- Seals and lubricants protect gearbox systems
- Steps to extend the life of gearboxes
- Heat Issues
- What about maintenance?
Bearings are at the heart of any power transmission system, and their makers have been toughening them up to take the pressure from hot washdowns. “Bearing manufacturers have improved the surface finish and geometric tolerance that increase bearing life and load carrying performance,” says Rick Wucherer, Boston Gear product manager. “Steel manufacturers have improved the cleanliness of the steel, which also increases bearing life.”
In washdown conditions, a better bearing does not correspond to longer life if the seals do not protect the lubrication and steel surfaces from corrosion. Upgrading to a stainless steel bearing insert increases the corrosion resistance of the steel surfaces and extends its life, according to Wucherer. Ideally, the bearing should include a shield or labyrinth to protect the rubber seal lip from damage due to high-pressure washdowns. For optimum performance, a stainless steel insert should be combined with a labyrinth-lip seal to keep the moisture out and protect the lubrication.
According to SKF data, metal fatigue is not usually a problem with the lifetime of a sealed bearing; it’s the grease life that is the limiting factor. Compared to standard SKF shielded deep-groove ball bearings, SKF energy-efficient deep-groove ball bearings can more than double the meantime between failure due to reduced heat generated by the bearings and the specially formulated SKF low-friction grease. In essence, the service life of the grease in the energy-efficient bearing defines the service life of the bearing. The grease life is defined as the period of time, at the end of which, 90 percent of a sufficiently large group of seemingly identical bearings are still reliably lubricated.1
A correct bearing maintenance program is one method to extend lifetime, explains Wucherer. “Unfortunately, this varies significantly from application to application,” he says. If properly applied, premium lubrication will last a very long time, but it can be degraded by water, chemicals, dirt or improper mixing of lubrications. Bearing re-lubrication should be completed with a grease of the same type of thickener; otherwise, the oil may separate from the grease and cause a lubrication failure. Over-greasing at high pressure can damage the seals, but under-greasing a bearing may lead to lubrication failure, warns Wucherer.
There are other potential issues to properly re-lubricating bearings, says Ian Rubin, director of marketing, mounted bearings, Power Transmission Solutions, Emerson Industrial Automation. “Re-lubricating with a grease gun can introduce contamination into a bearing if the fitting is not properly cleaned or the grease gun is not properly stored. Too much grease may also be added, and that can cause the bearing to run hot and possibly blow out the seals. Design matters too. For instance, we have been converting a customer to a Sealmaster reduced maintenance design, which does not require re-lubrication, and we are seeing increased bearing life as a result of the design change.”
Since seals keep lubrication in and foreign materials out of a gearbox, it’s necessary to make them last as long as possible. “It is important to use seals and lubricants that are tested and proven by the manufacturer to work together,” says Rich Mintz, Siemens process industries and drives (PD) division product manager, SIMOGEAR. “Look for a gearbox supplier that does this for you in its sizing software.”
“We use two proprietary techniques to preserve seals: roll burnishing and computer-aided gear centering,” says Rob Fuller, product manager—services, Emerson Power Transmission Solutions. Computer-aided gear centering ensures the load on the gear teeth is distributed equally, thereby reducing breakdown of the tooth material. The result of less material breakdown is less abrasive material contaminating the lubrication and acting as a grit to erode the seal lip. Roll-burnishing is a method of imparting a preferable surface onto the seal journal [i.e., the place on the shaft where the seal lip rides]. This “preferable” surface provides lubrication pockets that keep the seal lip cool and riding on top of a thin film of oil, adds Fuller.
“Most seal journals are ground, which leaves microscopic edges at the surface where the seal rides,” Fuller continues. “In these cases, as the new seal rides on the shaft, the sharp edges cut away at the seal lip, reducing its life and effectiveness. The roll-burnished seal is smooth and eliminates both of these problems to yield a long-lasting seal.”
“In general, a rolling gear—like a helical or bevel gear—is more efficient than a sliding gear, like a worm,” says Chris Wood, SEW Eurodrive industry account manager, food and beverage. “[However,] the heat generated by gear friction causes energy loss and reduces the life of oil and seals. Synthetic oil can be used, which has higher temperature ratings and longer life, usually 20,000 hours or more. Viton or PTFE seals also have better chemical resistance [than standard rubber seals] and can withstand higher temperatures and pressures from cleaning solutions.”
While tight seals are great for food processors, they are a challenge for gearbox designers and manufacturers. “Seal improvements continue to be made, but with increased sealing effectiveness comes increased frictional torque [drag],” says Rubin. This drag results in a lower speed capability that can also increase bearing operating temperature and break down the grease more quickly. So, the design becomes a balancing act to achieve the best specifications. “Many of the bearings we sell into washdown environments have extra caps and back seals to prevent direct spraying of the seal lips,” adds Rubin.
“Seals need to protect the lubrication and the bearing raceways, and prevent debris from entering the bearing,” says Wucherer. “To minimize bacteria growth, food processors are washing their equipment more frequently and with more aggressive chemicals. Lubrication that is degraded by water and washdown chemicals will not protect the raceways and will lead to shortened bearing life. Seals need a sealing lip as well as a metal shield to protect the rubber seal lip against high-pressure washdowns and prevent water and chemicals from entering the bearing. Incorrect chemicals or high concentrations of chemicals also can damage rubber seals.”
“The latest DODGE Ultra Kleen stainless steel gear reducers are completely sealed, with no paths for water to get in or lubrication to get out,” says Benjamin Hinds, Baldor-Dodge/ABB product manager. The DODGE Ultra Kleen Quantis reducer features highly efficient helical-bevel gearing in an all-stainless steel enclosure and stainless steel outputs for complete protection against washdowns in sanitary environments, says Hinds.
“Gear reducer seals in washdown environments typically consist of an oil lip seal with an outside excluder lip and, in some cases, two lip seals used in tandem,” explains Hinds. The DODGE XT harsh-duty output seal offers a two-piece design that prevents high-pressure washdowns from directly contacting the oil seal.
“Typical lip seals are made from rubber and are subject to time and environmental conditions,” adds Boston Gear’s Wucherer. “The rubber hardens with time and temperature, but it is a wear item. Premium mechanical face seals are available, but often are more expensive than a gearbox, so [processors] choose not to make the investment.”
The common solution is to use a spring-loaded lip seal, but caution must be taken to protect the seals from environmental issues, says Wucherer. For example, using an undersized gearbox will make it run hot, which will affect seal life since the rubber may harden more quickly. Therefore, the rubber used in every lip seal must be matched to the total operating range of the application. Seals fail quickly at temperatures above or below their operating range, but their life will be improved with temperatures approaching their limitations, says Wucherer.
Rubber seal materials have evolved significantly over the last 20 years, providing larger temperature ranges and lower friction, but the most common failure is still application induced, adds Wucherer. Protecting seals from dust/dirt will extend seal life because of reduced shaft/seal wear.
Food applications require easy-to-clean equipment and the elimination of confined areas that might allow bacteria to breed. The design of stainless steel gearboxes and seals should take into account CIP-type guidelines. Associations such as 3-A and NSF provide guidelines and certifications to manufacturers to help them build easy-to-clean, CIP-rated gearboxes.
Ratings for a device to be used in food plant washdowns would include, for example, IP69K, which indicates high-pressure water sprays (1,160-1,450 psi) at 176°F. According to Mitch Machelski, Cone Drive worm gear product manager, the company’s new line of IP69K-rated 316 stainless steel worm reducers is available in up to 184TC motor frame sizes with output torques up to 4,480 lb.-in. The devices are available in gear ratios from 5:1 to 60:1 in single reduction, and feature an aseptic design that is compliant with sanitary standards including FSMA, 3-A, HACCP and EHEDG. The reducers feature life-time lubrication with food-grade oil and double-enveloping worm gears.
“On the outside of the [gearbox], paints and coatings are always a problem in a heavy washdown environment,” says SEW Eurodrive’s Wood. “In many cases, NSF-3A specification paints and coatings are not acceptable. That only leaves stainless steel, which is a more expensive material, harder to machine and does not dissipate heat well. In most cases, a stainless steel product costs about two times the standard cast-iron product and gives no more reliability of life outside of having a non-corrosive surface.”
“Stainless steel gearboxes eliminate any exterior corrosion, but must be combined with stainless shafting to maximize seal performance,” says Wucherer. Many good coatings are available, but making one robust enough for every chemical, chemical combination and impact it might experience in the field is very difficult, he adds.
“Stainless steel gearboxes are definitely more expensive than comparable die-cast aluminum gearboxes,” says Matthew Roberson, senior director, gearmotor team, Brother International Corporation. “However, this high price is offset by their reliability in washdown environments vs. comparable powder-coated die-cast [boxes].”
There is one alternative to stainless steel gearboxes that provides high reliability while still being cost effective—electro-coated gearboxes. According to Roberson, these units go through the same process as many frames on automobiles. Each individual piece is submerged in electrically charged paint that clings to the metal and is baked onto the surface. This process provides full coverage of the aluminum, eliminating weak spots for rust to penetrate. Electroplating with acrylic paint also provides resistance to grease/oil, alkalis, acids and water/humidity.
Siemens has a coating system from Burke Industrial Coatings that inhibits the growth of a broad spectrum of bacteria, mold and fungi, and provides long-term protection against washdown chemicals, reports Siemens’ Mintz. Burke uses silver and copper ions embedded within the coating to inhibit these growths naturally.
“Case-hardened gears have a theoretical infinite life,” says Wood. “As long as the oil is between the gear teeth, there is no wear. The gear teeth only wear when the oil breaks down from contamination, and heat or an overload squeezes the oil from between them. Synthetic oils can increase life with higher temperature ratings and longer run times, [as said before] usually about 20,000 hours.”
“Gearboxes that run hot typically have poor efficiencies or involve high speed,” says Wucherer. In the last 10 years, gearing efficiency has improved through optimized designs and tighter manufacturing processes. Lubrication has a large impact on gearbox efficiency, but the ideal lubrication for a worm gear may not be ideal for a helical gear type. Using the optimum lubrication for each gearbox design is critical for getting the most out of the installed equipment. Simply because someone is using synthetic lubrication does not mean he/she is using the best lubrication available. Switching to a premium synthetic lubrication could increase system efficiency values by 10 percent or more, says Wucherer.
“The type of gearing is a significant factor in heat buildup,” says Emerson’s Fuller. “Worm gearing generally runs hotter than helical gearing at the same load. Also important is the selection of the correct reducer size. If a reducer is undersized, it may run hot.”
“Worm gears wear over time,” says Siemens’ Mintz. “It’s inherent in the sliding friction design of a bronze worm gear and a steel gear [vs. the rolling friction of helical or bevel gears].” This sliding friction, gear wear and the resulting inefficiency produce excessive heat. The combination of bronze particulate from the worn worm gears and excessive heat means lubricants must be changed regularly to prevent bearing damage and to keep gear wear to a minimum. “However, since the critical maintenance is often skipped, these units often have an even shorter life than expected,” adds Mintz.
“The life of a gearbox depends on many factors such as proper lubrication, correct application sizing and the internal design and construction of the gearbox,” says Wucherer. Helical bevel gearboxes typically have hardened gearing that has the potential to last forever if properly sized, lubricated, designed/manufactured and kept clean. The optimum design of spur and helical gearing maximizes the rolling between two gears and minimizes any slippage or loss. Unfortunately, some slippage will always be present, generating heat and reducing gearbox efficiency. Any slippage between gears can cause the hardened gears to wear, and that metallic wear debris will create more wear and shorten the life of the gears, bearings and seals. “Some gearbox manufacturers such as Boston Gear include metallic entrapment designs to remove the metallic wear debris from the oil sump, thus extending the life of the gearbox,” adds Wucherer.
During the selection of new equipment, one needs to research the efficiencies of potential gearbox solutions and weigh the differences in total cost of ownership vs. installation cost, says Wucherer. “For example, Boston Gear small worm gearboxes have efficiencies well over 90 percent at low ratios.” In comparison, three-stage helical bevel gearboxes have efficiencies constant throughout their ratio range, but have a higher installation cost. The monetary tradeoffs between these types of gearing require a full understanding of the intended application and exact mechanical efficiencies of each drive system, offers Wucherer.
“It’s pretty typical to see little or no maintenance in gearboxes in small [less than 5kNm] sizes like those used in many conveyor applications,” says Siemens’ Mintz. The most important thing to remember with gearboxes is that heat kills. “So, keep air circulation paths [free] by making sure equipment is clean and functioning. Remove the gunk and dust in motor fans and fins in non-washdown areas. Lubrication inside gearboxes and bearings take away heat; on-schedule oil changes are critical for long life,” observes Mintz.
But, what is a good maintenance program? “It really depends on the industry and whether the manufacturer has a strong reliability program,” explains SEW Eurodrive’s Wood. “Some units are supplied as ‘lube for life,’ meaning the synthetic oil supplied has a longer life than the bearings’ theoretical B10 life.” (B10 refers to the life at which 10 percent of the bearings in that application can be expected to fail due to classic fatigue failure or alternatively, the life at which 90 percent will still be operating.)2 “A lot of processors just run these until they fail. Most manufacturers’ recommendations involve proper oil levels and oil changing schedules. The more advanced manufacturers monitor vibration in gearing and bearings to determine when a gear reducer should be changed out,” says Wood.
“A good maintenance program operates predictively instead of reactively,” says Dirk Wernecke, manager—upstream marketing, The Timken Company. “Capturing the current status of bearings in the equipment via condition monitoring can detect potential problems at an early stage and allows for corrective actions before unplanned downtime occurs. Backed by industry-specific bearing inventory programs, a customer can have confidence in the operation of its assets when applying a predictive maintenance program.”
SKF recently released a bearing vibration monitor that allows users to safely, quickly and easily assess any problems with their machinery. The vibration assessment tool, the Wireless MicroVibe, is suitable for analyzing rotating equipment, and offers reliable and fast wireless (WiFi) data collection. “Anyone who owns a tablet can check the condition of a machine to ISO standards for bearing condition or by user-defined alarms,” says Torsten Bark, SKF product line manager. “They can even start a simple condition monitoring program for their rotating machinery with this simple entry-level solution.”
“Many food processors ignore the recommended maintenance schedule on small gearboxes and operate with MTBF [mean time between failures] in mind,” says Wucherer. “They run the gearbox right up to the time of failure and replace the unit. As the system becomes more critical and more expensive to replace, the maintenance is evaluated and a preventative maintenance schedule is often established.”
While a manufacturer’s recommended schedule might be adequate in some situations, it may not be ideal based on environmental conditions. Instead, a maintenance schedule should be designed to maximize gearbox life. This should include an oil chemistry evaluation to determine the optimum oil change frequency and whether any problems are developing, says Wucherer. For example, if a gearbox continuously has high water content, the root cause should be determined and a solution developed. Even low water concentrations inside a gearbox will attack typical steel shafting, bearings and gearing, and lead to shortened life.
“An aggressive cleaning process does more to reduce the life of power transmission equipment than its operation,” says Wood. “Making a gear reducer easy to clean reduces damage due to the cleaning process.”
Sizing gear motors
Don’t guess when it comes to sizing and selecting gear motors. Work with manufacturers with the expertise and tools to get it right the first time. According to Rich Mintz of Siemens PD division, here are some common mistakes to avoid:
The pressure’s on!
For many in the chicken processing industry, the Boston Gear SS700 stainless steel worm gearbox has become a reliable solution for installations where washdown challenges exist. The unit is designed to enhance processors’ sanitation efforts through the elimination of flat surfaces, holes and labels where moisture and bacteria could gather. The efficient stainless steel design includes a stainless axial face seal and food-grade lubrication. The gearbox carries NSF International certification.
For one leading chicken processing plant, however, the SS700 gearbox was not quite strong enough. Its pressurized washdowns far exceeded normal industry conditions.
After investigating the challenge, the Boston Gear engineering team developed a high-pressure seal option that could withstand the rigors of the intense washdowns. The new seal option incorporates an additional stainless exterior seal for rigidity, and has a housing modification that maintains the seal joint as well as the smooth exterior of the gearbox. This stainless cover rotates with the shaft while providing protection to the double-lipped seal in the unit.
This added protection has withstood pressures up to 1,000 psi for over 200 hours, without allowing water to damage the integrity of the oil. The new gearboxes have been installed and are successfully operating at the chicken processor’s facility.
For more information:
- SKF, “Bearing Service Life,” accessed 12-02-2014, SKF website (http://www.skf.com/group/products/bearings-units-housings/ball-bearings/deep-groove-ball-bearings/skf-energy-efficient-e2-deep-groove-ball-bearings/bearing-service-life/index.html).
- Wikipedia, “Bearing (mechanical);” http://en.wikipedia.org/wiki/Bearing_%28mechanical%29.