Bridges

The size of the bridge is dictated by the gap from the lift to the back of the truck bed. This is determined by the location of the bumpers or bollards that are used to stop the truck’s rearward progress toward the lift. When lifts are located inside of buildings with a wall between the lift and the truck, the bridge can rest against a wear strip on the wall and automatically reach into the truck as the lift is elevated and retract as the lift is lowered. These bridges are typically very long, but because no one needs to handle them, the extra weight is not a concern.

Bridge width: 72 in. is the maximum recommended width. The reason for the 72 in. is that trucks rarely back up perfectly centered on the dock. If the truck has a roll-up door and the bridge is wider than 72 in., the bridge can hit the framework on the trailer and prevent access to the truck.

Bridge length: The bridge should be long enough to project 4-6 in. beyond any truck bumpers to provide adequate purchase on the truck bed. 12 in. is generally the minimum recommended length.

Bridge weight: This is dependent on both length and capacity. Generally, manually operated bridge weight is a concern when bridges reach 24 in. or longer. Options to mitigate bridge weight include splitting the bridges into two sections, aluminum bridge plates and hydraulically operated bridges with push button controls.

Capacities

Capacities relating to dock lifts include lifting capacity, axle rating capacity and roll over capacity. See sidebar Calculating Lift and Axle Capacities.

Lifting capacity. Once a platform size has been determined, the following weights are added together to derive the lifting capacity: the maximum weight of goods being transferred, the weight of the transport equipment and the weight of an operator.

Axle rating capacity. While lifting capacity is the more significant specification, it is useful to understand the purpose behind manufacturers citing of axle rating capacities.

The axle capacity is the amount of weight placed on a lift as the load, transport vehicle and operator enter the lift. Because most transport equipment have four wheels and two axles (an exception is a hand cart with a single set of wheels), it would be reasonable to assume that the load pressure on the lift is evenly distributed when the transport equipment moves onto the lift. But the payload weight of most powered transport equipment is not evenly distributed (see “X” chart in sidebar Calculating Lifting and Axle Capacities).

This results in higher edge loading over the ends of the lift when the axle with the heavier load is rolling on and off. Dock lifts therefore have to have axle ratings that serve the heavier axle load.

The axle loads on manually propelled carts and pallet jacks are too light to be of concern. Manual pallet jacks divide payload weight evenly between two axles. But powered pallet jacks and straddle stackers, which can move heavier loads than manual equipment, often concentrate 60 percent of the combined payload and equipment weight on their rear axles. Fork trucks usually concentrate 80 percent of the maximum payload plus fork truck weight on the front axle; but some unusual trucks may run as high as 90 percent concentration on the front axle.

Most manufacturers have appropriate end load axle ratings for corresponding maximum lifting capacities. But specifiers should be aware that such ratings can be marginal if unusual equipment is used and/or if an application requires traveling over the side of the lift in any position other than the fully lowered position. It is wise to compare the axle loads produced by the actual equipment specified—particularly small fork trucks, see below—versus the rating of the dock lifts.

Roll over capacity. Roll over (or drive over) capacity applies only to recessed units. It refers to the maximum axle load allowable for vehicles driving over a fully lowered lift. Most recessed dock lifts are rated for 10,000-lb axle load slow speed drive over. This is usually more than adequate because of state mandated limits on truck axle loads. However, large specialty vehicles and high-capacity fork trucks with small hard wheels can exceed these limits. In these cases, the lift manufacturer should be notified so the lifts can be modified to accommodate larger loads by installing thicker deck plates and other forms of reinforcement. An example of an extra-large load is a truck that picks up and drops off dumpsters. A large full dumpster can weigh 30-40,000 lb and has two sets of very small steel wheels; these wheels present very high compact axle loads.

Power Units and Controls

The up/down controls must be controlled by the operator riding on the lift. Any other scenario is a sure recipe for an accident. Standard controls for most companies are National Electrical Manufacturers Association, NEMA 4 pushbuttons. (NEMA 4 enclosures, used in many applications where an occasional washdown occurs, are gasketed and the door is clamped for maximum sealing.) These are usually mounted on straight cords or coil cords. For security options the cords may be made longer so that they can be connected to boxes inside of a secure building or mounted with twist lock plugs so they may be stored in secure areas until needed.

In other instances the hand-held controls are replaced with key lock pushbuttons that are mounted on the platforms. Some retail chains specify that all the keys in their system are the same. Limit switches may be added to cause the lift to stop at fixed dock heights. Power units on top of ground units are usually mounted on the lifts and covered with weather protection. Recessed lifts typically have remote power units that are mounted on the inside of buildings; weather covers can be ordered for outdoor mounting. Oil immersion heaters are an important option for power units that will be exposed to cold winter outdoor temperatures.

Other Options

Half a dozen standard colors are typically available at no extra charge. Additional options can include special paint finishes, special platform surfaces, wheel chocks, gates and so forth.