# Pinion Failure Analysis of a Helical Reduction Gearbox in a Kraft Process
## 1 Introduction  
Gearboxes are widely employed for mechanical power transmission in automobiles, windmills, machine tools, plant machinery, and many other mechanical systems. Enhancement and reduction of the speed of the output shaft are achieved using gearboxes at reduced and increased torques, respectively. From the economical perspectives, the efficient and reliable functioning of gearboxes of machinery is extremely vital. The failure of gearboxes in industrial plants can cause malfunction in machinery, which leads to production loss, a costly affair in terms of loss of profit. It also involves the additional cost of repair/replacement of failed gears. It is worth noting here that gear failure may occur due to design error, manufacturing error (e.g., poor machining, faulty heat treatment of parts, poor gear-set assembly), or improper installation and/or operation. Design errors mainly include improper gear geometry, poor selection of materials, and inadequate lubrication arrangement [1, 2]. However, improper installation and operation involve faulty mounting, inadequate cooling, improper lubrication, and poor maintenance [3, 4, 5, 6]. When errors are present, as described in the previous paragraph, gears fail and show the symptoms of scoring, wear, pitting, plastic flow, and teeth breakage/fracture. Scoring occurs because of lubrication failure, which causes asperity-to-asperity contact between the teeth of the pinion and gear [7, 8, 9]. This results in micro-welding at the tip of the asperity followed by tearing, which leads to the continuous and rapid removal of material from the teeth surface. However, wear is a type of damage that occurs owing to the progressive removal of material from the interacting surfaces. Material removal in wear may be due to adhesion, abrasion, corrosion, or a combination of these. Wear causes increased surface roughness, enhanced clearance at the mating interface, and weakened teeth. Fitting occurs at the gear teeth surface because of repeated loading that results in the contact stress exceeding the surface fatigue strength of the material. Pitting failure takes place over millions ofcycles of gear revolutions. During the pitting process, if material is removed from the mating surfaces of teeth in the form of a flake, it is called flaking/spalling. High contact stresses at the interfaces of teeth under rolling/sliding motion can cause the plastic flow of teeth surfaces as well. This type of failure is normally found in softer gear materials. Moreover, teeth fracture/breakage also takes place because of high overload arising from either impact loading or static load itself [10; 11]. In many situations, the inspection of failed parts and analysis of data do not provide information regarding the cause of gear failure. In this case, gear design checks and laboratory tests are required to develop an understanding on the probable cause(s) of failure. It has been reported that if failure is influenced by the gear geometry, then checking for design and metallurgical defects should be performed. It is always recommended to perform non-destructive tests (e.g., measuring the surface hardness and roughness, magnetic particle inspection, acid etch inspection, gear tooth accuracy inspection) before performing any type of destructive tests (e.g., micro-hardness measurement, microstructural determination using acid etches, determination of grain size, inspection for non-metallic inclusions, SEM microscopy) on failed gears/components. Over the past several years, analyses have been conducted on the failure of gears of different mechanical systems. It has been reported that the majority of gear failures have occurred because of manufacturing and operational error of gears and gearboxes, respectively [12; 13; 14; 15; 16; 17; 18]. However, bending fatigue [19; 20], wear and surface contact fatigue [21; 22], and design faults [23; 24] have also contributed significantly to failure.