“Perfect” gears are defined as havi

“Perfect” gears are defined as having zero quasi-static transmission error at the nominal transmitted (design) mesh torque. They can only exist for a single load and, with proper modifications, have zero dynamic effects [e.g. zero transmission error (perfect conjugate action), zero excitation, no fluctuation at tooth mesh frequency and no fluctuation at rotational frequencies]. With zero excitation from the gears, there is zero response at any speed.6.1 Parameters affecting internal dynamic load and calculations6.1.1 DesignThe design parameters include the following:⎯ pitchline velocity;⎯ tooth load;⎯ inertia and stiffness of the rotating elements;⎯ tooth stiffness variation;⎯ lubricant properties;⎯ stiffness of bearings and case structure;⎯ critical speeds and internal vibration within the gear itself.20 © ISO 2006 – All rights reserved ISO 6336-1:2006(E)6.1.2 ManufacturingThe manufacturing considerations include the following:⎯ pitch deviations;⎯ runout of reference surfaces with respect to the axis of rotation;⎯ tooth flank deviations;⎯ compatibility of mating gear tooth elements;⎯ balance of parts;⎯ bearing fit and preload.6.1.3 Transmission perturbanceEven when the input torque and speed are constant, significant vibration of the gear masses, and resultant dynamic tooth loads, can exist. These loads result from the relative displacements between the mating gears as they vibrate in response to an excitation known as transmission error. The ideal kinematics of a gear pair require a constant ratio between the input and output rotations. Transmission error is defined as the departure from uniform relative angular motion of a pair of meshing gears. It is influenced by all deviations from the ideal gear tooth form and spacing due to the design and manufacture of the gears, and to the operational conditions under which the gears shall perform. The latter include the following.a) Pitch line velocity: the frequencies of excitation depend on the pitch line velocity and module.b) Gear mesh stiffness variations as the gear teeth pass through the meshing cycle: this source of excitation is especially pronounced in spur gears. Spur and helical gears with total contact ratios greater than 2,0 have less stiffness variation.c) Transmitted tooth load: since deflections are load-dependent, gear tooth profile modifications can be designed to give uniform velocity ratio only for one magnitude of load. Loads different from the design load will give increased transmission error.d) Dynamic unbalance of the gears and shafts.e) Application environment: excessive wear and plastic deformation of the gear tooth profiles increase the transmission error. Gears shall have a properly designed lubrication system, enclosure and seals to maintain a safe operating temperature and a contamination-free environment.f) Shaft alignment: gear tooth alignment is influenced by load and thermal deformations of the gears, shafts, bearings and housing.g) Excitation induced by tooth friction.6.1.4 Dynamic responseThe effects of dynamic tooth loads are influenced by the following:⎯ mass of the gears, shafts, and other major internal components;⎯ stiffness of the gear teeth, gear blanks, shafts, bearings and housings;⎯ damping, the principal sources of which are the shaft bearings and seals, while other sources includehysteresis of the gear shafts and viscous damping at sliding interfaces and shaft couplings.