Rigging, Lifting, and Load Calculations

Rigging, Lifting, and Load Calculations

Introduction to Safe Lifting in Excavation

As an excavator operator, you face lifting operations daily: placing culverts, handling pipes, moving concrete blocks, installing pumps, or removing submerged equipment. Unlike a crane, an excavator is not a lifting device designed for this primary purpose; its use for lifting is therefore subject to strict rules and rigorous calculations. This chapter covers all the knowledge required for the Red Seal exam: physical principles, types of slings, rigging methods, load calculations, angles, safety factors, and Canadian regulatory requirements.

Fundamental Principles of Lifting

Rated Load and Lifting Capacity

The rated load (or rated capacity) of an excavator is the maximum load the machine can lift under specific conditions, as determined by the manufacturer. This value is indicated on the nameplate and in the operator's manual. For an excavator, lifting capacity varies according to:

Lifting capacity is typically expressed in kilograms (kg) or metric tonnes (t). Load charts provided by the manufacturer indicate capacities for different configurations. The tipping factor (75% of the tipping load) and the structural factor (87% of the structural limit) determine the actual rated capacity.

Centre of Gravity and Overturning Moment

The overturning moment is the product of the lifted load multiplied by the horizontal distance between the machine's centre of rotation and the line of action of the load. Mathematically:

Overturning moment = Load × Horizontal distance

The unit is the newton-metre (N·m) or kilogram-metre (kg·m). When this moment exceeds the stabilizing moment (machine weight × distance from centre of gravity to the tipping axis), the machine tips over. The operator must always verify that the potential overturning moment remains below the machine's stabilizing capacity.

Safety Factor

The safety factor is the ratio between the breaking strength of a component and the maximum load it is permitted to support. For slings and lifting accessories, the minimum safety factor is 5:1 according to CSA standards. This means a sling with a breaking strength of 5,000 kg must never be used to lift more than 1,000 kg. This factor compensates for shock loads, wear, knots, and calculation inaccuracies.

Types of Slings and Their Characteristics

Chain Slings

Chain slings are made of heat-treated alloy steel. They offer excellent resistance to abrasion, heat, and cutting. Their main disadvantages are their heavy weight and lack of flexibility.

Links must be inspected for wear, cracks, deformation, and corrosion. The maximum allowable wear is 10% of the nominal link diameter.

Wire Rope Slings

Wire rope slings are composed of strands wound around a core. They offer a good compromise between strength, flexibility, and cost. Their strength depends on the number of strands, the number of wires per strand, and the type of core (fibre or steel).

The reduction factor applies when fabricating a sling with splices. A properly made splice reduces rope strength by 10 to 15%. Wire ropes must be inspected for broken wires, corrosion, deformation, and wear. The maximum number of broken wires over a length of 6 diameters is 6 for a 6×19 rope.

Synthetic Fibre Slings

Synthetic fibre slings (polyester, nylon, polypropylene) are lightweight, flexible, and will not scratch surfaces. However, they are sensitive to cutting, heat, and chemicals.

Synthetic slings lose strength when wet: nylon loses up to 15% of its strength, while polyester and polypropylene lose only 5%. Capacity labels must never be removed or covered.

Web Slings (Textile)

Web slings are flat, woven polyester. They offer a large contact surface, reducing the risk of damage to the load. Their capacity is indicated by a sewn-on label. They are available in single-eye, double-eye, and endless loop configurations.

Rigging Methods and Mode Factors

Vertical Hitch

The vertical hitch is the simplest method: the sling is attached directly to the load, perpendicular to the ground. The mode factor is 1.0. The load supported by the sling equals the weight of the load.

Choke Hitch

The choke hitch involves passing the sling around the load and then through its own eye. The mode factor is 0.75. This method reduces sling capacity by 25% due to bending and friction stresses at the choke point.

Basket Hitch

The basket hitch involves passing the sling under the load, with both ends attached to the hook. The mode factor is 2.0 in a vertical basket hitch (both legs working in parallel). However, this factor drops to 1.4 in a choked basket hitch (the leg passes around the load and returns to the hook).

Multi-Leg Bridle

A two-leg or multi-leg bridle distributes the load between several slings. The mode factor depends on the horizontal angle between the legs and the vertical. This factor is also called the angle factor.

Load Calculations and Sling Angles

The Fundamental Formula

The tension in each leg of a sling is calculated using the formula:

Tension per leg = (Load weight ÷ Number of legs) × Angle factor

The angle factor equals 1 ÷ cos(θ), where θ is the angle between the leg and the vertical.

Angle Factor Table

Golden rule: The horizontal angle between the legs of a sling must never be less than 30°. Below 30°, the tension in the legs becomes excessive and the risk of failure is very high. The ideal angle is between 45° and 60°.

Complete Calculation Example

Problem: You need to lift a concrete culvert weighing 4,500 kg using a two-leg sling. The angle between each leg and the horizontal is 45°. What is the tension in each leg?

Solution:

The tension in each leg is 3,172.5 kg. The selected sling must have a rated capacity of at least 3,172.5 kg, and ideally more, to respect the safety factor.

Calculating Maximum Allowable Load

To determine the maximum load a sling can lift in a given configuration:

Maximum load = Sling rated capacity × Mode factor × Angle factor

Example: A 10 mm chain sling has a rated capacity of 3,200 kg in a vertical hitch. Used in a basket hitch with a 60° angle to the horizontal:

Maximum load = 3,200 × 2.0 × 0.87 = 5,568 kg

Lifting Accessories

Hooks

Lifting hooks must be equipped with a locking device (latches) to prevent accidental disengagement of the sling. Hook inspection must verify:

Shackles

Shackles are classified by type (anchor, chain) and capacity. Their safety factor is 5:1. Pins must be fully threaded and cotter-pinned. The maximum allowable wear is 10% of the pin diameter.

Lifting Rings and Eye Bolts

Lifting rings (eye bolts) are attached directly to the load. Their capacity depends on load orientation: a ring designed for a vertical load can see its capacity reduced by 50% or more when subjected to an angled load.

Safe Lifting Procedures

Before the Lift

During the Lift

After the Lift

Standardized Command Signals

Hand signals for lifting operations are standardized according to CSA Z150 (Safety on Mobile Cranes). The essential signals every operator must know:

Canadian Regulatory Requirements

Canada Labour Code (Material Handling Regulations)

The Material Handling Regulations (SOR/88-64) under the Canada Labour Code require that:

CSA Z150 — Safety on Mobile Cranes

The CSA Z150 standard applies to mobile cranes, but its principles also apply to excavators used for lifting. It specifically requires:

CSA B149.1 — Natural Gas and Propane Installation Code

The CSA B149.1 standard is relevant when performing excavation work near gas pipelines. Section 4.8.1 requires that excavations near pipelines be carried out manually or with approved equipment, and that precautions be taken to avoid damaging pipelines. When lifting loads near pipelines, the operator must ensure the load cannot fall onto the pipeline.

Pipeline Systems Regulations (SOR/96-244)

These regulations, under the Canada Energy Regulator Act, apply to work near pipelines. Section 37 requires that any person performing work near a pipeline take measures to avoid damaging it, including during lifting operations.

Sling Inspection and Maintenance

Removal from Service Criteria

A sling must be removed from service if any of the following defects are found:

For chain slings:

For wire rope slings:

For synthetic slings:

Inspection Frequency

Common Pitfalls to Avoid

Summary

Mastery of load calculations and lifting procedures is essential not only to pass the Red Seal exam, but above all to ensure the safety of all workers on the job site. A poorly rigged load or an angle that is too tight can cause a sling failure, machine tip-over, or serious—even fatal—injuries. The professional operator integrates these principles into every action, without exception.