Engine Gear Train Design

Abstract

A drive system is generally located at the front of the engine and is used to drive essential engine components such as camshaft, fuel pump and oil pump. All gears are finally connected to the crank gear which provides the input power to all other gears. First the choice of spur or helical gear is made considering various aspects like noise, width allowed for the gear casing, machining simplicity ability to maintain accuracy and overall cost of the engine. Gear train can be simple or compound, the latter resulting more compact train. Contact ratio of engagement is studied for strength estimation as well as noise. For the complete design of engine gear train, first, loads from the fuel injection pump, engine camshaft, oil pump, water pump, air compressor and the hydraulic pump are estimated. The choice of the gear pressure angle is made considering its significance to the normal load on the shafts and the bearings. The calculation of gear consists of forces on gear, basic rack tooth, profile selection, material and heat treatment process. Then the safety factor for surface durability and tooth bending are calculated. The backlash of gear train is usually higher in engines using shell bearings than in, e.g., machinery using ball bearings, because of higher bearing clearances. While large backlash enables gear-train running free of interference, it can introduce rattle especially at those gear-pairs, where the load is alternating, e.g., at the gear driving high pressure plunger type fuel pump. Gear whine at high frequency stands out even when its sound power is relatively less than from other noise sources. The choice of gear material is made considering material availability, cost, fatigue limits, temperature limits, fracture toughness, load and torque carrying capacity, manufacturing requirements, weight, treatments, machinability, operational characteristics, damping characteristics, and corrosion and wear resistance. While designing the steel gear train, heat treatment is specified for strength against wear, pitting and bending.