Marking of WLL on Rubber Tarp Ties | Ancra
Date: November 21, 2011
RE: Marking of WLL on Rubber Tarp Ties
It has been brought to our attention that there are some suppliers of rubber tarp ties who are marking them with a WLL of 50 lbs. It is our position that this is a dangerous practice and will potentially result in reduced safety for the general motoring public.
Rubber tarp ties inherently have a large elongation under load. This is required for their design function of tensioning tarps. Tarps are installed over loads to protect the load from the elements and without these resilient tensioning devices will flap in the wind, reducing their effectiveness and service life. Rubber tarp ties are ideally suited to perform this function and in this function are not serving as a cargo restraint. A load cover has never been considered to be a piece of cargo requiring WLL rated tiedowns unless it is folded into a bundle where it's weight then becomes a concentrated load.
To use a rubber tarp tie as a cargo restraint is a completely different application than what it was designed for and one it is not suited for. It's inherently large elongation under load, while an advantage when used to tension a tarp, is a detriment if it used as a cargo restraint. Any piece of cargo secured by a rubber tarp tie will move, and potentially become loose from the transporting vehicle, if there is any significant dynamic force or wind generated force applied to the cargo.
The current FMCSA standards for cargo securement have no allowance for a rubber based product to use as a tie down. The fact that some rubber tarp ties can deteriorate rapidly from UV exposure and separate without warning puts them in a different category from other cargo securement devices. Without a suitable inspection criteria and "Out of Service" standard, rubber tarp ties should not be used as cargo securement devices.
Using the FMCSA formulas for cargo securement, two of these 50 lb. rated rubber tarp ties could be used to secure a 200 lb. piece of cargo. If they are stretched to their maximum recommended length to secure the cargo, they will have a tension of approx. 30 lb. applied to them. This results in a possible maximum down force of 120 lb. on the cargo giving a total contact pressure of 320 lb. between the cargo and the vehicle deck. Using a typical coefficient of friction of .2, the friction force holding the cargo against movement is 64 lb. This holding force will be overcome when there is a dynamic side load of .32 G. This is well below the required .5 G lateral and rear and .8 G forward required by the FMCSA standards. Once the friction holding force is exceeded by a dynamic impulse acting on the cargo, it will move, stretching the rubber tarp tie until the needed added restraining force is reached. Given the high elongation properties of the rubber material, the distance the cargo will move can be quite large. We see this as a dangerous situation that can result in cargo falling off a vehicle, endangering the public.
A typical polyester cargo securement strap has elongation of 5% when tension equal to the rated WLL is applied. The rubber tarp strap will have an elongation of 100% or more when the 50 lbs. WLL is applied. This drastic difference is significant and makes it dangerous to think that these rubber based products can be substituted for a typical cargo securement device.
In conclusion, the rubber tarp tie has traditionally been used to stabilize and tension tarps when they are used as load covers. For this application they are well suited. In this function the tarp is not a piece of cargo, but a load cover. Once the tarp is removed from the load and folded into a bundle and placed on the trailer deck, it becomes a piece of cargo with a concentrated weight. At this point a rated cargo securement device is needed to secure the tarp. If the folded tarp is secured to the trailer with rubber tarp straps, it will potentially become a dangerous piece of loose cargo on the roadway.
H. Tom Knox
Director of Engineering - Cargo Systems