Load Charts, Capacity Calculations, and Stability

Load Charts, Capacity Calculations, and Stability

Chapter Introduction

This chapter is the technical core of the mobile crane operator Red Seal exam. Mastering load charts, capacity calculations, and stability principles is not only essential for passing the exam but also for ensuring safety on job sites. In Canada, the requirements for mobile cranes are governed by the CSA Z150 standard (Safety of Mobile Cranes) and by federal and provincial occupational health and safety regulations. This chapter prepares you to correctly interpret manufacturer data, perform required load calculations, and apply stability factors in accordance with national standards.

1. Fundamental Principles of Mobile Crane Stability

1.1 The Stability Triangle

The stability of a mobile crane is based on a simple geometric concept: the stability triangle. For a crawler crane or a crane on tires with outriggers deployed, this triangle is formed by the following three support points:

The vertical projection of the center of gravity of the entire assembly (crane + load) must always remain within this triangle. If this projection moves outside the triangle, the crane will tip over.

> Exam Tip: The stability triangle is not an equilateral triangle. It is narrower at the rear than at the front. This is why lifting capacity is always lower at the rear than at the front, at equal radius.

1.2 The Stability Factor (or Tipping Coefficient)

The CSA Z150 standard requires that the crane be designed to withstand a minimum stability factor of 1.33 (i.e., 133%) when lifting with outriggers deployed. This means that the stabilizing moment (the weight of the crane multiplied by the distance from the center of gravity to the tipping point) must be at least 1.33 times greater than the overturning moment (the load multiplied by the horizontal radius).

Overturning moment formula (Mₒ):

Mₒ = Load (kg) × Horizontal radius (m)

Stabilizing moment formula (Mₛ):

Mₛ = Crane weight (kg) × Distance from center of gravity to tipping point (m)

Stability condition:

Mₛ ≥ 1.33 × Mₒ

1.3 The Center of Gravity

The center of gravity (CG) of a mobile crane varies depending on:

The manufacturer provides tables indicating the CG position for different configurations. This data is integrated into the load charts.

2. Reading and Interpreting Load Charts

2.1 Definition and Components

A load chart is a table provided by the manufacturer that indicates the maximum rated capacity of the crane for each combination of:

2.2 Typical Structure of a Chart

A load chart is generally presented as a two-entry table:

Note: The values shown are gross capacities. They include the weight of the load, hook, slings, spreader bar, and any lifting accessory suspended from the boom.

2.3 Essential Reading Rules

2.4 Net Capacity vs. Gross Capacity

Gross capacity is the value shown in the chart. Net capacity is the gross capacity minus the weight of all lifting accessories (hook, slings, spreader bar, etc.).

Formula:

Net capacity = Gross capacity − Weight of accessories

Example:

Gross capacity = 8,500 kg

Hook weight = 150 kg

Sling weight = 50 kg

Spreader bar weight = 200 kg

Net capacity = 8,500 − (150 + 50 + 200) = 8,100 kg

> Common Trap: Candidates often forget to subtract the weight of the hook and accessories. The chart indicates the gross capacity; the actual load to be lifted must be less than or equal to the net capacity.

3. Capacity and Load Calculations

3.1 Calculating the Total Load to be Lifted

The total load (or working load) includes:

Formula:

Total load = Payload + Weight of accessories

3.2 Calculating the Percentage of Capacity Used

The percentage of utilization (or load factor) is the ratio between the total load and the net capacity (or gross, depending on the method used by the manufacturer).

Formula:

% utilization = (Total load ÷ Net capacity) × 100

Example:

Total load = 6,500 kg

Net capacity = 8,100 kg

% utilization = (6,500 ÷ 8,100) × 100 = 80.2%

Golden Rule: The percentage of utilization must never exceed 100%. In practice, many companies impose an 85% limit for safety reasons, but this limit is not required by the CSA Z150 standard.

3.3 Calculating the Effective Radius

The effective radius is the horizontal distance between the crane's axis of rotation and the center of gravity of the load. It is calculated using trigonometry:

Formula:

Radius = Boom length × cos(elevation angle)

Example:

Boom length = 25 m

Elevation angle = 60°

Radius = 25 × cos(60°) = 25 × 0.5 = 12.5 m

Caution: This formula does not account for boom deflection (the boom bending under load). For precise calculations, you must use the values from the load chart, which already incorporate deflection.

3.4 Calculating the Lifting Height

The lifting height (or hook height) is the vertical distance between the ground and the load attachment point.

Formula:

Height = Pivot point height + (Boom length × sin(elevation angle))

Example:

Pivot point height = 2.0 m

Boom length = 25 m

Angle = 60°

Height = 2.0 + (25 × sin(60°)) = 2.0 + (25 × 0.866) = 2.0 + 21.65 = 23.65 m

4. Factors Affecting Lifting Capacity

4.1 Boom Angle

The higher the boom angle (more vertical boom), the greater the capacity, because the radius decreases. Conversely, a more horizontal boom reduces capacity.

4.2 Boom Length

At equal angles, a longer boom provides a greater radius but reduced capacity, because the overturning moment increases.

4.3 Outriggers

Deployed outriggers increase the width of the crane's base, which widens the stability triangle and increases capacity. Operating on tires (without outriggers) significantly reduces capacity.

4.4 Counterweight

Adding additional counterweights moves the crane's center of gravity toward the rear, increasing the stabilizing moment. Each counterweight configuration has its own load chart.

4.5 Wind

The CSA Z150 standard requires lifting operations to cease when wind speed exceeds the limits specified by the manufacturer (generally 20 to 30 km/h for light loads and large surface areas). Wind exerts a horizontal force on the load and the boom, increasing the overturning moment.

4.6 Ground Conditions

Stability also depends on the soil bearing capacity. Soft or uneven ground can cause outrigger settlement, altering the stability triangle. Outrigger pads are often required to distribute the load over a larger surface area.

5. Pre-Lift Calculation Procedures

5.1 The Five-Step Method

5.2 Complete Calculation Example

Data:

Step 1: Total load

Total load = 7,200 + 150 + 80 + 220 = 7,650 kg

Step 2: Radius

Radius = 25 × cos(50°) = 25 × 0.643 = 16.1 m

Step 3: Gross capacity (excerpt from chart)

At 25 m boom and 50°, the chart indicates: 8,200 kg

Step 4: Net capacity

Net capacity = 8,200 − (150 + 80 + 220) = 7,750 kg

Step 5: Verification

7,650 kg ≤ 7,750 kg → The lift is safe

% utilization = (7,650 ÷ 7,750) × 100 = 98.7%

> Warning: A utilization percentage of 98.7% is very close to the limit. In practice, it is recommended to plan for an additional safety margin, especially in case of wind or unstable ground.

6. The 75% Rule and Dynamic Loads

6.1 Dynamic Loads

Dynamic loads are additional forces caused by crane movement (swinging, lifting, lowering, braking). They can increase the effective load by 10 to 25% depending on conditions.

6.2 The 75% Rule (for Testing)

The CSA Z150 standard requires that load tests be performed at 75% of the rated capacity for new cranes or after major repairs. This rule does not apply to normal lifting operations, but it is often questioned on the exam.

6.3 The Wind Load Factor

For loads with a large surface area exposed to wind (e.g., panels, empty containers), the CSA Z150 standard requires reducing the rated capacity. The simplified formula is:

Reduced capacity = Rated capacity × (1 − (Exposed surface × Wind pressure) ÷ Stabilizing moment)

In practice, the manufacturer provides reduction tables for wind.

7. Safety Devices and Their Role in Calculations

7.1 The Load Moment Indicator (LMI)

The load moment indicator is an electronic device that continuously monitors:

It emits visual and audible alarms when the load approaches 90% of capacity and can interrupt dangerous movements at 100%.

Exam Rule: The LMI does not replace manual calculation. The operator must be able to verify calculations independently of the device.

7.2 The Anemometer

The anemometer measures wind speed. It must be used to determine whether weather conditions permit lifting.

7.3 Leveling Jacks

The crane must be perfectly level before any lift. A 1° inclination can reduce capacity by 5 to 10%. The crane's level must be checked using a bubble level or an electronic indicator.

8. Comparative Configuration Tables

8.1 Capacity Comparison by Configuration

8.2 Wind Reduction Factors

9. Pitfalls to Avoid

Here are the most frequent errors made by candidates on the Red Seal exam:

10. Summary

11. Final Exam Tips

Mastery of this chapter will not only help you pass the Red Seal exam but also practice your trade safely and professionally. Good luck with your preparation!