3.1 Types of Gears

Different gear types serve unique purposes in mechanical design, affecting efficiency, noise levels, and load capacity. Welding professionals involved in fabricating or maintaining machinery must grasp the distinctions among gear types to select or produce appropriate components that meet performance demands and durability standards.

3.1.1 Spur

Spur gears are among the simplest types of gear and are frequently encountered in fabrication workshops and industrial equipment maintenance. They have straight teeth cut parallel to the shaft axis and are used to connect parallel shafts. Their straightforward design makes them a fundamental study area for welding professionals who often repair or manufacture gear assemblies.

Design Characteristics of Spur Gears

Spur gears feature straight teeth parallel to the gear's axis of rotation. This alignment facilitates straightforward manufacturing processes such as milling or broaching, which are available in workshops serving the automotive and agricultural sectors. The simplicity of the design enables easy inspection and repair, an advantage in environments where downtime must be minimised.

Two meshing spur gears turn in opposite directions, and the speed ratio depends only on the numbers of teeth:

Gear ratio = number of teeth on driven gear ÷ number of teeth on driving gear

Speed of driven gear = speed of driving gear ÷ gear ratio

Example: a pinion with 20 teeth drives a gear with 60 teeth in the gearbox of a plate rolling machine in a fabrication workshop. The pinion turns at 1 440 rev/min.

Gear ratio = 60 ÷ 20 = 3

Speed of driven gear = 1 440 ÷ 3 = 480 rev/min

The driven gear turns three times slower but can deliver about three times the torque, ignoring small losses.

Load Transmission and Efficiency

Spur gears transmit power by direct tooth contact, resulting in efficient energy transfer with minimal power loss. Because the whole width of a tooth engages at once, the sudden contact can generate noise and vibration, which may be unsuitable for precision equipment such as the feed drive of a machine tool. Spur gears produce no axial thrust, so the bearings only need to carry radial loads. Their high efficiency and ease of production make them well suited to heavy-duty applications such as workshop conveyor and roller drives.

Applications in Welding and Fabrication

Spur gears are widely used in agricultural machinery such as maize milling machines and water pump drives. Welding professionals often fabricate or repair these gears, ensuring correct tooth profiles and alignment to prevent premature wear. Understanding the heat treatment processes used to enhance gear hardness is essential, because welding heat can soften or crack a hardened tooth and shorten its service life under heavy loads.

Maintenance and Failure Modes

Spur gears require regular lubrication to reduce friction and wear between teeth. Timely detection and repair of faults prevent cascading failures in complex gear trains, which could halt production in a fabrication plant. Technicians must be vigilant for these common failure signs:

• Tooth breakage: a tooth fractures at the root, usually after overload, shock loading or a fatigue crack that started at the root fillet.
• Pitting: small craters form on the tooth flank from repeated contact stress, often a sign of overload or poor lubrication.
• Abrasive wear: dirt or metal particles in the lubricant remove material from the tooth surface and change the profile.
• Misalignment: poor assembly or bent shafts cause contact on only part of the tooth width, which overloads one end of the tooth.
• Scoring: a broken lubricant film allows metal-to-metal contact, leaving scratches along the flank in the direction of sliding.

3.1.2 Helical

Helical gears present a more advanced gearing solution with their angled teeth, offering smoother and quieter operation compared to spur gears. Welding professionals engaged in precision engineering or high-speed machinery maintenance meet helical gears frequently, especially in vehicle transmissions and industrial compressors.

Tooth Geometry and Load Distribution

The teeth of helical gears are cut at an angle, called the helix angle, to the gear axis, creating a gradual engagement of teeth. Contact begins at one end of a tooth and spreads across its width as the gear turns. This spreads the load over several teeth at the same time, reducing stress concentration and increasing load-carrying capacity. For parallel shafts, the two gears must have the same helix angle but opposite hands, one right-hand and one left-hand.

Because the teeth are inclined, the tooth spacing measured across the face of the gear (transverse module) is larger than the module measured at right angles to the tooth (normal module):

Transverse module = normal module ÷ cos(helix angle)

Pitch diameter = transverse module × number of teeth

Example: a helical gear has a normal module of 3 mm, a helix angle of 20° and 30 teeth.

Transverse module = 3 ÷ cos 20° = 3 ÷ 0.9397 = 3.19 mm

Pitch diameter = 3.19 × 30 = 95.8 mm

A spur gear of the same module and tooth count would have a pitch diameter of 90 mm, so the helix angle must be allowed for when machining or checking a replacement gear.

Axial Thrust

The inclined teeth push the gear sideways along its shaft as well as turning it. This axial thrust increases with the helix angle and the transmitted torque, so the shaft must be supported by bearings that can carry end loads, such as tapered roller bearings or thrust bearings. Welders repairing a gearbox housing or bearing seat must restore these locations accurately so that the thrust is carried correctly.

Noise Reduction and Operational Smoothness

Due to the angled teeth, helical gears operate more quietly and smoothly than spur gears. This advantage is significant in machine tool headstocks and overhead crane hoists in a workshop, where reduced noise and vibration improve working conditions and finish quality. Welding professionals must ensure precise alignment during fabrication and assembly to maintain these performance benefits.

Manufacturing and Welding Considerations

Producing helical gears requires more complex machining techniques such as hobbing and grinding, which are found in well-equipped workshops. Welding repairs on helical gears demand careful heat control, including suitable preheat and slow cooling where the material requires it, to prevent distortion of the precise tooth angles. Distortion would compromise gear meshing and lead to premature failure.

Applications of Helical Gears in Industry

Helical gears are common in power transmission systems of road vehicles and manufacturing plants where smooth torque delivery is essential. A fleet workshop belonging to a transport SACCO, for example, will meet them in gearboxes and rear axle drives. Welding professionals tasked with maintenance must understand the interplay between gear material properties and welding techniques to ensure repairs restore original gear performance.

3.1.3 Double helical

Double helical gears, also known as herringbone gears, combine two sets of helical teeth of opposite hands on the same gear body. In some designs the two halves are separated by a central groove, and in others they meet in a V shape. This design is used in heavy industrial machinery and power plants, where high load capacities and minimal axial thrust are required.

Structural Design and Load Balancing

The opposing helical teeth of a double helical gear cancel out axial thrust forces, because each half pushes the gear in the opposite direction with equal force. This stabilises the gear during operation, reduces bearing loads and enhances the gear's lifespan. It is particularly important in large-scale equipment such as heavy rolling mill drives and large industrial pumps.

Advantages Over Single Helical Gears

By eliminating axial thrust, double helical gears prevent shaft deflection and associated wear in bearings and couplings. They keep the smooth, quiet action and high load capacity of a single helical gear, so they can use a steeper helix angle and transmit more power at the same size. Welding professionals working on heavy machinery benefit from this design as it reduces maintenance frequency and downtime, which is critical in continuous processing plants.

Fabrication Challenges and Welding Techniques

Manufacturing double helical gears requires precise machining and alignment to ensure the two sets of teeth mesh correctly and share the load equally. Welding repairs must maintain the gear's geometric integrity, because uneven heating or distortion can cause operational imbalance between the two halves. Workshops servicing large industrial clients therefore use specialised, carefully controlled welding procedures to address these challenges.

Industrial Applications of Double Helical Gears

Double helical gears are used in power transmission systems of hydroelectric plants and large-scale manufacturing equipment. Welding professionals engaged in maintaining these gears must combine metallurgical knowledge with precision welding skills to restore gear functionality without compromising strength or alignment.

Chapter Summary

This chapter explored various types of gears, beginning with spur gears known for their straightforward design and use in transmitting motion between parallel shafts. Helical gears were then examined, highlighting their angled teeth that provide smoother and quieter operation compared to spur gears. The chapter also covered double helical gears, which combine two sets of helical teeth to balance thrust forces and enhance load capacity. Moving on to gear trains, the simple gear train was introduced along with the necessary design calculations to determine speed ratios and torque transmission. Compound gear trains were discussed next, emphasizing their ability to achieve higher speed reductions within a compact arrangement, supported by detailed design calculations. The reverted gear train followed, noted for its configuration where the input and output shafts are aligned, with corresponding design calculations explained. The chapter concluded with an overview of epicyclic gear trains, recognized for their complex motion and high power density, and the importance of proper lubrication to ensure gear performance and longevity was also addressed.