Bridge Crane Capacity Selection Guide for Procurement

Why the Heaviest Load Drives the Selection

The starting point for any crane purchase is the maximum payload. This is the weight of the single heaviest object you will lift. It includes the object itself, its packaging, and the lifting gear. You must add the weight of the hook, block, and any attachments to this figure. The result is the rated load for the crane.

Many buyers make the mistake of selecting a crane based on typical daily loads. This approach fails when the occasional heavy item arrives. The crane will not move, or the structure will deform. The specification must state the maximum load clearly. If your process involves forklifts or other equipment, include those weights if they ride on the crane.

Consider a foundry that usually lifts 500 kg moulds. The average load is low, but the weekly delivery of a 2.5 tonne casting changes the selection entirely. A crane sized for 500 kg will have a beam that deflects under 2.5 tonnes. The trolley may jam on the track, and the structure may crack. The design load is not an average. It is a ceiling.

A second factor is the span. A 20-metre span requires a different beam depth and stiffness than a 5-metre span to carry the same tonnage. The longer the span, the more the beam deflects under load. Excessive deflection causes the trolley to stick or the load to swing. Engineers calculate the required beam section based on span and load.

Deflection is often measured in millimetres. If a 20-metre beam sags too far, the load may strike the floor or the surrounding equipment. The operator also feels the lag. The trolley drifts from the target position. This adds seconds to every cycle. For high-speed production, that lag is unacceptable. The beam must be stiff enough to keep the load aligned.

Duty Cycle and Operational Intensity

Duty cycle describes how hard the crane works. It is defined by the number of operating cycles per hour, the average load, and the travel distance per cycle. A crane used in a high-speed assembly line operates differently than one used in a warehouse for infrequent heavy lifts.

High-intensity duty requires stronger components. The trolley wheels, motor, and reducer must handle repeated start and stop cycles. If the duty cycle is high, the crane must be designed for continuous or near-continuous operation. This affects the thermal rating of the motor. An under-rated motor will overheat and shut down during peak production.

In a bottling plant, the crane moves a pallet every two minutes. The motor runs almost constantly. In a machine shop, the crane lifts a 10 tonne shaft once a week. The first crane needs a motor with a high duty class and a larger thermal margin. The second can run on a lower duty class. The price difference reflects this difference in usage.

The average load matters too. A crane rated for 10 tonnes is fine if the average load is 2 tonnes. It is also fine if the average load is 9 tonnes, but the wear on the components will be significantly higher. The manufacturer calculates the life of the gear based on the average load and the duty cycle.

Gearbox life is measured in equivalent operating hours. Running a 10 tonne crane at 9 tonnes average load for 24 hours a day will exhaust the gearbox life faster than running it at 2 tonnes average load for the same duration. The specification should state the average load used for the design calculation. Do not let the vendor assume a low average load if your production plan is dense.

Structural Requirements and Building Integration

The building structure must support the crane. The crane runs on beams, which rest on the building columns or rafters. Adding a heavy crane adds vertical load to the structure. You must verify with a structural engineer that the existing building can handle the added weight.

The crane also adds lateral loads during acceleration and braking. These forces push on the building frame. If the building is older or designed for light loads, you may need to reinforce the columns or add cross-bracing. This work can be expensive and time-consuming.

Floor space for the crane track also matters. The crane needs room for the end car and the trolley travel. If the crane cannot travel the full length of the bay, you must shorten the span or accept a reduced reach. This affects layout planning before the crane is ordered.

Check the column spacing. If the columns are spaced 6 metres apart, the track can run between them. If the columns are spaced 3 metres apart, the track may need to be supported by intermediate beams. This changes the installation cost. The vendor needs the architectural drawings to calculate the track support. Do not send only a floor plan. Send the structural drawings too.

The crane track itself has weight. A 20-metre beam for a 10 tonne crane can weigh several tonnes. The roof structure must support this static load. If the building was designed for a lighter roof, the rafters may need to be upgraded. This is a common discovery during the installation phase. It delays the project.

Performance Criteria for the Trolley and Hoist

Once the tonnage and duty are set, the performance specifications follow. The hoist speed determines how fast the load rises and lowers. For most general lifting, a moderate speed is sufficient. High-speed hoisting is used in repetitive assembly or packaging operations.

The trolley speed is the speed at which the crane moves along the beam. A faster trolley reduces the time between pick and place. However, high speeds require better braking and control. If the load is unstable or the floor is uneven, high speeds increase the risk of swinging.

Acceleration and deceleration are critical for load stability. A crane that stops abruptly will cause the load to swing. Modern controls allow for smooth ramp-up and ramp-down of speed. This reduces swing and improves operator comfort. The specification should state the maximum acceleration in metres per second squared.

In a machine shop, the operator moves a heavy block from the floor to the lathe. The lift height is 3 metres. A hoist speed of 15 metres per minute takes 12 seconds to lift. A speed of 30 metres per minute takes 6 seconds. In a high-mix environment, every second counts. In a low-mix environment, speed is less important than reliability.

The braking system must stop the load at a controlled distance. If the load is 5 tonnes, the kinetic energy is significant. The brake must dissipate that energy without skidding or slipping. The specification should require a braking distance limit. This ensures the load stops where the operator expects.

The Criteria Table for Evaluation

Use this table to compare options and ensure all requirements are met. It forces you to look beyond the headline tonnage and consider the full package.

Criterion	What to look for	Why it matters
Max Payload	Heaviest item plus hook weight	Determines beam strength and motor size
Duty Cycle	Cycles per hour and average load	Dictates component life and thermal rating
Span	Distance between end cars	Affects beam depth and deflection
Hoist Speed	Metres per minute required	Matches production pace of the line
Trolley Speed	Metres per minute required	Reduces cycle time between locations
Control Type	Manual or remote, with features	Impacts operator safety and swing control

Procurement and Documentation Best Practices

Clear documentation prevents disputes. The purchase order or technical specification must list every parameter from the table above. Ambiguity leads to the vendor quoting the cheapest option that technically meets the minimums.

Include the building drawings in the package. The vendor needs to see the column positions and rafter spacing. This allows them to propose the correct track mounting method. If the building is new, coordinate with the architect early. The crane track must be part of the structural design, not an afterthought.

Request a load test plan. After installation, the crane must be tested at rated load and proof load. The test certificate is part of the acceptance criteria. Do not sign off on the project until the test is complete and the documentation is filed.

The load test usually involves two stages. First, the crane lifts a load equal to its rated capacity. This checks the mechanical integrity. Second, the crane lifts a proof load, often 125% of the rated capacity. This checks the safety margin. The test must be witnessed by an independent engineer. The certificate should state the date, the load used, and the result.

A Decision Checklist for Final Selection

Before you sign the order, run through this checklist. It captures the most common failure points in crane procurement.

1. The maximum payload is defined, including hook and rigging gear.
2. The duty cycle is calculated based on actual production schedules.
3. The span and travel distance are confirmed against the floor plan.
4. The building structure is verified to support the crane weight.
5. The hoist and trolley speeds match the required cycle times.
6. The control system meets the safety standards for the site.
7. The vendor has provided a load test plan and acceptance criteria.
8. Lead time and delivery terms are agreed in writing.
9. Maintenance requirements and spare parts list are included.
10. The final price covers installation, testing, and commissioning.

If any item on this list is missing, the specification is incomplete. Do not proceed until every point is answered. The cost of a wrong crane is measured in downtime, not just the purchase price.

Conclusion

Bridge crane capacity selection is a technical process that requires precise data. The tonnage is not just a number. It is the sum of your heaviest load, your duty cycle, and your building constraints. By using the criteria table and the decision checklist, you can standardize your evaluation process.

The goal is a crane that fits the job. It should lift the heaviest item safely, move at the speed your production requires, and fit within your building structure. When the specification is tight, the bidding process becomes a comparison of quality and service, not a guessing game.