Gear Efficiency - Part 1

Summary

Professor Roland Ocean from the University of Technology in Sweden introduces the concept of gear efficiency in this lecture. He explains the sources of losses in gearboxes, including load-dependent and independent factors. The lecture delves into the mathematical expressions for efficiency, the influence of gear parameters on losses, and the measurement of gear efficiency. It also touches on friction in gear contact and its relation to elastohydrodynamic lubrication, aiming to provide a comprehensive understanding of gear performance and efficiency.

Takeaways

• 👨‍🏫 The lecture is led by Professor Roland Ocean, a professor of machine elements at the University of Technology in Sweden.
• 📘 The lecture covers the topic of gear efficiency, focusing on load-dependent and load-independent losses in gearboxes.
• 🔧 Losses in gear systems can be attributed to various sources such as oil churning, gear mesh friction, bearing drag, and windage.
• 📉 Load-independent losses are not influenced by the load but can vary with speed, including oil splash, windage, and churning losses.
• 📈 Load-dependent losses increase with the load and are primarily due to the contact between gear teeth, which can be influenced by the gear's shape and friction coefficient.
• 🛠 The lecture discusses the importance of reducing viscosity in lubricants to minimize load-independent losses, which is a current trend in the automotive industry.
• 🔄 The efficiency of gears is very high, often above 98%, but even a small percentage loss can represent a significant amount of power in high-power transmissions.
• 📐 Mathematical expressions are derived to understand the parameters influencing gear efficiency, such as sliding motion, gear geometry, and friction.
• 🔄 The slight rotation ratio, which measures the sliding motion in gear contacts, is crucial for understanding friction and efficiency in gear meshes.
• 🔧 The efficiency of a gear system is calculated as the power out divided by power in, and it varies along the engagement of the gear teeth due to sliding motion and friction.

Q & A

What are the two types of losses discussed in gear efficiency?

The two types of losses discussed are load-independent losses and load-dependent losses. Load-independent losses do not vary with load and include factors like oil splash, windage, and bearing drag, while load-dependent losses increase with increasing load and are primarily caused by the contact between the gear teeth.

How do oil splash and windage contribute to losses in a gearbox?

Oil splash occurs when gears dip into the lubricating oil, creating resistance, especially at high speeds. Windage, or air resistance, occurs when moving parts, like gears at high speeds, encounter resistance from the surrounding air, adding to overall losses.

What role does friction play in gear efficiency?

Friction occurs between the sliding surfaces of the gear teeth, causing energy losses. The friction coefficient depends on lubrication, and reducing this coefficient can improve efficiency. Surface treatments and coatings are used to minimize friction and enhance efficiency.

How does the speed and viscosity of the lubricant influence gear losses?

Higher speeds and higher viscosity of the lubricant increase losses due to more severe oil splashing and resistance. Many manufacturers try to reduce the viscosity of the oil to decrease losses, although this can lead to other issues in the gearbox.

What is the sliding-rolling ratio, and how does it affect gear efficiency?

The sliding-rolling ratio is a measure of how much the contact between gear teeth involves sliding versus rolling. A higher sliding-rolling ratio increases friction, while a pure rolling motion minimizes friction. The aim is to reduce the sliding motion to lower losses.

What is the significance of the base circle in gear design?

The base circle is crucial in defining how the gears roll and transfer torque and speed. It ensures a constant transfer of power between the gears, contributing to smooth motion and minimizing power losses.

How are forces and torques calculated in gear systems?

Forces and torques are calculated based on the normal and frictional forces acting at the contact point between gears. The torque around the center of the gear is determined by the force applied and the length of the gear's lever arm, adjusted for frictional forces.

What is the relationship between the friction coefficient and efficiency in gears?

The efficiency of a gear system decreases as the friction coefficient increases. Friction creates a small offset (Delta) from the pitch radius, which is related to the friction angle. This offset causes losses, and reducing the friction coefficient helps improve overall efficiency.

How does the gear helix angle influence efficiency?

The gear helix angle affects the efficiency by influencing the amount of sliding that occurs. A larger helix angle typically leads to more losses due to increased sliding, whereas a smaller angle tends to improve efficiency.

What are the key parameters that influence gear efficiency, as derived in the mathematical expression?

The key parameters that influence gear efficiency are the friction coefficient, gear helix angle, pressure angle, gear size (pitch radius), and the sliding motion along the line of action. These factors determine how much energy is lost due to friction and other resistances.