tip-proofing

A forklift maker takes control of the wheel.

by Paul Sharke

Forklift accidents in the united states cause nearly 100 deaths and another 20,000 serious injuries every year, according to the National Institute of Occupational Safety and Health. A quarter of the fatalities are attributed to injuries from overturning vehicles.

Differences in geometries make four-wheel forklifts more stable than three-wheelers. But, because they cost less and maneuver better, three-wheelers manage to dominate the market. Even with four wheels, a fork truck in the hands of a careless driver can struggle to keep its tires pressed firmly to the ground. Operator diligence is a key factor in forklift safety.

Concerns about overturning lifts prompted the Occupational and Safety Health Administration to mandate operator restraining means some years ago, according to industry consultant Robert N. Rogers, whose career spanned 37 years at Greene, N.Y.-based Raymond Corp. in the pursuit of forklift safety. As with safety rules, seat belts can't apply themselves. That's where "training, training, and more training" comes in, Rogers said.

Driver training and technology combine to alleviate some of the hazards of materials handling operations.

But when training fails, systems such as German forklift maker Jungheinrich AG's Curve Control step in to protect operators from the hazards of the job.

Jungheinrich approaches the question of safety by actively managing the speed at which a vehicle can negotiate a turn. Its Curve Control system watches truck speed and steering angle to determine when a vehicle is about to succumb to an overturning moment. A quick-responding drive motor then slows the truck to a safer speed.

According to Jungheinrich, conscientious drivers hardly notice the system because they are already slowing their trucks as they begin their turns.

Control Happens in Threes

Curve control has been around for several years, according to Dirk von Holt, president of Jungheinrich Lift Truck Corp., the company's Richmond, Va., subsidiary. The company applied it to four-wheelers originally and has since adapted it to its three-wheel counterbalance trucks and even some of its reach trucks, he said.

In the late 1990s, Jungheinrich asked Bundeswehr University in Hamburg to test the curve control system on its EFG four-wheel model. The university concluded that the forklift avoided tipping under all but a single circumstance. An experienced driver, turning the vehicle hard one way, then counter-steering hard and abruptly in the opposite direction, could force the truck over.

The university also looked at the case of fast driving with an elevated load. It found that the Curve Control system would step in on loads elevated above 30 inches and limit truck velocity to the maneuvering rate.

Overhead loads throw another variable into the stability mix, retired Raymond engineer Rogers said. And while OSHA rules ban traveling with loads carried overhead, sometimes operators are forced by the situation to do just that.

The development of speed control algorithms that work with sophisticated motor controllers for enhancing product safety has proven to be one advantage for ac-powered forklifts.

Automatic speed control on curves, while possible with dc machines, was more readily achieved with the controllers needed to operate variable-speed ac motors, von Holt said.

Outsiders to the materials-handling operation might be surprised to learn that ac-powered lift trucks have been around for some time, according to Martin Baumann, manager of product marketing at the Jungheinrich Counterbalance Truck Center in Moosburg, Germany.

The all-directional wheels on this lift help to render turns obsolete.

They originated in Italy in the 1980s, when boutique-scale forklift makers began applying ac propulsion to what had long been dc vehicles. Jungheinrich began its own foray into ac motors in 1993, introducing its first ac-powered truck in 1996. Today, the company continues building only a single dc truck model; its entire electric line has been switched over to ac, not only for traction, but for lifting as well.

Baumann cited three main advantages to ac motors over their dc brethren. First, there's dynamic response, characterized by immediate torque and rapid acceleration. "Drivers notice that immediately," he said.

Then there's the expanded environment into which ac motors can roll. Fully enclosed ac motors can operate in humid and dusty conditions that would doom their open-frame dc counterparts, he explained. Direct-current motors generate a lot of heat that must be vented, Baumann said.

Low maintenance, or none at all, is the ac motor's third strength. Without brushes to wear and cast off dust, ac motors are able to run for a longer period of time without needing service.

Popular in Europe, ac-powered forklifts have caught on in the United States as well. Yale, Hyster, Toyota, Nissan, and Crown are some of the forklift makers offering ac motor drives.

In addition, ac vehicles use 15 to 30 percent less power then dc-powered forklifts. This has ac lifts beginning to compete for some of the market long held by engine-powered trucks. Better performance in lifting and accelerating coupled with easier compliance with indoor air standards are also making some fleet managers take notice.

Forget Curves

As innovations in control and propulsion percolate through the forklift industry, another manufacturer is changing forklift design from the ground up. Airtrax Inc. of Hammonton, N.J., has begun making its first omnidirectional lift truck, calling it the Sidewinder.

Using a unique segmented wheel design, the truck can change direction without needing a corner to round. Whether it's moving forward, backward, or sideways, the vehicle changes direction by revolving its four independent drive wheels, according to sales director Peter Greenwood.

"The Sidewinder practically eliminates the K-turn," Greenwood said, referring to the forward-back-forward maneuvering that a driver of an ordinary forklift must execute in narrow aisles to switch directions. Backing up is the most dangerous part of any maneuver, he added.

There's more: The forklift's speed and direction are controlled from a single joystick. No pedals or steering wheels clog the cockpit. Because movement of the joystick exactly duplicates the motion of the forklift below him, a 10-year-old could drive it without training, Greenwood said.

Joystick control is something makers of traditional forklifts are waiting to introduce, Jungheinrich's Baumann said. The skills for guiding a conventional steered vehicle with a joystick might take time for the average driver to acquire, he added.

A generation fluent with Nintendo and Xbox could probably prove otherwise.

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© 2005 by The American Society of Mechanical Engineers