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ASME BTH-1 Design Categories and Service Classes: Rated Load Cycles

ASME BTH-1 classifies a below-the-hook lifting device on two axes: in Duerr's description of the then-new standard, the design category relates to the expected usage of the lifter, and the service class guides the design with respect to fatigue life assessment. UTEC Industrial designs, engineers, machines, fabricates, and installs custom material handling systems for aerospace and heavy industry from its Spokane Valley, WA facility, integrating Allen-Bradley PLC and motion control with in-house CNC machining, heat treating, and stress relief. This article sets out what BTH-1 covers and leaves to ASME B30.20, how the classification changed between editions, how a category and a class are chosen and converted into load cycles, and how they reach the welds, bearings, drives, and controls of a powered device. A lifting fixture's classification sits at the start of the build chain, design → engineering → parts machining → fabrication → assembly → weld fatigue → stress relief → drives → controls → tuning → monitoring: the category and class set the static margin and fatigue detailing at design, and the controls and records downstream show whether the device stays inside the cycle count it was designed for.

What does ASME BTH-1 cover, and what does it leave to ASME B30.20?​

ASME's product page says BTH-1 "provides minimum structural, mechanical, and electrical design criteria for ASME B30.20, Below-the-Hook Lifting Devices", and that its provisions "apply to the design or modification of below-the-hook lifting devices". It sets three limits on that scope:

  • Design only. "BTH-1 addresses only design requirements", it "should be used in conjunction with B30.20, which addresses safety requirements", and it "does not replace B30.20". Devices designed to BTH-1 "shall comply with ASME B30.20", which includes provisions on marking, construction, installation, inspection, testing, maintenance, and operation.
  • Minimums. The design criteria "are minimum requirements that may be increased at the discretion of the lifting device manufacturer or a qualified person".
  • Specialized service. "Compliance with requirements and criteria that may be unique to specialized industries and environments is outside the scope of this Standard."

The ANSI Blog's summary adds that BTH-1-2023 "calls for lifting devices to be designed by, or under the direct supervision of, a qualified person", and that because its provisions "address the most common and broadly applicable aspects of the design", "there may be design issues that are not explicitly covered in the standard. A qualified person is responsible for handling any specialized criteria." How bundle extractors, lifting beams, and valve fixtures in one heavy industry are classified as below-the-hook devices is worked through in Handling Exchanger Bundles, Valves, and Pipe Spools (ASME BTH-1-2023, publisher scope; Kelechava 2023).

Which edition applies, and how has the classification changed?​

ASME's product page lists BTH-1-2023 as the latest edition. Its public front matter gives a date of issuance of October 11, 2023, says the standard "will become effective 1 year after the Date of Issuance", and says "the next edition of this Standard is scheduled for publication in 2026". The ANSI Blog says the 2023 edition "updates and supersedes the 2020 version". The classification has changed across editions:

  • As newly published. Duerr's 2008 paper, written when ASME had newly published the standard (its abstract does not name the edition), says it "establishes two design categories and five service classes".
  • 2017. The ANSI Blog's list of 2017 changes ends with "Design Category C Lifters were added", and the same edition added Chapter 6, which the blog calls "Magnet Lifting Design" and the 2023 table of contents titles Lifting Magnet Design.
  • 2020. The illustrations in Table 3-4.4-1, "Fatigue Design Parameters", were revised, and Appendix C gained a note that allowable loads or stresses on connection elements, except bearing stress, express the design factor as 1.20Nd in the static strength design equations of Section 3-3.
  • 2023. ASME lists an "Updated Table 3-4.4-1 Fatigue Design Parameters" and "clarification of requirements to establish the rated load of a lifting device by calculation" among the key changes.

The sources read for this article do not agree on when the standard was first published, so this article states no first-publication year. As engineering reasoning, a device's drawings and data should record the BTH-1 edition it was designed to, since the category set and the fatigue table changed between editions, and the 2026 edition should be checked for before a new design is released (ASME BTH-1-2023, front matter and publisher key changes; Kelechava 2023; Duerr 2008, Abstract).

What is a design category, and how is it chosen?​

Duerr's abstract says the design category "relates to the expected usage of the lifter". The ANSI Blog's list of 2017 changes says "Design Category C Lifters were added", but it does not define the category, and no public source read for this article shows whether the 2023 text keeps it. The 2023 table of contents places Design Category at §2-2 on p. 10, and its commentary appendix lists four load-spectrum tables, Design Category A Static and Dynamic Load Spectrum and Design Category B Static and Dynamic Load Spectrum (Tables C-1.3-1 to C-1.3-4, pp. 61–62); no Category C spectrum table appears in the list.

The 2023 edition defines the two categories its load-spectrum tables cover:

  • Category A. As understood from BTH-1-2023 §2-2, Design Category A applies where the magnitude and variation of loads applied to the device are predictable and the loading and environmental conditions are accurately defined or not severe, with a nominal design factor of 2.00. Category A is restricted to Service Class 0.
  • Category B. As understood from the same clause, Design Category B applies where the magnitude and variation of loads are not predictable, or the loading and environmental conditions are severe or not accurately defined, with a nominal design factor of 3.00.
  • Load spectra. As understood from Appendix C, Tables C-1.3-1 to C-1.3-4 give the static and dynamic load spectra that the Category A and B design factors are based on.

As engineering reasoning, the category question for a buyer is how well the loads and conditions are known, not how heavy the load is: a fixture lifting the same machined casting on every cycle is a different case from a beam used across a plant on loads of uncertain weight and rigging (Duerr 2008, Abstract; Kelechava 2023; ASME BTH-1-2023, §2-2 and Tables C-1.3-1 to C-1.3-4).

What is a service class, and what load-cycle range do the classes cover?​

Duerr's abstract says the then-new standard established five service classes and that "the service class guides the design with respect to fatigue life assessment". The 2023 table of contents places Service Class at §2-3, with Table 2-3-1, "Service Class", on p. 10. As understood from BTH-1-2023 §2-3 and Table 2-3-1, the classes are set by the number of load cycles over the device's service life, from Service Class 0 at up to 20,000 cycles to Service Class 4 at over 2,000,000, with Service Classes 1 to 3 between them; the boundaries between the intermediate classes are given in Table 2-3-1 and are not reproduced here.

As engineering reasoning, a device whose usage estimate sits near a class boundary should be classed on the upper estimate. A one-class error at the top of the scale is the difference between a device designed for 2 million cycles and one in use far beyond that. Katona and co-authors make the same point for cranes: an incorrect service class "can lead to oversized components, but more importantly, it can result in undersized critical elements" (Duerr 2008, Abstract; ASME BTH-1-2023, §2-3 and Table 2-3-1; Katona et al. 2026, §2).

How is a service class converted into cycles per day and years of service?​

BTH-1's commentary on its classification chapter (Appendix B) includes Table B-3-1, "Service Class Life", on p. 59. As understood from BTH-1-2023, Table B-3-1 expresses each service class as combinations of load cycles per day and years of service; its values are not reproduced here.

The conversion itself is arithmetic. Katona and co-authors use the same form for a crane: cycles per day × working days per year × design life in years, which for their workshop crane gives 100 × 260 × 10 = 260,000 working cycles. Applied to three illustrative below-the-hook devices, the arithmetic below is UTEC Industrial's own, and the class column uses only the two end points stated above:

Illustrative deviceUse assumedLoad cycles over lifeClass
Turnover beam for an assembly built a few times a year2 lifts per week, 52 weeks, 30 years3,1200
Spreader on a production bay crane6 lifts per hour, 16 h/day, 250 days, 20 years480,0001 to 3; Table 2-3-1 sets which
Gripper fixture on a continuous process line30 cycles per hour, 24 h/day, 350 days, 15 years3,780,0004

The middle row shows why the usage estimate matters: a seventh lift per hour on the same pattern gives 560,000 cycles, about 17 percent more, and a count that sits near a boundary in Table 2-3-1 can move the device into the next class. A cycle in this arithmetic is one application of the load; whether a given handling motion counts as one cycle or more is set by the standard's own definition, which this article does not restate (ASME BTH-1-2023, Table B-3-1; Katona et al. 2026, §4.2).

When does BTH-1 call for fatigue design, and what does the service class change?​

The 2023 table of contents gives the fatigue provisions their own section and two tables: §3-4, Fatigue Design (p. 20); Table 3-4.3-1, Allowable Stress Ranges, ksi (MPa) (p. 21); and Table 3-4.4-1, Fatigue Design Parameters (p. 23), whose illustrations were revised in 2020 and which ASME lists as updated in 2023. Chapter 4 adds Table 4-7.6.1-1, Fatigue Stress Amplification Factors (p. 42), for shafting.

As understood from BTH-1-2023 §3-4, devices in Service Class 0 need no fatigue analysis, and for Service Classes 1 through 4 the allowable stress range for each stress category in Table 3-4.3-1 falls as the service class rises. As engineering reasoning, this is where the service class reaches the weld detail: the same fillet weld can pass static design at Service Class 0 and fail the fatigue check at Service Class 3, so the class has to be fixed before the detail is drawn.

Why welded details crack at stresses far below yield, and why stress range governs a welded frame, is covered in Fatigue vs. Static Strength: Why Heavy Machines Crack at Low Stress (ASME BTH-1-2023, §3-4, Tables 3-4.3-1, 3-4.4-1 and 4-7.6.1-1; Kelechava 2023).

How do service classes reach bearings, gearing, and shafts on a powered device?​

A rotator, a motorized C-hook, or a powered gripper has machinery inside it, and BTH-1's mechanical chapter covers it. The 2023 table of contents lists §4-4 Drive Systems, §4-5 Gearing (with Table 4-5.3-1, strength factors for calculating load capacity), §4-6 Bearings (with Table 4-6.2-1, L10 Bearing Life, p. 41), and §4-7 Shafting. As understood from BTH-1-2023 §4-6, Table 4-6.2-1 sets the required L10 bearing life by service class; its values are not reproduced here.

The bearing makers define the quantity the table specifies. Timken's engineering manual defines the rating life L10 as the life that 90 percent of a group of apparently identical bearings will complete or exceed before a fatigue spall develops, and, for bearings under radial or combined loading with a dynamic load rating based on one million cycles, gives L10 = (C ÷ Pr)^e × (10⁶ ÷ 60n) hours, with e = 3 for ball bearings and 10/3 for tapered, cylindrical and spherical roller bearings. For a load that varies through the cycle, the manual says selection "is often made on the basis of maximum load and speed. However … a more meaningful analysis may be made by examining the loading cycle to determine the weighted average load", while "it is still necessary to consider extreme loading conditions to evaluate bearing contact stresses and alignment". ISO 281:2007 describes the basic rating life as "the life associated with 90 % reliability, with commonly used high quality material, good manufacturing quality and with conventional operating conditions".

As engineering reasoning, the service class therefore reaches the drive train twice: once as a cycle count on the welded structure and again as an L10 requirement on every bearing (ASME BTH-1-2023, §§4-4 to 4-7 and Table 4-6.2-1; Timken Order No. 10424, pp. 48 and 55; ISO 281:2007, Abstract).

How does a BTH-1 service class compare with a CMAA crane class?​

The two classify different objects on different terms. The CMAA 70 (2020) classes that UFGS-41 22 13.14 states for DoD specifiers are qualitative descriptions in lifts per hour and share of lifts at rated capacity; for Class D, for example, "10 to 20 lifts per hour averaging 4.5 m [15 feet], not over 65 percent of the lifts at rated capacity". BTH-1, as above, classes the device by the total load cycles over its life. DOE-STD-1090-2020 keeps them separate in practice: the operation, inspection, maintenance, and testing of overhead and gantry cranes follow ASME B30.2 and B30.17 in addition to applicable OSHA standards, while below-the-hook devices are designed, constructed, installed, inspected, tested, operated and maintained per ASME B30.20 and BTH-1 (DOE practice, not a universal legal requirement).

As UTEC Industrial's own arithmetic, a spreader that goes on the hook for every lift of a crane averaging 15 lifts per hour, 16 hours a day, 250 days a year sees 60,000 load cycles a year, or 1,200,000 over 20 years, far above the 20,000-cycle limit of Service Class 0 and below the 2,000,000 at which Service Class 4 begins, so Table 2-3-1 places it in one of the intermediate classes. As engineering reasoning, the crane's class letter does not set the device's service class; the device's own cycle count does, and a spreader used on only some of the crane's lifts can sit several classes lower (UFGS-41 22 13.14, Chg 1 2021, §1.3.1.2 note, p. 13; DOE-STD-1090-2020, §6.1 and §12.1; ASME BTH-1-2023, Table 2-3-1).

Does the proof load test come from BTH-1?​

No. BTH-1 is a design standard; ASME's own page says it "addresses only design requirements" and that ASME B30.20 includes the provisions on testing. Federal practice documents point the test to B30.20 as well. NASA-STD-8719.9C §13.3.2.5 says the below-the-hook proof test is "as specified in the applicable ASME standard (B30.20 or B30.26)", and DOE-STD-1090-2020 §2.2.5 says all rigging equipment used in critical lifts (slings, below-the-hook lifting devices and rigging hardware) "shall be proof load tested in accordance with applicable ASME standards". Both are agency practice: NASA's applies to NASA lifting operations and DOE's is a technical standard for DOE sites, not a universal legal requirement.

As engineering reasoning, the design category's design factor and the proof test are two separate checks on the same device: the first is a margin built in at design under BTH-1, and the second is a demonstration before use under B30.20. A device can pass its proof test and still be under-designed for fatigue if its service class was set too low, because a proof test applies the load once (ASME BTH-1-2023, publisher scope; NASA-STD-8719.9C, 2024, §13.3.2.5; DOE-STD-1090-2020, §2.2.5 and §12.1).

What controls and sensing does a powered below-the-hook device need?​

BTH-1 has its own electrical chapter. The 2023 table of contents lists Chapter 5, Electrical Design: §5-2 Electric Motors and Brakes, §5-3 Operator Interface, §5-4 Controllers and Auxiliary Equipment, §5-5 Grounding, §5-6 Power Disconnects, and §5-7 Batteries. Chapter 6 is Lifting Magnet Design, and Chapter 4 carries §4-10 Vacuum Lifting Device Design and §4-11 Fluid Power Systems. The ANSI Blog lists a new Table 4-10.2-1, "Intended Use Type Summary", for 2023, and ASME lists the incorporation of ASME B30.1-2020 into the fluid power requirements. Those clause contents are not stated here.

NASA's lifting standard adds a daily pre-use inspection of below-the-hook devices for cracks, deformations, gouges, galling, kinks, crushed areas, and corrosion, plus a configuration check (NASA practice, §13.4.4).

As engineering practice built on these sources, not a requirement of them, the intelligence layer on a powered device can be built to protect the classification:

  • Cycle counting. The device controller counts each load application, for example from a load pin or a grip-closed and load-on signal pair, so the accumulated count can be compared with the service-class range the device was designed to.
  • Load measurement. A load pin or load cell in the device's load path records the load on each cycle, which shows whether the loads are as predictable as the design category assumed.
  • Interlocks. Grip-confirmed or vacuum-confirmed signals interlocked with the hoist so the crane cannot lift before the device has the load.
  • Records. Cycle and load logs stored with the device's rated-load documentation for the owner's inspections.

UTEC Industrial builds UL 508A control panels with Allen-Bradley ControlLogix and CompactLogix controllers, which is where counting and interlock logic of this kind is implemented (ASME BTH-1-2023, Chapters 4–6; Kelechava 2023; NASA-STD-8719.9C, 2024, §13.4.4).

Who assigns the category and class, and what belongs in the purchase specification?​

Duerr's abstract says successful use of the standard requires that designers, manufacturers, purchasers, and users of lifting devices "all understand the meanings of these design categories and service classes and apply that understanding to the proper specification of lifter design requirements", and that his paper gives guidance to purchasers and users "in specifying the correct design category and service class". DOE-STD-1090-2020 §12.2.2 requires rated load markings on below-the-hook devices at DOE sites and, where security classification or other approved reasons prevent marking, an identification number with the capacity and weight kept in the documentation (DOE practice).

As engineering practice, a purchase specification for a below-the-hook device should state:

  • Use and loads. What is lifted, its weight range and centre of gravity, and how well each is known, which drives the design category.
  • Cycle estimate. Lifts per hour, hours per day, days per year and design life in years, which drive the service class, with the upper estimate stated.
  • Classification. The design category and service class required, and the BTH-1 edition.
  • Environment. Temperature, corrosion, and anything outside general industrial service, since requirements that may be unique to specialized industries and environments are outside BTH-1's scope.
  • Powered functions. Motors, brakes, fluid power, vacuum or magnets, and the controls and interlocks required.
  • Proof test and markings. The B30.20 test, rated-load marking, and documentation.

Without the cycle estimate, the designer has to assume one, and the assumption becomes the device's design life (Duerr 2008, Abstract; DOE-STD-1090-2020, §12.2.2; ASME BTH-1-2023, publisher scope).

Where does a below-the-hook device sit in the design-to-monitoring chain?​

Each link of the build chain acts on the category and class:

  • Design and engineering. As understood from §2-2 and §3-4, the category sets the static design factor and the class sets the fatigue check, so both are fixed before detailing.
  • Machining and fabrication. As engineering reasoning, pin holes, padeyes, and weld toes are where a fatigue-classed device concentrates stress, and their finish and weld profile are set in the shop.
  • Stress relief. What thermal and vibratory relief do to a welded frame before final machining is covered in Stress Relief for Machine Bases and Frames Before Final Machining.
  • Drives and controls. On a powered device, the bearings carry their own service-class requirement (Table 4-6.2-1), and, as engineering practice, the controls count the cycles the class allows.
  • Monitoring. As engineering reasoning, the cycle log and the pre-use inspection are how an owner sees a device approaching the cycle count it was designed for.

UTEC Industrial inspects the welded structures it builds with NDT and CMM inspection and runs factory acceptance testing before shipment (Duerr 2008, Abstract; ASME BTH-1-2023, §§2-2, 2-3 and 3-4; NASA-STD-8719.9C, 2024, §13.4.4).

Related Articles

References​

  • ASME BTH-1-2023: Design of Below-the-Hook Lifting Devices. ASME, 2023.
  • Duerr, D. (2008). "Design Category and Service Class Selection for Below-the-Hook Lifting Devices." Practice Periodical on Structural Design and Construction, 13(2), 43-47.
  • Kelechava, B. ASME BTH-1-2023: Design of Below-the-Hook Lifting Devices. The ANSI Blog, American National Standards Institute, December 5, 2023.
  • Katona, M., Zelić, A., Živanić, D., Đokić, R., Jojić, T., Ilanković, N. (2026). "Comparative overview of determining service classes of industrial cranes according to relevant standards and guidelines." Proceedings of the XII International Triennial Conference Engineering TODAY (ET 2026), A25-A31.
  • Timken Order No. 10424: Timken Engineering Manual. The Timken Company, 2024.
  • ISO 281:2007: Rolling Bearings — Dynamic Load Ratings and Rating Life. International Organization for Standardization, 2007.
  • UFGS-41 22 13.14: Bridge Cranes, Overhead Electric, Top Running. U.S. Army Corps of Engineers / Naval Facilities Engineering Systems Command / Air Force Civil Engineer Center, November 2019 (Change 1, February 2021).
  • DOE-STD-1090-2020: Hoisting and Rigging. U.S. Department of Energy, 2020.
  • NASA-STD-8719.9C: Lifting Standard. National Aeronautics and Space Administration, 2024.

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