Equipment Type

Stability Triangle Determines if Telehandlers Tip

There are many facets to safe operation of reach forklifts, but keeping them upright is key to keeping them productive

February 01, 2006

In the late 1990s, OSHA required that telehandler operators be trained, but didn't specify what that training should include. Mike Popovich, training director at JLG, uses an all-encompassing five-step approach to teaching accident avoidance that asks five important questions:

Five Steps

  1. Walk-around inspection — Is the unit safe, visually?
  2. Worksite assessment — Is the worksite safe to operate in?
  3. Function test — Is the unit safe, functionally?
  4. Proper operation — Am I operating safely?
  5. Proper shutdown — Is the unit in a safe place and shut down properly?

At the center of this regulation, and the five steps, is a machine that stands on a triangular footprint and lifts heavy weights to impressive heights. Of course, there are other dangers, but the key objective is to keep the telehandler upright.

The two front wheels and the rear-axle pivot point form a telehandler's triangular base. The rear wheels are not part of the base because the rear axle typically oscillates. As long as the machine's center of gravity (the point in three dimensions around which the machine's weight is balanced) remains oriented inside this stability triangle, the telehandler remains upright.

Adding a load to the forks while the boom is down moves the center of gravity forward and down. Raising the load will move the center of gravity up and to the rear, while at the same time causing the stability triangle to shrink. The higher you lift a load, the less margin for error you have because the stability triangle becomes smaller.

A small stability triangle leaves less room for the center of gravity to wander left or right if the frame is not perfectly level. Imagine the center of gravity as a plumb bob hanging from the boom. You'll always find the center of gravity somewhere on a perfectly vertical line between a point on the boom and the center of the Earth. If the frame's not level, the center of gravity will not be oriented over the machine's centerline. But the stability triangle is always aligned with the telehandler centerline.

With the boom raised and extended (the stability triangle very small), the frame only has to be slightly out of level to make the center of gravity drift to either side of the triangle. The laws of physics are as certain as gravity. The moment the machine's center of gravity moves beyond the boundaries of the stability triangle, the telehandler begins to tip.

At any jobsite, there are things affecting the stability of a telehandler other than where you put the boom. You may be on a 1-degree side slope, in a 5-mile-per-hour wind. There may be a little ice under your wheels, and one tire a little low on air pressure. If the machine goes over, there may be no single cause. It was a combination of factors that are all things you can catch and correct or avoid if you do all of the Five Steps.

A safe lift starts well before putting the boom in motion, or even before the operator assesses the site for hazards. Most of today's telehandlers are designed with a quick-attach coupler at the end of the boom. Before installing an attachment, be sure it's approved by the telehandler manufacturer and that its specific capacity chart is in the operator's station.

"It's an OSHA violation to run a machine if the proper load chart for that particular combination of machine and attachment isn't in the machine," Popovich says.

Make sure forks haven't been tampered with. Other than block forks, all forks should be used in matched pairs. Block forks are used in matched sets.

"Putting on and taking off attachments is critical," adds Marty Turek, curriculum developer/instructor, Operating Engineers Local 150. "You have to make sure it's secure."

The maximum capacity of a telehandler-and-attachment combination will be the lightest of these: capacity stamped on the attachment identification plate, the fork capacities stamped on the side of each fork (fork capacity is multiplied by the number of forks on the attachment, not to exceed the maximum on the attachment ID plate), maximum capacity indicated on the proper load chart, or the load rating of the telehandler.

Referring operators to the capacity chart can be easier said than done on many jobsites because the weight of the load is not always known. Construction sites require a lot of general lifting that obviously doesn't approach a telehandler's limits, though — miscellaneous materials and tools that need to be unloaded and kept out of the way. As long as the telehandler is properly equipped and maintained, the operator assesses the site for hazards and makes these lifts carefully, work can continue without knowing the exact weight of each load.

"But every experienced operator knows when they're about to lift something that's going to test a machine's limits," Popovich says. "The key is to find out how much loads weigh."

Truckers can be a reasonable source of information, as can project supervisors, and sometimes the subcontractor or crew chief that will be working with whatever's on the pallet. If general inquiry fails, though, there are few options for determining the safe boundaries for a lift.

Equipment manufacturers can't recommend picking up the load and extending the boom forward until you feel the rear wheels getting light. It is intentionally destabilizing the machine. It's not likely to result in immediately recognizable damage, but the practice is discouraged because if the machine tips forward and lands on the pallet, it can damage the load and accelerate boom wear.

But by extending the load to the point where it reaches the machine's stability limit, you can use the load chart to estimate its weight. Find the boom angle on the chart on the following page and follow it out to the zone matching the boom extension. The load weighs more than the weight limit printed in that load zone.

Handle this estimate with extreme caution. You've exceeded the safety factors built into the load chart, so the margin for error becomes very slight. Don't plan a lift at the furthest reaches of the stability range using this estimate, and observe stability-enhancing procedures dealing with frame level, tire pressure, load center and the like meticulously.

"Whenever you test the limits of the load chart you should do a test pick without a load on the forks — position the telehandler as you plan for the actual lift and run the forks up to where you plan to place the load," Popovich says. By noting the boom extension and boom angle at the extreme point of the lift, this exercise confirms the vertical height from ground level where the load is to be placed, and the horizontal distance from the front tires to where the load will be placed.

On the capacity chart, find the horizontal line for the height of the lift and follow it over to where it intersects the line for the distance. The point where the two lines meet should fall within a load zone marked on the chart. If it doesn't, the machine and attachment can't complete this lift. The weight indicated in the zone where the height and distance lines intersect is the maximum capacity for this lift. If the two lines meet on a boundary between zones, use the lighter of the two weights for this lift's limit.

Check the boom-extension and boom-angle limits of this load zone on the capacity chart. When the load is in the air, no matter what happens, do not exceed those boom-extension and angle limits or the machine will tip.

All of the loads shown on capacity charts are assigned with the assumption that the machine is on firm ground with the frame level; forks positioned evenly on the carriage; load centered on the forks; proper size tires properly inflated, and the telehandler in good operating condition. Failing to comply with any of these conditions could tip the machine over.

Once you've determined that the lift is within the telehandler's and attachment's capabilities, it's time to set up the machine. Never engage a load or lift a load more than four feet above ground unless the telehandler is level.

"One of the things we always emphasize in our training classes is, when you get ready to place a load: Park Brake; Neutral; Level," Popovich says. "We drill it into people just like that."

Stop the machine on a stable surface in the best place to lift and place the load, set the parking brake and shift the transmission into neutral. Before raising the boom, check the level indicator to see if the frame needs to be leveled from side to side. Many telehandlers have hydraulically tilting main frames that allow you to compensate for uneven terrain. Some have outriggers. Set the boom in the "carry" position — forks less than four feet off the ground — and level the machine. The machine should be ready to make the lift.

Never use the leveling system (sometimes called "frame sway") or outriggers to level the telehandler after the load is more than four feet off the ground. After the load is in the air, if you discover that the telehandler is not level, bring the load back down before leveling the frame.

"We instruct operators not to use the frame sway to position the load with the boom elevated," Popovich says.

The higher up you go, the closer the sides of the stability triangle are — you don't have to go very far to move the center of gravity outside the triangle. If you use the frame sway with a load high in the air, the boom only has to move a couple of degrees right or left and the machine may tip over.

If the telehandler isn't level when you reach the limits of the frame-leveling system, don't attempt the lift until the problem is solved. Reposition the telehandler on more-level ground, or have the surface where it is standing leveled.

Always lift the load slowly, watching closely for changes in the footing or other conditions that could cause the frame to shift out of level. Don't move the machine once the load is more than four feet off the ground.

Placing a load at height requires a careful combination of multiple functions — boom down, boom out, while holding the forks level.

"One of the most important things to remember is to maintain constant engine rpm so the hydraulic flow remains the same," says Turek, from the Operating Engineers. "People in tense situations often want to let off the throttle, but the change in engine speed slows down the hydraulic flow, which changes what the levers do. You want to run at a high rpm all the time so the hydraulic performance is predictable, and feather the controls to control the lift."

Machines with the carriage-transfer feature, which slides the entire boom forward hydraulically, simplify the procedure. But the machine's load capacity is derated for any carriage position forward of the rear-most position.

Turek says most of Local 150's telehandler operators take the OSHA 10-hour safety awareness training and learn a lot about their job from the section on scaffolding. He recommends that program or any of OSHA's scaffolding-specific programs to all telehandler operators.

Once the lift is complete, the boom should be retracted and lowered to the carry position before the telehandler is moved.

Telehandlers are designed to handle rough terrain, but that doesn't mean they can be driven without regard for the terrain. Risk of tipping or load loss is much greater when traveling on slopes. If you must drive on a slope, keep the load low and proceed slowly, with great care. Before you get on the slope, downshift to a lower gear and four-wheel drive. Engine braking will help control the telehandler's speed. Avoid turning on a slope, but if you must turn, make the turn as wide as possible and use extreme caution.

Never drive across excessively steep slopes under any circumstances. Ascend and descend slopes with the heavy end of the telehandler pointing up the incline. When there's no load on the forks, the counterweighted rear of the machine is the heavy end, so you should back up slopes. When the telehandler is loaded, the front of the machine is the heavy end, so you should back down slopes

Operator training becomes very important on a mixed jobsite — with rear-pivot machines and coordinated-steering machines on the same jobsite where everyone is allowed to run all the equipment. Someone accustomed to operating a coordinated-steer machine jumps on a rear-pivot machine. One really significant difference between the way those two machines operate has to do with what part of the machine extends outside the turning radius.

The load or the forks cut the widest swath in a tight turn on a four-wheel-steer machine. But on the rear-pivot machine, it's the counterweight or the outermost rear wheel that's at the edge of the turning radius. In either case, the operator has to look in the direction of travel to be sure the path is clear. But in cases where the operator is using a machine with rear-pivot steering, it's especially important to be aware of the counterweight.

The things an operator does to ensure safe telehandler use don't take a lot of time. Accidents tend to happen when people neglect basic safety requirements in the name of speed.

Shrinking Stability Triangle




Telehandlers stand on a triangular base formed by the two front wheels and the pivot point on the rear axle. As long as the machine's center of gravity remains within the stability triangle, the telehandler is stable. Loading the forks moves the center of gravity forward and down. Raising the load will move the center of gravity up and to the rear, while at the same time causing the stability triangle to shrink. The greater the boom angle and extension, the smaller the stability triangle. If the frame's not level, the center of gravity wanders toward the edge of the triangle. If the machine is far enough out of level, the center of gravity drifts beyond the boundaries of the stability triangle and the telehandler begins to tip.

Always a Perfect Match

Before installing an attachment, verify that it is approved and the telehandler is equipped with the proper capacity chart (NOTE: Never use an attachment without the appropriate capacity chart installed on the telehandler.) To determine the maximum capacity of the telehandler and attachment, use the lightest of the following capacities:

  • Capacity stamped on the attachment identification plate
  • Fork capacities stamped on the side of each fork. Fork capacity is multiplied by the number of forks on the attachment (not to exceed the max. capacity stamped on the attachment ID plate)
  • Maximum capacity as indicated on the proper capacity chart
  • Load rating of the telehandler and the capacity of the forks or attachment

The boom angle indicator and boom extension indicators will be crucial to determining that the lift remains within the load zone identified on the capacity chart.

Read the Load Chart

To use a capacity chart, the operator needs:

  • An attachment approved by the telehandler manufacturer
  • The right capacity chart for that telehandler model and attachment
  • Weight of the load to be lifted
  • Vertical height from the ground where the load is to be placed
  • Horizontal distance from the front tires to where the load will be placed

On the chart, find the horizontal line for the height of the lift and follow it over to where it intersects the line for the distance. The weight indicated in the load zone where the two lines meet is the maximum capacity for this lift. Determine the boom-extension and boom-angle limit.

Watching Front or Rear?

On front-wheel- and four-wheel-steer telehandlers, the forks cut the widest circle when turning, so you have to keep an eye forward to avoid obstacles. But the rear wheels of a pivot-steer Gradall telehandler turn the widest circle on those machines, so you have to look to the rear to be sure the wheels and counterweight don't collide with anything.












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