Lubrication

History and Etymology of Gears - Gears date back to the 4th century BC in China and were preserved at the Luoyang Museum of Henan Province. - The Antikythera mechanism, built between 150 and 100 BC, is an early example of a complex geared device used to calculate astronomical positions. - Aristotle mentioned gears around 330 BC, specifically in wheel drives in windlasses. - Gears were used in water raising devices by Philon of Byzantium in ancient Greece. - Gears were developed by Archimedes in 3rd-century BC Ptolemaic Egypt and were connected to the mechanics of the Library of Alexandria. - The word 'gear' is derived from Old Norse 'gørvi,' meaning apparel or gear. - The specific mechanical sense of gears as parts by which a motor communicates motion originated in 1814. - The term 'cog' was first used in the 14th century to refer to a tooth on a wheel. - Historically, cogs were made of wood and were used when large metal gears couldn't be cut. - Wooden cogs were often made of maple wood and were used in paper mills and grist mills. Comparison with Drive Mechanisms - Gears provide an advantage over other drives in precision machines like watches due to their definite ratio. - Gears require fewer parts compared to other drives when the driver and follower are proximal. - However, gears are more expensive to manufacture and have higher lubrication requirements, leading to higher operating costs. Types of Gears - External gears have teeth formed on the outer surface of a cylinder or cone. - Internal gears have teeth formed on the inner surface of a cylinder or cone. - Bevel gears have an internal gear with a pitch angle exceeding 90 degrees. - Spur gears, also known as straight-cut gears, are the simplest type of gear. - Helical gears have teeth that are not straight-sided but have a helix shape. External versus Internal Gears - External gears cause output shaft direction reversal. - Internal gears do not cause output shaft direction reversal. - Internal gears are used for bevel gears with a pitch angle exceeding 90 degrees. Helical Gears and Configurations - Helical gears offer a refinement over spur gears. - The teeth are set at an angle, forming a segment of a helix. - Can be meshed in parallel or crossed orientations. - Engage more gradually, resulting in smoother and quieter operation. - Used in high-speed applications and for noise abatement. - Parallel configuration: shafts are parallel to each other, most common orientation for helical gears. - Crossed configuration: shafts are non-parallel, gears are sometimes known as skew gears. - Disadvantages of helical gears include resultant thrust along the axis of the gear and sliding friction between meshing teeth. - Herringbone gears and double helical gears can address these disadvantages. - Double helical gears overcome axial thrust presented by single helical gears by using a double set of teeth slanted in opposite directions.