4 predictive tools to improve gearbox performance

Falk Gearbox attached to a Marathon Electric motor

Like most machines, gearboxes fail over time. Typically, these failures can be traced to a few problems. Luckily, some basic preventive maintenance practices minimize gearbox breakdowns. There are numerous PdM programs available, but this article from Plant Services breaks the maintenance down to four areas: lubrication, temperature, noise, and vibration.

Lubrication

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.

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.

One of the key attributes to gear oil is viscosity. Providing an oil film between the gears, proper viscosity actually keeps the gear teeth from touching other by a very thin film. If the film is too thin – or no film at all – scoring will occur.

If the gearbox is exposed to extreme temperatures over the course of a year, oil viscosity becomes affected by temperature. As each season approaches, changing lubricants for the appropriate viscosity is a must.

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.

To effectively monitor the health of your gearbox, develop an oil-sampling program. By examining the oil periodically, you will be able to ascertain whether or not the oil is breaking down or if there’s moisture present.

Ensure that your gearbox is filled with enough oil by checking levels regularly. If there isn’t enough oil, gearing and bearings will end up damaged. If you notice a low level of oil, check the apparatus for a seal or gasket leak.

Conversely, if the gearbox if overfilled with oil, operating temperatures will jump and operating efficiency falls because of the extra energy required to churning losses from the gears and bearings. The increased operating temperatures reduce both oil and seal life, which inevitably lead 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.

Temperature

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.

Noise

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

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 up-time, 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 up-time is in the hands of the maintenance personnel that service the gearboxes. Following preventive maintenance procedures will increase machine up-time and life, reduce operating costs, improve product quality, protect the environment and provide a safer working environment.

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