How to Choose an Overhead Crane: Capacity, Span & Duty Class

Selecting an overhead crane is not simply a matter of choosing a lifting capacity from a catalogue. Two cranes rated for the same maximum load can have very different structures, hoists, motors, brakes and expected service lives.

A reliable overhead crane specification must account for the actual load spectrum, number of operating cycles, span, lifting height, travel distance, environment and required positioning accuracy. This guide explains how these factors work together and what information a crane manufacturer needs before proposing a safe, economical solution.

1. Determine the Required Lifting Capacity

The crane’s rated capacity must cover the total suspended load—not just the nominal weight of the workpiece. Include every item carried below the hook:

• The heaviest workpiece or machine component
• Slings, shackles and lifting chains
• Spreader beams, grabs, magnets or other below-the-hook devices
• Any fixtures that remain attached during lifting

For example, a 9,200 kg component lifted with a 500 kg spreader beam and 300 kg of rigging produces a total suspended load of 10,000 kg. Specifying the crane solely from the workpiece weight would understate the actual load.

Important: Do not add an arbitrary percentage to compensate for uncertain load data. Establish the real maximum suspended load and let a qualified crane engineer determine the appropriate design margin.

The U.S. Occupational Safety and Health Administration defines rated load as the maximum load for which the crane or hoist is designed and built, as shown on its nameplate. OSHA also states that a crane must not be loaded beyond its rated load except during an authorized test. See OSHA 29 CFR 1910.179.

If the load is occasionally much heavier than normal, provide both figures in the request for quotation. The maximum load determines capacity, while the distribution of light, medium and heavy lifts helps determine the duty classification.

2. Measure the Span and Building Geometry

Crane span is normally the horizontal center-to-center distance between the runway rails. It is not simply the clear width of the workshop.

The following dimensions should be measured or confirmed from structural drawings:

• Runway rail center-to-center span
• Total runway length
• Distance from floor to rail top
• Distance from rail top to the lowest roof obstruction
• Required hook approach at both sides
• Columns, ducts, lighting, pipes and other obstructions
• Available installation and maintenance access

Longer spans increase bending demand, girder self-weight and wheel loads. A longer bridge may therefore require a deeper single girder or a double-girder structure even when lifting capacity remains unchanged.

Building suitability must also be checked. Existing runway beams, columns and foundations may need verification by a qualified structural engineer. Increasing crane capacity without checking the supporting structure is not an acceptable upgrade strategy.

As one jurisdiction-specific example, OSHA references minimum clearances of 3 inches overhead and 2 inches laterally between a crane and obstructions. Local regulations and the project design standard must always take precedence.

3. Calculate Lifting Height and Headroom

Lifting height is the vertical distance the hook must travel between its lowest and highest working positions. Headroom is the space consumed by the crane and hoist above the highest hook position.

Before selecting a hoist, define:

• Lowest required hook position
• Highest required hook position
• Floor-to-runway height
• Roof and service-clearance restrictions
• Depth of pits, machines or process vessels

A low-headroom wire rope hoist or optimized European-style crane can provide additional hook height in an existing building. In some facilities, recovering even a modest amount of vertical space can avoid the far greater cost of raising a roof or modifying the supporting structure.

4. Select the Correct Crane Duty Class

Capacity tells you the heaviest permitted load. Duty class describes how intensively the crane and its mechanisms are expected to work.

ISO 4301-1:2016 classifies cranes and mechanisms primarily by the total number of working cycles during the specified design life and the load spectrum. This is why two 10-ton cranes may require different motors, gearboxes, brakes, ropes and structures.

Information Needed for Duty Classification

• Lifts per hour
• Operating hours per shift
• Shifts per day
• Operating days per year
• Expected service life
• Percentage of lifts at different load levels
• Average lifting distance
• Required lifting and travel speeds

Simple Cycle Estimate

A preliminary lifetime cycle estimate can be calculated as:

lifts per hour × operating hours per day × operating days per year × design life

For a crane making 8 lifts per hour, operating 8 hours per day, 250 days per year for 15 years:

8 × 8 × 250 × 15 = 240,000 lifting cycles

This is only an initial estimate. The engineer must still consider how heavy the loads are, how far they travel and how each mechanism operates during a complete work cycle.

Typical CMAA Service Classes

Class	General service level	Typical application
UN	Standby or infrequent	Powerhouses, equipment installation and long idle periods
B	Light service	Repair shops and light warehousing
C	Moderate service	Machine shops, assembly and general manufacturing
D	Heavy service	Heavy fabrication, foundries, steel storage and production handling
E	Severe service	Scrap yards, cement mills and high-frequency process operations
F	Continuous severe service	Specialized cranes operating continuously near rated capacity

The categories above are a practical overview, not a substitute for an engineering calculation. Detailed requirements are contained in CMAA Specification No. 70. A useful industry explanation is also available in Demag’s crane and hoist duty classification overview.

5. Choose the Crane Configuration

Pont roulant monopoutre

A single-girder crane is often suitable for light-to-medium capacity, moderate spans and applications where lower self-weight and project cost are priorities. The hoist commonly travels beneath the bridge girder.

Double-Girder Overhead Crane

A double-girder crane is commonly considered when the project involves high capacity, long span, demanding duty, high lifting height or specialized lifting attachments. A top-running trolley can provide improved hook height and easier access to service platforms.

For a detailed comparison, read KRC’s single-girder vs double-girder overhead crane guide.

Grue suspendue

An under-running or suspension crane travels beneath the runway structure. It can be useful where the building layout supports suspended runways or where floor columns would interfere with production.

Gantry or Semi-Gantry Crane

A gantry crane carries its bridge on legs rather than relying entirely on an elevated building runway. It is frequently selected for outdoor yards, construction areas and buildings that cannot support conventional runway loads.

See KRC’s European gantry and semi-gantry crane solutions.

Système de grue légère KBK

For repetitive workstation handling and lighter loads, a modular crane system may be more efficient than a conventional bridge crane. KRC’s Systèmes de grues légères KBK are intended for assembly lines, production cells and workstation logistics.

6. Define the Operating Environment

Environmental conditions can change the required motors, electrical enclosures, brakes, coatings, ropes and controls. Include the following information in the specification:

• Indoor or outdoor operation
• Minimum and maximum ambient temperature
• Humidity, rain, snow, wind and solar exposure
• Dust, steam, oil mist or corrosive chemicals
• Hazardous gas or combustible-dust zones
• High-temperature materials or molten metal
• Cleanroom, food-processing or hygiene requirements
• Altitude and available power supply

Explosion-protected equipment must be selected from the actual hazardous-area classification, gas or dust group and temperature class. “Explosion-proof” should never be treated as a generic product option.

For hazardous applications, see KRC’s ATEX, IECEx and GB certification comparison.

7. Worked Overhead Crane Selection Example

Consider a fabrication workshop with these preliminary requirements:

• Maximum workpiece: 8.5 tons
• Rigging and lifting beam: 1.0 ton
• Total maximum suspended load: 9.5 tons
• Runway span: 18 meters
• Required lifting height: 8 meters
• 6–10 lifts per hour
• One 8-hour shift, 250 days per year
• Most loads between 3 and 6 tons
• Occasional lifts close to maximum capacity
• Indoor, non-hazardous environment

A 10-ton crane may appear suitable from capacity alone. However, the final selection must also consider the lifetime cycle count, load spectrum, required speeds, building runway capacity and available headroom.

Depending on those calculations, the engineering proposal might use a single-girder crane for an economical moderate-duty solution or a double-girder arrangement where hook height, service access or future process demands justify it. The correct answer comes from the complete operating profile—not from capacity alone.

8. Overhead Crane RFQ Checklist

Providing complete information at the quotation stage reduces redesign work and produces a more accurate proposal.

• Maximum total suspended load
• Typical load and load distribution
• Span and runway length
• Required lifting height
• Floor-to-rail and rail-to-roof dimensions
• Lifts per hour and shifts per day
• Desired design life
• Lifting, trolley and bridge speeds
• Indoor or outdoor conditions
• Temperature, dust, corrosion and hazardous-area details
• Power supply
• Pendant, radio remote, cabin or automated control
• Required attachments and below-the-hook devices
• Applicable national or project standards
• Building and runway drawings

Safety and Compliance Considerations

Crane requirements vary by jurisdiction and application. In the United States, ASME B30.2 covers the construction, installation, operation, inspection and maintenance of specified top-running overhead and gantry cranes.

OSHA requires rated loads to be visibly marked and states that test loads should not exceed 125% of rated load unless otherwise recommended by the manufacturer. These provisions should not be interpreted as permission to operate routinely above rated capacity.

Final design, installation, testing and commissioning should be completed according to the applicable local regulations, project specifications and manufacturer instructions.

Conclusion

Effective overhead crane selection requires five connected decisions:

1. Establish the true maximum suspended load.
2. Measure span, lifting height and building clearances.
3. Calculate the load spectrum and expected operating cycles.
4. Select a crane and hoist configuration suited to the duty.
5. Specify environmental, control and compliance requirements.

A crane that is oversized without reason increases structural and project costs. A crane that is underspecified may experience excessive wear, downtime and safety risk. The most economical solution is the one correctly matched to the real operating profile.

Frequently Asked Questions

How much crane capacity should I choose?

Capacity must cover the heaviest workpiece plus all rigging, lifting beams and attachments. Do not base the decision on workpiece weight alone.

Does a higher-capacity crane automatically have a higher duty class?

No. Capacity and duty class describe different requirements. Capacity concerns maximum load, while duty class considers operating cycles, load spectrum and frequency of use.

When should I choose a double-girder crane?

Double-girder cranes are commonly evaluated for higher capacities, longer spans, demanding duty, improved hook height, maintenance access or specialized attachments. Final selection requires project-specific engineering.

What information is needed for an overhead crane quotation?

Provide maximum and typical loads, span, lifting height, runway length, operating frequency, environment, speeds, controls, power supply and available building drawings.

Can an existing crane be upgraded to a higher capacity?

Only after the crane, runway, supporting structure and relevant mechanisms have been evaluated by the manufacturer or a qualified engineer. The modified crane must then be tested and correctly marked under the applicable requirements.