How to Calculate Mechanical Load and Tension for Reeling Cable

Reeling cables form the basic infrastructure of dynamic power supply systems in industries. The machinery, which operates continuously on the go, includes portal cranes, container handlers, mining machinery, and material conveyors. Contrary to static cables, which are safely placed within static conduits, reeling cables are always in the process of being reeled out, reeled in, and operated in constant motion.

The fact that the cables are in constant motion means they are continually subjected to various external forces. Failure to accurately determine the mechanical loads of these cables during installation results in excessive cable tension, core destruction, outer covering tearage, and even downtime in operations. When selecting a reeling cable for your machinery, mechanical load evaluation is important.

Large blue reeling cable drum mounted on a yellow industrial portal crane.

What Factors Affect Mechanical Load on a Reeling Cable?

Tension calculation involves analysis of certain operating parameters. Mechanical loading of the reeling cable is primarily influenced by three core input parameters: mass of the cable, motion of the equipment, and physical configuration.

1. Cable Weight and Total Travel Distance

The easiest but at the same time the most important factor is the weight of the cable itself. Each additional meter of its length increases the overall weight and thus the force needed to pull, reel, and hold it.

For instance, a 100-meter installation using the light cable weighing 3 kg/m will have a base weight of 300 kg. In the case of the use of heavy cable weighing 8 kg/m, the base weight will be equal to 800 kg. The difference lies in the influence on the loading of motor drives, guide rollers, and internal conductors.

Two basic factors should be determined at the very beginning:

Unit weight (W): Weight of the cable per unit length, usually in kg/m or N/m.
Length of travel (L): Overall active length that the cable covers during its installation.

2. Equipment Movement and Operating Conditions

The operating speed influences the tension. If an industrial machine does not move or is moving at a very low speed, the load on the cable is static in nature. This static condition is composed mostly of the mass of the cable and surface friction.

In reality, any equipment is continuously being started, stopped, reversed, and accelerated. This gives rise to the dynamic load.

When a gantry crane suddenly accelerates, the inertia of the motion raises the tensile load on the cable. In fast-moving cases, such as cranes moving more than 200 m per minute, the dynamic loads in acceleration can exceed even the static mass of the cable.

3. Pulling Direction and Installation Layout

The route that the cable follows will determine the way that gravity and friction affect its mass:

If the movement is horizontal: Surface friction between the sheath of the cable and either the tray or the ground or guide rails will dominate the tension.
If it involves vertical lifting: Only the force of gravity will oppose the cable. The full tension becomes a permanent tension at the highest anchor point.
If there are inclined routes: The tension needs to be determined based on the combined effect of gravitational force and friction along the guide rails.

Large yellow cable reel drum installed on a harbor crane structure.

How to Calculate Reeling Cable Pulling Force and Tension?

Once the physical parameters are defined, you can estimate the actual pulling forces and select a cable capable of handling the operational load.

Basic Pulling Force Calculation Method

In the case of a standard horizontal winching machine, there is a possibility to estimate the practical pulling force (F) by means of unit weight, total length of the cable, and friction coefficient.

The fundamental relationship is expressed as:

F = Pulling force (N or kgf)
W = Cable weight per unit length (kg/m or N/m)
L = Maximum deployed cable length (m)
μ = Friction coefficient of the resting surface or guide channels

With the growing weight of the cable or length of its travel, the value of the needed pulling force grows proportionally.

While this basic formula provides a starting point, practical engineering calculations apply an additional safety factor (typically 1.3 to 1.5) to account for dynamic acceleration spikes, motor torque fluctuations, and environmental drag from dirt or weather.

Understanding Maximum Allowable Cable Tension

Buyers may come across conflicting tensions while assessing manufacturers' datasheets. Such misunderstandings may lead to failure of the cables.

Tensile Strength (Break Load Ultimate): It is the highest amount of force that causes breakage or permanent deformation of the copper conductors, aramid fibers, or outer sheath.
Maximum Allowable Pulling Tension: It is the maximum weight that the cable is able to carry without elongating the conductors or damaging the insulation.

While selecting the cable, it should always be considered on the basis of the allowable pulling tension limit rather than the ultimate tensile limit.

Example: Mechanical Load Evaluation for a Port Crane

Consider a port crane operating on a dockside track:

Suspended/Deployed Cable Length (L): 80m
Cable Unit Weight (W): 5 kg/m

1. Calculate Base Static Weight:

Total Mass = 80 m × 5 kg/m = 400 kg
Converted to gravitational force (where g ≈ 9.81 m/s²):
Static Force ≈ 3,924 N

2. Account for Travel Mechanics and Dynamic Loads:

Assuming a horizontal track with a guide channel friction coefficient (μ) of 0.3, the basic friction force is:

F = 3, 924 N × 0.3 = 1, 177.2 N

Next, add an acceleration factor for rapid movement cycles plus a safety margin (1.4× multiplier):

Target Working Load Capability ≈ 1,177.2 N × 1.4 = 1,648 N

In this scenario, selecting a cable rated only for the basic sliding friction (1, 177 N) would cause structural fatigue over time. The system requires a reeling cable explicitly rated for a continuous tensile load of at least 1,650 N to ensure reliable long-term service.

How to Avoid Excessive Mechanical Load on Reeling Cables

Calculating tension is only the first half of the process; preventing excessive mechanical strain during actual operation requires proper equipment matching and design margins.

Action Item	Operational Benefit	Risk Avoided
Complete Spec Share	Ensures cable design fits real system mechanics	Under-specifying tensile support members
Maintain Tension Margin	Absorbs unexpected torque spikes and inertia	Core stretching, conductor necking, sheath cracking
System-Wide Alignment	Keeps motor torque and reel speed balanced with cable feed	Cable snap, excessive catenary sag, track derailment

Match Cable Specifications with Equipment Requirements

Before making the procurement request, obtain full information about the mechanical loads rather than focusing on the electrical loads only. Give the manufacturers all necessary information concerning:

Type of machine (gantry crane, hoist, stacker-reclaimer)
Total distance covered and the route of installation
Maximum running speed and maximum rate of acceleration
Number of duty cycles that can be made in an hour

Giving this operational information enables the manufacturers to incorporate special internal parts such as a high-tensile aramid center core and a heavy-duty PUR outer jacket in the cable.

Avoid Operating Close to Maximum Pulling Force

If the cable is run constantly at its rated tensile strength, this will lead to cumulative stress. The internal copper core will experience elongation, which is referred to as necking. This leads to the generation of heat and increased resistance, and eventually the core will fail.

To avoid this:

It is necessary to keep a safety margin.
Ensure the working tension of the cable is within 70%-80% of the rated tension of the cable.
Reserve a safety buffer to account for any changes in the operational system.

Consider the Complete Reeling System

A reeled cable is not used independently, but the mechanical forces acting upon it are directly connected with the neighboring mechanical elements:

Motorized Reeling Drum Drive: Settings of the motor torques need to take into account the rate of the deployment/retrieval process as well as avoid any dragging force acting on the cable guide.
Roller Guides Packages: Dimensions of the rollers, their alignment, and the radius of curvatures affect local friction and the lateral force on the cable.
Braking/Tension Control Mechanisms: Soft couplings or VFD systems eliminate mechanical shock loads in case of any directional change of the equipment.

Matching your reeling drum, electronics of the drive, and guide channels will minimize mechanical impact on the cable.

Conclusion

Mechanical load and pull strength calculation is an important task during the selection of reeling cables for large machines. Engineers consider the weight of the cable, the distance over which it travels, the speed of movement, dynamic acceleration, and its physical setup to pick cables designed to endure operational strain.

Load calculations help avoid stretching of copper conductors, avoid damaging the outer layer, and minimize unplanned breakdowns.

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about HEBEI- HUATONG

Founded in 1993, Hebei-Huatong is a global cable manufacturing enterprise with production facilities located in Tangshan (Hebei Province, China), Busan (South Korea), Panama, Kazakhstan, Tanzania, Cameroon, and Angola. Its core product portfolio includes submersible pump cables for oil extraction, flexible moving cables for harbor cranes, cUL/CSA listed cables for AI PDU and marine shipboard cables. The company provides robust support for the continuous, safe, and efficient operation of industrial sectors worldwide, including offshore and onshore oil & gas exploration, and material handling via port cranes.