How Is the Duty Class of an Overhead Travelling Crane Determined?
How Is the Duty Class of an Overhead Travelling Crane Determined?
Selecting the Correct Crane Duty Class According to Usage Frequency, Load Spectrum and Operating Conditions
When selecting an overhead travelling crane, lifting capacity, crane span and lifting height are among the primary technical criteria. However, another critical factor directly affects the crane’s service life, maintenance requirements, operating performance and investment cost: the crane duty class.
Two overhead travelling cranes with the same lifting capacity should not necessarily be manufactured with the same technical specifications if their operating intensities are different.
For example, a 10-ton crane may be used only a few times per week in a maintenance workshop. Another 10-ton crane may complete hundreds of lifting movements throughout the day in a steel service centre. Although both cranes have the same maximum lifting capacity, the fatigue loads, motor operating times, braking cycles, rope movements and mechanical wear they experience are completely different.
For this reason, crane selection should not be based solely on the following question:
“How many tonnes will the crane lift?”
The following questions must also be answered:
- How many hours will the crane operate per day?
- How many lifting operations will be performed per hour?
- What will the average load weight be?
- Will the crane frequently operate at maximum capacity?
- How often will lifting and travelling movements be repeated?
- Does the facility operate one, two or three shifts?
- Under which environmental conditions will the crane operate?
- How critical would crane downtime be for production?
- Is precise load positioning required?
- Is production capacity expected to increase in the future?
Based in Konya, Eser Vinç analyses not only the lifting capacity but also the actual operating scenario of each overhead travelling crane project. The appropriate duty class is determined by evaluating the technical site survey, operating intensity, load distribution, shift system, environmental conditions and future production targets together.
This comprehensive guide explains what overhead crane duty class means, which data are used to determine it, the problems caused by incorrect classification and how the correct crane class should be selected for different industries.
What Is Crane Duty Class?
Crane duty class is a technical classification describing the operating intensity of a crane, its working time under load, the number of lifting cycles it performs and the total mechanical stress it experiences throughout its service life.
The duty class does not only indicate how much weight the crane carries. It also defines:
- How frequently loads are handled
- How long loads remain suspended
- At what speed loads are moved
- How frequently the crane starts and stops
- How close the handled loads are to maximum capacity
This classification directly affects the selection of the following components:
- Main bridge girders
- Hoisting motor
- Gearbox
- Brake system
- Rope drum
- Steel wire rope
- Hook block
- Travelling motors
- Wheels
- Bearings
- Electrical control panel
- Contactors and variable frequency drives
- Control system
- Power supply system
There are significant differences between a crane operating at low intensity and a crane working under heavy-duty conditions, not only in terms of motor power but also in mechanical durability and component service life.
Are Lifting Capacity and Duty Class the Same?
No.
Lifting capacity defines the maximum load a crane can safely carry. Duty class defines how intensively and under what operating pattern the crane will carry those loads.
For example, two cranes may both have a lifting capacity of 10 tonnes.
The first crane:
- Operates for one hour per day
- Is used only a few times per week
- Usually lifts loads of 2–3 tonnes
- Rarely reaches maximum capacity
The second crane:
- Operates for 16 hours per day
- Performs many lifting operations every hour
- Usually carries loads of 8–10 tonnes
- Works as part of a continuous production line
These two cranes cannot have the same duty classification.
The second crane requires a more durable motor, a stronger braking system, a heavy-duty gearbox, suitable rope reeving and mechanical components designed for a longer operating life.
Why Is Crane Duty Class Important?
Selecting the correct duty class is critical for crane safety and economic service life.
An incorrectly specified duty class may cause:
- Motor overheating
- Premature brake wear
- Gearbox failures
- Reduced rope service life
- Wheel and rail wear
- Frequent electrical faults
- Increased production downtime
- Higher maintenance costs
- Reduced crane performance
- Fatigue damage in the load-bearing structure
Selecting a higher duty class does not automatically mean that the crane is more suitable. An unnecessarily high classification may also increase investment costs.
The objective is to determine the duty class that accurately matches the facility’s real operating requirements.
Which Information Is Used to Determine Crane Duty Class?
Accurate classification requires detailed information about the site and the operating process.
Daily Operating Time
The number of hours the crane is actively used each day is one of the fundamental criteria.
A crane may operate:
- Intermittently
- In a single-shift facility
- In a two-shift facility
- In a three-shift facility
- As part of continuous production
However, the duration of the shift is not the same as the actual motor operating time.
For example, a facility may remain open for eight hours, while the crane may actively move for only one hour in total. Actual hoisting and travelling times must therefore be analysed.
Number of Lifts per Hour
How many times does the crane lift and lower a load within one hour?
Each lifting operation creates an operating cycle for the mechanical and electrical components.
As the number of lifting operations increases, the following also increase:
- Number of motor starts
- Number of brake opening and closing cycles
- Rope movement
- Drum rotation
- Use of contactors and drives
A crane operating only a few times per hour cannot be classified in the same way as a crane performing repeated lifting movements every few minutes.
Average Load Weight
A crane may not always operate at maximum capacity.
For example, a 10-ton crane may usually carry loads of:
- 1 tonne
- 3 tonnes
- 5 tonnes
- 8 tonnes
Duty class calculations should consider not only the maximum load but also the average load distribution expected throughout the crane’s service life.
Frequency of Maximum-Capacity Operation
How often the crane reaches maximum capacity is extremely important.
A crane lifting its maximum load once a month requires a different design from a crane performing multiple maximum-capacity lifts during every shift.
Cranes operating continuously near their rated capacity may require:
- A stronger hoist unit
- A more durable gearbox
- A suitable braking system
- Greater thermal capacity
- Heavy-duty mechanical components
Lifting Distance
How many metres does the hook travel during each cycle?
There is a significant difference in rope movement, drum operation and motor running time between a crane performing short lifts and one operating over a long lifting height.
A 2-metre lifting movement does not produce the same operating cycle as a 12-metre lifting movement.
Bridge and Trolley Travel Distance
A crane does not only move the load vertically.
The load is also transported:
- Along the crane bridge
- Along the factory bay
- Between loading and unloading points
Frequent long-distance travel produces greater operating loads on wheels, motors, gearboxes and the runway system.
Starting and Stopping Frequency
Motors and brakes that start and stop repeatedly are exposed to greater thermal and mechanical stress.
Particularly in short-distance production operations, the crane may:
- Start frequently
- Stop after a short movement
- Reverse direction
- Accelerate again
This type of operation may be more demanding than continuous operation at a constant speed.
Shift Pattern
A facility operating one shift has different crane requirements from a facility running continuously over three shifts.
In three-shift operations, the following factors must be evaluated more carefully:
- Thermal capacity
- Lubrication requirements
- Brake durability
- Motor insulation
- Maintenance accessibility
What Is Load Spectrum?
Load spectrum describes the weight ranges handled by the crane and how frequently each load level occurs throughout its operating life.
Consider a simplified example.
A 10-ton crane may operate as follows:
- 50% of lifts at 2 tonnes
- 30% of lifts at 5 tonnes
- 15% of lifts at 8 tonnes
- 5% of lifts at 10 tonnes
Another 10-ton crane may operate as follows:
- 20% of lifts at 5 tonnes
- 50% of lifts at 8 tonnes
- 30% of lifts at 10 tonnes
Although both cranes have the same rated capacity, the second crane operates under a much heavier load spectrum.
As the load spectrum becomes heavier, the structural and mechanical components are exposed to greater fatigue effects.
What Is a Crane Operating Cycle?
A typical crane operating cycle may include the following movements:
- Lowering the empty hook to the loading point
- Attaching the load
- Lifting the load
- Moving the trolley
- Moving the crane bridge
- Lowering the load at the destination
- Releasing the load
- Returning the hook to the starting point
This entire sequence may be considered one operating cycle.
The number of cycles completed each day determines how frequently the motors, brakes, ropes and travelling systems operate.
In production lines with short cycle times, crane operating intensity may increase rapidly.
Light-Duty Cranes
Light-duty cranes are generally used in applications where lifting operations are infrequent.
Typical applications include:
- Maintenance workshops
- Small manufacturing facilities
- Spare-parts areas
- Machine maintenance departments
- Low-intensity warehouses
- Training and testing areas
In these applications:
- Operating time is limited
- Maximum load is rarely lifted
- The number of cycles per hour is low
- Production does not directly depend on the crane
However, if a light-duty crane is used on a heavy production line, it may show premature wear and failure.
Medium-Duty Cranes
Medium-duty cranes are preferred for regular but non-continuous production operations.
Typical application areas include:
- Machinery manufacturing plants
- Metalworking factories
- Agricultural machinery production
- Steel construction workshops
- General production lines
- Assembly facilities
In these applications, the crane operates regularly throughout the day but does not continuously carry loads close to maximum capacity.
Heavy-Duty Cranes
Heavy-duty overhead travelling cranes are designed for intensive and continuous production conditions.
Common applications include:
- Iron and steel plants
- Foundries
- Steel service centres
- Port and logistics operations
- Heavy machinery manufacturing
- Continuous production lines
- Metal storage areas
- Coil handling facilities
In these applications:
- Daily operating time is long
- Lifting cycles are repeated frequently
- Loads are often close to rated capacity
- Production continuity depends on the crane
- The financial impact of breakdowns is high
Mechanical and electrical components must therefore be selected to provide higher durability.
Very Heavy-Duty and Special Operating Conditions
Some cranes operate under conditions far more demanding than standard production environments.
Examples include:
- Molten metal handling
- High-temperature operation
- Continuous ladle handling
- Scrap handling
- Intensive coil handling
- Hazardous-area operation
- Extremely high lifting-cycle frequency
- Continuous automated production
In these applications, special safety systems and redundancy solutions may be required in addition to the correct duty class.
Depending on the project, the following systems may be used:
- Dual brake systems
- Redundant motors
- Heat-resistant electrical equipment
- Special cable systems
- Load monitoring
- Overload protection
- Automatic control
- Remote monitoring
- Special control-panel cooling
Do Environmental Conditions Affect Duty Classification?
Yes.
Even when two cranes operate at the same intensity, environmental conditions may affect component service life.
The following factors should be considered:
- High temperatures
- Low temperatures
- Heavy dust
- Humidity
- Chemical vapours
- Corrosive environments
- Outdoor operation
- Rain and snow
- Wind
- Risk of explosive atmospheres
In dusty facilities, motor cooling and electrical-panel protection become more critical.
For cranes operating at high temperatures, motor and brake thermal capacities must be selected carefully.
Outdoor cranes require weather-resistant equipment and effective corrosion protection.
Effect of Duty Class on Single-Girder and Double-Girder Selection
The required duty class may also influence the choice between a single-girder and double-girder crane.
Single-Girder Overhead Travelling Crane
Single-girder systems are generally preferred for:
- Light and medium loads
- Medium spans
- Low and medium operating intensity
- Cost-effective projects
However, with suitable engineering and component selection, they can also be designed for different operating conditions.
Double-Girder Overhead Travelling Crane
Double-girder cranes generally provide advantages for:
- High lifting capacities
- Long spans
- High lifting heights
- Heavy-duty operating conditions
- Special lifting attachments
As operating intensity increases, a double-girder structure may be more suitable in terms of maintenance access, structural strength and trolley design.
However, the final decision should not be based only on duty class. Lifting capacity, span, lifting height and building structure must also be evaluated.
Motor Selection According to Duty Class
A motor is not selected solely according to the weight of the load.
The following criteria are important when selecting the hoisting motor:
- Maximum lifting load
- Lifting speed
- Frequency of motor starts
- Daily operating time
- Number of operating cycles
- Ambient temperature
- Braking frequency
- Speed-control system
A motor exposed to frequent starting and stopping may experience greater stress than one operating continuously at a constant speed.
Insufficient motor thermal capacity may cause:
- Overheating
- Insulation damage
- Frequent activation of protective systems
- Production interruptions
How Is the Brake System Selected According to Duty Class?
The brake is one of the most critical components for crane safety.
In cranes operating intensively, the brake:
- Opens and closes more frequently
- Generates more heat
- Wears more quickly
- Requires more maintenance
Under heavy-duty conditions, brake torque, lining design, switching life and thermal resistance must be selected appropriately.
A secondary or redundant braking system may also be used in special applications.
Rope and Drum Selection According to Operating Intensity
The steel wire rope moves during every lifting cycle, winds onto the drum and passes over sheaves.
As operating intensity increases, the following also increase:
- Number of rope bending cycles
- Wire fatigue
- Surface wear
- Lubrication requirements
The rope diameter, rope construction, drum diameter and sheave dimensions must therefore be selected according to operating conditions.
An inadequate rope system may create premature deformation and safety risks.
Importance of Duty Class for Wheel and Rail Systems
Bridge and trolley wheels are continuously exposed to mechanical loads during crane travel.
Under intensive operating conditions:
- Wheel surfaces may wear
- Irregular rail wear may occur
- Bearings may be damaged
- Flange contact may increase
- Travel motors may be overloaded
The correct wheel diameter, material hardness, rail type and travelling system must be selected according to the crane duty class.
Rail alignment errors may also develop into serious failures much more quickly under heavy-duty conditions.
How Do Variable Frequency Drives Affect Crane Performance?
Variable frequency drive systems allow crane movements to accelerate and decelerate in a controlled manner.
Their main advantages include:
- Smooth starting
- Controlled stopping
- Reduced load sway
- More precise positioning
- Reduced mechanical shock
- Lower brake wear
- Improved operational safety
Particularly in high-intensity applications, frequency control may reduce mechanical stress on the crane.
However, the drive, motor and braking system must be selected together according to the project requirements.
Consequences of Selecting the Wrong Duty Class
Selecting a Duty Class Lower Than Required
Using a low-duty crane in intensive production may cause:
- Frequent motor failures
- Brake wear
- Gearbox damage
- Reduced rope life
- Electrical-panel failures
- Production downtime
- Higher maintenance costs
- Safety risks
Selecting a Duty Class Higher Than Required
Choosing a much heavier-duty crane than necessary may:
- Increase the initial investment cost
- Increase crane self-weight
- Increase loads transferred to the building
- Require a stronger building structure
- Create unnecessary equipment costs
The objective is not to select the cheapest crane or the highest duty class. It is to determine the correct system for the facility’s operating conditions.
Should Future Production Growth Be Considered?
Yes.
If the crane project is designed only for current production conditions, the system may become insufficient after production capacity increases.
The following questions should be evaluated:
- Will another shift be added?
- Will production capacity increase?
- Will heavier products be manufactured?
- Will new machinery be installed?
- Will the number of lifting operations increase?
- Will the crane be integrated with automation?
Expected future growth should be considered with a reasonable engineering margin when selecting the duty class.
However, an unnecessarily expensive system should not be designed based on unrealistic or uncertain growth scenarios.
Can the Duty Class of an Existing Crane Be Increased?
Modernisation may be possible in some cases.
When crane operating intensity increases, the following components may be renewed:
- Hoisting motor
- Gearbox
- Brake system
- Rope and drum
- Travelling motors
- Electrical panel
- Variable frequency drives
- Control system
- Wheels
- Bearings
- Power supply system
However, increasing motor power alone does not automatically increase the crane’s duty class.
The following must also be inspected:
- Main bridge girders
- Trolley frame
- Connections
- Wheel loads
- Building support structure
A comprehensive engineering analysis is required before modernisation.
Duty Class Evaluation by Industry
Machinery Manufacturing
Loads may vary significantly in machinery production. Precise movement and controlled speed become important during assembly operations.
Agricultural Machinery Manufacturing
Facilities handling frames, bodies and large machine components may experience medium or intensive operating conditions.
Steel Construction
Profiles, plates and welded assemblies are handled. Crane usage frequency varies according to production capacity.
Iron and Steel Plants
High temperatures, intensive cycles and a heavy load spectrum may require heavy-duty crane solutions.
Foundries
Ladle and mould handling may require special safety systems and heavy-duty design.
Marble and Natural Stone
Loads may be heavy and dense. Precise movement, braking safety and appropriate lifting attachments are important.
Logistics and Warehousing
The number of crane cycles may increase rapidly when loading and unloading operations are frequent.
Maintenance Workshops
Cranes are generally used intermittently, but critical and heavy components may be lifted during maintenance operations.
Correct Duty Class Analysis with Eser Vinç
An overhead travelling crane should not be selected solely according to catalogue values or lifting capacity.
Every facility has different:
- Production processes
- Shift systems
- Load distribution
- Number of lifts
- Building structure
- Environmental conditions
- Future production targets
Eser Vinç analyses the facility’s actual operating conditions before the project and determines the appropriate crane duty class.
Project services include:
- On-site technical survey
- Operational analysis
- Load spectrum evaluation
- Daily operating-time calculation
- Lifting-cycle analysis
- Capacity and span calculation
- Duty class determination
- Single-girder and double-girder selection
- Motor and gearbox selection
- Brake-system design
- Rope and drum calculation
- Electrical and automation engineering
- Overhead travelling crane manufacturing
- Installation and commissioning
- Operator training
- Periodic maintenance
- Modernisation and technical service
With its manufacturing capability in Konya, Eser Vinç develops overhead travelling crane solutions tailored to different capacities and operating intensities throughout Türkiye and international markets.
Frequently Asked Questions
What is crane duty class?
It is a technical classification defining the crane’s usage frequency, load distribution, operating cycles and overall mechanical stress.
Is duty class the same as lifting capacity?
No. Lifting capacity defines the maximum load, while duty class defines how intensively the crane will be used.
Can a crane operating only one hour per day be heavy-duty?
It depends on the load spectrum, number of lifts and operating conditions. Daily operating time alone is not sufficient.
Does the duty class change if maximum load is rarely lifted?
Yes. The frequency of maximum-capacity lifting affects the load spectrum calculation.
Which crane class should be used in a three-shift factory?
A single class cannot be specified without analysing actual motor operating time, lifting frequency, load weights and the operating process.
Does duty class affect crane price?
Yes. It directly affects the selection of motors, gearboxes, brakes, ropes, wheels and electrical systems.
Is a heavy-duty crane safer?
A properly designed heavy-duty crane is more suitable for intensive operating conditions, but it is not necessary for every application.
Can the duty class of an existing crane be increased?
Some mechanical and electrical components may be modernised after a technical inspection. The load-bearing structure must also be evaluated.
Can variable frequency control extend crane service life?
Smooth acceleration and deceleration may reduce mechanical shock. Correct equipment selection and regular maintenance are still necessary.
Which information is required for a quotation?
The following information should be provided for an initial evaluation:
- Maximum lifting capacity
- Average handled load
- Daily operating time
- Number of lifts per hour
- Shift pattern
- Lifting height
- Crane span
- Load travel distance
- Environmental conditions
- Factory location
Conclusion
The duty class of an overhead travelling crane is one of the primary engineering criteria determining crane safety, durability, maintenance requirements and total operating cost.
Cranes with the same lifting capacity may be exposed to completely different operating loads under different production conditions. Lifting capacity alone is therefore not sufficient for correct crane selection.
The following factors must be evaluated together when determining duty class:
- Daily operating time
- Number of lifting cycles
- Average load weight
- Frequency of maximum-load operation
- Lifting and travelling distances
- Starting and stopping frequency
- Shift pattern
- Environmental conditions
- Future production targets
A duty class lower than required may cause premature wear and production downtime. A class higher than required may increase the investment cost and the loads imposed on the building.
Eser Vinç manages overhead travelling crane, gantry crane and overhead crane projects from technical site survey and engineering design to manufacturing, installation, commissioning, maintenance and modernisation. By determining the duty class according to actual operating conditions, the company develops safe, efficient and long-lasting crane systems.
Overhead Travelling Crane Solutions Tailored to Your Facility by Eser Vinç
For new overhead travelling crane investments, duty-class analysis of existing cranes, heavy-duty crane design or modernisation projects, companies can obtain technical support from the specialist engineering team at Eser Vinç.