Specifying inplant trailers
Trailers may look alike at first glance, but there are some significant differences that greatly affect performance and cost. The wise purchaser will study the differences and select the system that makes the best sense for the specific application. Obviously, there is no universal right answer.
Design load factors
The intended load is the primary consideration in specifying a trailer. If all loads are about the same, the selection process is easier. If not, then one must work with worst-case scenarios or “typical” loads.
The size of the load surface, or deck size, is usually the first choice to be made. The load or loads to be carried will factor heavily in the determination of a deck size. And the size of the deck will impact required aisle widths and practical length of trains that can be safely pulled. Clearly there are tradeoffs in this process.
The total weight of a load unit is important, but the center of gravity must also be considered. Turning presents a serious risk for taller loads, and the presence of ramps or uneven floor conditions may completely rule out stacking loads.
The capacity rating of a trailer is based on the running gear and the frame or structure. This rating considers the maximum load as a uniform load spread over the entire deck area. In addition to the maximum weight of the anticipated loads, consider the possibility of shock loading due to abusive loading or unloading techniques and obstacles on the floor.
Design engineers should try to anticipate how trailer systems might actually be used by the operators.
The running gear, or wheel system including the steering portion, is crucial to performance and useful life. Stability, tracking, and pulling force are some of the factors affected by running gear.
There are five basic categories of steering systems: caster steer, fifth-wheel steer, four-wheel steer, two-wheel auto steer, and four-wheel auto steer. All of them share the inherent capability requisite for trailers that they “track” in a consistent pathway (each trailer tracks roughly in the preceding one’s footprint). The differences are in cost and performance. Each category of steering system is discussed in the accompanying sidebar.
Accurate tracking is crucial to an effective trailer system. The benefits are numerous, ranging from minimal aisle space (cost savings) to fewer collisions and corner damage during turns (more cost savings). Generally the accuracy in tracking coincides with the steering type and complexity. Caster steer and fifth- wheel steer trailers track least accurately, while four-wheel steer trailers track the best.
Depending on how a system of trailers is used, the maneuverability of any one trailer may be important. Ironically, the ranking order of maneuverability is opposite to that of tracking. That is, the more accurately a trailer tracks when being towed, the less maneuverable it is when handled individually.
The number of trailers that can be safely towed is another significant factor. There is a mathematical interdependency between trailer length, width, aisle width, and number of trailers (see Fig. 1).
High productivity normally demands that maximum loads be towed in longer trains. On the other hand, longer trains require wider intersecting aisles, and wider aisles reduce available storage space in a warehouse. For this reason there is seldom a “right” or “wrong” answer, but a maximum of five trailers in a given train is a good rule of thumb.
The type of coupler used is an important factor. The relatively wide “bail” on an automatic coupler, for example, affords significant lateral movement of the trailer jaw. This movement can degrade the trailability, forcing extra aisle widths or reduced train lengths.
Running gear placement or orientation is the first determinant of stability. Any trailer with fixed running gear poses a stability risk during loading. For proper trailing, the fixed running gear must be set well forward of the rear edge of the deck. The risk here is that someone may overload the rear deck overhang and create a tipping situation.
Loads stacked high raise the center of gravity of a trailer, creating a tipping tendency during turns. Soft wheels such as pneumatics are particularly susceptible to this. Production pressures might motivate material handlers to stack loads too high and/or travel too fast. The combination of the two is particularly hazardous.
Ramps pose problems for fifth wheel, for auto-four-wheel-steer trailers, and for trailers connected with automatic couplers. It is possible for automatic couplers to twist in such a fashion as to become disconnected, posing an obvious safety risk. Connecting steering rods are vulnerable to ramps and may bind against the pavement in extreme cases. If ramps must be negotiated, consider employing only straight-on approach and departure routes, and strict enforcement of slow speeds during travel up or down. Experimentation may be necessary to determine how steep a grade can be safely negotiated.
Fifth-wheel trailers, particularly those with four-wheel steering, pose their own unique stability challenge. Despite the obvious fact that the wheels are at the outside edges, the axle assembly is essentially “pinned” along the centerline of the trailer. A single fifth-wheel plate assembly located on this centerline provides the only lateral support for the front end. For four-wheel steer trailers, lateral stability comes only from a fifth wheel at both ends.
To a casual observer, the size of side and end frame members may seem to be the most important aspect of the “strength” of a trailer. But only by checking the entire bolstering structure underneath the deck can one discover the truth.
A center rail is a necessity for handling the longitudinal pull on any trailer. There must be adequate support to withstand the tremendous longitudinal pulling forces at both ends of a trailer. Extraordinarily heavy loads may warrant two center rails, located at or near the centerline.
Side loading represents another potential problem. Forklift operators may occasionally misjudge the height of their load and the trailer deck height, hitting the trailer side rail with either their forks or the load itself. These collisions put incredible force on the axle supports, or the plates on either side of the wheels.
Actual movement of trailers, particularly when loaded, may also introduce side forces. Simply turning a corner creates lateral forces on a loaded trailer. In some instances, corner posts designed to protect rack uprights at the ends of large warehouse aisles become “rubbing posts” for trailer trains, and extreme side forces suddenly emerge.
Axle supports, or steel plates welded between cross members, are generally more resistant to these forces than rigid casters. Casters may be strengthened with gussets, and even axle supports may have lateral reinforcing members to add strength. Prudent planning normally involves “worst case” scenarios, and the wise specifier anticipates stresses such as side forces.
Fifth-wheel plate size, particularly in four-wheel steer trailers, impacts lateral stability. Smaller diameter fifth-wheel plates may be adequate for a given load in static conditions, but they can become woefully inadequate when trailing a full capacity load. Generally, a larger fifth-wheel plate translates into more lateral stability.
Rigidity can be described as the trailer’s resistance to twisting and flexing. Construction of the deck and undercarriage plays the largest role in rigidity. Open frame trailers (no deck) typically employ structural tubing in the frame. Steel deck trailers, the most laterally rigid design, more frequently use structural channel in their frames. Wood deck trailers feature structural angle frame members, although the heavier duty versions typically use channel frames with the deck boards bolted on and secured with angle hold-down strips. Different sizes and placement of structural members can cause key differences in competing trailer bids, although not apparent to the eye.
By definition, trailers have the capacity to be coupled together and then uncoupled as required. The physical effort required for these activities varies by coupler design (see Fig. 2).
The automatic coupler, as its name implies, allows trailers to be coupled “automatically.” The loop, or bail, can actually engage the jaw of the adjacent trailer simply by pushing the two together. Uncoupling involves stepping on a jaw pedal and simultaneously pushing the two trailers apart. It is the manual pushing or pulling of a trailer that carries the risk of injury.
Manual couplers require a person to raise or lower a wishbone tongue into or out of an eye mounted on an adjacent trailer. Jockeying trailers into position while lowering the tongue can create an ergonomic problem. Attaching convenient handles to tongues permits easier grasping by personnel.
Proper choice of wheels may be the most important aspect of everyday trailer performance. Load capacity and maximum speed are the primary factors to consider. Noise, anticipated side loads, usage, floor and load protection, and shock loading are important, too. Constraints usually include deck height and cost.
Wheels with straight roller bearings are typically inadequate. They provide virtually no resistance to side forces, present in every trailer application. Also, they are generally not intended for continuous duty and higher speeds typically experienced by trailers.
Tapered roller bearings are the best choice for trailer wheels. They are designed for both higher speeds and the rigors of impact loads and side loads encountered when cornering. These are the best choice for severe towing conditions.
Precision ball bearings feature the ergonomic advantage of minimal rolling resistance. Like tapered roller bearings, they are suitable for both horizontal and vertical loads as well as higher speeds and sustained use. For trailers that may see manual movement, these bearings offer the least rolling resistance, although their capacities are more limited.
Softer treads on wheels, either pneumatic or solid rubber, provide load cushioning, quieter running, and better traction. Harder treads, like poly-urethane, provide more capacity while still protecting floors and offering some noise reduction over the hardest materials.
Disadvantages of each major tread type can complicate the selection. Rubber tread wheels have reduced capacities. Polyurethane is susceptible to heat buildup during sustained speeds. Steel wheels are noisy, abusive to plant floors, and have a greater tendency to slide around corners (particularly when the trailer is unloaded).
Desired deck height impacts the wheel selection. Generally, larger wheels are preferable for trailer applications.
A trailer system inherently offers economy and flexibility. When that system is professionally designed and built from quality components selected to fit a specific application, it can offer much more.
The authors have prepared a white paper that provides additional information. For a copy of “Design Principles of Inplant Trailers,” contact Dave Lippert at 513-863-3300 or firstname.lastname@example.org .
Trailer steering systems
Caster steer trailers have two swivel casters at one end and two fixed load wheels or rigid casters near the other end. Placement of the fixed wheels is critical to the trailing characteristic. Typically, each succeeding trailer tracks “in” slightly from the preceding one, making slightly wider aisles a necessity. This type is the most economical due to its inherent simplicity.
Fifth-wheel steer trailers ,like a child’s wagon, feature a single pivot point for the front axle/wheel assembly and fixed rear wheels. Again, placement of the rear wheels is critical to trailing performance. The steering axle is “pinned” along the centerline of the trailer, and typically contacts the trailer body through a fifth wheel plate assembly. The trailer width and load capacity determine the appropriate size of the plate assembly. Typically there are “stops” (that limit the degrees of available turning) to prevent oversteering the trailer. These trailers will track “in” from preceding ones, requiring slightly wider aisles for turns.
Four-wheel steer trailers have no fixed axles. Both axles pivot for steering and are connected to each other by a steering rod. This arrangement enables a tighter turning radius and more accurate tracking than two-wheel steer models. Cost is higher than for single fifth-wheel trailers, and there may be a slight decrease in lateral stability. One quality feature to look for is proper coupler structure. Optimal tracking and strength is achieved when the rear coupler is tied to the frame.
Auto-steer trailers (two-wheel steer) have a steering system that mimics an automobile. The turning wheels pivot similar to automobile wheels, maintaining more front corner stability than the fifth wheel steering trailer. As before, the rear wheels do not steer but must be located precisely for accurate tracking. Due primarily to the complexity of the steering mechanism, these trailers are considerably more expensive than caster steer trailers.
Auto-steer trailers (four-wheel steer) have both front and rear steering as described previously, connected by a steering rod to coordinate turns. Tighter, more accurate turns result while maintaining solid lateral stability, albeit at a price. A trailer with this steering type can cost 21/2 times a caster steer trailer of the same deck size.