Licensed Professional Engineers
FORENSIC CLUES # 22 - "Articulated Ladders " by John L. Ryan and L.D. Ryan
A newsletter dedicated to keeping attorneys informed of the technical side of product liability cases.
Issue 22: Vol. 1 December/January 2007
By John L. Ryan and L.D. Ryan
Articulated ladders are becoming more popular. With increased use has come increased accident rates.
Articulated ladders are ladders that have multiple configurations combined in one ladder. Articulated ladders can be used as straight ladders, stepladders, scaffolding, and other uses. These ladders allow users to access difficult-to-reach areas, as well as to more efficiently reach elevated regions. Articulated ladders can often reach elevations normally only reachable by extension ladders. These ladders can also be configured into scaffolds, when working on a larger area of elevated space, such as painting a wall.
Figure 1: Example of articulated ladder being used to work on stairs
Accidents on articulated ladders occur in different manners, the end result is a ladder user falling to the ground. Some accidents on articulated ladder are due to material failure of the ladder. If a ladder rail or supporting member fails, the ladder is likely to become highly unstable and/or collapse. Poor ladder material selection, manufacturing defects, damage to a ladder, inadequate load support, all can cause material failure.
Hinge failures also cause articulated ladder accidents. When a hinge fails with a person on the ladder, the ladder is very likely to collapse, sending the user falling to the ground.
Hinge failure can occur because of overloading of the hinge. Overloading means that the forces that act on a hinge during use exceed some component’s strength. This occurs when a manufacturer oes not design a hinge correctly. A structural device is only as
strong as the weakest member. If pins or bolts are used in a hinge that are of inadequate size and/or strength, these connection members can fail, effectively incapacitating the ladder hinge.
Hinges can also fail without experiencing component failure. Hinges on articulated ladders must rotate through a large angle of rotation in order for the ladder to be used in the different configurations that these ladders are used in. Articulated ladder accidents occur frequently where a ladder hinge collapses, sending the ladder user falling to the ground. Upon further investigation, some of these accidents reveal that certain manufacturers have not designed their hinges correctly, and these hinges may come unlatched and rotate freely under certain loading conditions.
This leads to another problem with articulated ladder hinges, false-locking of the hinges. When using articulated ladders, the hinges must be somehow locked in each position to configure the ladder in the desired position. Various ladder manufacturers use a variety of hinge designs. Some ladder manufacturers use hinges that have little room for error. These hinges use mechanisms that can unlock and cause hinge rotation. Other manufacturers use different kinds of locking mechanisms at the hinge. These locking mechanisms include spring loaded detents, locking levers, and locking knobs. Some of these locks are effective, others are not. Another issue that arises with hinges when locks are used, is whether the lock is engaged or disengaged. Many ladder manufacturers use some sort of coding system to communicate when the hinge is locked and unlocked. Some manufacturers use color-coded mechanisms that show green when the ladder is locked and ready to go and red when the hinge is unlocked.
Figure 2: An example of an articulated hinge
One of the better hinge designs we have seen uses a large red knob for the human interface of the hinges. If the hinges are unlocked, the ladder will rotate freely about the hinges, preventing the ladder from being erected into a useable configuration. This can prevent false-lock accidents since this articulated ladder does not have a false-lock position.
Figure 3: Unlocked hinge
The position of the red knob indicates an unlocked state when the red knob is next to the ladder rail, which exposes the three locking pins of the hinge. When locked, the locking pins are no longer visible, and the red knob sets away from the rail.
Figure 4: Locked hinge
Another feature of this articulated ladder is the requisite unlocking of the ladder hinges when unfolding the ladder from the stored position. This shows the ladder user how to properly engage and disengage the ladder since the ladder user must do both procedures for the ladder to be erected in a useable configuration.
Another issue with articulated ladder hinges is hinge friction. If the internal friction caused by the hinge mechanism is too high, the ladder user may think the ladder is locked when it is not, creating another type of false-lock scenario. These defective hinges can be erected without locked hinges, and may even hold the ladder user*s weight for a short time, making a severe accident likely when the unlocked hinge rotates freely. Standardized tests exist that can determine if the internal friction of a ladder hinge is too high.
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