Mechanical Power Transmission Systems

Mechanical Power Transmission Systems

Chapter Introduction

This chapter covers all mechanical power transmission systems you must master for the Red Seal exam. As an industrial mechanic (millwright), you will be called upon to install, align, troubleshoot, and replace these components. The exam evaluates not only your ability to identify components, but also your understanding of physical principles, speed and torque calculations, alignment procedures, and safety requirements. This chapter is structured to follow a logical progression: from fundamental principles to practical applications, with special attention to common pitfalls.

Fundamental Principles of Power Transmission

Definition and Role

Mechanical power transmission is the set of devices that transfer mechanical energy from a source (electric motor, internal combustion engine, turbine) to a receiving machine (pump, compressor, conveyor). The role of the industrial mechanic is to ensure that this transmission occurs with optimal efficiency, minimal wear, and maximum safety.

Fundamental Parameters: Speed, Torque, and Power

Three physical quantities govern all transmission systems:

The fundamental relationship is:

P = T × Ω

Where Ω (omega) is the angular velocity in radians per second (rad/s). The conversion between RPM and rad/s is:

Ω = (2 × π × N) / 60

In practice, for quick calculations, you will often use:

P (kW) = T (N·m) × N (RPM) / 9550

P (hp) = T (lb·ft) × N (RPM) / 5252

Transmission Ratio

The transmission ratio (i) is the ratio between the input speed and the output speed of a system:

i = N₁ / N₂ = D₂ / D₁ = Z₂ / Z₁

Where:

Golden rule: in a transmission system, the product of torque × speed is constant (neglecting losses). If speed decreases, torque increases proportionally.

Drive Belts

Types of Belts

V-Belts: Designations and Dimensions

V-belts are designated by their cross-section (A, B, C, D, E) according to standards. The nominal dimensions are:

Belt Length Calculation

For a V-belt, the pitch length (Lp) is calculated as:

Lp = 2C + (π/2) × (D₁ + D₂) + (D₂ − D₁)² / (4C)

Where:

Common pitfall: the pitch length is not the outside length. Belts are measured on their pitch diameter, which is located approximately at the centre of the cross-section.

Belt Tension

Correct tension is essential. A belt that is too loose will slip and overheat; a belt that is too tight will overload bearings and reduce service life.

Deflection method: apply a perpendicular force at the midpoint of the free span. The deflection should be approximately 1/64 of an inch per inch of free span (approximately 1.5 mm per 100 mm). The force to apply depends on the belt cross-section.

Rule of thumb: the deflection should be approximately equal to the belt thickness for correct tension.

Pulley Alignment

Pulley alignment is critical. Poor alignment causes premature wear, vibration, and power loss. Use:

Requirement: pulley faces must be in the same plane with a tolerance of ±0.5 mm for V-belts.

Drive Chains

Types of Chains

Roller Chain Designations

ANSI chains are designated by a number: the first digit indicates the pitch in eighths of an inch, the second digit indicates the type (0 = standard, 1 = light, 5 = heavy).

Examples:

Chain Length Calculation

Chain length is expressed as the number of links. For a chain drive:

L = (2C/P) + (Z₁ + Z₂)/2 + (Z₂ − Z₁)² / (4π² × C/P)

Where:

Important rule: the number of links must be a whole number. If the calculation gives a fractional number, round up to the next whole number and adjust the centre distance.

Chain Tension and Alignment

The recommended sag for a chain is approximately 2 to 4% of the centre distance (measured at the midpoint of the free span). A tensioner may be necessary if the centre distance is not adjustable.

Alignment: sprockets must be in the same plane with a tolerance of ±0.25 mm. Angular misalignment of more than 1° is unacceptable.

Chain Lubrication

Lubrication is essential for service life. Methods range from manual lubrication (low speed) to oil bath or spray lubrication (high speed). The general rule:

Gears

Types of Gears

Gear Ratio and Torque

For a pair of gears:

i = Z₂ / Z₁ = N₁ / N₂

The output torque is:

T₂ = T₁ × i × η

Where η is the efficiency (typically 0.95 to 0.98 for spur gears, 0.85 to 0.90 for worm gears).

Backlash

Backlash is the space between the tooth flanks of two meshing gears. It is necessary to:

Typical backlash is 0.05 to 0.15 mm for standard industrial gears. Excessive backlash causes noise and shock loads; insufficient backlash causes overheating and rapid wear.

Gear Installation

When installing a pair of gears:

Couplings

Role and Classification

A coupling connects two shafts to transmit torque while compensating for alignment defects. Types include:

Coupling Alignment

Alignment is one of the most critical operations in the trade. Methods:

Typical tolerances (according to ISO 1940 and manufacturer recommendations):

Dial Indicator Alignment Procedure

Common pitfall: never align a machine cold when it operates hot. Thermal expansion moves the shaft; you must account for a "thermal offset" (usually provided by the manufacturer).

Gearboxes and Speed Reducers

Types of Reducers

Calculating Reducer Output Torque

T₂ = T₁ × i × η

Example: a 5 kW motor at 1750 RPM drives a 20:1 reducer with 92% efficiency.

Checking Thermal Capacity

A reducer may be limited by its thermal capacity (heat dissipation) rather than its mechanical capacity. If the oil temperature exceeds 90 °C, you must:

Transmission Shafts and Bearings

Shafts

Transmission shafts transmit torque and support loads. Design criteria include:

Torsional stress:

τ = (16 × T) / (π × d³)

Where τ = shear stress (Pa), T = torque (N·m), d = shaft diameter (m).

Bearings

Bearing Installation

Hot installation: heat the bearing in an oven or oil bath to 80-120 °C (never above 125 °C). Never use a torch directly on the bearing.

Cold installation: use a press or a mounting tube that presses on the inner race (never on the outer race for a shaft mounting).

Internal clearance: a bearing's internal clearance decreases when mounted with interference. Negative clearance (preload) can cause overheating.

Applicable Standards and Codes

Canadian Electrical Code (CE Code)

The Canadian Electrical Code, Part I (CE Code) (CSA C22.1) applies to electrical installations, including motors and their protection. Relevant points:

Although this code is electrical, the industrial mechanic must know these rules because you work in interface with motors.

CSA B149.1 (Natural Gas and Propane Installation Code)

This code applies to gas installations, including gas engines and compressors. Relevant points:

CSA Machinery Standards

Relevant ISO Standards

Safety Procedures

Lockout/Tagout

Before any intervention on a transmission system:

Machine Guards

All transmission elements (belts, chains, gears, couplings) must be protected by guards compliant with CSA Z432:

Rotor Balancing

Balancing is necessary for pulleys, rotors, and flywheels. The balancing quality G is defined by ISO 1940:

Pitfalls to Avoid

Summary

Self-Assessment Questions

(Answers: 1. 65.9 N·m; 2. 480 RPM; 3. 1275 N·m; 4. 12 to 24 mm; 5. 0.05 mm parallel, 0.05 mm/100 mm angular; 6. 125%)