Rigging Accessories, Sling Selection, and Load Control
Introduction to the Chapter
This chapter covers the essential knowledge required for the Interprovincial (Red Seal) certification exam for tower crane operators. The correct selection and use of rigging accessories are critical responsibilities: a sling failure can result in serious injuries, property damage, and legal consequences. You must master load calculation principles, safety factors, sling angles, and the working limits of various types of accessories. This chapter is organized according to the requirements of the Canada Occupational Health and Safety Regulations (COHSR), Part XII and the CSA Z150 (Safety Code on Mobile Cranes) and CSA Z248 (Safety Code on Tower Cranes) standards, which are the primary references for the exam.
1. Definitions and Fundamental Principles
1.1 Essential Terminology
Rated Load: The maximum load that an accessory can support under specific working conditions, as determined by the manufacturer and certified according to applicable standards.
Safe Working Load (SWL): The maximum load that an accessory can safely lift under normal working conditions. It is always less than the breaking load.
Safety Factor (SF): The ratio between the minimum breaking load and the safe working load. For steel wire rope slings, the safety factor is generally 5:1 (unless otherwise specified by the manufacturer). For synthetic slings, it is also 5:1, but may vary depending on the material.
Sling Angle (β): The angle measured between the sling leg and the vertical (or horizontal, depending on the convention used). This angle is critical because it changes the tension in each leg.
Sling Load: The tensile force exerted on each leg of a sling, which increases as the angle between the legs increases.
1.2 Load Calculation Principles
The tension in each leg of a two-leg sling is calculated using the formula:
T = (P / n) × (1 / cos β)
Where:
T = tension in each leg (kg or lb)
P = total weight of the load (kg or lb)
n = number of load-bearing legs
β = angle between the sling leg and the vertical (in degrees)
Worked Example: A 2,000 kg load is lifted by a two-leg sling with a 45° angle from the vertical.
T = (2,000 / 2) × (1 / cos 45°) = 1,000 × 1.414 = 1,414 kg per leg
Tension increases exponentially with the angle. At 60° from the vertical (120° included angle), the tension in each leg equals the total weight of the load. At 90° (180° included angle), the tension is theoretically infinite — which is why sling angles greater than 90° (included angle) are prohibited.
1.3 Load Factor Table for Sling Angles
| Angle from Vertical (β) | Included Angle Between Legs | Multiplier Factor (1/cos β) | Tension per Leg (for 1,000 kg load, 2 legs) |
|---|
| 0° | 0° | 1.00 | 500 kg |
| 15° | 30° | 1.04 | 520 kg |
| 30° | 60° | 1.15 | 575 kg |
| 45° | 90° | 1.41 | 705 kg |
| 60° | 120° | 2.00 | 1,000 kg |
| 75° | 150° | 3.86 | 1,930 kg |
| 90° | 180° | ∞ (theoretical) | Prohibited |
Rule of Thumb: Never use an included angle greater than 120° (β > 60°). Beyond this point, the effective load on each leg exceeds the weight of the load, increasing the risk of failure and loss of control.
2. Types of Slings and Their Characteristics
2.1 Steel Wire Rope Slings
Steel wire rope slings are the most common for tower cranes. They are manufactured according to CSA G4 (Steel Wire Rope) and must be inspected in accordance with CSA Z150.
Technical Characteristics:
Typical construction: 6 × 19 (6 strands of 19 wires) or 6 × 37 (6 strands of 37 wires) with a fibre core (FC) or an independent wire rope core (IWRC).
Independent Wire Rope Core (IWRC): Stronger, better resistance to crushing, recommended for heavy loads and abrasive environments.
Fibre Core (FC): More flexible, but less resistant to heat and abrasion.
Working Limits:
Maximum working temperature: 90 °C for ropes with a fibre core, 120 °C for ropes with an independent wire rope core.
Minimum bending radius: Do not bend a rope around a radius less than 6 times the rope diameter (for eye slings, the eye radius must be at least 3 times the diameter).
Rejection Criteria (Inspection):
Number of broken wires: 6 broken wires over a length of 6 diameters, or 3 broken wires in one strand over a length of 3 diameters.
Wear: Diameter reduction of more than 10% from the nominal diameter.
Corrosion: Visible pitting, discolouration, loose wires.
Deformation: Kinks, loops, crushing, birdcaging, core protrusion.
2.2 Chain Slings
Chains are used for abrasive loads, high-temperature applications, or loads with sharp edges. They are manufactured according to CSA G4.1 (Lifting Chains) and must conform to ASTM A391 for Grade 80 chain.
Characteristics:
Grade 80: Minimum breaking strength of 800 MPa.
Grade 100: Minimum breaking strength of 1,000 MPa (lighter for the same capacity).
Maximum temperature: 200 °C for Grade 80 chain (above this, capacity is reduced by 25% at 300 °C).
Rejection Criteria:
Elongation: Increase of more than 5% in link pitch.
Wear: Diameter reduction of more than 10%.
Cracks, nicks, deformations, open links.
Illegible or missing markings (each chain must bear the grade and manufacturer).
2.3 Synthetic Slings (Webbing and Rope)
Synthetic slings are lightweight, flexible, and do not damage delicate surfaces. They are manufactured according to ASTM D5720 (webbing) and ASTM D4268 (rope).
Material Types:
Polyester (PES): Resistant to acids, but sensitive to alkalis. Maximum temperature: 90 °C.
Polyamide (PA / Nylon): Resistant to alkalis, but sensitive to acids. Maximum temperature: 90 °C.
Polypropylene (PP): Lightweight, resistant to acids and alkalis, but sensitive to UV radiation. Maximum temperature: 80 °C.
Rejection Criteria:
Cuts, tears, excessive abrasion.
Damaged or missing stitching.
Illegible markings (each sling must bear the SWL, manufacturer, and lot number).
Exposure to chemicals that have degraded the material.
2.4 Comparative Table of Sling Types
| Characteristic | Steel Wire Rope | Chain | Synthetic Webbing |
|---|
| Abrasion resistance | Good | Excellent | Poor |
| Heat resistance | Good (up to 120 °C) | Excellent (up to 200 °C) | Poor (up to 90 °C) |
| Flexibility | Moderate | Poor | Excellent |
| Surface protection | Moderate | Poor | Excellent |
| Chemical resistance | Good (except strong acids) | Good (except acids) | Varies by material |
| Visual inspection | Easy | Easy | Difficult (internal defects) |
| Cost | Moderate | High | Low |
3. Rigging Accessories
3.1 Hooks
Hooks are the connection points between the sling and the load or the crane. They must conform to CSA Z150 and bear the manufacturer's mark, capacity, and serial number.
Types of Hooks:
Clevis Hook: General purpose, with or without a safety latch.
Eye Hook: The eye is aligned with the direction of the load.
Lifting Hook: Designed to support loads in a specific direction (often vertical).
Usage Rules:
The safety latch must be in place and functional.
The load must be applied to the centre of the hook bowl, never on the tip.
The hook throat opening (measured between the two sides of the bowl) must not exceed 15% increase from the original opening (or 10% depending on the manufacturer).
A deformed, cracked, or worn hook must be removed from service immediately.
3.2 Shackles
Shackles are used to connect slings to anchor points or to the load. They are classified according to their shape and closing method.
Types of Shackles:
Anchor Shackle (or screw pin shackle): The most common, with a threaded pin.
Chain Shackle: Used for heavy loads, with a forged steel bow.
Cotter Pin Shackle: The pin is secured by a cotter pin or clip.
Usage Rules:
The pin must be fully threaded and secured (cotter pin or safety wire).
The load must be applied to the centre of the shackle bow, never on the sides.
Never replace the original pin with a pin of smaller diameter.
The capacity of a shackle is reduced if the load is applied at an angle (see table below).
Table of Reduction Factors for Shackles (Angled Load):
| Load Angle Relative to Shackle Axis | Reduction Factor |
|---|
| 0° (axial load) | 1.00 |
| 45° | 0.70 |
| 90° (transverse load) | 0.50 |
3.3 Lifting Rings and Thimbles
Lifting Ring: Used to connect multiple slings to a central point. Must conform to CSA Z150 and bear the maximum capacity.
Thimble: Used to protect a rope or webbing at a connection point. The thimble must be matched to the diameter of the rope or webbing.
3.4 Swivels
Swivels allow the load to rotate without twisting the sling. They are used for loads that must be oriented during the lift. The swivel capacity must be at least equal to the maximum load, and it must be lubricated regularly.
3.5 Lifting Clamps
Clamps are used to lift plates, sheets, or structural shapes. They are classified by type:
Vertical Lift Clamp: For vertical plates.
Horizontal Lift Clamp: For horizontal plates.
Lever Clamp: For loads with sharp edges.
Usage Rules:
The clamp must be matched to the thickness and material of the load.
The load must be centred in the clamp.
Never use a damaged clamp or one with worn jaws.
4. Sling Selection
4.1 Selection Procedure
Selecting an appropriate sling follows a systematic procedure:
110.Determine the weight of the load: Use drawings, labels, or calculate from dimensions and material density.
111.Identify the anchor points: Verify that the anchor points are designed for the load and are in good condition.
112.Choose the type of sling: Based on the material, shape, temperature, and environment (abrasion, chemicals).
113.Calculate the tension in each leg: Taking into account the number of legs and the sling angle.
114.Verify the sling capacity: The SWL of the sling must be greater than or equal to the calculated tension.
115.Verify the accessories: Hooks, shackles, rings — all must have sufficient capacity.
116.Inspect the sling and accessories: Before each use, according to the rejection criteria.
4.2 Calculating Load Weights
The weight of a load is calculated by multiplying the volume by the density of the material.
Formula: Weight (kg) = Volume (m³) × Density (kg/m³)
Table of Common Densities:
| Material | Density (kg/m³) |
|---|
| Steel | 7,850 |
| Cast iron | 7,200 |
| Aluminium | 2,700 |
| Concrete | 2,400 |
| Wood (pine) | 500 |
| Water | 1,000 |
Example: A steel plate measuring 2 m × 1 m × 0.05 m.
Volume = 2 × 1 × 0.05 = 0.1 m³
Weight = 0.1 × 7,850 = 785 kg
4.3 Capacity Reduction Factors
The capacity of a sling may be reduced under certain conditions:
Elevated temperature: Capacity reduction beyond specified limits (see table below).
Sharp edges: A sharp edge can cut the sling. Use edge protectors (load protectors) if the contact angle is less than 90°.
Knots: A knot in a wire rope sling reduces capacity by 50% (prohibited in most cases).
Twisted or kinked slings: Capacity reduction of 10 to 15%.
Table of Capacity Reductions for Temperature (Steel Wire Rope):
| Temperature | Capacity Reduction |
|---|
| ≤ 90 °C | 0% |
| 90 °C – 120 °C | 10% |
| 120 °C – 200 °C | 25% |
| > 200 °C | Prohibited |
5. Load Control
5.1 Control Principles
Load control is the operator's ability to keep the load stable and predictable during lifting, moving, and positioning. The key principles are:
Centre of Gravity (COG): The load must be rigged so that the COG is directly below the lifting point. If the COG is offset, the load will tip.
Stability: A load is stable if its COG is within the support base (the area between the bearing points).
Swing: Swing is caused by horizontal forces (wind, crane movement, acceleration). It must be controlled through slow, gradual movements.
5.2 Control Methods
4-Leg Bridle Sling: Used for stable and symmetrical loads. The load is distributed over 4 points, but if the load is rigid, only 2 or 3 legs actually bear the load (depending on geometry). Always calculate with 2 legs for rigid loads.
Basket Hitch: The sling passes under the load and both ends are attached to the hook. The capacity is doubled compared to a single sling (if the angle is 0°).
Choker Hitch: The sling makes a full turn around the load and tightens on itself. The capacity is reduced by 25% compared to a single sling.
5.3 Table of Rigging Configurations
| Configuration | Number of Load-Bearing Legs | Capacity Factor (relative to 1 leg) | Remarks |
|---|
| Single vertical | 1 | 1.0 | Load directly below the hook |
| Two legs (0°) | 2 | 2.0 | Included angle of 0° |
| Two legs (90°) | 2 | 1.4 | Included angle of 90° |
| Four legs (0°) | 4 (theoretical) | 4.0 | In practice, 2 legs for rigid loads |
| Basket (0°) | 2 | 2.0 | Sling passes under the load |
| Choker | 1 | 0.75 | Sling tightens on itself |
5.4 Safe Lifting Procedures
147.Before the lift:
Verify that the load is free (no bolts, welds, or ties).
Verify that the slings are correctly positioned and that the angles are within limits.
Verify that the lifting path is clear.
Establish communication with the signaler (standardized hand signals according to CSA Z150).
152.During the lift:
Lift slowly until the slings are taut.
Verify that the load is balanced (no tipping).
Lift a few centimetres, then stop and check stability.
Move the load slowly, avoiding sudden movements.
Keep the load as low as possible above the ground.
158.After the lift:
Set the load down on a stable, level surface.
Slacken the slings before removing them.
Inspect the slings and accessories for any damage.
5.5 Communication and Signals
Communication between the operator and the signaler is essential. Standardized hand signals are defined in CSA Z150 and must be known by all workers. Voice or radio signals must be clear and unambiguous.
Basic Signals:
Hoist: Forearm vertical, index finger pointing up, making small circles.
Lower: Forearm horizontal, index finger pointing down, making small circles.
Stop: Arm extended horizontally, palm facing down.
Emergency Stop: Both arms extended horizontally, palms facing down.
6. Inspection and Maintenance
6.1 Inspection Frequency
Pre-use inspection: Quick visual check (obvious defects, damage, markings).
Periodic inspection: Detailed inspection performed by a competent person, at least once per year (or more frequently depending on usage and environment).
Post-incident inspection: Any sling involved in a shock load, overload, or damage must be removed from service and inspected.
6.2 Inspection Procedure
176.Cleaning: Remove dirt, grease, and debris to allow a complete visual inspection.
177.Visual examination: Look for cracks, deformations, wear, corrosion, broken wires, damaged stitching.
178.Measurements: Measure the diameter (rope, chain) or thickness (webbing) to detect wear.
179.Marking verification: Ensure that the SWL, manufacturer, and lot number are legible.
180.Documentation: Record the inspection results in a log.
6.3 Removal from Service
A sling must be removed from service if:
It has a defect that reduces its capacity below the SWL.
It has been overloaded (even without visible damage).
It has been exposed to excessive temperatures.
It has been exposed to corrosive chemicals.
Its markings are illegible.
7. Applicable Standards and Regulations
7.1 Canadian Standards
CSA Z150: Safety Code on Mobile Cranes — covers general requirements for slings and accessories.
CSA Z248: Safety Code on Tower Cranes — specific to tower cranes, includes requirements for slings and accessories.
CSA G4: Steel Wire Rope — specifications for hoisting ropes.
CSA G4.1: Lifting Chains — specifications for chains.
Canada Occupational Health and Safety Regulations (COHSR), Part XII: General requirements for cranes and lifting devices.
7.2 Key COHSR Rules
Section 12.10: Slings must conform to applicable standards and bear the manufacturer's mark.
Section 12.11: Slings must be inspected before each use and periodically by a competent person.
Section 12.12: Damaged slings must be removed from service.
Section 12.13: Slings must be stored in a manner that prevents damage.
7.3 CSA Z248 Requirements (Tower Cranes)
Section 8: Slings and accessories — inspection, selection, and usage requirements.
Section 9: Lifting operations — lifting procedures, communication, load control.
Section 10: Maintenance and inspection — frequencies and procedures.
8. Common Pitfalls to Avoid
Here are the most frequent errors on the exam (and in practice):
208.Forgetting the sling angle: Never calculate tension without accounting for the angle. A 60° angle from the vertical doubles the tension compared to a 0° angle.
209.Using the theoretical number of legs: For a 4-leg sling on a rigid load, calculate with 2 legs only.
210.Confusing the angle from vertical with the included angle: The included angle is twice the angle from vertical. Limits are expressed as included angle (max 120°).
211.Neglecting reduction factors: Temperature, sharp edges, choker hitch — all reduce capacity.
212.Ignoring rejection criteria: A rope with 6 broken wires over 6 diameters must be removed, even if the load seems light.
213.Using a shackle with an unsecured pin: The pin must be fully threaded and secured.
214.Not checking the markings: A sling without markings cannot be used — its capacity is unknown.
215.Lifting a load with an offset COG: The load will tip, even if the slings are correct.
216.Confusing units: Always verify whether calculations are in kg or lb, and in metres or feet.
217.Forgetting edge protectors: A sharp edge can cut a synthetic webbing sling in seconds.
9. Summary
Tension in a sling increases with the angle: Use the formula T = (P/n) × (1/cos β) and never exceed an included angle of 120°.
Three types of slings: Steel wire rope (strong, rigid), chain (abrasion, heat), synthetic (lightweight, protects surfaces). Each has its limits and rejection criteria.
Accessories must be compatible and in good condition: Hooks, shackles, rings, swivels, clamps — all must bear the capacity rating and be inspected.
Sling selection follows a systematic procedure: Weight → anchor points → type → tension → capacity → inspection.
Load control is based on the centre of gravity and stability: Lift slowly, check balance, keep the load low.
The reference standards are CSA Z150, CSA Z248, and COHSR Part XII: Know the inspection and removal-from-service requirements.
Inspection is mandatory before each use: Know the rejection criteria for each type of sling.
10. Exam Tips
Memorize the key formulas: Tension per leg, weight = volume × density, reduction factors.
Practice calculations with various angles: 30°, 45°, 60° — know the values of cos β (0.866; 0.707; 0.5).
Learn the rejection criteria by heart: 6 broken wires over 6 diameters, 10% wear, 5% elongation for chains.
Understand the difference between theoretical and actual capacity: Reduction factors always apply.
Read the questions twice: Examiners often include traps in the wording (units, angles, number of legs).
Use the density table: It is provided in the exam, but know how to use it quickly.
This chapter provides you with the essential knowledge to pass the section on rigging accessories, sling selection, and load control. Review the formulas, tables, and rejection criteria, and practice the calculations until they become automatic. Good luck with your preparation!
