator working on U.S. Forest Service land was killed after the rough-terrain crane that he was driving downhill tumbled off the road. Or in 2006, also in Colorado, when an operator died in a rough-terrain crane tip-over accident that resulted in large part because the crane was operating outside the manufacturer's load-chart specifications.

Or again in 2007 in Gilbert, Va., when a 49-year-old operator technician chose not to use the outriggers on his 20-ton rough-terrain crane to lift a section of a pipeline at an iron-ore mine, ending in yet another tip over. The man jumped or fell from the crane as it was tipping and was crushed when it landed on him.

According to Bud Wilson, president of and crane instructor at The Crane School, most rough-terrain-crane accidents occur because operators fail to scrutinize the load chart.

The Bible of cranes

There are three different methods of lifting loads with a rough-terrain crane: stationary and over the front; stationary and 360 degrees; and pick and carry.

The first method — stationary and over the front — often provides higher load capacities. For this type of lift, the boom must stay in between the front two outriggers or tires.

Take note, however: The maximum load capacities for outriggers are different with tires, so operators must make sure to study the appropriate load chart or risk tipping the crane. Also, moving the boom to the outside of the two front outriggers or tires could also make the crane unstable.

The second method — stationary and 360 degrees — means the boom can be in any load radius and can rotate around the base, while the crane itself remains motionless. The boom can move outside the two front outriggers or tires, but load capacities will be reduced compared with the over-the-front method. Again, load capacities for outriggers and tires are different.

The first two methods allow for operators to make lifts on rubber tires in addition to outriggers, provided that the appropriate load chart — either for tires or for outriggers — is used to determine maximum capacity.

Still, the crane must not be driven. The third lifting method was designed for pick-and-carry operations.

When transporting loads on a rough-terrain crane, operators must drive in “creep mode,” meaning the crane should keep below 1 mile per hour and not exceed 200 feet of movement in a 30-minute period.

Rough-terrain cranes on wheels can be so unstable at certain weights or boom angles that Link-Belt's TLL load chart, used by the National Commission for the Certification of Crane Operators to instruct operators, lists this warning: “When operating on tires, do not exceed 76-degrees maximum boom angle. Loss of backward stability will occur causing a tipping condition.”

In other words, rough-terrain cranes can tip at extreme boom angles even if they are not loaded at all.

Driving with the boom too high “creates a very high center of gravity and will cause sideways and rearward stability problems,” says Steve Fryer, training coordinator at Northern Crane Services. “This is especially dangerous with jib or extension erected. The ideal boom angle is as low as possible and still be able to see under the boom with the block or ball hanging free and as low as possible to prevent excess swinging.”

Tipping is not the only danger when a rough-terrain crane operates outside the rated capacities. Structural damage is possible as well. But unlike tipping, operators have no warning that a section of the crane is about to break.

“When you're in the tipping range of the crane, the crane will start to get light and the operator can actually feel it,” says Wilson. “It will feel like it's not stable anymore, and when that happens, then you know to stop and set the load down. It's the seat of your pants, it'll get light.

“But in structural, there is no indication of overload; it'll just fail, and that's dangerous. You can have a catastrophic failure and not have any advanced warning that you've overloaded the crane.”

Determining the structural and tipping ranges of the crane is illustrated in the load chart. Under the “360 Degrees” and “Over Front” columns of the load chart, capacities below the bold line indicate tipping. So, for instance, if the load chart shows an over-the-front limit of 32,300 pounds — listed below the bold line — at a load radius of 35 feet and boom angle of 47 degrees, then exceeding that 32,300-pound limit could tip the crane.

If, however, the same load chart also shows an over-the-front limit of 38,000 pounds — listed above the bold line — at a load radius of 30 feet and a boom angle of 47 degrees, the capacity may not seem much different, but exceeding that load limit would not tip the crane. Rather, it could break the boom or another part of the crane without warning.

In addition to being listed above a bold line in the load chart, structural ranges sometimes are indicated by a shaded background.

Today, manufacturers are making rough-terrain cranes lighter by spreading out the crane base, Wilson says. It also helps to boost stability. This means, though, that the structural range is expanded, and operators who don't abide by the load chart are increasingly likely to structurally damage the crane rather than tip it.

Now, electronic load moment indicators also help operators determine load capacities, but they should not be relied upon, as they are sometimes improperly calibrated and thus fail to shut down the crane when the real load limit is exceeded.

“LMIs are getting better and better,” Wilson says. “But they are an operator aid not to be depended upon. You have to give way to actual measurement.”

Furthermore, according to Fryer, crane LMIs often cannot distinguish between fully extended and slightly extended outriggers.

“Regardless of outrigger extension, out-of-level outriggers is strictly prohibited under any conditions at Northern Crane and is very dangerous,” Fryer says.

Ground support

Rough-terrain crane engineers, like those at Grove Worldwide or Link-Belt, have determined exactly how much weight the materials in their cranes can handle through rigorous testing.

Grove, for example, examines and defines rough-terrain crane capacities at their test facility in Shady Grove, Pa., Wilson says. “They will hang a weight on the crane, and they will take it up until a point where the engineers say: 'That's as far as you can go. The metal itself, the internal structure, and the molecules in the steel are telling you that you can't go any further.'”

Those results are manifested in the load chart. But since rough-terrain crane operators work on none other than rough terrain, they lack the perfect conditions that engineers in a testing facility have.

“When engineers test these things, they set up on a concrete pad that is 10-feet thick and 40-feet square, solid as it gets,” Wilson says. “When you take your crane out on the jobsite, set it up, and run your outriggers out, you have to know the density of what's underneath the underlying soil.”

If done wrong, all of the weight could end up on one outrigger, throwing off the load-chart capacities. Or on tires, if the ground surface has some deflection, the tires could begin to sink. Work-site supervisors are responsible for determining the condition of the ground before any lifting is done on it.

Ground support is the most important part of setting up a rough-terrain crane, Wilson says. Loss of ground support has resulted in a large number of crane turnovers.

To prevent accidents in the future, Wilson advises that site supervision research the site and study the ground to find out if there are fill areas or underground utilities, such as fiber-optic boxes. Accordingly, blocking or crane matting should be laid out underneath the crane to reduce ground pressure and even out the forces exerted on the outriggers or tires.

Maximum Allowable Lifting Capacities
( in Pounds on Fully Extended Outriggers )
Load Radius(feet) Loaded Boom Angle (degrees) 360° (pounds) Over Front(pounds)
10 77.0 79,700 79,700
12 75.0 72,400 72,400
15 71.5 63,500 63,500
20 66.0 52,300 52,300
25 60.0 44,200 44,200
30 53.5 37,800 38,000
35 47.0 27,900 32,300
40 39.0 21,500 24,900
45 29.0 17,000 19,700


Maximum Allowable Lifting Capacities
( in Pounds on Tires )
        Pick & Carry
    Stationary   Creep 2.5 mph
Load Radius (feet) Max. Boom Length(feet) 360° Straight Over Front(pounds)
10 35 46,600 74,000 56,300 47,600
12 35 31,200 64,400 48,800 41,100
15 35 19,800 53,600 40,200 33,600
20 35 12,800 33,000 30,300 25,000
25 50 8,900 20,800 20,800 19,200
30 50 5,200 13,300 13,300 13,300