Caldwell clamp

Design Category C for Clamps

Photo: Clamps for lifting come in many designs and are used in a myriad of applications, including lifting plate steel, ingots, slabs, concrete barriers, pipe, coil steel, paper bales, and paper rolls. Photographs courtesy of The Caldwell Group.

Did you know that Caldwell is the only company in the world who manufactures clamps that meet ASME BTH (below-the-hook) Design Category C?

Tom Eicher, director of engineering; and Dale Kelly, engineering manager at The Caldwell Group Inc., explain why the industry should better understand the new benchmark for clamps.

Design Category C for Clamps

Industry should sit up and take note of the fact that, at least to the best of our knowledge, J.C. Renfroe, a subsidiary of The Caldwell Group, is the only clamp manufacturer in the world that can meet ASME BTH (below-the-hook) Design Category C.

Think about that. It means that, despite the abundance of suppliers and products in the below-the-hook market, only one makes (over 200) clamps to the most rigorous standards in the land. This is of particular relevance to heavy duty industries, such as forging/forming houses, shipyards, heavy construction, etc.

The C design category was introduced only two years ago but our clamps were always designed with the appropriate ruggedness so it was a no-brainer for us to reclassify them.

Manufacturers that joined the market later, followed the previous standards and designed a clamp that was just good enough, which resulted in clamps that will have to be redesigned to meet this higher category class. This takes time and explains the lack of clamps meeting the new standard.

References to standards can get complicated, which doesn’t always help the situation, because people can misperceive them or turn a blind eye. It’s especially difficult for end users and those at the coalface to understand every standard or each product they use. Often, they are experts in a specialized field, but lifting equipment is just something that they need.

Put simply, the C class has a higher design factor than the current B category, which provides a larger margin of safety to the end user against failure and industrial accidents due to clamp failure. The B class has a design factor (what the clamp can withstand/rated load) of 3:1, whereas the C class has a 6:1 design factor minimum. That should mean something to anyone in real terms. And many of our clamps are manufactured to a much higher design factor than that.

The new standards provide a measure of clamp quality. It reflects a clamp’s ability to do its job over the long haul. Inescapably, higher ratings equals a better clamp. For example, an A0 clamp is a low rated clamp and could be of inferior quality. A C4 rated clamp is the best clamp there is.

Keep in mind that this is self-regulated so the manufacture rates his or her own clamps. In our case we back it up with laboratory testing. Many Renfroe clamps were designed to withstand severe duty before the standard was written and in that sense the standard is catching up with Renfroe. There have been many changes over the years related to manufacturing processes and materials; we are retesting each clamp that is reclassified to ensure the integrity of the clamp is still valid.

This provides distributors and end users with an immediate indication as to how good a clamp is. The customers can now look at this rating and immediately determine—if they know standard—the relative robustness of any clamp. This provides them with a quick assessment tool without a lot of research.

It’s hard to say how other manufacturers will respond. Truth is, many clamps do not hold up well to the environments we cater to. We have dominated this sector of the market and many users try other clamps and return to Renfroe. We have performed independent lab testing on our and competitors’ clamps to validate the relationship of this standard to the strength and robustness of their clamps. Unfortunately, these tests are expensive and time consuming and many will not do these to confirm the designs.


Looking at clamps more generally helps us understand the importance of Design Category C.

Clamps for lifting come in many designs and are used in a myriad of applications, including lifting plate steel, ingots, slabs, concrete barriers, pipe, coil steel, paper bales and paper rolls, to name just a few. Clamps can be used for attaching loads to a hoist, or for other load handling purposes such as anchoring or positioning. However, despite being common tools in everyday use, clamps are often misused and remain widely misunderstood. Further, knowledge on inspection, repair, testing and maintenance is often lacking.

ASME B30.20 and ASME BTH-1 standards are recognized in the United States as the industry benchmarks. The design category established in the BTH-1 design criteria is based on category A (2:1 Yield 2.4:1 Ultimate), Category B (3:1 Yield 3.6 Ultimate), Category C (6:1 Yield 7.2:1 Ultimate). The factor of safety and design of a lifter is based on the customer’s requirements such as 1) number of lifts per day 2) application 3) severe environment 4) specific industry requirements such as nuclear, military, or energy. These are only a few considerations to establish the category needed.

The relevance of ASME is in its name: The American Society of Mechanical Engineers. Much of the focus on lifting, rigging and other equipment is on its safe application in industry. We talk a lot about inspection, maintenance, best practices, etc. However, what makes ASME so relevant is that its “mission is to serve diverse global communities by advancing, disseminating and applying engineering knowledge for improving the quality of life; and communicating the excitement of engineering.” In other words, “ASME aims to be the essential resource for mechanical engineers and other technical professionals throughout the world for solutions that benefit humankind.”

As we put it regularly to our distributors and end users, ASME ensures that certain steps are taken into consideration in the design and manufacturing of lifting equipment. No product can ever be applied safely if the highest possible safety standards haven’t been adhered to during its design and manufacturing process. What can the best rigger in the world do with a faulty sling?


Updates to the BTH-1-2017 includes the new service class C. The following are design requirements, including the original A and B service classes:

  • Service class A is a 2:1 design to the material’s yield strength or 2.4:1 to the fracture strength of the material and connections.
  • Service class B is a 3:1 design to the material’s yield strength or 3.6:1 to the fracture strength of the material and connections.
  • Service class C is a 6:1 design to the materials yield strength or 7.2:1 to the fracture strength of the material and connections, using BTH-1-2017 design classifications.
Clamp tag
Photo: A clamp tag with Design Category C clearly marked in the bottom right-hand corner. The tag also carries information about the Service Class (4; bottom left), capacity, and other detail.

Clamps can be designed for light loads to loads exceeding 100,000 lbs. or more. As we’ve explored, some manufacturers have clamps that meet design category A, service class 0, while others meet design category B, service class 4. The difference between these two scenarios is a design category A, service class 0 (A0) is a 2:1 minimum design to the material’s yield strength for use up to 20,000 cycles, while a B4 clamp is a 3:1 design to the material’s yield strength for use over 2,000,000 cycles.

It should be noted that a design category A should only be specified when the loading and environmental conditions are not severe, and the loading is known and predictable. Design category A is limited to a service class 0 (maximum of 20,000 cycles):

The service class of a lifter establishes the design parameters of the lifting device and are defined as follows:

  • Service Class 0: 0 to 20,000 cycles
  • Service Class 1: 20,001 to 100,000 cycles
  • Service Class 2: 100,001 to 500,000 cycles
  • Service Class 3: 500,001 to 2,000,000 cycles
  • Service Class 4: over 2,000,000 cycles

As discussed, ASME B30.20-2018 was recently revised to include a new chapter (20-6) for clamps, as follows:

  • Chapter 20-1: Structural and Mechanical Lifting Devices
  • Chapter 20-2: Vacuum Lifting Devices
  • Chapter 20-3: Close Proximity Operated Lifting Magnets
  • Chapter 20-4: Remotely Operated Lifting Magnets
  • Chapter 20-5: Scrap and Material-Handling Grapples
  • Chapter 20-6: Clamps

Within Chapter 20-6, clamps are categorized as follows:

  • Load supporting clamps.
  • Friction-type pressure-gripping clamps.
  • Indentation-type pressure-gripping clamps.
  • Anchoring and positioning clamps. Anchoring clamps can be used for suspending or attaching detachable rigging hardware. Positioning clamps are used to hold or locate parts during assembly operations.
  • Hybrid clamps use a combination of categories above.

It’s worth noting that other items that influence the design of a lifter is extreme cold or hot environments. The current standard is applicable at temperatures from 25 degrees F to 150 degrees F (-6 degrees C to 66 degrees C). Temperatures beyond these limits require additional design considerations and may require special materials that are better suited for extreme cold or extreme heat.


We urge users and industry generally to proceed with great caution when it comes to considering modifications to clamps. We’ve looked at the many different scenarios that use clamps—plate steel, ingots, slabs, concrete barriers, pipe, etc.—and each of these applications present their own specific requirements for the clamp. However, it is never ok to modify a clamp without consulting the manufacturer—period. Even at the very first consideration that a modification might enhance productivity or efficiency as it relates to that particular application, the manufacturer of the clamp should be contacted.

We’ve seen scenarios where a user has welded a chain or a handle onto a clamp, thinking that it is safe to do so. Practically, it might have aided operation, but it did so at great compromise to the integrity of the clamp itself. When we do work with a user to modify a clamp, we produce a drawing and are particular about the materials used and type of weld wire, for example, and offer other guidance such as not to disturb the base metal. Additionally, we require written or photographic evidence that the modification was completed in line with that instruction.

As a general rule of thumb, always consult the manufacturer about better tailoring or adapting a product for a specific application. In some cases, an alternative solution already exists. As stated, so much of safe operation starts with selecting the right tool for the job in hand. Modifying the right tool can turn it into the wrong one.


End users should be more aware of the manufacturing origins of the products they use to generate a clamping force perpendicular to the surface of the load during lifting. They could even use available information when competing with a lesser design factor imported lifter that does not meet the ASME criteria.

Remember, not all clamps will meet Category C. We’ve established that Caldwell and Renfroe has clamps that have historically met the criteria, but the new Category C has only recently been implemented into the ASME design specification. Therefore, as a manufacturer, we are only looking at updating the existing clamp category to C on the clamps that currently meet the requirements of Category C and always have.

For information, currently no other Caldwell products are planned for Category C at this time. Future products will ultimately be designed to a Category C if required per a customer’s specification requirements. Feedback from sales in the future may redirect our current focus for this category based on market response.

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