Gearing & gear drives

June 1, 2002
The mechanized world is dominated by rotating shafts, each designed to turn at a particular speed and torque

The mechanized world is dominated by rotating shafts, each designed to turn at a particular speed and torque. On one end of the shaft, quite often, is an electric motor — the prime mover. On the other end is the load. In between, keeping both sides happy, are gears. Keeping gears happy is the focus of this month’s discussion on productivity. Hear why gears are likely to fail and what you can do to prevent it. Also learn where gear technology is headed, and what this means for designers and end users alike. It’s all on the following pages — waiting for you.


What is “productivity” and how do gears contribute to it?

Philip/Thomson: Machine “up” time and component life determine productivity. Compared to belts, pulleys, and other reduction methods, gear drives last longer and are easier to maintain, increasing the “productivity” of each motion axis.

Dave/SEW Eurodrive: Productivity is the volume of products produced in a specified time.

John/Alpha: It’s the ability to process goods in a timely manner at the lowest possible costs. In drive components, it’s tied to speed and power requirements.

Roger/Poly Hi Solidur: Webster defines “productive” as “yielding or furnishing results, benefits or profits.” Gear drives, in and of themselves, contribute to productivity by efficiently transmitting rotary power from one location to another and by reducing or increasing speed and torque. Rack and pinion systems have the added benefit of converting rotary motion to linear or vice versa.

Fred/Forest City Gear: In a world driven to reduce costs and come to market quicker, we are all dependent upon mechanisms that get us to the end result faster and more accurately. This translates to more precise mechanical products, servodrives, stepper motors. It follows that gear accuracy must improve commensurately. Also, at higher speeds inaccuracies generate more noise, which certainly doesn’t help productivity.

Paul/Falk: Productivity is the ability to accomplish more work with fewer resources. Something as subtle as how a component mounts can be a factor. A shaft-mounted gear drive, for example, doesn’t require a significant foundation — no large investment to build a support structure — saving time and money.

Tom/Mijno: Productivity is performing a process quickly and precisely. Gear drives contribute by converting power (force and motion) into a more usable form than the source is able to provide.

Steve/Danfoss Bauer: The simplest definition is the economical conversion of resources, whether human, energy, financial, space, or material into profitable sales, be it goods or services. Gear drives have held a place in “productivity” since an ancient carpenter hammered pegs into a wooden wheel to produce a cog.

Mike/Emerson: The term productivity generally implies “doing more with less” or “getting more for less” and this has certainly been the trend in the gearing industry. “Power density” has become a popular term referring to squeezing more torque from a smaller envelope. This has been accomplished primarily through optimization of gear tooth geometry, improvements in materials and heat treatment, and post heat treatment finishing of gear teeth to achieve higher quality levels.

Limiting factors

What elements most often limit machine productivity?

Philip/Thomson: What typically limits planetary gearheads are the bearings. Bearing life, in turn, is a function of loads and speeds associated with the application.

Dave/SEW Eurodrive: Gear failures are usually caused by overloads, shock loads, poor maintenance, and incorrect sizing.

John/Alpha: Limiting factors include heat, radial loading on output bearings, speed at input bearings, and torque for planetary bearings. Heat from oil churning and seal friction worsens with increasing speed.

Roger/Poly Hi Solidur: There are two modes of failure that affect spur gears; surface and fracture fatigue. Because individual spur gear teeth act as cantilevered beams, power capacity is based on material bending strength. With heavily loaded all-polymer gears, keyway failure may also be a concern.

Keith/Rockwell Automation: Many gear failures are due to applying the incorrect reducer to the required load. A gear reducer must be sized according to required torque based on speed and motor horsepower. Shock loads must also be considered. Other common causes of failure include contamination, seal wear, and improper installation and maintenance.

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Greg/Groschopp: There are a variety of failure modes. Heat can cause lubrication breakdown, where the grease or oil solidifies and ruins the gears. It can also damage gearbox seals, allowing the lubricant to spill out. Another mode of failure, over-torquing, can break gear teeth. Excessive over-hung shaft loads are also a problem, causing bearing failure.

Steve/Danfoss Bauer: In terms of efficiency, old designs are a big problem. Many of today’s gears are based on designs that predate the electronic computer. In some cases, power transmission efficiencies are below 60%. It is amazing how many of these gearboxes are coupled to high efficiency motors.

Fred/Forest City Gear: Looking at productivity in terms of cost, engineers have been very conservative in their design standards for safety reasons. Many are saddled with costly products because they copied old designs or have not really optimized their gears for load carrying and power transmission properties. Much can be gained through preliminary testing to define life and strength parameters.

Gary/GAM Gear: There are many elements that may limit machine productivity: heat (generated from speed), torque, backlash, stiffness, efficiency, and the number of starts and stops.

Tom/Mijno: Grease-lubricated geardrives generally do not have shaft seals. Oil lubricated geardrives have seals, which are usually the first components to wear out. Elastomer lip type seals wear out because of a lack of lubrication and associated friction heat, which softens the lip material. Excessive ambient heat is also a concern as it can prematurely age (harden) the seal material.

Paul/Falk: Many manufacturers use economical lip type seals, which are great for the price, but if grit or abrasive dusts are present, it can imbed on the seal lips, causing oil weepage over time. Grease purgeable seal cages that protect seal lips are one way to combat this.

Greg/Brevini: Productivity is most often hampered by poor oil maintenance and a lack of protection from environmental factors.

Mike/Emerson: Premature seal failure may be due to improper installation, poor shaft finish, and broken garter springs, but the most common cause is environmental extremes.

Bob/Mitrpak: In caustic environments, the failure mode could be the seals. Under widely varying and abusive load conditions, it could be the bearings.

Making it better

What can component MANUFACTURERS do to offset limitations?

Greg/Groschopp: Design a better product. Stronger worm and worm wheel material, for example, can give a smaller gearbox more torque carrying capability. Likewise, better drive bearings can improve over hung load capabilities.

Philip/Thomson Micron: Giving users a choice in bearings is a good place to start. On the output shaft, dual deep ball groove bearings are sufficient in most cases, but tapered roller bearings should be an option for applications with significant radial or axial loading. For planet gears, needle bearings are the best bet because planets cycle the most and need the greatest support.

Dave/SEW Eurodrive: Gear manufacturers can alleviate many problems by properly sizing transmission components. Components should also be easy to remove and service to minimize downtime.

John/Alpha: Proper lubrication is the key to productivity. Some lubricants applied to gear teeth can boost input speeds by 20% . Integration also helps. Incorporating a pulley into a gearbox, for example, can raise an actuator’s radial load capacity by a factor of four.

Paul/Falk: In larger drives, a double sealing arrangement may be necessary to keep oil in while letting abrasive particles escape during purge cycles. Using tapered bushings can also raise productivity. Normally, fretting corrosion causes drive bores to seize to the shaft. Tapered bushings solve this by gripping the shaft so tightly that it prevents relative motion between bore and shaft, the cause of fretting corrosion.

Roger/Poly Hi Solidur: Productivity is a matter of optimizing trade-offs. Polymers, for example, have less material strength than metals, but because of greater flexibility, adjacent pairs of teeth can sometimes share the load. Polymers also are lighter, requiring less power than metal gears to start and stop. If stress in the keyway is a concern, carbon or stainless steel hubs are a dependable solution.

Fred/Forest City Gear: Gearmakers need to invest more in new equipment. We put 25 to 40% of gross sales back into new equipment each year, and the results are mind boggling in terms of productivity and profitability. Ancillary equipment and inspection instruments must be upgraded as well. That’s been our approach for years, and we’ve experienced 10 to 20% growth (in productivity) every year without adding people.

Tom/Mijno: Gear makers should be more informative. It would help if they provided temperature limits and accurate “design life” data based on “normal” operating conditions.

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Oscar/Hub City: Lubricating oils and shaft-sealing technology contribute to life and reliability. Synthetic lubricants have much greater functional life than conventional lubricants at the same operating temperature. They also increase gear drive efficiency, in turn, reducing operating temperatures and further extending the functional life of the lubricant. Shaft seals are generally the lowest life element in an enclosed gear drive. But new seal materials and designs, coupled with a greater understanding of seal-shaft interface requirements, have significantly extended seal life on enclosed gear drives. Unlike conventional seals that require precise control over internal pressure, environmental conditions, and shaft sealing surfaces, newer seals, require no internal pressure control, function in any environment, and contain all sealing surfaces within the sealing element itself.

Steve/Danfoss Bauer: One way to improve productivity is to reduce the number of parts and increase reliability. Offering a wider variety of mounting and configuration options, as well as quieter, more precise, and easier-to-use gears also helps.

Greg/Brevini: Clean oil can increase gear life dramatically, so it’s important to let users monitor oil contamination and make oil changes more easily and quickly.

Mike/Emerson: Manufacturers need to more thoroughly test oil seals with regard to the impact of various shaft finishes on seal life. Some new finishes — particularly those having an “orange peel” surface texture — can extend seal life by a factor of four or five.

Keith/Rockwell Automation: From an engineering standpoint, gearmakers need to review each application to ensure that the particular speed reducer is right for the job. Things to focus on include load carrying capability, accuracy, oil contamination, sealing systems, and materials. A systems approach to sealing that acknowledges environmental factors, lubricants, seal materials, and seal installation is essential, as is extensive testing prior to product introduction.

Intelligent design

What can DESIGNERS do to optimize productivity?

Keith/Rockwell Automation: The most important thing a designer can do to ensure uninterrupted productivity is to make sure the geared drives are sized correctly for the application and specified for existing environmental considerations.

Greg/Groschopp: Select the right gear drive for the application. Proper sizing eliminates many failure modes. For example, lubrication won’t break down when gears operate at the designed temperature. Gear teeth will last when loads are within torque limits. And bearings won’t fail when overhung load ratings are sufficient for the application. Designers can best optimize their selection by working closely with the technical staff of the gear drive supplier and providing detailed information about the application.

Philip/Thomson Micron: Designers can start by identifying the requirements of the application in terms of loads, speeds, external forces, and environmental concerns. This information can then be analyzed to determine the best gear drive. In the case of extreme radial loading, for example, a helical geartrain with tapered roller bearings would provide the highest productivity.

John/Alpha: Component knowledge is critical. Some gearboxes are designed not to leak. Some are designed to run fast, letting machines reach speeds of 1,500 rpm with a 9:1 reduction. This translates to lower power requirements, reducing the size and cost of associated motors, drives, cables, and power supplies. Inexpensive gear alternatives to direct drive motors and mechanical indexers are also available. An integral gearmotor with a large output shaft, for example, can provide a torsionally rigid solution in a small package.

Roger/Poly Hi Solidur: Designers need to understand material properties. Oil filled polymer gears, for example, can often run without externally applied lubricants. Oil filled nylon’s weight is only about 14% that of steel, so inertia is substantially reduced. This can be critical in repeating start/stop applications. Polymer gears are also quieter.

Paul/Falk: Make sure equipment is sized per the manufacturer’s guidelines, accounting for any unusual circumstances. Related components like brakes must also be taken into account. An oversized brake, for example, could cause gear failure by applying repeated torque spikes.

Fred/Forest City Gear: Don’t lock yourself into a source just because it’s close; it may not be the best producer. Compare various sources and the gearmaking methods they use. You might find there are huge cost savings with carbide rehobbing, for example, in lieu of gear grinding. Likewise, crowning (in a helical gear set) may help alleviate spline wear, improving fit and alignment while reducing noise. Designers must also start measuring the quality called for on their prints. Gear measuring equipment is expensive, but in the age of ISO 9000, we should be going further to verify that what we have purchased is actually correct.

Gary/GAM Gear: Look to new technology — high-ratio gearing, hollow outputs, and integrated couplings. In addition to increasing performance, the newer technology may also eliminate the need for other components, saving cost and space. Likewise, higher end gearboxes, though they cost more up front, may save money over the life of the machine.

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Steve/Danfoss Bauer: Designers should accept all the help they can get. Some manufacturers have engineering staff, or at least software, that can assist in selection, application, and integration. Mounting options, such as hollow shafts and torque arms, also afford considerable savings in design and assembly.

Oscar/Hub City: Don’t forget efficiency. Helicalbevel and helical worm gears increase productivity being right angle, high-ratio reducers. Traditionally, their function was accomplished with single reduction drives, which were inherently inefficient. Efficiencies ran as low as 50%. Today’s helical-bevel and helical worm reducers incorporate multiple gear sets, accomplishing the same overall ratio, but at efficiencies as high as 97%.

End user advice

What can END USERS do to increase productivity?

Oscar/Hub City: With the exception of a few extendedlife reducer designs, the oil in most geared drives should be changed regularly. Otherwise, oil will become contaminated possibly causing bearings and/or gears to fail. Upgrading seals may also prove beneficial.

Paul/Falk: Also change breather elements when they become plugged, as breathers prevent abrasive particles from being pulled into the oil sump. Store gear drives responsibly, in a dry and protected location. Make sure equipment is properly aligned — bad alignment places extreme forces on shafts and bearings, causing failure to the drive or associated couplings. If there is vibration, find the source and eliminate it. Lastly, end users should make sure they specify minimum acceptable service factors and methods of sealing when ordering equipment.

Fred/Forest City Gear: End users need to avail themselves of industry knowledge. To that end they should be sending their engineers to AGMA (American Gear Manufacturers Association) seminars, classes, and expositions; or better still, have them work on committees that write the gear standards. Also, it is of paramount importance for engineers to gain some field experience and seasoning to understand what can be manufactured and how much it costs, so that they do not carelessly specify something unnecessary or impractical. By the same token they should also know what is achievable through world-class manufacturing techniques — things that their competitors may be using to gain advantage.

Keith/Rockwell Automation: End users should pay particular attention to the selection of the correct reducer, as well as installation and maintenance procedures. As a safeguard against lost productivity, proper supply and maintenance of spares is also important.

Philip/Thomson Micron: Understand rating techniques (based on industry) and accepted standards (i.e. AGMA, DIN, ISO). End users can also help by sharing application information with gear suppliers.

On the horizon

What’s new and promising for the future of gear drives?

Philip/Thomson Micron: New materials such as vacuum-melted steels and special alloys hold promise, as do shot peening and other treatments that reduce friction and improve wear. Another area where improvement lies is in electronics, including on-board sensors and fieldbus-based communications. Integrated sensors can reduce unnecessary maintenance and shut downs as well as failures by simply monitoring vibration and temperature. The addition of a fieldbus is a natural extension, speeding up installation and reducing startup expenses. New gear designs are also in the pipeline, including straddle-mounted needle bearing systems that more than triple bearing life and hence productivity.

Keith/Rockwell Automation: Advances in gear materials and manufacturing methods are steadily increasing power density. Modular designs are another breakthrough, offering users, designers, and manufacturers greater inventory flexibility.

Roger/Poly Hi Solidur: Polymer science is continually improving, turning out new polymers that are stronger, more wear resistant, and dimensionally stable. These new materials, alone and in combination, let polymer gears move deeper into what once was “steel only” territory.

Greg/Groschopp: Gear technology continues to evolve. Better, stronger, lighter, and cheaper materials are constantly being tested and approved for gears. The use of powdered metal also shows promise, primarily in keeping costs low and eliminating the need for precision machining.

Fred/Forest City Gear: There are many positives. Inspection equipment is much faster today and more accurate, providing better documentation of product quality. Statistical measuring methods are now simpler, making it easier to predict quality. Modern gear equipment and cutting tools are also improving, benefiting from faster computers and higher-grade materials with better coatings. However, we must not become complacent thinking we have reached the limit.

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