Just like typical cars, gearboxes have gotten smaller over time. The catch phrase is ‘power-dense’, meaning that from equal sized gearboxes, the newer model has a more available rating. Due mostly to technological advances, current model gearboxes weigh an average of 25% less than their 30 year-old old counterpart while delivering the same amount of torque, and 35-40% less than their 40 year-old old counterpart. Since applications typically dictate the Hp requirement, this means that the gearbox has gotten smaller over time. A smaller gearbox means that less oil and surface area is available to lubricate and cool the rotating elements. Both of these items are paramount to the long-term operation of a gearbox.
Small, power-dense gearboxes are often considered more economical than their older brethren. Because of their size, they require smaller foundations and structures to support them; use smaller quantities of lubricant to operate; and use less raw material to construct (and inevitably dispose of, which becomes more prevalent in our ever increasing “go-green” society). With this economical head-start it’s easy to think of many gearboxes as disposable or “throw-away.” Many people trying to justify why they don’t have or can’t make the time needed to properly maintain a gearbox claim, “It’s cheaper to buy a new one.” Don’t be fooled! It is easy for a plant to spend more money on repairing and replacing gearboxes in a few short years than to maintain one properly over the course of a many years.
We could spend countless hours pouring over a basic total cost of ownership (TCO) spreadsheet, researching wages, lost profit, lubrication costs, various fees, and service charges to name just a few of the costs. Each operation would then have to customize their own TCO spreadsheet to suit their particular process, only to support what common sense already tells us. Let us just agree on principle that it costs less to buy one gearbox in 10 years than five gearboxes for the same period. If you know that buying one new car and having it for 10 years is more economical than buying five new cars in 10 years, you should be able to see a correlation.
So what’s the purpose of this article, you ask? It’s not to convince you that you need to take care of your gearboxes; you already know that. What we are going to do is give you a few pointers and some ideas on HOW to properly maintain your gearboxes.
Sumitomo’s Top Ten List!
Here are the top ten tips to keep your power transmission equipment operating at peak performance; at highest efficiency; smooth and quiet; longer between repairs and rebuilds:
10. Ensure the gearbox is sized correctly to avoid inevitable repairs and costly replacements.
9. Store the unit properly according to manufacturer’s recommendations until it’s time to install, in order to avoid rust and dry rot.
8. Align the gearbox and components to the driven equipment to prevent excessive shaft or bearing loads.
7. Ensure the mounting sub-base is suitable for the weight and forces of the gearbox assembly to keep natural frequency vibrations from shortening the life of rotating parts.
6. Lubricant is a major contributing factor to the life of the gearbox.
• Confirm the unit is filled with lubricant.
• Fill the unit with the proper grade, type, and quantity of lubricant if required, meeting manufacturer’s requirements.
5. Replace the lubricant according to manufacturer’s recommended intervals, including start-up periods. Pay close attention to units with oil in addition to grease requirements, e.g. oil lube gears and grease lube bearings, as they might be on different schedules.
4. Monitor the operation. The cooler the lubricant, the longer it will last!
– Keep the oil COOL – CLEAN – DRY.
• Consider supplemental cooling from a water or air cooler system.
• Consider filtering if the environment is dusty or humid.
• Prevent water ingress to avoid rust.
– Data points and trend analysis to ‘predict’ concerns.
• Oil analysis
• Gearbox housing or oil temperature
3. Inspections are required to confirm severity of trending issues. Plan for regular external [and internal] inspections, if the gearbox type allows. Raw data is good, but nothing beats seeing or feeling the parts first hand and understanding the condition of its severity.
2. Establish a preventative maintenance plan.
And the top item is…The Big Commitment
1. A commitment to maintaining your gearbox is paramount for it to work economically. When you devise and stick to a plan to maintain your gearbox, your ROI (return on investment) will be realized sooner and your TCO will be reduced. If possible, make it part of an ISO procedure to ‘force’ the implementation.
Ok, you’re comfortable with the gearbox selection. One potential problem exists if it’s delivered ahead of the installation schedule. This also applies to critical spares for inventory. It’s the first offense many make: Protect your investment.
Unless the gearbox is going into installation and operation immediately, you’ll have to store it. If you have the option, store the gearbox inside. It will at least be protected from the elements. Most gearboxes shipped from reputable manufactures will ship the gearbox with some rust inhibitor inside the unit and modifications for short-term exposure. We have seen gearboxes succumb to rust in under a week of exposure, so you need to use some common sense when determining what ‘short-term’ means to you.
The ideal conditions for storage are dry, low humidity, small temperature variances and non-corrosive atmospheres. When the temperature fluctuates greatly between daily highs and lows, condensation can build up inside the gearbox. If exposed to enough condensation build-up, the water will overpower the factory rust inhibitor. In response you might say, “So fill up the gearbox completely with oil!”
Yes, metal submerged in oil will not oxidize. In some cases, that’s the best option. In other cases, it’s the only option. In a few cases, you’ll negatively affect the gearbox’s operating performance (and maybe not even know it). With every action comes a trade-off. For smaller gearboxes this typically won’t be a problem. The best argument against completely filling large units with oil is that the standard oil seals may not be designed to have such a large head of oil behind them. Contrary to popular belief, most seals are only capable of handling a few PSI’s of pressure. The amount of oil required to generate enough PSI varies depending on the size and orientation of the gearbox. Temperature can also affect the pressure of the oil. As a unit full of oil bakes in the sun, the heated oil will want to expand. In certain instances, only a few gallons of oil provide enough weight to weep oil through the seals. The issue is scalable. Larger units required more oil.
So you have a large unit that costs a lot of money and because you don’t have the indoor space for it, you store it outside and probably out-of-mind. Units are usually stored outside because of limited space or outdoor projects. The units probably won’t be looked at as part of a general preventive maintenance procedure, so as the unit weeps oil you have a) an environmental problem and b) air backfilling the gearbox. This air condenses, and we’re back to the problem addressed earlier, and you now have an oil spill to clean up.
Consider a temporary structure or “lean-to” in order to protect your investment from the elements. Follow the gearbox manufacturer’s recommendations for storage and the schedule for reapplying rust preventative.
Is there really such a thing as a quick start guide?
Installation starts by securing the gearbox firmly to a rigid foundation if floor-mounted, or securing it to the shaft for shaft-mounted installations. Follow the gearbox manufacturer’s recommendation for mounting bolt grade or strength and use all of the available mounting points. If mounting to a structure, your structure must be capable of supporting the weight of the gearbox assembly and oil without deflection. If there is deflection in the mounting structure, it must not exceed the allowable limits of the shaft connections. Couplings, chain and sprocket, or V-belts and sheaves typically make shaft connections. Each of these shaft connections has their own recommended alignment tolerances.
Not all gearboxes are equivalent — some will have different installation parameters. It’s important to read and understand the complete manufacturer’s maintenance manual for your equipment. Sometimes there might even be different manuals if you are installing different product models. For example, the mounting surface needs to be level to within tenths of a degree on some gearboxes. Figure 1
After the gearbox is level according to specifications, alignment of the motors, chains, and v-belts are the next critical item on your new checklist. Shaft connections need to be as accurate in parallel and angular alignment as possible. Depending on your installation configuration, it is strongly suggested to fix one component and align all other components to it. If you are installing separate motor and gearbox components, align the gearbox to the driven shaft of your equipment, and then align the motor to the gearbox. If you are installing a completed bedplate assembly from Sumitomo, the gearbox is mounted to the bedplate and motors and couplings are aligned to it. This allows the entire bedplate to be aligned to the driven equipment.
Trending is the Key…
The external critical items to monitor on a normal gearbox application would be 1) temperature, 2) vibration, and 3) noise. An increase of any (or all) of these is a significant signal that something is wrong. We are talking about even the beginning stages of a wearing item, not necessarily an immanent catastrophic event. Gear or bearing wear can progress for months or years depending on the speed, load condition, and lubrication.
These external ‘signals’ are indicators of the operating condition of internal components. The gears, bearings, seals, and lubrication are often trying to ‘say’ something to the operator. A temperature rise can be caused by any of the elements of a gearbox. Noise can also be generated from all of the elements of the gearbox. Noise can also be caused by vibration. Vibration can be caused by all of the elements in the gearbox. There is no single cause and effect critical path when it comes to the signals being put out by the internal components. This is where a bit of experience and “CSI” methodology require operators to determine root-cause problems. All reputable gearbox manufactures will have personnel at the factory or local warehouse who can offer technical assistance.
The main root cause of bearings, gears, and oil degredation are typically:
a) overload operating conditions (too much throughput or E-stops)
b) improper oil viscosity
c) oil cleanliness (dusty, dirty or wet environments? Your last oil change was when?)
d) oil temperature
There are a number of ways to combat the monitoring of various gearbox components. Trending these items is the key. If we assume that the plant is responsible for overload conditions imposed on the gearbox and these are minimized, we can focus on actually monitoring the gearbox. Figure 2
Lubrication is the lifeblood of a gearbox. You want to keep the oil clean, cool, and dry.
Just as you would never run your car without oil, so it is the same for the gearbox. Sounds simple! Make sure the gearbox is filled to the proper level with the proper lubricant. Some gearboxes come factory filled. In this case, visual inspections are required to ensure no leaks develop. For many other cases, the lubricant type and quantity is the responsibility of maintenance personnel. Check the gearbox operating and maintenance (or Installation) manual for the proper grade of lubricant. Sometimes this will also be on the nameplate. Usually different ambient temperatures require different lubrications, so be aware of outdoor operations that span winter and summer seasons. Lower temperatures require thinner oil to flow effectively and conversely thicker oil for higher temperatures to compensate for heat breakdown. Check the gearbox for the actual oil fill level. Most operating manuals list approximate oil quantities. Always fill the gearbox to the level indicated by a sight gauge, dipstick, or fill level plug.
In a perfect world, properly selected lubricant will protect all the compents The reality is that all manufactured components will have some normal wear. These normal wear metal particulates have to go somewhere, and into the oil bath or lube system they go! Left alone, these particles will act as abrasives and further wear the gearbox or lube system components until an inevitable failure. This is why most gearbox manufactures recommend an initial oil change somewhere in the first 500 hours (or six months of operation). Grease-filled units don’t often have the same change requirements, since the grease has a tendency to suspend the particles rather than splash them around the gearbox and into critical components.
This is magical to some people, but there is a science behind it. Automobile manufactures typically recommend between a 3000 to 5000 mile oil change interval depending on the type of driving you do. To be more specific, this is an average life span of the oil viscosity. Automobile manufacturers base this estimate on countless hours of raw testing and it means we are changing oil on a convenience schedule instead of when it is actually required.
During operation, the oil will cycle through the various components. At each load point on the gears and bearings, the oil is put under extreme pressure. The oil is designed to absorb this extreme pressure, by providing a thin film of lubricant, but not infinitely. After an estimated time period, our experience indicates that the oil will start to lose its ability to provide adequate protection against pressure. Inadequate viscosity can lead to scuffing which creates wear particles. Wear particles only lead to more wear. Too high of a viscosity leads to ‘skidding’ which generates heat. Too low of a viscosity leads to ‘flow’ problems (or inadequate lubrication during splash) generating more heat. Theoretically, if you maintain the proper grade of oil, the gears and bearings should last for a very long time. There are a few exceptions to the rule and sometimes a multipronged approach is best. In our experience, maintaining the oil quality is the first priority.
Your first option to protect against viscosity breakdown is to change the oil at the designated intervals, per manufacturer’s instructions. This option, while convenient, may have you throwing away ‘good’ oil. Arguably the next best option is to determine the oil viscosity and trend it through oil sampling and testing (also known as tribology). There are many labs across the country that specialize in testing oil. Chances are your lubricant supplier can do it or recommend someone. All major lubricant manufacturers have some testing capabilities and strongly recommend this option to predict your oil life span. After only a few tests you will be able to accurately predict the oil change interval for your gearbox. If any problems do develop with the gearbox and it is returned to the manufacturer, you’ll have the proper documentation to illustrate when and why you changed the oil – despite the operating manual recommendations. If you have complete records, I predict that the gearbox manufacturer will be more interested in the condition of the oil over time, then whether or not you followed the operating manual to the letter. How can they argue against using the proper viscosity of oil? Just be sure to maintain your records – or you won’t have a ‘leg-to-stand-on’ when they inevitably question you about your oil changes! Sumitomo engineers will consider your accurate laboratory records during a warranty evaluation without penalizing you for changing the oil before the oil goes bad, not just during the maintenance manual schedule.
Cleanliness is Next to Godliness…
Particulates in the oil are a close second problematic area after viscosity breakdown. As your car has an oil filter, sometimes your gearbox should too. A magnetic drain plug is low end technolog, and used in conjunction with other methods, is an inexpensive and relatively effective method of keeping metallic wear particles out of rotating elements. If your gearbox is forced-lubricated (through a shaft or motor driven pump), always make sure the lubricant is filtered. If you have an extreme environment, like severe dust or dirt, consider adding a circulation pump and filtering the oil. On larger units where breathers are required, filtered breathers for dusty and dirty environments help. Most breathers require some form of regular maintenance to ensure they are not clogged, or that the desiccant is still active. A clogged breather will pressurize the gearbox and the path of least resistance is usually the seals. So if your gearbox is weeping oil at the seal, check to ensure the breather is not clogged before scheduling a rebuild. Additionally, gearboxes buried in dirt, coal, or dust do not transfer heat as effectively as a clean gearbox. Keep the housing surface clean to avoid oil contamination and heat buildup.
Keeping it cool…
Heat is a natural component of all mechanical devices. It’s this heat that helps to break the oil down. Once the oil starts to break down it usually is a very fast decline to ‘worn’ oil. This ultimately leads to metal-to-metal contact. Some experts would recommend trending this temperature rise. When you trend the temperature of the gearbox, the idea being able to determine if ‘something’ is going wrong with the gearbox. Unfortunately, a rise in temperature can be, among other things, a bearing going bad or from ‘worn’ oil. Ultimately you are left to determine the cause of the temperature rise. While this is a relatively inexpensive up-front alternative, it usually leads to more questions.
Heat will break down the viscosity of the oil, preventing it from properly lubricating the gearbox components. You can keep the oil cool in a number of ways. Usually the most common and cost effective is to keep the gearbox within its thermal limitations. All gearboxes have a thermal rating. For some units, thermal rating is not a problem; it may exceed the mechanical rating by design (like most Sumitomo Cyclo Drives) or because high reduction ratios (above 90:1) correlate to more gears and bigger housings. For units with gear teeth, like worm or hardened and ground helical or bevel gearing, as the unit ratio gets smaller the unit is more thermally limited. Simply put, this has to do with surface area. The more surface area, the more heat transfer (convection) can occur.
The Torque-HP-RPM formula says that for constant torque (like a gear), Hp will vary with speed. Therfore high speed equates to high horsepower. The lower the ratio, the faster the gearbox, and the higher the HP capacity for an equivalent-sized gearbox. Lower ratios are typically 1, 2, or 3-stage units with high mechanical Hp ratings but a relatively small housing surface area. These units can be boosted thermally by an economical shaft driven fan that in many cases is a standard option. For units where the fan doesn’t provide enough boost, you should consider a separate cooling system. This is similar in principle to your car cooling system, except the oil is pumped in directly through a radiator and cooler oil is returned back to the gearbox. Gearbox radiators or heat exchangers can be oil-to-water or oil-to-air coolers. Oil-to-air is just like your car radiator. The oil-to-water uses plant water to more effectively transfer heat.
Keeping it dry
Everyone knows that oil and water don’t mix. Besides interfering with the proper lubrication of the rolling elements, metal corrosion can be a problem. Corrosion will lead to particulates. For outdoor or humid environments, there are a couple of tricks to slow down water ingress into a gearbox. It sounds weird, but an immersion oil heater (usually used to keep oil warm in cold weather starts) can keep the oil at a constant temperature between day and evening temperature swings where condensation might be a problem. For a unit running 24/7, condensation might not be an issue, since the gearbox is at a relatively constant temperature. For units that only shut down for periods of time, like second and third shifts, condensation build-up might be an issue for you. An immersion oil heater can help.
Another method is to install a hydroscopic breather. These breathers are filled with desiccant to absorb moisture. Some hydroscopic breathers come with plugs to allow air to filter through the desiccant. In a humid environment, it’s imperative to only remove one or two plugs (typically) so that the breather isn’t trying to absorb moisture from the atmosphere, which can lead to shortened breather life.
State Inspection Now Due
If you have scheduled outages, you need to plan and execute an inspection of critical equipment like gearboxes. Just like your car is (supposed) to be inspected once a year to maintain a minimum safety standard for operation, your plant equipment should be inspected to identify items that should be closely monitored, so that they can operate effectively until the potential problem is rectified. An external visual inspection should indentify proper oil level, signs of leakage, or cleanliness. An internal inspection is even more important and should be scheduled on a regular basis. Internal inspections should identify the surface condition of the components and any abnormal wear.
If you’re operating 24/7, internal inspections are not practical. In these cases, alternative monitoring equipment should be considered to trend the operation of the gearbox. Vibration accelerometers are a common example. If you are able to monitor each bearing, you’ll likely be able to pinpoint the actual element in question (i.e. bearing or gear) due to operating frequency of that particular component. There are many third party companies that specialize in this type of monitoring — consider this alternative if you don’t have the staff to monitor and evaluate frequency calculations. They have the software and focused expertise to evaluate the gearbox effectively. In most cases, they will even contact the manufacturer for all the gear tooth counts and bearing numbers, which are required to identify the frequencies based on operating speeds.
The Big “C” in TCO
A commitment to maintain your gearbox is paramount for this to work economically. When you devise a plan to maintain your gearbox, your ROI (return on investment) will be realized sooner and your TCO (total cost of ownership) will be reduced, if you stick to the plan.
Keeping the oil in proper condition will by far ensure a properly sized and operating gearbox, running longer, reduce your repair costs and ultimately reduce your inventory for spare parts.
Fred Mitchell, Sr. equipment builder engineer, ExxonMobil
Jeff Rybak, field application engineer, PCB Piezotronics
Joe Medici, account manager, Bently Tribology Services