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The system characteristics of the entire train must be known so that the selection of the gear will be proper. The major points affecting the systemare: (1) couplings， (2)vibration， (3) operation conditions， (4) thrustloads， and (5) mounting type.
Couplings are a constant source of unbalance vibration， and critical speed changes can be attributed to spacer shift and wear. Coupling lockeys canalso cause severe housingvibration， while shaft vibration can remain low.Therefore， it is important to monitor vibration with accelerometers in add-ition to proximity probes. Gear failure from high vibration is common when the gear and pinion teeth operate within a few hundred microinches of each other. Accelerometers can also monitor gear mesh frequencies and thus act as early warning devices. Operating conditions must be known in detail.
In manycases， the gear manufacturer is provided with only the design horsepower of the machine. Actual transmitted loads can be much higher due to the proximity of torsional or lateral critical speeds. Surge in centri-fugal compressors can cause severe overload and result in failures.
External thrust loads are another major problem， and in many cases they lead to double-helical gear selection. The gear housing and the mounting type of the gear train are very important considerations in the overall life oftheunit， since improper mounting and expansion of the gear housing can lead to misalignment problems.
A substantial tosupport the gear drive weight，thrust， and torquereactionswithministructuremum load deflections must be provided. At leasttwo dowels for locating each gear housing are required， and it is necessaryto minimize housing vibration from whatever source.Ideally， the structures should be reinforced concrete of steel filled with grout. The inclusion of oil reservoirs in the structure supporting major train components should beavoided， since unavoidable thermal changes will have adverse effects onalignment. If a reinforced concrete or a filled structure cannot beprovided， resonance due to train component mass and structure stiffness at system rotational frequencies or harmonies should be avoided.
Gear Types
The choice between single-and double-helical gearing is sometimes diffi-cult. Both gearing types can be made to equal limits of accuracy as control of the gearing accuracy is only a function of the accuracy and maintenance ofthe gear-generating machines， machining techniques， and operator skill. A hobbed gear is generated in a continuous process by a simple and easilymaintained rackfromcutter， which will produce gearing of extremely high-profile accuracy with virtually immeasurable spacing errors and uniform lead. Where both helices of a double-helical gear are cut at the sametime， orsequentially without a change insetup， apex position error will be virtually unobservable inoperation， and axial vibration excitation from the mesh will be negligible. The same basic equipment can be used for generating either single-ordouble-helicalgears， although the continuous finishing processes used for double-helical gears produce a higher order of accuracy of lead and tooth spacing than does grinding if it is used for finishing a single-helical type.
Thrust loading is a significant problem in the design of any gearunit， and effects differ based on the choice of single-or double-helical gearing. In either case an accurate estimate of the thrust loading is required to make anintelligent compensation for it. With double-helical gearing， continuous axial loading can be accommodated by a slight increase in capacity to account for the helix load imbalance. API 613 and 617 specifications require that single-helical gears must have a thrust bearing. It is also recommended (but not required) that double-helical gears have thrust bearings.
The increase in cost and the reduction of efficiency thus caused is only afraction of that incurred when a large-diameter， high-velocity thrust bearing must be mounted on a single-helical pinion shaft.
　
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