Tuggers, carts work together to increase productivity and ergonomics

Manufacturing facilities are continuously looking to get lean while meeting the increasing expectations of consumers for mass customization of products. Couple those challenges with an aging workforce and—in many parts of the country—a difficulty attracting and retaining qualified workers, the resulting solution for some companies has been the implementation of a designed lean material flow system.

The key to those systems, according to Larry Tyler, vice president of sales and marketing for K-Tec/Kinetic Technologies, is the reduction of a facility’s current fleet of fork trucks by replacing some of them with tugger and cart systems. “A fork truck is a vital component of a complete materials handling system: It’s not the bad guy,” Tyler says. “It’s just one way to move materials, and as long as 15 years ago, fork trucks were the most efficient way.”

Today, however, tuggers and carts have evolved to the point where they can be flexibly engineered to support some, or all, of a facility’s handling needs. Both manned and unmanned tuggers work with powered and non-powered carts in a virtually limitless range of configurations. Their features and flexibility allow these systems to address production complexities previously only handled by fork trucks. In turn, fork trucks can be limited to the perimeter of a facility—for shipping and receiving, and for inventory placement into and out of racks—to increase the safety of personnel in the area.

In manufacturing, tugger and cart systems replace one-load-at-a-time deliveries by fork trucks to the production floor by acting as trains, or a linked series of multiple carts, carrying multiple loads in one trip. The carts themselves may be loaded by a fork truck, but a tugger handles the deliveries. Unlike a fork truck, automatic guided vehicle (AGV) style tugs travel along a pre-determined route and at a limited speed, while manned-style driven tug operators have greater visibility because the loads trail behind instead of riding on elevated forks in front of the driver.

Changing roles of carts

Delivering smaller lots more frequently in kitted and sequenced cart orders is an engineered response to the complexity of mass customization. Building multiple product lines on the same production floor or offering a selection of options to customize a standard product, require the flexibility that carts can provide, Tyler explains. The carts have evolved into specialized tools used in engineered processes to move materials as efficiently as possible.

“For parts deliveries to a production line, horizontal space is at a premium,” Tyler says. “Production engineers can be reluctant to build fixed solutions, such as a conveyor, because the process may need to change going forward. That’s why the flexibility of cart-based solutions can make more sense from a return-on-investment perspective.”

Further, “I think of conveyor as being ideal for situations where you’re moving more than 60 loads per hour,” says Chuck Russell, vice president of sales at Transbotics. “If you have a really long run, a conveyor breaks up the plant, crosses aisles, requires power—it can be really expensive. But to make a long run of anything more than 300 feet, or if you have to snake in and out of existing facility structures, then a tugger might be a better choice.”

Most tuggers can handle a total maximum capacity of 10,000 pounds, including the load and the carts, when hauling across a flat surface, says Russell. While, in theory, the number of carts pulled behind is limitless, the actual number of carts that can be towed is limited by the hauling capacity of the tugger. If you require the tug to travel up or down an incline, across a rough surface, or outside from one facility to another, a whole host of additional complicating factors come into play.

“It’s very important to consider the stopping distance required by a tugger pulling a train of carts, particularly down a ramp. Traveling down an incline produces more kinetic energy, and the tugger’s brake system has to be able to handle it,” Russell says.

Another limitation to the number of carts a tugger can pull is aisle width. Whenever a train of carts pulled by a tugger has to make a turn, the more carts in the train, the more clearance the system will need. That’s because of cart drift.

“If you put two carts in a train, they’ll follow the tugger on the same path,” explains Russell. “If you add a third, the last cart has a slightly different turning radius; the more carts you add, the wider your aisles will need to be.”

In response, four-wheel steer carts have been engineered mechanically so when the front wheels turn, the back wheels do, too. This design minimizes the radius issue, keeping all the carts more in-line with the tugger when it turns. These systems cost more, but for a facility with narrow aisles, they might be necessary.

Carts and the worker

In addition to productivity improvements, in the past five years more manufacturers have emphasized proper worker ergonomics when creating tugger and cart system solutions. The combination of an aging workforce and the challenge of filling production and warehousing jobs have driven this trend.

Once the carts are delivered line-side by a tugger, they typically have to be finessed into position by a worker, says Dave Lippert, president of Hamilton Caster, Cart & Trailers. “If a person is going to be pushing or pulling a cart—even just a short distance—it’s probably not going to weigh much more than 3,000 pounds loaded,” he says. “So it has to be safely maneuverable. That makes the wheels an important part of the cart’s design.”
Wheels come in myriad different polymers, sizes and treads that offer varying degrees of rolling resistance based on their hardness, shape and contour. The choice of trailer steering mechanisms and handles also impacts the ability of a worker to maneuver the cart and its load manually, Lippert says.

Once the cart is in position, a worker has to be able to access the parts stored within it easily and ergonomically, adds Lippert. To minimize bending, stretching, reaching and strains as much as possible, customized carts account for multiple of ergonomic factors. Today’s carts can elevate, rotate, tilt and present materials to the user at the ideal height and distance through cubicles, gravity-fed shelves, rollers, integrated carousels that spin, and pneumatic-assist lifting devices. On-board power lets some carts maintain its contents at a certain temperature if required.

Cart concepts

Separating a cart from a train can be a challenge. If the cart needed is between two others, it has to be disconnected from both, manually dragged away, and the other two carts reconnected. Mother/daughter cart solutions have grown in popularity as a means to address that problem, says Ed Brown, CEO of Topper Industrial.

“These are two-part cart concepts. The mother cart is a larger unit that corrals the smaller carts inside and they all move together in a train,” he explains. “At each stop along the production line, the worker just pulls out the smaller cart wherever it’s needed from either side of the mother cart, which remains connected in the train.”

In certain cases, existing carts can be converted to a mother/daughter system, says Brown. He also notes that this type of system can often be less expensive than adding tow-bars and hitch mechanisms to a standard towed cart design: “That’s because there’s more engineering into fewer mother carts, while the process deploys more of the simpler, less expensive daughter carts.”

To ergonomically move the heaviest loads up to 250,000 pounds, manual power movers can be deployed in place of a motorized vehicle. The units are built to produce torque rather than speed, supplying the initial push force required to get such a heavy load to move in the first place, says John Adams, vice president of sales for Nu-Star’s Power Pusher division. “Plus, because these manual tugs are designed as walk-behind systems, they don’t require a licensed driver,” he says.

As much as 75% of the time, this style of tug is employed purely to convert a manual process into an assisted one for ergonomic reasons, Adams says. For example, due to their compact size, “some users will implement a manual tug at each workstation on an assembly line to help operators move the cart to and from the aisle, where it can be picked up by a larger tugger in a train, or by a forklift,” he says.

Powered, manual movers typically use electricity, further enhancing worker safety and environmental comfort because they don’t create emissions. “About 25% of our installations do replace forklifts because of safety and emissions concerns,” Adams adds.

Companies mentioned in this article

Hamilton Caster, Cart & Trailers: cartsandtrailers.com
K-Tec/Kinetic Technologies: ktecinc.com
Lightning Pick, a part of Matthews Fulfillment Systems: lightningpick.com
Nu-Star’s Power Pusher division: powerpusher.com
Topper Industrial: topperindustrial.com
Transbotics Corp.: transbotics.com