Meaning of Working Load Limit, Breaking Strength, Tensile Strength, Safety Factor & More

Lifting heavy equipment onto your flatbed truck is hard work. At times, it can be dangerous. That’s why it’s important to understand the terminology surrounding rigging and lifting procedures. Understanding concepts like Working Load Limit (WLL), Breaking Strength and Tensile Strength can help you stay safe and prevent accidents on the job. In today’s blog, we’ll go over the key terms that you should know for every rigging and lifting job.

What Does Working Load Limit Mean?

Working Load Limit (WLL) is one of the most important concepts to understand in the industrial and trucking industry. It is important to apply WLL for safe cargo control. It refers to the maximum weight your crane or turnbuckles can withstand. Before you start using any lifting or rigging device, you have to first determine its WLL based on the intended use and the force applied. In other words, the WLL must be greater or equal to the weight you’re lifting.

Other factors to consider are how the load is positioned over your flatbed, how many attachment points you have over your trailer and the condition of your equipment. While the WLL is usually predetermined by the manufacturer, you can calculate it on your own as well. Simply divide the Breaking Strength by the Safety Factor. These terms will be explained later on in the blog.

What Does The Breaking Strength Mean?

On the other hand, the Breaking Strength refers to the maximum weight that a lifting device can handle before it breaks. Apart from understanding the WLL, it will always be ⅓ of the Breaking Strength. If a ratchet strap has a Breaking Strength of 10,000 lbs, then it’ll have a WLL of approximately 3,333 lbs. You’ll hear these terms being used interchangeably on a job. One way to tell the difference between the two is that the Breaking Strength is always the higher value.

Also, the Breaking Strength is determined through destructive testing of your equipment to determine its maximum strength. Now that you have a deeper understanding of Breaking Strength, you’ll be able to calculate the WLL of your rigging equipment. For instance, if the Breaking Strength is 10,000 lbs. and the Safety Factor is a ratio of 5:1, then the WLL will be 2,000 pounds.

Tensile Strength Vs. Breaking Strength

Similarly, the Breaking Strength and Tensile Strength often coexist with one another. While they share similarities in meaning, they do have differences. The Tensile Strength puts a laser focus on how much pressure the material of your lifting device can handle. When building durable structures, engineers have to take into account both the strength of the materials they’re using and the loads the lifting device will have to handle. Understanding these concepts will help engineers and truckers like you apply them to real-life situations. You’ll even become a pro at lifting heavy equipment.

What Is A Safety Factor?

The Safety factor, also known as the Design Factor, refers to the ratio between the WLL and Break Strength and determines the safety of your equipment. Knowing the difference between the WLL and Breaking Strength will help determine the safety factor. The standard safety ratio for any lifting device, such as chain slings, ratchet straps, and shackles, is typically 3:1. That means the Breaking Strength has to be at least five times the WLL. If the safety factor is high, you have the green light to use the load-bearing device. Moreover, the Safety Factor permits Shock Loading, G force and can even predict other contributing factors.

At the end of the day, the Safety Factor is a crucial part of rigging and lifting operations. Not only does it help determine if your equipment meets safety standards, but also it can help you complete a job effectively.

What Is Safety Working Load?

Working load - Tie Down

The Safety Working Load, also referred to as the Normal Working Load (NWL), is similar to WLL, as it details the maximum amount of force that a chain sling can use to lift or lower cargo in a safe manner. It factors in the strength of the material, the design, safety measures, and the condition of your lifting device.

The SWL offers a guideline for workers and truckers to help them determine the correct amount of weight that can be safely applied to a crane. However, the concept has gone extinct in the industrial and trucking industry. Even more pressing, SWL is often confused with Working Load Limit, as they share very similar meanings. More often than not, workers and trucking companies will use WLL during lifting and rigging jobs.

What Does The Manufacturer’s Rated Capacity Refer To?

Furthermore, the Rated Capacity or Manufacturer’s Rated Capacity (MRC) basically refers to the amount of weight a lifting device’s attachment piece can handle. Also, the term details the greatest lifting capacity of any hoist or Flat Eye Sling when the lift is a straight line pull. As the name suggests, the maximum rate is predetermined by the manufacturer.

The rate factors in engineering calculations, testing and stay in line with industry standards. Moreover, the MCR and SWL were once used interchangeably. Though, the key difference between the two is that the MCR is the value provided by the manufacturer in relation to your equipment’s load-bearing capacity. On the other hand, the SWL showed how that capacity could be demonstrated on the job.

Breaking Strength Vs. Working Load Limit

Previously, we discussed the terms Breaking Strength and Working Load Limit. While we know the definitions of both and how they relate to one another, we have yet to discuss their key differences. Understanding how they differ from one another will help you understand the ins and outs of a lifting job.

For starters, the Breaking Strength shows the amount of force applied to your crane before it gives out. In other words, this value is the Minimum force or Breaking Strength that a lifting device can withstand before breaking. According to industry standards, the manufacturer must provide the Minimum Breaking Strength on the lifting device, not the WLL. For instance, on a pulley, the load-bearing value is often shown on both ends of the rope, with the addition of the Breaking Strength.

On the flip side, the Working Load Limit basically refers to the maximum amount of force that can be regularly applied to a crane, host, or ratchet strap. At the end of the day, understanding these concepts and their differences can help you excel at your job.

What Is A Shock Load?

Next on our list comes the Shock Load. This term refers to the condition of a load after a rapid change of movement, which can stem from the impact, jerking or swinging of a static load. Also, the quick release of tension is another type of Shock Loading. Though, they’re greater than static loads. It’s important to note that Shock Loads can cause tons of problems during a lifting job.

They can lead to equipment failure, dislodge loads, and even place higher stress levels on your shackles. To avoid these problems on the job, use proper rigging and lifting techniques to handle Shock Loads effectively. One way involves using certain lifting equipment, such as dampeners, that can help reduce the negative impact a Shock Load can have on your cargo and device. Making sure that you stay safe on the job is most important. You’ll want to protect your materials as well.

What Is A Proof Test Load?

Lastly, the term “Proof Test Load” or Proof Load states that a quality control test was performed on your lifting device to detect defects in its design. Also, the Proof Load refers to the cargo your lifting equipment could withstand under pressure and in testing without breaking. Moreover, continuous force is applied onto the device at a certain speed on a pull testing machine. Also, this value of the Proof Test Load is much higher than the WLL. However, that doesn’t mean the value of the load should exceed the Working Load Limit. In any case, nothing should ever go over the WLL.

The Power Of Knowledge

Should you ever find yourself lifting a heavy piece of machinery, you’ll be able to rise to the occasion. Understanding concepts that relate to the amount of force your crane, shackles or chains can lift, is the first step in gaining the experience you need for a lifting job.

FAQs

Can The WLL Be Exceeded?

It would be bad news for all parties involved if you attempted to go over the WLL of your lifting device. It could lead to your equipment failing and cause an accident.

Can The Breaking Strength Be Used As The WLL?

Unfortunately, the Breaking Strength can’t be replaced with the Working Load Limit. It doesn’t provide the necessary safety measures needed for your lifting equipment.

What Is A Safety Factor In OSHA?

The safety factors in the Occupational Safety and Health Administration (OSHA) help ensure that your lifting and rigging equipment can withstand heavy cargo in a safe manner. One major safety factor that the organization details is that employers promote on-the-job safety measures, especially on lifting jobs.

Is The Working Load Limit The Same As The Safe Working Load?

The two terms are associated with the maximum amount a crane or sling can lift. However, they hold slightly different meanings, with the Working Load Limit being the more commonly used term.