Last month I examined the effects of using cheater bars when tightening tie downs, both chain and web.This time I going to try to answer the question-How tight should they be? Before I do that, I want to absolutely convince you that cheater bars have no place in the tightening tie-downs.
If you remember, I was able to reach and exceed the Work Load Limit of 3/8 Grade 7 chain using a 36” long cheater bar and minimal effort. What would happen if someone was to use considerable effort? I was able to nearly EXCEED the breaking strength of the chain. How?
When I saw the loads that were being reached with minimal effort I realized two things: it might be possible to exceed the break strength of the 3/8” Grade 7 chain (26,400 LB)in a vertical hitch, and this was becoming unsafe and I needed to rerig my test set up. I changed the chain from a vertical hitch to a basket hitch. The chain was looped around one hook of the tester, then around the other hook of the tester then hooked back to itself. The lever load binder was attached to one leg of the chain so that the load was divided between the two legs. In this configuration the test bed would read the entire load, while the chain and binder would only see half of the load.
Chain in basket hitch for load binder testing Disclaimer time: The tests are conducted with new, unused products in a controlled environment. Extreme care was used to prevent personal injury and property damage. The data presented is for informational purposes only. All load binder manufacturers, including B/A Products, state that cheater bars are NOT to be used. As always, NEVER exceed the Work Load Limit of your tie-downs.
So what load was reached? 25,803 lbs. Yes, you read that right, 25,803 lbs. Less than 1000 lbs below the breaking strength of the chain. Now, remember that in the basket set up, the chain and binder are only seeing half that load, roughly 12,901 lbs, or just shy of twice the chains WLL. Could I have broken the chain if I had stayed with a vertical hitch? I doubt it, and here is why:
In order to reach the load of 25,803 lbs I had to strain. With half that load on the binder the threads of the load binder were beginning to bind up (squeaking and resisting turning) and the body of the load binder was getting warm. The signs were evident to me that I had tightened enough. Hopefully this is evidence that cheater bars have no place on your trucks.
So this month I’m going to talk about web tie downs. How tight should they be? This should be easy, right? Well, not so much. I contacted a few people in the chain and tie down industry. I got answers like “that’s a good question”, “no one has ever asked that before”,” I don’t know”, and “as tight as you can without exceeding the Work Load limit”. At least one of these answers is useful.
I looked through my usual sources, and found very little, sometimes conflicting information. I started with the Web Sling Tie Down Associations WSTDA-T-1 “Recommended Standard Specifications for Synthetic Web Tie Downs”, chapter 4 Recommended Operating Practices. There is a large amount of information presented, including not to exceed the WLL of your tie down, how the angle of the tie downeffects the WLL, and so on, but no guide to tightening. There are two bits of information worth noting: first, you must take into consideration the anchor point of your tie down. If your anchor point, be it a D-ring, keyhole slot or rub rail is not rated the same or higher than your tie down, the tie down MUST be de-rated to match. A chain (or tie-down) is only as strong as its weakest link (or anchor point). Second, it refers to the Federal Motor Carriers Safety Administration Standard for Protection Against Shifting and Falling Cargo. Guess where I’m going next?
I went to the FMCSA web site (www.fmcsa.dot.gov), found Regulations, then cargo securement, then Drivers handbook on Cargo Securement, Chapter 9-Automobiles, light trucks and vans. This must be the place! Nope. But I did find some useful information. In brief, the chapter covers securing vehicles under 10,000 lbs. it states:
“At least two tie downs at the front and rear of the cargo to prevent movement side to side, forward and rearward, and vertically.
The sum of the working load limits from all tie-downs must be at least 50% of the weight of the cargo”
I also looked at Chapter 1, “Fundamentals of Cargo Securement”. Once again, no instructions on tightness, but I did find this:
“Each cargo securement system must be able to withstand a minimum amount of force in each direction.
Forward force: 80% of cargo weight when braking while driving straight ahead.
Rearward force: 50% of cargo weight when accelerating, shifting gears while climbing a hill, or braking in reverse.
Sideways force: 50% of cargo weight when turning, changing lanes, or braking while turning.
Upward force: 20% of cargo weight when traveling over bumps in the road or cresting a hill.”
A diagram is included showing these forces with the definition the 80% of the cargo weight is considered .8g (as in g forces).
So far, we know what we have to restrain, and how much of it we have to be able to hold, but not how tight the tie-downs should be. On to the next reference, “Practical Cargo Securement, Guidelines for Drivers, carriers & Shippers”, published by the Commercial vehicle Safety Alliance. This book as lots of useful information on the proper ways to secure many things, including vehicles. In chapter 5, section 5.4.B they discuss tie-down position and tightness. The following statement is made:
“When tightening direct tie-downs you want them snug but not over tightened. You can make them weaker by over tightening them. For example a chain with a WLL of 5000 lbs that is tightened so that it has 1000 lbs of tension will only have 4,000 lbs remaining to resist force acting against it.”
Lets step away for a minute and talk about direct vs. indirect tie-downs. A direct tie-down is one that goes from the deck of the transport vehicle to an attachment point on the cargo, a cluster strap for example where the chain on the ratchet is in a keyhole slot on the deck of a carrier and the T hook in in the frame slot of the towed vehicle.
An example of a direct tie down. The tie down is connected to the load and the transport vehicle.
An indirect tie-down attaches to the transport vehicle, goes over or through the cargo and then reattaches to the transport vehicle. A crushed car on a flat bed with chains over the hood and trunk areas would be an example of an indirect tie-down. The angle of the tie-down to the cargo has and effect of the usable WLL of the tie-down. For our purposes we are discussing direct tie-downs only.
An example of an indirect tie down. The straps hold the load in place with pressure only; they are not connected to the load.
Lets go back to the statement “…will only have 4000 lbs remaining to remaining to resist force acting against it”. This just didn’t sound right to me. If it was reversed, say the chain had a WLL of 5000 lbs and was tensioned to 4000 lbs (which we know is possible) it will only have 1000 lbs remaining to resist force acting against it, I might have accepted it and moved on.
I contacted a friend, Mike Green and posed that question to him. Mike is a Captain in the Montgomery County MD Fire Department Special Operations, has a general engineering degree, and runs his own business, Mid Atlantic Technical Rescue, which teaches rescue techniques, including how to figure loads and forces. Mike consulted with one of his instructors, Mike Johns, who is also in Special Operations with the Montgomery County Fire Department and holds an engineering degree. Their answer, in short, is that the original tension on the tie-down would have no effect on the end loading.
They explain it this way: Lets say you put 3000 lbs of force on your tie down. The only way that 3000 lbs of force would ever be increased beyond 3000 lbs is if the car were to surge forward with sufficient acceleration or deceleration to create a force greater than 3000 lbs. At that point the initial tension you placed on it is irrelevant, because the load itself has exceeded your initial force.
I asked Mike about the CVSA statement that the tie-downs must be able to restrain .8g of the load. He did some quick calculations and found the deceleration rate to achieve .8g. The rate would be going from 53 mph to 0 in 3 seconds. Short of hitting a 4’ thick concrete wall, not likely to happen, as trucks do not have the ability to brake that quickly.
What about damage cause by over tightening, is that a possibility? I called a friend at a body shop that has a frame machine and asked if he could tell me how much pressure they used when straightening frames or bodies. He answered that there are a lot of factors, but forces as low as 4000 lbs are used. If 4000 lbs can straighten a frame, I’m betting it can also bend, or damage one. I think I’m starting to get somewhere.
Lets take a little break and recap where we are at. So far, I have not found an answer to how tight a tie down should be. I have found:
The tie-down should not be tightened above its WLL. Depending on your tie-down, this could be as low as 2000 lbs. A frame machine uses 4000 lbs of force, so we definitely want to stay below 4000 lbs.
The attachment points of the tie-downs must be equal or greater to the tie-down WLL, or the tie-down must be derated.
The tie-downs need to restrain .8g of the load (defined by FMCSA as 80% of the loads weight).
I looked through several instruction manuals for car carriers and tow trucks to see if specific forces were recommended. I did not find specific forces, but saw the phrase “sufficient force” mentioned several times. In addition, I saw the statement “sufficient force has been applied if the tire sidewalls begin to bulge”. How much force does it take to bulge a side wall? Do different sizes of tires bulge at different loads? What about tire pressures? Guess what I’m going to try and find out next.
One afternoon I gathered a roll back, several vehicles with different sizes of tires, a load cell and some tie-downs and attempted to find out. The first question that came up was what if the tire pressure is low? Sure enough, the first car we put on the bed was supposed to be at 52 psi was at 35 psi. The nextquestion was what defines a bulge? There were 3 of us trying to work it out, and I could already see there were too many variables and opinions for this to be accurate. Since everything was there, I trudged on. What we found out was that whether the tire pressure was at 35 or 52 psi, it did not make much of a difference with a direct tie down. We really were not getting a noticeable bulge, and the load on the tie-down was anywhere from 300 to 600 lbs.
We switched to an over the tire strap, as used in our Roll Back Tie Down System. The loop around the tire was indenting the tire when we had between 400 and 600 lbs of force at the ratchet side of the assembly. The issue was that the three of us each had a different opinion of the indentation. Back to the drawing board.
I stepped away from this to work on another project for a customer. The customer had asked for tie-down assemblies to be labeled to a European standard, EN 12195-2. I located the standard, printed it out and among other things discovered it called for a SHF (standard hand force) and a STF (standard tension force) to be on the label. Per the Standards definitions the SHF is a “hand operating force of500N” and the STF is “residual force after release of the handle of the ratchet”. I had found a standard for tightness, but what is a N and how can I measure it?
From my other testing work I knew that N stands for Newtons, the metric measurement of force. I Googled “convert Newtons to pounds”, plugged in 500N in the Newtons box, hit enter and found that 500 Newtons is equal to 112 lbs. So the standard read that the Standard Hand Force to be applied to the ratchet is 112 lbs. Next I made a section of strap covered with Velcro, and adjusted the overlap of the until until regularly came apart at between 110 and 115 lbs. This was done by repeatedly lifting a bucket filled with a known weight and measuring the overlap.
I then attached the Velcro to the handle of a ratchet strap assembly in the test bed, and repeatedly tightened the ratchet by pulling on the Velcro strap until it came apart, and recorded the load on the test bed. The average load was 1181 lbs. Now, I’m the first to admit that this was not the most scientific method, and may not be 100 percent accurate. But, an EN 12195-2 rated tie down assembly with a 5000daN break strength (11,240 lbs) has a standard tension force of 1124 lbs, so I am not too far off.
So now we have a target load for our web tie-downs. Time to see how close some operators are, and if I can teach them to load the assembly to 1124 lbs.
I picked three employees at random to do my control tests. TJ, Andy and Keith were brought to the test bed. The only instructions they were given was to tighten the web assembly until they thought it was tight, and that there was no right or wrong way to do it. They each tightened the strap three times, and the loads were recorded. Average loads were 1442 for TJ, 872 for Andy, and 1615 for Keith. More interesting was the difference between each employees high and low: TJ was 931 lbs, Andy’s was 11 lbs and Keiths was 417 lbs. As each of them finished their turns, I asked them if they thought they tightened the strap evenly each time. Andy answered yes, TJ said the first two times yes but on the third he “put more leg into it”, which was the high load of the test at 1976 lbs. Keith thought he did it evenly all 3 times.
Now when I said I picked the employees at random, I was not completely honest. I picked one person who I was pretty sure had never used a tie-down before (Andy), someone who I knew for sure had used them (Keith) and one who I was not quite sure of (TJ). I also picked employees of varying height and weight.
I then explained the purpose of the testing to my group, and told them how I was going to try and teach them some techniques. The first thing we discussed was “Body English”. If you remember in part 1 I defined Body English as “body motions made in a usually unconscious effort to influence the progress of a propelled object”. What I wanted them to be conscious of was to not use their body or legs, but to attempt to use arm strength only. I pointed out TJ’s admission that he “put more leg into it” and the difference it made in the load applied.
I also showed them the technique I wanted them to use. I had them pull the slack out of the assembly so the strap was snug before they started tightening the ratchet. I asked them to operate the ratchet until they began to feel tension build in the assembly, and then to tighten the ratchet two more “clicks”.
Over the day we practiced four times, with each employee tightening the strap three times in each session. I did not tell them the loads until all three had completed their turn. I then reviewed the load each applied, and the difference between the high and low for each of them.
By the end of the day TJ was averaging 1046 lbs, with an 11 lb difference between the high and the low. Andy averaged 798 lbs, with a 35 lb difference between the high and low. Keith averaged 922 lbs, and had a 50 lb difference between the high and low. So far so good. TJ is pretty close to the target of 1124 lbs, Keith and Andy are a little low. I gave them a day off, then did one more test to see how much they remembered and how close they would be to the target. TJ averaged 852 lbs, 86 lb difference between high and low, Andy averaged 627 lbs, with a 55 lb difference, and Keith averaged 707 lbs, with a 90 lb difference.
So do I call this experiment a success? I would say it was marginal. While we missed the target, especially after a day off, we were able to tighten the strap more evenly. Consider that the difference between the high and low load in the first test was 1107 lbs and in the final test it was 225 lbs, all 3 operators were became much more consistent. With a little more practice, who knows?
In addition to my earlier disclaimer, I need to say that a number of factors were ignored during this testing. Operator position and stance were not taken into account. The setup of the ratchet strap assembly in the test bed does not accurately reflect how a direct tie-down acts against a cars suspension. My point of this exercise was to try and find a tightness standard, and to see how close I could get a group of employees to get to it.
I would be interested to hear from you, and get your input. Is there a standard for tightness that I was unable to find? Do you teach your employees a specific method to tighten tie-downs? What other information would you find helpful? I can be reached at Fritz@baprod.com.