- Many gearbox failures can be traced to a few problems, and basic preventive maintenance practices will minimize these failures. The four areas in this article are lubrication, temperature, noise, and vibration.
- Viscosity at a temperature is a key attribute of the gear oil. Periodic oil analysis indicates if the oil is still suitable, if there is gear or bearing wear and helps establish oil change intervals.
- Temperature readings should be compared against previous baseline values to confirm the gearbox is operating as efficiently as it once was, but thermal data alone won’t tell where the problem is located.
- Gearboxes rated in accordance with AGMA standards have a sump oil temperature limit of 200° F. Synthetic oils can operate to 225° F, but are more expensive than mineral oils.
Gearboxes are a key component of many industrial power transmission systems. Industrial gearboxes are highly engineered machines. Many components in gear drives have tight tolerances and optimized gear geometry, which are required for transferring working loads as smoothly and efficiently as possible. As with other industrial machines, the successful operation and long life of a gearbox is directly related to proper maintenance.
Fortunately, most gearboxes are easy to maintain. Moreover, many gearbox failures can be traced to a few problems, and basic preventive maintenance practices will minimize these failures. While there are many preventive maintenance tools available, we will focus on four areas to keep your applications running: lubrication, temperature, noise and vibration.
Lubrication is one of the most important components of a gearbox. Oil has two main purposes. It keeps components from wearing and also keeps them cool. Most gearbox failures can be attributed to improper lubrication.
Figure 1. Checking oil levels on a regular basis should be part of your preventive maintenance program. Too little oil can damage gearing and bearings. Too much oil may cause operating temperatures to increase, causing decreased efficiency and reduced oil and seal life.
Many gearboxes are not supplied with oil, so it is important to follow the manufacturer’s recommendations when purchasing oils. If the wrong type of oil or incorrect viscosity is used, the result can be gear or bearing failures. If the gearbox is factory-filled with oil, again follow the manufacturer’s recommendations to ensure that additional oil added in the field is compatible.
Viscosity is a key attribute of the gear oil. The proper oil viscosity will provide an oil film between meshing gear teeth. This oil film is very thin and keeps the gear teeth from actually contacting each other. With too thin of a film or no film, failures such as scoring or wear will occur.
Temperature changes affect the oil viscosity. If your application is exposed to extreme temperature variations throughout the year, lubricant viscosity grades should be changed for the season. For example, an ISO 320 grade viscosity mineral oil is a typical selection for ambient temperatures of 50° F to 125° F. If this oil is used in the winter and the temperature is 10° F, the oil will be too viscous to flow and properly lubricate components. Synthetic oils are suitable for a wider temperature range but may have compatibility issues with seals. Again, it is best to follow manufacturer’s recommendations for oil selection based on ambient temperature.
Like the oil in your automobile, it is essential that the gearbox oil is replaced regularly. Oil and grease will break down over time. As the oil breaks down, it is less effective at maintaining the proper oil film. Also contaminants that build up in the sump will be flushed out during the oil change.
An oil-sampling program is very effective at monitoring the health of your equipment. Periodic oil analysis will indicate if water is getting into the oil or if the oil is breaking down. It will also tell if there is gear or bearing wear. Sampling can also be used to establish oil change intervals based on the lubricant’s actual condition.