- 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.
Oil levels should be checked on a regular basis. If a gearbox is not filled with enough oil, gearing and bearings can be starved resulting in damage. A low oil level may also indicate a seal or gasket leak. On the other hand, if the gearbox is overfilled with oil, operating temperatures will increase and efficiency will be reduced due to churning losses from the gears and bearings. Elevated operating temperatures can lead to reduced oil and seal life. High oil levels often contribute to oil seal leakage.
When inspecting the gearbox, check for leaks at shaft seal areas. Seals will wear over time, but leaks can also be an indicator of contaminated or degraded lubricant. Leaking seals should be replaced as they can allow contaminants to enter the gearbox, can cause the oil level to drop or can have a negative environmental impact if the oil leaks are not contained. Also, many gearboxes have a grease cavity outboard of the shaft seal to keep dirt and dust out of the gearbox. These cavities should be purged by pumping grease through them with a grease gun when changing oil and more frequently for highly contaminated environments.
Filters and strainers are important for removing contaminants when the gearbox is equipped with a pressurized lubrication system. They should be replaced or cleaned when changing oil or when the filter’s bypass indicator warning appears. Clogged or dirty filters will allow contaminated oil to bypass the filter and go directly to bearings and gears, causing debris damage and shortened life.
Like filters, vents are used to keep contaminants out of the gearbox. They also allow the gearbox to breathe. Vents should be inspected and cleaned periodically to ensure they are not clogged. A plugged breather can cause a pressure buildup in the gearbox leading to seal failure and oil leakage. Desiccant type breathers are used to remove moisture from air entering the gearbox and should be replaced when they become saturated. Most manufacturers have added dye to the media, which changes colors when saturated.
Figure 2. A heat gun is one of several tools to measure and monitor the temperature of equipment. A gearbox that’s running too hot will degrade oil, leading to wear on gears and bearings, along with shortening the life of seals.
Temperature measurement is another useful tool in the maintenance toolbox. Thermography, heat guns, gauges, RTDs or thermocouples can detect changes in gearbox operation. If temperature is monitored, a baseline reading should be established. Subsequent readings should be compared against previous values and that data trended. A rise in temperature or localized hot spots can indicate that the gearbox is not operating as efficiently as it once was due to a problem with either the gears or bearings. Thermal data alone may not tell where the problem is located, but it can provide enough information to warrant a deeper investigation into the gearbox health and spare the user from costly downtime due to a catastrophic failure.
Temperature control is also important for oil life. Gearboxes rated in accordance with AGMA standards have a sump oil temperature limit of 200° F. For sump temperatures above 200° F, R & O mineral oils start to degrade rapidly and gear and bearing wear may occur along with shortened seal life. Synthetic oils have been used successfully in operations up to 225° F, but are more expensive than mineral oils. If gearboxes are running close to or above this limit, cooling devices should be used. Most manufacturers offer cooling packages such as shaft-driven fans, electric-motor-driven fans or heat exchangers to keep gearboxes running at lower temperatures.
The machine operator and maintenance person’s ears are also useful preventive maintenance tools. Abnormal sounds are often the first indicator that something is wrong with the gearbox. An increased sound level may indicate worn or damaged gears and bearings. Knocks can be the result of broken teeth or bearings. Rattles may be caused by loose fasteners or high vibration. Squeals can be an indicator of loose bearings or metal rubs. A machinist’s stethoscope or screwdriver are good for locating problems.
Vibration readings can be a good indicator of gearbox health. Vibration analysis can help detect coupling misalignment, improper foundation support (soft foot) and gear or bearing damage. A baseline reading should be taken with the gearbox installed and connected to the driven equipment. Like temperature measurements, vibration trends can help tell what’s happening inside the gearbox. Vibration readings should be reviewed by a trained analyst. Most gearbox manufacturers can provide gear tooth counts and bearing numbers to aid the vibration analysis.
With today’s ever increasing demands on machinery uptime, it is important to keep equipment in good operating condition. The cost of machine downtime can be in the tens of thousands of dollars per hour. The key to obtaining long life and improved uptime is in the hands of the maintenance personnel that service the gearboxes. Following preventive maintenance procedures will increase machine uptime and life, reduce operating costs, improve product quality, protect the environment and provide a safer working environment.