It is important that end users understand the design categories and service class of clamps and other lifting products, says Tom Eicher, director of engineering at The Caldwell Group Inc.
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.
How well do you know ASME BTH-1, Design of Below-The-Hook Lifting Devices? Did you know that ASME B30.20-2018 is a new version that references the BTH-1-2017 design specification? Many end users don’t realize that the manufacturer must meet BTH-1 design specifications to meet the ASME B30.20 standard.
ASME BTH-1 provides designers and engineers of clamps and other ‘below-the-hook’ lifting devices with the design criteria, standards and structural requirements needed to manufacture reliable products for various lifting applications. ASME BTH-1 also provides fabrication requirements for welding processes and procedures.
ASME recently released the new specification for B30.20, dated 2018, while BTH-1 was released a year earlier.
What defines a clamp?
It’s important to establish the product to which this guidance applies. Clamps are designed to generate a clamping force perpendicular to the surface of the load during lifting. These clamping forces are generated from the clamp geometry by the pulling force on the lifting shackle of the clamp or tightening of a screw.
Many clamps have features that allow for specific lifting conditions. When the wrong clamp design is used for the application, it can lead to a lost load or clamp damage. A number of factors lead to such misuse, such as not following the instruction manual, unapproved modifications to the lifting device or a lack of maintenance or inspection.
Importantly, the load being lifted must be able to support the amount of squeeze that the clamp applies to the load. One area where we see repeated misuse is picking the wrong clamp for the application. Some clamps are designed for vertical lift only, some are for upending plate, some are for 180 degrees plate flipping, or for flat surface versus curved surface. It is necessary to understand the application before selecting the clamp.
New Service Class C
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.
Clamps can be designed for light loads to loads exceeding 100,000 lbs. or more. 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.
Modifying a Clamp
We urge users and industry generally to proceed with great caution when it comes to considering modifications to clamps. At the outset of this article we 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.
Director of Engineering
The Caldwell Group Inc.
Tom Eicher is director of engineering at The Caldwell Group Inc. He started his career at Beloit Corporation, designing equipment for the paper industry, and joined Caldwell in 2004. He contributes to a number of industry groups, including the ASME B30.20 subcommittee. Tom is also chair of the BTH-1 committee for Associated Wire Rope Fabricators (AWRF) and the below-the-hook committee for the Crane Manufacturers Association of America (CMAA). Prior to his professional career, he graduated with a Bachelor of Science degree in Mechanical Engineering from Michigan Technological University in 1987. Tom is married to wife Dorothy and has four children and eight grandchildren.