Plants today can no longer think of the loading dock in isolation. Instead, companies will benefit by treating the dock as part of a material transfer zone (MTZ) that plays a critical role when working to improve safety and productivity in the dock area.
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Plants today can no longer think of the loading dock in isolation. Instead, companies will benefit by treating the dock as part of a material transfer zone (MTZ) that plays a critical role when working to improve safety and productivity in the dock area.
MTZ: the critical valve
The material transfer zone can be thought of as a valve that regulates the rate at which goods flow in and out of a facility. In this context, an efficient, productive loading dock becomes a competitive necessity. When everything flows smoothly, delays are avoided, accidents are minimized, product damage is prevented, schedules are met, and customers are satisfied.
Fig. 1.
The MTZ reaches from the drive approach well into the shipping, receiving, and staging area (Fig. 1). In the past, many simply thought of this area as the loading dock. However, the MTZ has evolved to become much more. As such, it presents plant engineers with challenges and opportunities.
Complex interactions abound
In the world of supply chain logistics, profitability is largely achieved through efficiencies. The drive for supply chain efficiency increases the importance of the MTZ. Yet at the same time, the challenges associated with the MTZ have never been greater.
The seemingly simple process of moving material between plant floor and inbound or outbound vehicles is not what it appears. It involves a complex interplay concerning:
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Flow of trucks in and around the facility and loading dock approach
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Material handling vehicles
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Widely varied tractor/trailer types
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Variable loads and load configurations
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Loading dock door openings
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Loading dock equipment selection
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Safety initiatives.
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Trucking trends key
Trucking is one area that is profoundly impacted by recent trends. Within the trucking industry, physical changes in trailers increasingly affect receiving and shipping efficiency. The overriding trend is toward larger trailer dimensions (Fig. 2). To maximize interior space, trailer walls have also become thinner. Some trailers have as much as 101 in. of clear width inside.
Fig. 2. Truck and trailer dimensions have changed over the last few years. In general, trailers may be higher, longer and lower, necessitating changes to the loading docks and doors.
As shown, some trailer manufacturers have also introduced low-profile tires to squeeze out more cube space within the trailer. These tires allow the box to be 6 to 8 in. higher while overall truck height remains the same. Additionally, increasing numbers of trailers use air suspensions that are designed to minimize cargo damage and increase trailer life. These suspensions cause bed heights to fluctuate by several inches at the dock as loads are added or removed and as lift trucks move in and out.
Challenges of reconfigured loads
As trailer dimensions have increased, shippers have reconfigured loads to take advantage of increased payload space. In particular, 102-in.-wide trailers enable 48-in. by 40-in. pallets to be pinwheeled (turned 90 degrees) to increase the payload. If pallets are stacked two high and double-pinwheeled (meaning two pinwheeled rows side by side) the total payload can increase by as many as 12 pallets.
Trailer widths aside, some shippers have opted for full-width loads to achieve fewer lift truck cycles and save time. Full-width loads may travel on wood or plastic pallets. Or, as is common in the automotive industry, goods may travel on specially designed racks.
Changes in trucks, trailers and loads can boost shipping efficiencies. But they also create bottlenecks at the MTZ.
Mismatches clog the valve
Many existing loading docks are undersized or ill equipped to handle today’s larger loads and wide range of trailer designs. Additionally, many new dock designs rely on outdated rules of thumb for dock height, dock leveler size and capacity, door opening size and dock-door sealing configurations. The result is a mismatch at the dock, which translates into a clogged valve in the supply chain.
Fig. 3. Doors and seals should allow lift truck access to the full width of trailers.
Consider a 102-in.-wide trailer in position at a traditionally designed dock, filled to the rear sill with double-pinwheeled, 48-in.-wide pallets. Also factor in a foam dock seal with a 96-in.-wide overhead door, which is common at many loading docks. As shown in Fig. 3, pallet loads fill the width of the trailer, leaving virtually no maneuvering space. A lift truck can remove the first loads (end loads) only by pulling them straight off. But this is difficult at best because the foam dock seal and door jam obstruct the load on each side.
Even with no dock seal, the building wall may interfere on one side or the other, unless the truck has backed in perfectly on center. If the forklifts can’t remove the end loads, employees must unload those pallets by hand, wasting time and money.
Fig. 4. Truck floors that are below dock height can make unloading of end loads difficult for forklift drivers.
More complications come into play when the same truck rides on low-profile tires, because the trailer bed rests 4 to 6 in. below the typical 48 to 50-in. dock height (Fig. 4). Lift trucks are now constrained by the walls of the 6-ft-wide leveler pit, which doesn’t allow for straight-in access to end pallets. As such, the lift truck must spend a great deal of time to carefully extract end loads.
The situation becomes even more difficult when double-pinwheeled and double-stacked pallets are involved. Now the upper loads cannot pass through the door opening and hand unloading is virtually unavoidable.
Other pitfalls lurk
Fig, 5. Dock ramps that are not as wide as the trailers they service encourage damage to both forklifts and docks.
Many additional pitfalls exist when servicing modern trailers with conventional dock designs. As shown in Fig. 5, drivers may violently strike the corner of the building wall when backing out of a trailer. Another example involves lift trucks servicing trailers with low beds (6 to 12 in. below dock) by way of 6 or 8-ft-long levelers. Drivers may then face excessive platform slopes (Fig. 6), forcing drivers to accelerate sharply when backing out with loads in order to make the incline, causing load instability.
Fig. 6. Dock ramps need to be long enough to prevent excessive slope when low trailers are being loaded or unloaded.
Mismatched dock designs often present a number of significant problems (see sidebar). Unfortunately, these problems are magnified in a finely tuned supply chain that depends on the smooth flow of materials at all points.
Ensuring a free-flowing valve
While MTZ efficiency may pose complex challenges, solutions are readily available. The key lies in intelligent facility design and in equipment designed specifically to suit current trends in trailers and load configurations.
Facility layout, dock locations, and dock design represent critical components for a fluid and productive MTZ. Of equal importance is proper selection of loading dock equipment. Key considerations include:
Longer, wider dock levelers: Forklifts must have straight-in, straight-out, full-width access to even the largest loads on high-cube trailers. Conventional dock levers that measure 8-ft long and 6 or 7-ft wide may not be equal to the task. To ensure unobstructed access to all loads, even below dock level, companies should consider levelers 81/2-ft wide to eliminate pit wall interference. Platform lengths of 10 or 12 ft ensure a gentle grade for lift trucks servicing below-dock trailers.
Truck levelers: Even a 12-ft-long dock leveler platform may be inadequate for servicing very low-bed trailers, such as “super vans.” In these cases, truck levelers can easily and safely raise the trailer to dock level for servicing, eliminating pit wall interference and providing a level loading surface. Today’s truck levelers come surface-mounted for easy installation and simple service, with no pit, trench, or drain. Low collapsed height makes it easy for trailers to back into position.
Hydraulic leveler operation: Unlike mechanical and air-powered levelers, hydraulic dock levelers follow the movement of the trailer bed above and below the dock level during all stages of loading or unloading. Hydraulic levelers also eliminate mechanical problems and ergonomic concerns associated with mechanical levelers. Additionally, they eliminate safety leg interference that often prohibits air-powered leveler platforms from effectively reaching low trailer beds.
Full-access environmental control enclosures: Foam pad dock seals provide a tight weather seal against trailers, but they may not fit all trailer sizes. Thus, they may obstruct loads by encroaching on the trailer opening. Conversely, shelters wipe against trailer sides, providing an effective seal without obstructing loads.
Soft-sided impactable shelters eliminate the costly problem of traditional rigid-frame shelters being damaged or destroyed by trucks that back in off-center. Some shelters also have side curtains with rubber hooks that cover the hinge gaps on trailers with swing-out doors, maximizing weather protection and energy savings.
Impactable dock doors: Ideally, properly sized doors should have impactable sections. If accidentally struck by a lift truck, these panels break away without damage and can be easily reattached. These kinds of doors can greatly minimize related plant downtime and maintenance expenses.
Fully automatic vehicle restraints: Vehicle restraint systems should have fully automatic operation for positioning, locking/unlocking, and warning light communication functions. Automatic restraints also reduce the potential for accidents at the dock, such as premature trailer separation. The immediate payoff is improved forklift driver confidence, which translates to increased productivity. In the long run, restraints contribute to overall plant efficiencies by accident avoidance and optimal uptime at the loading dock.
Don’t abandon safety, think ahead
In any plant, safety must be a given, and it’s important not to put employees at risk for the sake of speed. It’s also important to think ahead.
Safety and productivity are inexorably linked. When drivers know their work environment is safe, they are more productive. And accidents of any kind — whether they cause personal injury or just product damage — are a drain on resources. A well-designed MTZ is safe and productive.
The prudent strategy is to make wise choices in dock design and equipment today. Doing so provides relatively quick payback and a long-lasting, solid return on investment.
More Info:
The author is available to answer questions about material transfer zone concepts. Mr. Swietlik can be reached at 414-362-0631. Additional information on equipment used in material transfer zones is available in the “Material Handling” and “Construction & Buildings” channels on plantengineering.com . Article edited by Richard L. Dunn, Editor, 630-288-8779, [email protected]
Consequences of dock design mismatches
Here’s how docks improperly designed for today’s needs can erode supply chain efficiencies and profitability. Product damage: Product damage is not uncommon when forklifts need to maneuver through inadequately sized door openings or up steep leveler grades, which often results in jostled and spilled loads. The losses can go well beyond the value of damaged goods alone. In many industries, recipients are intolerant of damaged pallets or cartons. Instead of simply filing freight claims, they may issue chargebacks for damage, handling, and disposal.
According to reports issued by Food Distributors International, Food Marketing Institute, and Grocery Manufacturers of America, damaged products cost the grocery industry alone an estimated 1% of annual sales of $2.4 billion in 2000. The total cost of unsaleables as a percentage of net profit was 8.56%.
Delayed shipments or delayed production: In virtually all just-in-time environments, delays in loading and unloading trailers can be tremendously expensive. Slowdowns are inevitable if full access to loads is not possible.
Increased labor rates: Loads that can’t be adequately accessed by a forklift due to insufficient door opening size, intruding dock seals, or pit wall interference must be hand unloaded.
Dock equipment damage: Lift trucks traversing steep slopes place excessive strain on dock levelers and may cause premature failure. Pallet boards can catch and tear dock seals while loads are squeezed through tight door openings. Jostling of pallet loads can bend or break roll-up doors.
Building damage: Forklifts reaching for loads on platforms too narrow for the application can damage dock leveler pit walls. Lift trucks backing into pit wall corners also can cause structural damage, eventually requiring high-cost repairs. Dock bumpers can be damaged or torn off completely by air-ride trailer fluctuations.
Trailer damage: If double-stacked or other tall loads have too little “head room,” the loads may strike the trailer roof. Jostling of loads also can stress trailer side walls. Puncturing the trailer skin or breaking a support strut can weaken the trailer’s entire structure, leading to a catastrophic failure.
Excess shipping costs: Delays of any kind in the MTZ can add up quickly in the form of demurrage charges for driver waiting time.
Workplace injuries: Hand unloading of end pallets forces employees to lift and bend, placing them at risk for back injuries. Forklift drivers can sustain back and neck injuries when their vehicles strike exposed pit wall corners or absorb the shock of accelerating up steep leveler ramps.
