Working cycles are one of the most important factors in selecting a double girder bridge crane because they determine how frequently the crane starts, lifts, travels, stops, and repeats these operations during its service life. A crane lifting 30 tons once a day has a very different operating requirement from a crane lifting 30 tons every few minutes across several shifts.
Working cycles affect the crane's duty class, load spectrum, motor capacity, brake selection, gearbox loading, electrical equipment, thermal performance, fatigue life, maintenance intervals, and overall structural design. This means lifting capacity alone cannot determine whether a double girder bridge crane is suitable for an application.
A practical selection should define not only the maximum load but also cycles per hour, operating hours per shift, shifts per day, percentage of rated-load lifts, trolley travel distance, crane travel distance, and expected service life. These operating characteristics allow the crane to be matched to the actual workload rather than simply selecting a crane with a higher nominal capacity.
What Is a Working Cycle in a Bridge Crane?
A working cycle is one complete sequence of crane operations required to handle a load.
Depending on the application, a cycle may include:
Pick up → hoist → trolley travel → crane travel → position → lower → release → return
For example, in a steel fabrication workshop, one cycle might involve:
- Hooking onto a 20-ton steel assembly
- Hoisting it 6 m
- Moving the trolley 12 m
- Traveling the bridge 30 m
- Lowering the load
- Releasing the lifting attachment
- Returning the trolley and bridge to the next pickup location
The exact definition of a cycle can vary between applications, so procurement specifications should clearly describe what constitutes one complete handling cycle.
Why Do Working Cycles Matter More Than Maximum Capacity Alone?
Consider two double girder bridge cranes, both rated at 50 tons.
Crane A handles a 50-ton load approximately five times per day.
Crane B handles loads every five minutes during two 8-hour shifts.
Although both cranes have the same rated lifting capacity, Crane B experiences substantially more:
- Hoisting cycles
- Motor starts
- Brake applications
- Trolley acceleration and braking
- Bridge travel
- Gearbox operation
- Structural stress cycles
- Electrical switching
- Thermal loading
The second crane therefore requires a design appropriate for significantly more intensive service.
This is why a crane's capacity and working intensity must be evaluated together.
How Do Working Cycles Affect Duty Class?
Working cycles are closely related to crane duty classification.
Standards and classification systems such as FEM and ISO consider factors including operating frequency and load spectrum when determining the appropriate service class.
The basic concept is straightforward:
More cycles + heavier average loads = higher operating duty
However, cycle count alone does not determine the duty class.
A crane that performs 100 cycles per day at 20% of rated capacity is not necessarily equivalent to a crane performing 100 cycles per day at 90% of rated capacity.
The load spectrum matters because high loads create greater mechanical and structural stresses.
Therefore, crane selection should consider both:
Number of working cycles + distribution of loads within those cycles
How Do You Calculate Daily Working Cycles?
A simple estimate can be made from the operating schedule.
For example, assume a crane operates:
- 8 hours per shift
- 2 shifts per day
- 20 minutes of active crane operation per hour
- 4 minutes per complete handling cycle
The estimated number of cycles per day would be:
8 × 2 × 60 ÷ 4 × (20 ÷ 60)
This gives approximately 80 cycles per day under these simplified assumptions.
A more useful calculation uses actual observed cycle time.
If a crane completes one complete handling sequence every 5 minutes during 12 hours of active operation:
12 × 60 ÷ 5 = 144 cycles/day
If the facility operates 300 days per year:
144 × 300 = 43,200 cycles/year
Over a 10-year operating period, the simplified total would reach approximately:
432,000 cycles
This calculation illustrates why a crane that appears lightly loaded on a daily basis can accumulate a substantial number of operating cycles over its service life.
How Do Working Cycles Affect Hoisting Equipment?
The hoisting mechanism is directly affected by cycle frequency.
Each cycle can involve:
- Motor starting
- Acceleration
- Steady lifting
- Deceleration
- Braking
- Lowering
- Reversal
Frequent starts and stops increase thermal and mechanical loading.
The hoisting motor must therefore be selected based not only on the maximum load but also on the expected starting frequency and duty cycle.
For example, a 40 ton bridge crane used occasionally may have much lower motor thermal requirements than a 40-ton crane repeatedly lifting 30–40 tons throughout a production shift.
Motor selection should therefore consider:
- Rated power
- Starting frequency
- Duty rating
- Hoisting speed
- Load spectrum
- Ambient temperature
- Cooling method
- Required acceleration and deceleration
Working Cycles Also Affect Brake Selection
Brakes are repeatedly applied whenever the load is stopped or held.
A high-cycle crane may perform thousands of braking operations over its operating life.
Brake selection should therefore account for:
- Maximum lifted load
- Hoisting speed
- Stopping frequency
- Brake torque
- Thermal capacity
- Emergency stopping requirements
- Fail-safe requirements
- Maintenance accessibility
For a production crane with frequent positioning, brake wear can become an important maintenance factor.
Selecting a brake based only on maximum load without considering stopping frequency can underestimate the actual service requirement.
What About Trolley and Bridge Travel?
Working cycles affect more than the hoist.
Every handling cycle may require the trolley and bridge to accelerate, travel, decelerate, and stop.
For example, a production crane could perform:
- 100 hoisting cycles/day
- 100 trolley movements/day
- 100 bridge movements/day
But if the process requires multiple repositioning movements per load, the number of motor starts and travel operations may be much higher.
This matters because repeated acceleration and braking affect:
- Travel motors
- Gearboxes
- Wheels
- Bearings
- Couplings
- Rails
- End carriages
- Electrical drives
A crane with high travel frequency should therefore be evaluated based on movement frequency, not just lifting cycles.
- How Do Working Cycles Affect Crane Motors?
Motor duty is closely associated with the ratio between operating time and rest time.
A motor that runs continuously has a different thermal condition from one that repeatedly starts, stops, and restarts.
During operation, electrical energy is converted into mechanical work and heat. Frequent starting can temporarily produce higher current and additional thermal stress.
For high-cycle bridge cranes, engineers may consider:
- Motor duty class
- Starts per hour
- Running time
- Acceleration time
- Deceleration time
- Ambient temperature
- Cooling conditions
- Variable-frequency drive operation
This is particularly important in applications where positioning accuracy requires frequent acceleration and deceleration.
- How Do Working Cycles Affect Structural Fatigue?
Working cycles also influence the structural components of a double girder bridge crane.
The crane bridge repeatedly experiences changing stresses as loads are:
- Picked up
- Accelerated
- Moved
- Lowered
- Released
A crane operating for many years can accumulate a large number of stress cycles.
Structural fatigue considerations can involve:
- Main girders
- End carriages
- Trolley frame
- Connections
- Welded joints
- Wheel assemblies
- Runway interfaces
This is one reason duty classification should not be treated as a label added after the crane has been selected. It is part of the structural design process.
- Working Cycles and Load Spectrum Must Be Considered Together
Suppose a 50-ton double girder bridge crane has the following daily workload:
- 20 cycles at 50 tons
- 50 cycles at 30 tons
- 80 cycles at 10 tons
The crane does not experience the same stress on every cycle.
The load spectrum describes how frequently different portions of the rated capacity are used.
This information is more useful than saying simply:
“The crane performs 150 cycles per day.”
A crane with 150 mostly light-load cycles may have a different service requirement from one with 150 cycles dominated by near-rated loads.
For procurement, the buyer should provide an estimated distribution of typical load levels.
How Do Working Cycles Affect Crane Selection in Different Industries?
Steel Fabrication
A steel fabrication shop may use a crane throughout the production process to move plates, beams, assemblies, and finished structures.
The crane may perform many short-distance movements with varying loads.
Important selection factors include:
- Frequent starts and stops
- Variable load spectrum
- Positioning accuracy
- Trolley travel frequency
- Multiple shifts
- Precast Concrete Production
A precast plant may repeatedly move molds, reinforcement cages, concrete components, and finished products.
The loads may be relatively predictable, but repeated handling can create a high number of cycles.
The crane should therefore be selected around the actual production rate.
Machinery Manufacturing
A machining or assembly plant may require accurate load positioning rather than extremely high cycle rates.
In this case, cycle frequency and positioning requirements should be considered together.
Steel Mill or Heavy Industrial Plant
Heavy industrial environments can combine:
- High loads
- High temperatures
- Frequent operation
- Long operating hours
- Multiple shifts
The crane may require a higher-duty design with components selected for both load severity and operating frequency.
- What Information Should You Give a Crane Manufacturer?
Instead of providing only:
“We need a 50-ton double girder bridge crane.”
A more useful specification would include:
- Rated capacity: 50 t
- Span: 20 m
- Lifting height: 10 m
- Average load: 25–30 t
- Maximum load: 50 t
- Cycles per hour: 10
- Operating hours: 16 h/day
- Shifts: 2
- Working days: 300/year
- Hoisting distance per cycle: 6 m
- Trolley travel per cycle: 15 m
- Bridge travel per cycle: 40 m
- Outdoor or indoor environment
- Required service life
This information gives the crane designer a much clearer basis for selecting the duty class and mechanical components.
A Practical Working-Cycle Selection Example
Consider two applications for a 30-ton double girder bridge crane.
Application A
The crane operates in a maintenance workshop.
- 10 cycles/day
- 250 operating days/year
- Average load: 10 t
- Maximum load: 30 t
- One shift/day
Annual cycles:
10 × 250 = 2,500 cycles/year
Application B
The crane operates in a production facility.
- 80 cycles/hour
- 16 operating hours/day
- 300 operating days/year
Annual cycles:
80 × 16 × 300 = 384,000 cycles/year
Both cranes have a 30-ton rated capacity, but their operating intensity is dramatically different.
Using the same mechanical configuration for both without evaluating duty requirements could result in either unnecessary overspecification or insufficient service capability.
What Happens If Working Cycles Are Underestimated?
Underestimating working cycles can affect the crane over its service life.
Potential consequences include:
- Excessive brake wear
- Motor overheating
- Premature gearbox wear
- Increased wheel and rail wear
- Higher maintenance frequency
- Structural fatigue accumulation
- Reduced component service life
- Unexpected production downtime
The problem may not appear immediately after commissioning. A crane can operate normally for months or years before accumulated wear becomes significant.
That is why expected service life and operating cycles should be considered during initial selection.
What Happens If Working Cycles Are Overestimated?
Overestimating the workload can also have economic consequences.
If a crane is designed for substantially higher duty than the application requires, it may involve:
- Larger motors
- Heavier structural components
- Higher equipment cost
- Higher electrical consumption
- Larger runway loads
- Higher foundation requirements
The objective is therefore not simply to choose the highest available duty class.
The objective is to match the crane's duty and components to the actual load spectrum and operating cycle.
Working Cycle Checklist for Double Girder Bridge Crane Selection
Before finalizing a crane specification, determine:
- Maximum lifting capacity
- Typical lifted load
- Minimum lifted load
- Cycles per hour
- Operating hours per shift
- Number of shifts per day
- Operating days per year
- Expected service life
- Hoisting distance per cycle
- Trolley travel distance
- Bridge travel distance
- Number of starts and stops
- Percentage of lifts near rated capacity
- Required lifting and travel speeds
- Indoor or outdoor operating conditions
These parameters provide a much more accurate picture of the required crane duty than rated capacity alone.
Final Takeaway
Working cycles affect double girder bridge crane selection because they determine how frequently the crane's structure, hoist, motors, brakes, gearboxes, wheels, electrical drives, and runway system are subjected to repeated loading and movement.
The key relationship is:
Rated capacity + load spectrum + working cycles + operating hours → duty class → component selection → service life and maintenance requirements
For this reason, a 50-ton crane operating a few times per day should not automatically be specified in the same way as a 50-ton crane operating continuously across multiple shifts.
When selecting a double girder bridge crane, provide the manufacturer with both the maximum load and the actual operating pattern. Cycles per hour, average load, load distribution, travel distances, shifts, and expected service life are essential engineering inputs for selecting an appropriate crane configuration and duty classification.
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