Final Drive, Wheels, and Tire Systems

Wheel Hubs, Wheels, and Tire Systems

Introduction to Wheel and Tire Systems

The wheel and tire system of heavy equipment is a critical assembly that transmits power from the drivetrain to the ground, supports the machine's load, and ensures stability and steering. For a Heavy Duty Equipment Technician (HDET), mastering this system is essential, as it represents a frequent point of failure and a major safety concern. This chapter covers fundamental principles, diagnostic procedures, load calculations, applicable Canadian standards, and common pitfalls to avoid on the Red Seal exam.

You must understand the interaction between the hub (the center of the wheel that supports the bearings), the rim (the metal part on which the tire is mounted), and the tire itself. A failure in any of these components can result in loss of control, fire, or serious accident. Precision in torquing, mounting, and balancing is therefore non-negotiable.

Wheel Hubs

Definition and Function

The hub is the central part of a wheel that mounts onto the axle. It contains the bearings that allow the wheel to rotate freely around the stationary axle (or to rotate with the axle in the case of a drive axle). The hub also supports the brake drum or disc and provides the mounting surface for the rim.

There are two main hub configurations:

Tapered roller bearing hubs: Used on non-drive axles (trailers, front steering axles). They are adjustable and require periodic maintenance.

Cylindrical roller bearing hubs: Used on drive axles (planetary or non-planetary). They are typically installed with an interference fit and are not adjustable.

Hub Bearings

Bearings are the components that reduce friction between the hub and the axle. Their proper operation depends on lubrication, clearance adjustment, and alignment.

Bearing Type	Typical Application	Key Characteristic	Adjustment Required
Tapered (cone and cup)	Front axles, trailers	Supports radial and axial loads	Yes, 0.03 to 0.13 mm (0.001 to 0.005 in) end play
Cylindrical roller	Drive axles, planetary hubs	Supports high radial loads	No, press fit
Ball	Small wheels, light-duty applications	Low friction	Variable
Thrust ball or roller	Steering axles	Supports steering axial load	No

Operating principle of tapered roller bearings: The tapered bearing is designed to support combined loads (radial and axial). The contact point between the cone and cup is a line, which distributes the load over a larger surface area. Clearance adjustment is critical: too much end play causes clicking noise and premature wear; too little end play causes overheating and rapid failure.

Tapered Bearing Adjustment Procedure

16.Cleaning: Thoroughly clean the hub, spindle, and bearings with an appropriate solvent.

17.Inspection: Inspect the raceways, rollers, and cage for any signs of spalling, scoring, or discoloration (bluing indicates overheating).

18.Lubrication: Apply a high-quality grease specified by the manufacturer (typically NLGI #2).

19.Assembly: Install the inner bearing into the hub, then mount the hub onto the spindle. Install the outer bearing and washer.

20.Initial adjustment: Torque the adjusting nut to 200 N·m (150 lb-ft) while rotating the wheel to seat the bearings properly.

21.Back off: Loosen the nut by one-quarter turn (90°).

22.Final adjustment: Tighten the nut by hand or with very low torque (approximately 5 N·m) until contact is made. Then, install the cotter pin or locking device.

23.End play verification: Axial end play should be 0.03 to 0.13 mm (0.001 to 0.005 in). You can verify this with a dial indicator.

Exam trap: Never torque the bearing nut to the final specification without first backing it off after the initial tightening. This would crush the bearings and give a false adjustment.

Drive Axle Hubs (Planetary)

On heavy drive axles, the hub is often integrated with a planetary gear reduction. This system is mounted inside the wheel hub and provides final speed reduction and torque multiplication. The hub is then bolted to the axle and contains the planet carrier, planet gears, and ring gear.

Key service points:

The oil level in the planetary hub must be checked regularly. A leak at the axle seal can contaminate the brakes.

The torque on the hub bolts is critical and must follow manufacturer specifications (often between 400 and 600 N·m).

Using the incorrect oil can cause premature wear of the planetary gears.

Wheels and Rims

Types of Rims

The rim is the metal part that supports the tire. There are several types of rims, each designed for specific applications.

Rim Type	Description	Application
Drop center rim	The center groove is deeper to facilitate tire mounting.	Cars, light trucks, some equipment
Semi-drop center rim	Compromise between drop center and flat base rims.	Medium trucks, trailers
Flat base rim	The base is flat; the tire is held by locking rings.	Heavy trucks, construction equipment
Multi-piece rim	Composed of several parts (base, locking ring, retaining ring).	Heavy equipment, construction machinery
O-ring lock rim	A rubber ring provides the seal between the rim and the tire.	Mining equipment, large machinery

Safety rule: Multi-piece rims are extremely dangerous if disassembled or inflated incorrectly. The locking ring can detach with explosive force. It is imperative to follow complete deflation procedures before any disassembly and to use an inflation cage.

Wheel Bolts and Nuts

The wheel is secured to the hub using bolts or nuts. Tightening must be done in a star or cross pattern to ensure uniform pressure.

Typical torque specifications:

Cone (tapered) nuts: 400 to 600 N·m (300 to 450 lb-ft) for heavy trucks.

Flange nuts: 500 to 700 N·m (370 to 520 lb-ft) for heavy equipment.

Planetary hub bolts: 400 to 600 N·m.

Tightening procedure:

43.Clean the threads and contact surfaces (hub and wheel).

44.Apply a small amount of oil or anti-seize lubricant to the threads (unless otherwise specified).

45.Tighten the nuts by hand.

46.Tighten with a torque wrench following a star pattern, in three passes: 50%, 75%, then 100% of the final torque.

47.Re-check the torque after 50 to 100 km of service (or 1 to 2 hours of operation).

Exam trap: Never use grease on the threads or contact surfaces of cone nuts, as this can skew torque readings and cause loosening.

Calculating Load per Wheel

You must be able to calculate the static load on each wheel to verify that the tires and rims are adequate.

Basic formula:

Load per wheel = (Total machine weight) / (Number of wheels)

Example: A wheel loader weighs 20,000 kg and has 4 wheels.

Load per wheel = 20,000 kg / 4 = 5,000 kg.

However, weight distribution is not always uniform. You must consider front/rear and left/right distribution.

More precise formula:

Load on front wheel = (Total weight × % load on front axle) / (Number of wheels on front axle)

Example: A truck weighing 30,000 kg has 60% of its weight on the front axle (2 wheels) and 40% on the rear axle (4 wheels).

Load on front wheel = (30,000 × 0.60) / 2 = 9,000 kg.

Load on rear wheel = (30,000 × 0.40) / 4 = 3,000 kg.

Important: These calculations are static. Under dynamic conditions (braking, cornering), the load can increase significantly. You must verify the tire's load index (see next section).

Tire Systems

Tire Anatomy

A tire is composed of several layers of materials:

Tread: The rubber layer in contact with the ground.

Carcass: The reinforcement structure (plies) that gives the tire its shape.

Bead: The part that engages with the rim.

Bead wire: A steel cable in the bead that holds the tire onto the rim.

Sidewall: The lateral part between the tread and the bead.

Belt plies: Reinforcement layers under the tread (often steel).

Tire Construction Types

Type	Description	Advantages	Disadvantages
Bias ply	Plies are arranged diagonally, crossing over each other.	Sidewall more resistant to impacts, lower cost.	Higher heat generation, faster wear on road.
Radial	Plies are perpendicular to the direction of travel, with steel belts.	Better grip, more uniform wear, lower rolling resistance.	Softer sidewall, more susceptible to cuts.

Application: For heavy equipment, radial tires are increasingly common, but bias ply tires are still used on construction machinery where the risk of cuts is high.

Tire Markings and Load Indices

The markings on the tire sidewall provide crucial information. You must know how to interpret them.

Example marking: 445/65R22.5

445: Tire width in millimeters.

65: Aspect ratio (sidewall height / width × 100). Here, the height is 65% of the width.

R: Radial construction.

22.5: Rim diameter in inches.

Load index: A number that corresponds to a maximum load in kg. This table is standardized.

Load Index	Load (kg)
100	800
110	1060
120	1400
130	1900
140	2500
150	3350

Speed symbol: A letter that indicates the maximum speed for which the tire is designed (e.g., L = 120 km/h, M = 130 km/h, N = 140 km/h). For heavy equipment, speed symbols are often lower (F = 80 km/h, G = 90 km/h).

Exam trap: Do not confuse the load index with inflation pressure. The load index is a maximum capacity at a reference pressure (often 700 kPa or 100 psi). A lower pressure reduces the load capacity.

Inflation Pressure

Inflation pressure is essential for performance, safety, and tire service life.

Too low pressure: Overheating, shoulder wear, possible bead unseating, increased fuel consumption.

Too high pressure: Center tread wear, risk of blowout, reduced comfort.

General rule: Pressure should be checked cold (tires not driven for at least 3 hours). Pressure increases with operating temperature. An increase of 10 to 15% over cold pressure is normal.

Recommended pressure: Always refer to the equipment and tire manufacturer's specifications. For heavy equipment, pressure can range from 350 kPa (50 psi) to over 700 kPa (100 psi).

Tire Wear and Diagnosis

Tire wear is a valuable indicator of alignment, suspension, or pressure problems.

Wear Pattern	Probable Cause
Center tread wear	Inflation pressure too high.
Shoulder wear	Inflation pressure too low.
Sawtooth wear (one shoulder)	Poor alignment (excessive camber or toe).
Uneven wear on one side	Excessive camber.
Flat spots (cupping)	Wheel imbalance, worn shock absorbers, loose bearings.

Minimum tread depth: Canadian law requires a minimum tread depth of 1.6 mm (2/32 in) for road vehicles. However, for heavy equipment, it is recommended to replace tires before reaching this minimum, especially for steering tires.

Canadian Standards and Codes

You must be familiar with the standards governing wheels and tires in Canada.

Canadian Electrical Code, Part I (CE Code)

Although this code deals with electricity, it is relevant to heavy equipment. Rule 8-200 of the CE Code concerns grounding and bonding requirements for mobile vehicles and equipment. This includes grounding the chassis and metal components, which can affect wheel mounting (for example, aluminum wheels may require special precautions to ensure electrical continuity).

CSA B149.1 - Natural Gas and Propane Installation Code

This standard applies to vehicles and equipment operating on natural gas or propane. Section 6 of CSA B149.1 deals with vehicles and engines. It specifies requirements for the installation of tanks, piping, and regulators. Although this does not directly concern tires, a technician working on propane equipment must be aware that work on wheels must not damage nearby gas lines.

Tire Safety Standards

The Motor Vehicle Tire Safety Regulations (SOR/2013-198) is a federal standard that specifies performance requirements for tires sold in Canada. It is based on the National Highway Traffic Safety Administration (NHTSA) standards from the United States. You must ensure that the tires you install carry the appropriate conformity marking (DOT for American tires, or an equivalent marking).

Motor Vehicle Safety Regulations (MVSR)

The MVSR is a set of federal standards governing the manufacture and importation of vehicles. Standard 120 of the MVSR deals with tire and rim selection for motor vehicles. It specifies load and pressure requirements for tires. You must ensure that the tires and rims you install meet the original equipment manufacturer's specifications.

Diagnostic and Maintenance Procedures

Checking Bearing End Play

107.Raise the wheel with an appropriate jack.

108.Grasp the tire at 12 o'clock and 6 o'clock.

109.Move the wheel up and down. Noticeable play indicates loose bearings.

110.Grasp the tire at 3 o'clock and 9 o'clock.

111.Move the wheel side to side. Noticeable play may indicate loose bearings or worn spindles.

Note: Slight axial end play is normal for tapered roller bearings. Radial play (up-down) should not be noticeable.

Diagnosing Vibrations

Vibrations in the steering wheel or cab can have several causes:

Wheel imbalance: Vibrations at certain speeds.

Flat-spotted tires: Constant vibrations.

Worn bearings: Grumbling noise that changes when cornering.

Incorrect alignment: Vehicle pulls to one side.

Diagnostic procedure:

120.Check tire pressure.

121.Visually inspect tires for bulges, cuts, or abnormal wear.

122.Check bearing end play.

123.Spin the wheel on a balancer to check static and dynamic balance.

124.If vibrations persist, check alignment.

Tire Removal and Installation Procedure

Safety first:

127.Completely deflate the tire by removing the valve core.

128.Place the wheel-tire assembly in an inflation cage or use a demounting protector.

129.Use a hydraulic or manual tire changer.

130.Lubricate the tire beads with an appropriate lubricant.

131.Never use sharp tools that could damage the bead wire.

Installation:

133.Inspect the rim for cracks, corrosion, or damage.

134.Install the tire onto the rim, ensuring the beads are properly positioned.

135.Inflate the tire inside the inflation cage to the recommended pressure.

136.Verify that the locking rings (if applicable) are properly installed.

137.Torque the wheel nuts to the specified value.

Exam trap: Never inflate a tire beyond the maximum pressure indicated on the sidewall to attempt to seat the beads. Use a quick-inflation device with a pressure regulator.

Pitfalls to Avoid

140.Confusing axial end play and radial play: Axial end play is the up-and-down (or side-to-side) movement due to clearance in the bearings. Radial play is lateral movement. Excessive radial play is always a problem.

141.Forgetting to back off the bearing nut after initial tightening: This crushes the bearings and gives a false adjustment.

142.Using an impact wrench for the final tightening of wheel nuts: This can damage threads and produce uneven torque. Always use a torque wrench.

143.Not checking tire pressure cold: Hot pressure is higher and will give a false reading.

144.Ignoring the tire's load index: A tire with too low a load index can overheat and blow out.

145.Confusing torque specifications for cone and flange nuts: The torques are different, and an error can cause loosening or breakage.

146.Not using an inflation cage: This is a serious negligence that can result in fatal injuries.

147.Forgetting to check the tire's manufacturing date: Tires age and degrade even if unused. The date is indicated by the DOT code (e.g., 2319 = 23rd week of 2019).

148.Confusing pressure units: kPa, psi, and bar are different units. 1 bar = 100 kPa = 14.5 psi. A conversion error can be catastrophic.

149.Neglecting rim inspection: A cracked or corroded rim can break under load. Always inspect before mounting.

Summary

The hub is the center of the wheel, containing the bearings. Tapered roller bearings require precise end play adjustment (0.03 to 0.13 mm), while cylindrical roller bearings are installed with an interference fit.

Rims can be drop center, flat base, or multi-piece. Multi-piece rims are dangerous and require extreme precautions.

Wheel nut tightening must be done with a torque wrench, in a star pattern, in three passes, to the specified torque.

Tires are characterized by their construction (bias ply or radial), load index, and speed symbol. Inflation pressure must be checked cold.

Tire wear is an indicator of alignment, pressure, or suspension problems.

Relevant Canadian standards include the CE Code (Rule 8-200) for grounding, CSA B149.1 for gas vehicles, and the MVSR (Standard 120) for tire and rim selection.

Safety is paramount: always use an inflation cage, completely deflate before disassembly, and never exceed the maximum pressure.

Mastering these concepts will not only help you pass the Red Seal exam but also enable you to perform safe and professional work in the field. Precision and rigor are the hallmarks of an excellent technician.