Mobile Crane Systems and Components

Mobile Crane Systems and Components

Chapter Introduction

This chapter covers all the mechanical, hydraulic, electrical, and safety systems you must master for the Red Seal exam. As a mobile crane operator, your ability to diagnose, operate, and maintain these systems determines not only your efficiency but, most importantly, your safety and that of your crew. The interprovincial exam tests your understanding of fundamental principles, Canadian standards, and safe operating procedures. This chapter is structured to follow the logical progression of systems, from the load-bearing structure to load-limiting devices.

Load-Bearing Structure and Chassis

Chassis and Outriggers

The chassis of the mobile crane supports the entire superstructure and transmits loads to the ground. There are two main configurations: rubber-tired chassis (carrier) and crawler (track) chassis. The choice between these configurations depends on soil bearing capacity, required mobility, and lifting capacity.

Outriggers (stabilisateurs) are hydraulic devices that widen the crane's ground footprint and level it. They are essential for stability because they increase the allowable overturning moment. Common types include:

Ground bearing pressure calculation: The pressure exerted by an outrigger is calculated as follows:

P = F / A

Where P is the pressure (kPa or psi), F is the vertical force (N or lb), and A is the contact surface area of the pad or cribbing (m² or in²). The standard requires that the ground pressure not exceed the allowable bearing capacity of the soil. For example, soft clay soil can support approximately 50 kPa, while rocky soil can support more than 500 kPa.

> Rule of thumb: Always use wooden cribbing or load-spreading plates under outriggers when working on soft ground. The cribbing surface area must be sufficient to reduce the pressure below the soil's bearing limit.

Counterweights

The counterweight is a mass attached to the rear of the superstructure that counterbalances the load moment. It can be fixed or removable. The overturning moment is the product of the load and its distance from the pivot point. The counterweight creates an opposing moment that stabilizes the crane.

The moment formula is:

M = F × d

Where M is the moment (N·m or lb·ft), F is the force (N or lb), and d is the perpendicular distance (m or ft). For a crane in equilibrium, the sum of moments around the tipping point must be zero.

Counterweight configuration comparison table

Hydraulic System

Fundamental Principles

The hydraulic system is the heart of the modern mobile crane. It converts the engine's mechanical energy into hydraulic energy, then back into mechanical energy at the hydraulic cylinders and motors. Pascal's law states that pressure applied to a confined fluid is transmitted uniformly in all directions. This law is the basis for how hydraulic cylinders operate.

Hydraulic pressure is measured in kilopascals (kPa) or pounds per square inch (psi). The relationship between force, pressure, and area is:

F = P × A

Where F is the force (N), P is the pressure (Pa), and A is the piston area (m²). For example, a cylinder with a 0.1 m² piston and a pressure of 20,000 kPa (20 MPa) generates a force of 2,000 kN.

Hydraulic Circuit Components

Hydraulic pumps: These convert mechanical energy into hydraulic energy. Common types are gear pumps, vane pumps, and piston pumps. Axial piston pumps are the most efficient for large-capacity cranes.

Hydraulic cylinders: These convert hydraulic energy into linear motion. They are used for outriggers, the telescopic boom, and boom hoisting.

Hydraulic motors: These convert hydraulic energy into rotary motion. They drive the winch, slewing, and sometimes the travel function.

Control valves: These direct hydraulic flow to the various actuators. Directional spool valves are the most common.

Safety valves: These protect the system against overpressure. The relief valve limits the maximum circuit pressure. Pilot-operated check valves (counterbalance valves) prevent a load from falling in the event of a line failure.

Maintenance and Diagnostics

Hydraulic fluid must be kept clean and at the proper temperature. Contamination by water or particles is the leading cause of hydraulic failures. Daily checks include:

Cavitation: A phenomenon where vapor bubbles form in the fluid at the pump inlet, then implode at the discharge. It causes premature wear and a characteristic noise. It is caused by a restricted inlet, fluid that is too viscous, or a pump running too fast.

> Exam trap: You will often be asked to identify the cause of hydraulic power loss. The most frequent causes are: low fluid level, clogged filter, relief valve set too low, or a worn pump. Always check the fluid level and filters before replacing a pump.

Boom and Lifting Equipment

Boom Types

The telescopic boom is the most common on hydraulic cranes. It consists of nested sections that extend hydraulically. Lifting capacity decreases as the boom extends because the overturning moment increases.

The lattice boom is used on cable cranes and large crawler cranes. It is assembled in sections and offers a better strength-to-weight ratio than the telescopic boom but requires assembly time.

The fly jib is an extension attached to the end of the main boom to increase lifting height or reach.

Angles and Radii

The boom angle is the angle between the boom and the horizontal. It is measured using an inclinometer or angle indicator. The radius is the horizontal distance between the crane's axis of rotation and the load's center of gravity.

The relationship between lifting height and radius is given by trigonometry:

H = L × sin(θ)

R = L × cos(θ)

Where H is the height (m), L is the boom length (m), θ is the boom angle (degrees), and R is the radius (m).

Example: A 30 m boom at a 60° angle gives a height of 30 × sin(60°) = 30 × 0.866 = 25.98 m and a radius of 30 × cos(60°) = 30 × 0.5 = 15 m.

Wire Ropes and Sheaves

The hoist rope is a critical component. Wire ropes are classified by their construction (e.g., 6 × 19, 6 × 37), which indicates the number of strands and wires per strand. The rope must be inspected regularly to detect:

A rope's removal criteria are reached when the number of broken wires over a given length exceeds the values in the standard. For example, for a 6 × 19 rope, removal is required if 6 broken wires are visible over a length of 6 rope diameters, or 3 broken wires in a single strand.

Parts of line (reeving): The number of rope parts supporting the load. Single-part line (1 part) is used for light loads. Multiple-part line (2, 4, 6 parts or more) reduces the tension in each part but reduces the hoisting speed. The tension in each part is:

T = W / (n × η)

Where T is the tension per part (N), W is the load weight (N), n is the number of parts, and η is the reeving efficiency (typically 0.85 to 0.95 per sheave).

> Exam rule: Reeving efficiency decreases with each added sheave. For a 4-part reeving with 3 sheaves, the total efficiency is approximately 0.85³ = 0.61. Never neglect efficiency in capacity calculations.

Limiting and Safety Devices

Load Moment Indicator (LMI)

The Load Moment Indicator (LMI) is a mandatory device on most mobile cranes. It continuously monitors the load, boom angle, boom length, and radius, and compares the actual load to the rated capacity. It emits visual and audible alarms when the load approaches the limit and can interrupt dangerous movements.

The LMI calculates the load moment:

M = W × R

Where M is the moment (N·m), W is the load weight (N), and R is the radius (m). The moment is compared to the maximum allowable moment for the given configuration.

Angle and Length Indicator

The boom angle indicator displays the boom angle relative to the horizontal. The boom length indicator displays the total boom length. These two parameters are essential for determining lifting capacity from load charts.

Limit Switches

Limit switches automatically stop a movement at a predetermined position. The most common are:

Brakes and Anti-Drop Devices

Winch brakes are disc or band brakes that hold the load in position. They must be capable of supporting 125% of the rated load. The safety brake (ratchet) prevents the load from falling in the event of main brake failure.

The anti-two-block device is a system that prevents the hook from coming into contact with the boom tip sheave. It is mandatory on cranes equipped with an LMI. Its activation cuts the hoist-up and boom-down functions.

Electrical System

Electrical Components

The mobile crane's electrical system includes:

The standard CSA C22.1, Canadian Electrical Code, Part I, Chapter V applies to crane electrical installations. Key requirements include grounding, overcurrent protection, and the use of approved components.

Electrical Safety

Proximity to power lines is a major hazard. The standard requires a minimum distance of 3 m between the crane and power lines under 75 kV, and 6 m for lines over 75 kV. These distances increase with voltage.

Rule 8-200 of the Canadian Electrical Code, Part I, Chapter V, addresses the minimum distances to be maintained when working near power lines. The crane operator must know these distances and communicate them to the crew.

> Exam trap: The minimum 3 m distance applies to the entire crane, including the load and the rope. A swinging rope can reduce the effective distance. Always add a safety margin.

Load Charts and Calculations

Reading Load Charts

The load chart is the most important document for the crane operator. It indicates the maximum lifting capacity for each boom configuration, angle, and radius. Charts are specific to each crane model and each configuration (boom length, parts of line, counterweight presence).

Example of a simplified load chart

Calculating the Effective Load

The effective load is the total weight being lifted, including:

The effective load must be less than or equal to the rated capacity from the chart, taking into account reduction factors (e.g., wind, sloping ground).

Load factor: The ratio of the effective load to the rated capacity. A load factor of 0.8 means the crane is working at 80% of its capacity. Most standards recommend not exceeding 90% of the rated capacity.

Wind Effect

Wind exerts a force on the load and the boom. The wind force is calculated as:

F_w = 0.5 × ρ × v² × A × C_d

Where F_w is the wind force (N), ρ is the air density (approximately 1.2 kg/m³), v is the wind speed (m/s), A is the projected area (m²), and C_d is the drag coefficient (typically 1.2 for a rectangular load).

Example: A 10 m² load exposed to a 15 m/s wind (54 km/h) experiences a force of 0.5 × 1.2 × 15² × 10 × 1.2 = 1,620 N, or approximately 165 kg. This force can be significant for light loads with large surface areas.

Inspection and Preventive Maintenance

Daily Inspection

The daily inspection is mandatory before each use. It includes:

Periodic Inspection

Periodic inspections are performed according to a schedule defined by the manufacturer or regulations. They include more thorough checks:

Removal from Service Criteria

A crane must be removed from service if:

Applicable Canadian Standards

CSA Z150

The CSA Z150, Safety Code on Mobile Cranes, is the primary standard for mobile crane operations in Canada. It covers:

CSA B149.1

The CSA B149.1, Natural Gas and Propane Installation Code, applies to cranes equipped with natural gas or propane engines. It covers the installation, maintenance, and operation of gaseous fuel systems.

Canadian Electrical Code, Part I, Chapter V

The Canadian Electrical Code, Part I, Chapter V (CSA C22.1) applies to crane electrical installations. Relevant rules include:

Canada Occupational Health and Safety Regulations

The Canada Occupational Health and Safety Regulations apply to cranes used in workplaces under federal jurisdiction. They require, among other things:

Pitfalls to Avoid

Summary

Final Exam Tips

Mastery of mobile crane systems and components is the foundation of your trade. Every component has a specific function, and every procedure has a purpose. By understanding the principles, you will be able to answer exam questions with confidence and work safely in the field.