Selecting the right dock leveler
Hydraulic, power-assisted, or mechanical? Choosing a dock leveler and deciding whether it should be powered or not is more complicated these days, but here are some guidelines for your specific application.
By Tom Feare -- Modern Materials Handling, 1/1/1998
Poorly designed or equipped loading/unloading docks can slow, stall, or stop altogether the flows of inbound materials or outbound shipments. Less obviously, perhaps, these deficiencies may also create a "dock danger zone," resulting in worker injuries and damage to goods or property.But you can make a world of difference in higher productivity, greater worker safety, and less damage by:
- Insisting on good design for new (or remodeled) receiving and shipping dock areas, and then
- Selecting the right devices- including dock levelers-to meet your company's specific needs and receiving/shipping throughput conditions.
Let's assume, for this overview on levelers, that the dock area is well designed. Thought has been given to such factors as well-planned truck approach and turning areas, a minimal incline or decline leading up to dock bays, ample truck waiting and parking spaces, and the like.
Here's a key question then: How do you bridge the gap between the facility's dock bays and the beds of over-the-road trucks arriving for loading or unloading? (We won't cover rail freight docks here.)
For openers, you'll need to measure dock height(s) as well as the heights of the beds of the different trucks, semitrailers, and other vehicles that you typically service.
In many cases, your facility will have roughly 4-ft-high docks. But highway vehicle bed heights can vary as much as from a low of 20 in. above grade to 62 in. high off the road. Low-profile tires on more of today's trucks also lower bed heights. Trailers with air-ride suspensions, when the air is released for safer loading, can drop their beds up to 8 in. from the "normal," over-the-road height, moreover.
For operations where freight truck beds are always higher than the height of the dock and, more importantly, very low volume loading is the rule, you might consider using low cost dock boards. Generally made of reinforced steel or aluminum, these portable boards will need to be positioned by hand or forklift over the gap.
Edge-of-dock levelers (hy-draulic or mechanical) are another inexpensive choice; they are mounted on the front wall of the dock.
Levelers for busy docks
In contrast, let's say that your plant, warehouse, or distribution center is the typically busy one. It has a high volume of inbound and/or outbound over-the-road truck traffic. Then it's time to consider selecting from among the various types of dock levelers.
Pit-style levelers are the most commonly used design for high-volume docks. Vertical-storing, hydraulically-powered levelers see service most often in refrigerated or other food warehouses.
You'll next need to determine the appropriate leveler capacity, length, and width for your needs. Capacity is measured in several ways and the complexities of the different methods are beyond the scope of this article. Just be sure you provide any potential leveler supplier with realistic figures from your dock operations for forklift load capacity, heaviest loads, weight(s) of any forklift attachments, expected number of trips by forklift (or pallet jack) from truck to dock per dock position per shift or per day, and the like.
When it comes to specifying dock leveler length and width, longer and wider-within limits- are the rules of thumb to follow. In general, the greater the vertical distance between truck bed and dock height, the longer (horizontally) the dock leveler should be that you select.
Greater leveler length lowers the percentage of the grade of the dock-to-truck incline or decline that forklifts, pallet jacks, and other loading equipment will traverse, up or down.
An 8 ft long dockboard or leveler, for example, bridging the gap from a 48 in. high dock to a truck bed only 38 in. off the pavement, creates nearly a 12% grade. (With a 12 ft long board or leveler, the grade is about 7%.)
Some kinds of load-handling equipment won't readily climb a 12% grade, nor are they necessarily very safe when moving back down that steep a decline. The longer the dock board or leveler, the less the grade and the easier and safer the move.
Depending upon the type of equipment, here are recommended maximum grades: power operated hand trucks (3%); powered platform trucks (7%); low-lift pallet-skid trucks (10%); electric lift trucks (10%); and internal combustion lift trucks (15%).
Wider levelers, meanwhile, provide better accessibility to loads on the trailer bed and give your forklift operator more maneuvering room.
Powered or manual?
Now comes the hard part: selecting from among the manually operated and powered types of levelers. In general, there are four methods of leveler activation: mechanical, hydraulic, low-pressure air bag, and electric "power assist." The latter two methods can be grouped together as "power assisted."
Up until some two years ago, dock leveler selection was rather straightforward for most applications. After deciding on appropriate leveler length, width, and capacity, a specifier compared mechanical and hydraulic levelers on performance features and costs.
In general, hydraulic levelers cost more than the up-front price of mechanical models. But there will be less need for maintenance. And hydraulics deliver greater safety, better ergonomics, and more productivity to their users over an expected life of 10 years or more. Hydraulics are push-button simple to operate with the hydraulic system providing the power to raise the steel leveler platform or deck into position.
A mechanical leveler, in contrast, depends upon the upward force of a spring to raise its deck. Manual activation is required and an operator will have to perform a "walk-down" routine to lower the leveler onto the trailer bed. Though their initial cost is low, and perhaps a plus, with mechanical levelers you'll need to adjust the hold-down, lip-control, and counterbalance mechanism regularly, adding to lifetime cost.
More recently, the choice of equipment became more complicated when two new types of dock levelers were introduced. One type uses the lifting power from a high-volume, low-pressure air bag to raise the leveler deck. Another type accomplishes a similar lifting action through the "power-assist" provided by an electrically-driven linear actuator motor working together with a counterbalancing spring.
The costs of these newer kinds of levelers are intermediate between the prices of low-end mechanical levelers and the costs of upper-end hydraulic levelers.
Cost isn't the only consideration, however. You also should compare different leveler designs on their operational, performance, and safety features. Included should be questions such as (1) how the unit performs if the trailer bed being serviced is below dock level, (2) what happens if a trailer suddenly moves away from the dock with a forklift load on the platform, and the leveler goes into an uncontrolled platform drop, or free-fall, (3) what safety measures are there on non-hydraulic levelers to stop or prevent free-fall. (Most hydraulics stop free-fall after just a few inches of drop from any position above or below dock.) and (4) is there is a mechanical override if electrical power fails?
To guide you to the suppliers of the different leveler types, we have included a table (above) of the major manufacturers.
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