Gears tooth contact

This paper briefly reviews some current research on gear tooth contact lubrication that will lead to a better understanding of the problems of micropitting, scuffing and gear and transmission lubrication in general. 

Figure 8. Pressure and film thickness at a particular timestep in a micro EHL solution for gear tooth contact in an FZG test. Corresponding maximum Hertzian pressure = 1.4 GPa. Also shown are contours of z/GPa, the sub-surface maximum shear stress.

To promote even wear, a hunting tooth design is desirable. In this design, a pinion tooth does not contact a given gear tooth more than once until it has meshed with all the other gear teeth. 

Figure 52.10 Comparison of the apparent viscosities of a gear oil and a gear grease. The grease flows readily in the tooth-contact region, where shear stress is high, but resists leakage at seals and joints...

The elements of gear teeth common to all gears are tooth surface and profile, flank, top and bottom land, crown, root and pitch circle, gear center, line of centers, pitch point, line of action, line of contact, and point of contact. Figure 57.27 labels many of the common gear tooth elements. Figure 57.28 labels the common rack tooth elements. 

The problem with thin fluid film lubrication occurs when the relative motion of the solid surfaces either stops completely, stops at reversal in reciprocating motion or the dynamic loading of a cam on its follower, one gear tooth on another or on a journal within a bearing such that this lubrication mechanism fails and the surfaces make contact. Under boundary lubrication conditions the role of adsorbed molecular films of protective additives is crucial in protecting against wear. 

Figure 15-17. Block diagram of the thermal flow in a lubricated gear and bearing system. Heat sources.—A oil film at tooth contact B churning of bulk oil C oil film in bearings and bulk churning D oil film at Seals E external sources. Heat transmission.—F m G c H c, m I c J f K m L f M f N m P f Q c, f, r R c S n, f, r T c. c = conduction f = forced convection m = mass transport n = natural convection r = radiation.

It is a steel carriage which holds the test gear. It will integrate several parts as the fixing system for the micro gear, the pressure system which will assure the tooth contact without backlash among both gears teeth once the nominal centres position is reached, and a capacitive sensor. 

The contact coils, translating the signal to the gas capsule, were housed in PC. The hub and gear tooth system of the coil was made of acetal and PTFE, with a centring device in an ABS graft polymer. 

In addition to gear tooth wear, center-to-center distance between shafts will create an erratic spacing and amplitude. If the shafts are too close together, the spacing will tend to be at input shaft speed, but the amplitude will drop drastically. Because the gears are deeply meshed, i.e., below the normal pitch line, the teeth will maintain contact through the entire mesh. This loss of clearance will result in lower amplitudes but will exa erate any tooth profile defect that may be present. 

These curves, based on experimental gear failure results, establish the specific lubricant film thickness A) for which the probability of tooth contact failure is 5% and 80%, as a function of the gear pitch line velocity. 

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