Failure Prevention

Much of this site is devoted to information that can be used to diagnose the root cause of a failure, and to steps that can be taken to prevent a reoccurrence of the failure. This section is intended to provide information for preventing failure from occurring in the first place. Since there is so much variation in the design and usage of gearboxes, not all of this information will be applicable in all cases.

Before Purchase of Gearbox

One of the most effective ways of preventing a gearbox failure is to ensure that the design of the gearbox is appropriate for its usage environment. If the gearbox is a custom design for the application, the best means to ensure that the resulting design will provide reliable service is the development of a good procurement specification for the gearbox. The procurement specification should include interface requirements, lubrication requirements (i.e viscosity, additive content, base oil formulation) desired ratio, gearbox load information, minimum gear and bearing design margin, required calculation methodologies for gear and bearing life, material cleanliness, gear accuracy, required system life and reliability, minimum and maximum operating temperature, allowable maintenance intervals, acoustic noise requirements, and other such information. ANSI/AGMA/AWEA 6006-A03 provides guidelines for design and specification of gearboxes for wind turbines. It can be used as a model for procurement specifications for other applications. If the application requires a very high reliability, or if the consequence of failure is high, a due diligence review of the gearbox design should be performed by an independent third party expert. For information on recommended third party experts, please submit a question through our Ask an Expert page. If the gearbox is an off-the-shelf design, the suitability of the design for the intended application should be reviewed. To the extent possible, the same factors that are considered for a custom design should be considered for an off-the-shelf design.

During Fabrication of Gearbox

If the gearbox is a custom unit, all of the gearbox quality documentation should be reviewed. See ANSI/AGMA/AWEA 6006-A03 Annex C for more information on quality assurance. Typical QA documentation includes material cleanliness certifications, gear metallurgical reports, gear charts, deviation reports, vibration and noise test data for assembled gearbox, and other such documents. The gearbox should also be run-in in stages, with a suitable run-in oil. The cleanliness of the oil during break in should be monitored, and the load not increased until the oil has reached a predetermined cleanliness level. Once break in is complete, the gearbox should be drained and flushed and refilled with the oil to be used in service. See the article on oil cleanliness  on our “Resources” page.

During Transportation of Gearbox

Although some gearboxes are huge and capable of transmitting massive amounts of torque, a gearbox is in many ways a delicate precision component, and must be handled accordingly. The biggest risk while transporting a gearbox is fretting damage to gears and bearings due to relative motion between the mating surfaces, combined with a lack of rotation and lubricant film in the contact interface. See the article on fretting and false brinelling  on our “Resources” page. Care should be taken to minimize the amount of vibration that the gearbox is exposed to during transportation, and to ensuring a good lubricant film is present when the gearbox first begins it journey. Special equipment can also be designed to continually rotate the gearbox to maintain the oil film in the contact interface, and to prevent repeated oscillatory motion at the same location on gears and bearings.

During Installation of Gearbox

Proper alignment between the gearbox, the prime mover, and the driven piece of equipment is critical for the proper functioning of the gearbox. If the alignment is not within specified limits, the gears and bearings can be subject to loads for which they were not designed, and can fail prematurely. Depending on the gearbox speed, balancing might be important as well. A gearbox that operates at high speed that is not properly balanced can develop high levels of vibration, which can also result in premature failure.

During Operation of Gearbox

Even if all of the best practices regarding specification, fabrication, transportation, and installation of the gearbox were followed, a gearbox is not likely to give reliable service unless it is operated and maintained properly. Important operational parameters include keeping gearbox loads within the design operating envelope, and minimizing the number of start and stop cycles and speed excursions to the extent possible. Proper maintenance is also critical to reliable gearbox operation. The most important preventive maintenance activities involve keeping the gearbox lubricant clean, dry, and cool. Many of the failure modes discussed on this website (micropitting, macropitting, scuffing, abrasion) can be caused in whole or in part by lubricant condition, and properly maintaining the lubrication system is the best means of preventing these failure modes. Periodic lubrication condition monitoring is an extremely important part of a preventative maintenance program. The monitoring frequency should be set such that lubrication problems are detected well before significant equipment damage has occurred. Monitoring frequencies vary from application to application, but typical frequencies for gearbox lubrication monitoring range from 300 to 2000 operating hours, and tests typically include water content, viscosity, acid number, FTIR (Fourier transform infrared) spectroscopy, elemental analysis, particle count, and oxidation level. Information provided by oil condition testing can be used to take action to improve lubricant condition prior to the occurrence of a catastrophic failure or to identify and repair a failing component before it fails and causes extensive collateral damage to other gearbox components. Regular change out of filter elements is important to maintain oil cleanliness, which is critically important to gear and bearing life. Maintenance of the oil cooling system is also important, as oil viscosity is a strong function of temperature, and a gearbox that is operating with oil at elevated temperatures will have lower lubrication film thickness and increased rates of gear and bearing life consumption. See ANSI/AGMA/AWEA 6006-A03 Annex F for more information on condition monitoring. Vibration monitoring can be a useful means of identifying potential reliability issues before a failure occurs, providing time to take corrective action. For questions about your specific application, please use the “Ask an Expert” link for more information.  Also see the articles on How to Inspect a Gearbox and Best Practices for Analyzing Gear Failures on our “Resources” page.