by | Mar 21, 2020 | Recovery

Calculating Working Load Limit for Recovery Gear

The most common rating you will find on recovery gear is Minimum Breaking Strength (MBS).  While MBS is great for comparing products it doesn’t help you size recovery gear for your vehicle.  That’s where Working Load Limit (WLL) comes in since the WLL of a piece of recovery gear should always be greater than the Gross Vehicle Weight.

While MBS is determined through testing, WLL is a calculated number.  It is derived by applying a Safety Factor (SF) to the Minimum Breaking Strength (also called Mean Tensile Strength, MTS).  See formula below for calculating WLL*.

*Note: The WLL for synthetic winch line is calculated differently than other recover gear.  This will be covered later in the article.

Once you understand the formula for calculating Working Load Limit you will be able to:

  • Evaluate what Safety Factor a company is using, and no they don’t all use the same SF.
  • Compare a product rated in MBS to one rated in WLL, which I see all the time.
  • Make an informed opinion of the quality of a company’s recovery gear.

Formula for Calculating Working Load Limit

MBS ÷ SF = WLL

MBS – Minimum Breaking Strength
SF – Safety Factor
WLL – Working Load Limit

Gross Vehicle Weight

Since the Working Load Limit of our recovery equipment should exceed the weight of our vehicle, we’ll first cover determining Gross Vehicle Weight.

The easiest way to do this is to use the scale at a truck stop or the local dump. This isn’t always practical though.  For a rough, but fairly accurate, GVW we can use the manufacturer’s Curb Weight and add fuel, passengers and cargo.

A 4dr Jeep Wrangler has a curb weight of 4,200lb.  Jeep uses an estimate of 600lb for fuel, passengers and gear when calculating GVW.  Therefore our back of the envelope calculation of Gross Vehicle Weight would be 4,800.

That seems a little high to me, but we also need to add any major modifications done to the vehicle.  I won’t go through all the numbers, but in my case, I felt adding 600lb to curb weight was reasonable if I included mods.

If you have an overland setup you can easily be much higher.  For example, venture4wd weighed his Jeep at a truck scale and it came in at 6,400lb.

It is recommended to always size recovery gear to the vehicle being recovered1.  You can adjust for that if you plan on using your gear to pull out other vehicles.  In the groups I off-road with everyone has their own recovery gear so this is a non-issue.  I just size my gear to my vehicle, which covers most off-road vehicles, other than trucks, anyways.

1It is argued there is one exception to that rule, kinetic recovery rope, which we’ll discuss later.

Resistance

There are three types of resistance that affect the load placed on our recovery gear during operation.  They are rolling resistance, mire resistance, and gradient resistance.  In extreme recovery situations, we adjust for these resistances by applying a multiplier to the weight of the vehicle being recovered.

Rolling resistance is the force it takes to put a vehicle in motion.  On a hard, flat surface (such as concrete or asphalt), you should multiply the vehicle weight by 1.05.  On grass or gravel, use a factor of 1.15.

Mire resistance accounts for how deeply the tires are buried.  If the vehicle is mired in mud to a depth that covers the lower part of the wheel, use a factor of 1.75.  If wheels are mired in mud up to the bottom of the wheel rims, double the amount of the vehicle’s weight.

Gradient resistance accounts for the force of gravity pulling against the stuck equipment.  The steeper the slope, the greater the stress exerted on the towing vehicle.  For a 15 degree angle add 25% to rolling resistance, for 30 degrees add 50% and for 45 degrees add 75%.

Minimum Breaking Strength

Minimum Breaking Strength is the minimum rating at which the piece of recovery gear will break.  It is determined by destructive testing, pulling a sample until it breaks and noting the force required to break it.  After a number of tests the company can calculate Average Breaking Strength and Minimum Breaking Strength.

Minimum Breaking Strength is the best measure of rope strength.  Some manufacturers will use Average Breaking Strength since it makes their rope appear stronger1.  Others may list Breaking Strength, which is not a standard and means absolutely nothing—other than avoid that company.

All recovery gear should come with a listed MBS.  My personal preference is the gear comes with a tag showing MBS and a serial number2.  If you’re unsure about a company ask them to provide the test data for MBS.  Any legitimate supplier will be more than happy to oblige.

So you might wonder why if a rope’s Minimum Breaking Strength is 18,000lbs you can’t use it safely to recover a 5,000lb vehicle.  The MBS is determined via destructive static pull, which means the rope is put under a steadily increasing load until it breaks.  A rope in the field though is often subjected to dynamic or shock loading, rapid cyclic loading, improper rigging or other unexpected/adverse conditions (resistances, for example) while in use.

You will also stress the recovery component without a sufficient Safety Factor thereby shortening it’s life or risking it breaking while in use.

In order to account for these issues a Safety Factor, also called design factor (DF) or Factor of Safety (FoS) is used.

1One company I came across listed their 1″ rope at 32,750lb, which seemed about right since the top-tier companies rate their 1″ rope at 33,500lb. Upon closer inspection I found that number was the Average Breaking Strength.
2The serial number is used to track any failures back to the specific lot of materials used in manufacturing.  My attitude is that this is a good indication of a vendor with high quality control standards.

Safety Factor

Safety Factor is the ratio between the Minimum Breaking Strength and the Working Load Limit.  Generally, with the exception of winch lines and kinetic ropes, the top-tier manufacturers all use a Safety Factor of 5:1.

This number comes from the Cordage Institute, which sets standards for the industrial overhead lifting industry.  They recommend a Safety Factor “in the general range between 5:1 and 12:1”.  Obviously off-road recovery is a lot less demanding application than lifting a tow truck off a barge so off-road companies use the lower end of the range.

The American Society of Mechanical Engineers also recommends a Safety Factor of 5:1.

Unfortunately, some companies will use a Safety Factor lower than 5:1 to make the Working Load Limit of their product look better.  You can easily determine what SF they are using by dividing MBS by WLL.  Obviously companies like this should be avoided.

Detailed information on each category of recovery gear:

Hard Shackles, Winch Hook and Pulleys

The American Society of Mechanical Engineers recommends a minimum 5:1 Safety Factor for detachable rigging hardware.  Most quality manufacturers in the off-road industry will use an SF of 5:1 to 6:1.  Crosby, for example uses a 6:1 SF.

Kinetic Rope

Although the Cordage Institue stands as a general standard many different markets have evolved to create their own SF standards based on unique applications.  One of these is kinetic recovery rope.  The argument for using a different Safety Factor is based on the increased elasticity of kinetic rope and the resultant slingshot effect.

While some companies do use the 5:1 standard others use a lower ratio like 3:1 because, according to them, you start to lose the kinetic effect above that ratio.  I have been told this is because the tow vehicle is not able to stretch the rope enough to get the slingshot effect.

There is no publically available testing to support this opinion at the moment, but I can tell you that I have seen people use oversized rope (1″) and it did not provide the expected kinetic effect.  Several industry experts and synthetic rope suppliers I spoke with confirmed this perspective.  Two of the rope suppliers I spoke with had experienced returns on “oversized” rope because of this issue.

Following is the Safety Factor ratings used by some top-tier companies.  The list is not meant to include all of the trustworthy companies, just the ones I’m familiar with.

Factor 555:1
Advanced Synthetic Rigging3:1
Masterpull5:1
Bubba Rope3.5:1 – 4:1

*Their spokesman said it depends upon the application, and could be lower or higher, but the median is 5:1.

Winch Line

The Safety Factor and sizing for winch lines is calculated differently.  In this case, SF is in relation to the pulling power of the winch.

The Army Test Operations Procedure 02-2-618 requires a safety factor of 2:1 (not surprisingly, the military typically has more stringent standards for recovery gear).   Master Pull recommends using a 1.5:1 to 2:1 safety factor with the winch that you are installing the winch line on. This means the winch line should have a breaking strength of 1.5 to 2 times the pulling power of the winch. 

Using Masterpull’s Safety Factor, if you have a 9,000 lb pulling capacity the line should have a breaking strength in the range of 13,500 to 18,000 lbs or higher.

Summary

Working Load Limit is the best way to compare and evaluate recovery gear.  It is the rating you need to know to properly size equipment for your vehicle’s weight.  Since some companies will use an SF less than industry standards, you need to know what SF was used in calculating the WLL. 

You can use the Working Load Limit formula to:

  • Determine the SF used if you know MBS and WLL
  • Determine WLL based on the MBS and industry-standard SF

References