Know what you’re designing for. Extend the life of your gearing system. Minimize repair costs. Improve efficiency. All gearing systems undergo failure over time. However, there are preliminary and preventative best practices to prolong performance, minimize repair costs and boost productivity.
This presentation covers the fundamentals of gear, bearing, shaft and housing design for efficient, long-lasting gearing systems. It will explain how torque, size limitations, shaft orientations and backlash calculations affect wear and how you can prevent this early on.
Learn gear design best practices and receive practical guidance on gear, shaft and bearing ratings to verify they have an ideal configuration for their intended application. An explanation of the AGMA Gear Quality Scale and some examples of applications associated with the level of quality required for specific industry sectors are also discussed.
Key Takeaways (Jump to a section)
Gearbox Application Factors
Application constraints are the single most important factor for determining the correct design. Ask yourself:
- Why do you need a gear system?
- Are you concerned most with positioning? If so, how precisely does that position need to be held?
- Are you looking to magnify the torque?
- Are you trying to change the orientation of the shaft?
- How much power do you need to transfer?
Some of the most important constraints to understand when designing are:
- What are the input & output torques?
- What speed do you have (either input or output)?
- What envelope size & orientation will the gearbox be?
- What is the allowable overall backlash of the system?
Temperature and environmental considerations should also be identified to ensure your application has the correct lubrication and sealing methods. Next, know the duty cycle:
- How often either on an hourly, daily, weekly, monthly cycle basis will the system run?
- Is there a decibel rating you cannot exceed?
These will be important factors to consider when choosing the types of gears within the assembly. Getting answers to these questions upfront helps to make the design process that much easier. Be proactive, and protect against gear drive wear by choosing a design that properly fits your application’s requirements.
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Gear Type Selection
Berg manufactures many different types of gears for many different applications. These include:
- Straight spur gears, our strongest and most common type of gear
- Helical gears, similar to spur gears, but quieter running
- Bevel gears are used in many applications where the input & output shafts are not parallel
- Worm gear systems, which are primarily used for higher-ratio reductions in relatively small package sizes
All of the gear types that Berg makes are also available with an anti backlash feature designed to significantly reduce, if not eliminate, the backlash in the system. We can also create custom gears if need be.
Please note it is very common for a gearbox to contain multiple types of gears within it. For example, a bevel set may be used to create a right-angle orientation of the input and output shafts, while spur gears are used for torque multiplication.
For each gear type, there are specific tools that can be used to determine the capability of the gear set. We’ll discuss some of the more critical aspects of gear design and the tools used for them.
Gear Materials
This table gives general properties of the materials most commonly used in gear applications. Selecting the correct material is of critical importance, as is selecting the correct material combination. Choosing poorly here can result in high costs.
For example, the gear material may have too high performance capability for the application. Or in some cases, a seized gear set where improperly mated materials cause actually welding between the gears as a slide.
Gear Strength
This is one of the many equations for calculating gear tooth strength: the Lewis Gear Strength Formula. This formula is for straight spur gears. There are other tools for the different types of gears. This equation assumes that only one tooth is transmitting the load and is essentially a simple cantilever. It does not account for the root fillet or other key aspects that may otherwise influence the design.
That said, we like to use this tool as a quick way to help identify the pitch and width of a gear required to transmit a given force. Please note that this is based on force. If torque is given, it should be converted to force at the tooth using the diametrical pitch.
Gear Oil Temperature Limits
The design of a gear system also includes an analysis of the thermal performance. As the force is transmitted from one gear tooth to the other, there are multiple types of deflection and contact that occur.
For example, with a worm gear system, the worm slides against the worm wheel, typically for one complete revolution of the worm. This sliding contact deflects the worm as well as the tooth of the worm gear, and there is friction throughout this contact. All of the stored energy is released in the form of heat into the gearbox. The lubrication will have a maximum temperature before either the oil breaks down or the oil drops out of the grease. It will be important to stay well below this.
Additionally, each gear system should be rated for maximum torque and speed. Operation above the specifications will violate the thermal model and cause premature failure. This heat needs to be balanced with what the gearbox housing is capable of emitting. The designer has many options, including orienting the gearbox to maximize free convection, adding cooling fins, or finding other methods to extract the heat.
Thermal Ratings
Variables in determining thermal capacity of a gear drive are:
- Gearbox size
- Input speed
- Torque
Thermal Limits Best Practices
Gear drive designers should be aware of the following:
- Maximum lubricant temperature
- Surface temperature of teeth
- Maximum output torque & speed
Effective Cooling Methods
- Gearbox orientation to maximize free convection
- Small size gearboxes: fans, cooling fins
- Larger gearboxes: circulation of cooling medium through tubes
- High speed/high load systems: oil bath
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AGMA Gear Quality Levels
One area that we often see confusion is in the use of the AGMA gear quality levels. Many times this gets overspecified with the belief that a higher quality level will result in stronger gears or will eliminate backlash from a system. Care needs to be taken as costs can rise almost exponentially as gear quality is increased.
AGMA Q14/Q15 gears are typically only used in very high-precision applications, or as calibration artifacts for gear testing. These gears are precisely ground to shape in very controlled environments and are some of the most expensive gears available for that size.
Examples:
AGMA Q12/Q13 gears are used in applications where efficiency is of high concern, for example in wind turbine drives. The use of the Q12 gears helps ensure that the maximum amount of input power is converted to electricity and that waste due to inefficiency is minimized.
Examples:
- Aerospace applications such as gyroscopes & engine parts
- Marine applications such as naval warships
- Wind turbine drives
AGMA Q10 gears are a balance of precision and cost, and are used in many industrial applications. All of the Berg catalog gears are produced to a Q10 standard except for those made from polymers.
Examples:
- Semi-tractor transmission gears
- Paper machinery (main, dryer and press drives)
- Marine applications such as commercial and naval supply ships
- Steel mills (mill pinion stands, roller drives)
- Cement industry (open gearing, kiln drives)
AGMA Q8 gears are the most common and are used in applications where torque, transmission and cost are more important than precision. Typical automotive transmissions use Q8 gears.
Examples:
- Auto & motorcycle transmission gears
- Paper & pulp machinery (chippers, mixers, box machines)
- Food industry equipment (mixers, conveyors)
- Mining crushers
AGMA Q6 gears tend to be more of the injection molded or powdered metal gears—still very capable of transmitting good amounts of torque, but cost is of primary importance in the design.
Examples:
- Power tools (drills, saws, etc)
Like a chain, the gear system is only as strong as its weakest link. That is, if you specify a Q12 gear for one part of the system for precision, but have Q8 gears throughout the rest, you are not likely to get the performance benefit you expect. It is important to look at the whole system to determine what quality level is needed.
Shaft Type Selection
Once the gear set has been identified, the next step our engineers follow is to determine what shaft should be used to transmit power. The obvious parameters are identified, for example:
- Shaft elements
- Amount of torque applied to the shaft
- Shaft supports
- Shaft modifications
- Actual torsional deflection & bending deflection
As well as the not so obvious ones:
- Will there be keyways, or will items be press-fit onto the shaft?
All of this information is then used for examining the shaft deflections, as well as the strength, to determine what material should be used and what safety factor will be applied.
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Bearing Type Selection
Next is the selection of bearings. The type of gear you are using will often drive you to choose a particular bearing type. For example, helical gears produce thrust loads and the selected bearing will often need to react this.
Once the bearing type is identified, the bearing is sized based on:
- Radial & axial loads
- Rotational speeds
- Life expectancy
- Safety factor
- Envelope available for bearing to fit into
If a rolling element bearing is too large, Berg also offers a number of bushings that are capable of thrust and radial loading to ensure you can stay within the package size requirements.
Berg provides a handy bearing reference for inner and outer ring tolerances based on the ABEC rating of bearings to help you in the design process:
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Gearbox Housing Design
Designing the housing itself is typically the last thing that is done. Key factors include:
- Envelope size:
- How much room do you have in which to fit the gearbox?
- Are there areas that you need to avoid due to other components?
- Gearbox mounting:
- Will it be shaft mounted or base mounted?
- Do you need threaded holes that support the gearbox & react torque?
- How much precision is required in the actual gearbox placement?
- Center distance tolerance:
- How closely does the center distance need to be maintained for the desired backlash?
- What impact will material properties have on this?
- Gearbox sealing:
- Is the design adequate for the lubrication system needed?
- Will it keep the grease or oil in and any contaminants out?
- Environment:
- Will the gearbox be in an environment where corrosion can occur?
- How well-protected will that gearbox need to be?
From all this information, you can then choose the appropriate material for the gearbox housing. Good engineering calculations will be important to ensure that the housing has sufficient strength to transmit the power required.
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Gear Center Distance & Backlash
As we discussed previously, in precise applications, backlash can be a critical parameter in the performance of the gear system. Backlash occurs due to teeth not making perfect contact and being able to move freely in the system. As the operating center distance increases—noting that operating center distance may not be the same as the designed center distance—the teeth have more clearance, and hence backlash increases.
However, while decreasing, the operating center distance will have the opposite effect. That is: backlash will decrease. Care needs to be taken to ensure that binding does not occur as the teeth bottom out. Given enough torque, this can either cause wear of both the tooth and root of the gear, or fracture of the tooth.
How Center Distance Affects Backlash
- Errors in center distance influence the gear drive’s ability to run efficiently
- If center distance increases: backlash increases, & gear teeth contact ratio decreases
- If center distance decreases: backlash decreases until binding occurs
- Minimum & maximum backlash values between mating gears are based on ‘standard’ center distance mountings
These tables are a reference for the impact of the center distance on backlash of a Q10 C gear set with a 20° pressure angle, with inches on the left and millimeters on the right.
Change in Center Distance Calculation
As you can see, small changes can have large impacts on the backlash of the gear set. This is why, most often, the Berg engineering team works to evaluate a gear system instead of increasing gear quality. Many other parameters influence backlash and need to be addressed differently. If the center distance tolerance is too wide, no higher-quality gear will resolve the backlash issue.
Calculating Center Distance
When designing a gear system, here are some common practices for calculating center distance with given inputs.
First: if you know the chordal tooth thickness of the gears and the pinion is fixed in place, the center distance at which the gears will mesh properly and meet the backlash requirement can be calculated.
Alternately: if you know the center distance and tolerance, the chordal tooth thickness of the gears can be calculated.
This can be compared to the AGMA quality information to select the appropriate gears for the system.
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How Gearbox Design Affects Operating Temperature
Next, we will discuss how the design parameters can influence the temperature and lubrication of the gear system. This discussion will assume that the appropriate materials have been chosen, and that the gearbox has been adequately rated for thermal performance. The latter is another key parameter ensuring that the heat generated within the gearbox will be sufficiently transferred to the environment so that the maximum temperature of the gear set is not exceeded.
Improper alignment of the gears and related components can result in distortion of the gear teeth. As the gear teeth come out of mesh, this energy is released in the form of heat, much of which would not be accounted for in the design process. This can add up quickly. The Berg engineering team has helped many customers realize that even small misalignment issues can result in big thermal problems.
Going back to the application constraints: it is critical to identify as much as possible regarding the operating environment of the gearbox. Items such as motor fans and other sources of air movement can significantly help performance. However, some common polymeric coatings that can give significant corrosion protection can also act as a blanket and restrict the flow of heat. Review this information and results of this carefully. It will be of critical importance.
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How Gear Design Affects Lubrication Performance
Much like running temperature, improper gear alignment can have significant negative effects on the performance of lubrication in the gearbox. This alignment can cause the contact area to exceed the maximum force the hydrodynamic lubrication layer can support. This can result in an indirect metal-to-metal contact and, as you would expect, significantly accelerated wear.
Gear tooth accuracy, more specifically the roughness of the tooth surface, also has an impact on lubricant performance. Too rough a surface will allow metal-to-metal contact, while too smooth a surface can add significant cost. A happy balance needs to be found to get the correct amount of performance for the targeted cost.
Gear Oil Selection
Load, speed and expected operating temperatures are the three factors that are used for selecting gear lubricants. There are many good resources online and through your local lubricant distributor. Please leverage these resources to ensure you use the appropriate lubricant.
Once the lubricant is chosen, the designer must decide how to ensure that it is present on the gear faces when contact is made. For lightly-loaded low-speed systems, a light coating of grease during assembly may prove to be enough. Because the system is moving slowly, the grease is not likely to be flung off the teeth. Additionally, because it is lightly loaded, degradation of the grease may take a longer period of time. Care will need to be taken to ensure that the thermal capacity of the system, including the lubricant, is not exceeded.
For high-speed or highly-loaded systems, an oil bath may be the preferred solution. This system will have higher thermal capacity because the oil will be in contact with the gearbox housing and can conduct heat away more easily. However, turning the gears through the oil will take additional power and generate more heat. The oil bath may also require a change following the run-in period, or through a regular schedule.
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Quality Control in Gear Manufacturing
One of the critical items to ensuring gear system performance is quality control throughout the gear-making process. Berg ensures the quality of our gears is maintained by using the double flank test method, a process that has proven very reliable over the 50+ years Berg has been manufacturing gears.
In order for both our operations and quality teams to know what they are testing, a setup sheet is completed for each production lot. Information regarding both the gear being produced as well as the master gear that will be used is entered into the data sheet.
One key piece of information that is needed—but many times overlooked—is the arbor size. Arbor size is chosen to accommodate the bore size and allow the gear to be mounted, but tight enough tolerance to not affect the results.
We talked before about the high cost of AGMA Q12 and higher gears. The master gear is one of the drivers of this cost. In order to ensure an accurate test, the master must be of higher quality than the gear that is being produced.
Gear Testing
Once the setup information has been collected, the tests are performed. From this data, we look for the results from two critical factors. Both of these types of error are considered test radius errors that are shown when doing composite testing on the test rig against a master.
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- Total composite error is the difference of the highest center distance & the lowest center distance over one complete revolution of the gear
- Tooth error is the largest deviation of any single tooth that is crest to the adjacent root on the graph
These are important to the designer because AGMA defines these two measurements as the readings necessary to give a gear a quality rating with this testing method. The theory is that a perfectly-formed tooth will show no center distance deviation. Therefore, this test accounts for profile, pitch, lead, tooth thickness and total run-out.
Gear Design, Application Constraints & Quality Specs
To sum up, here is how we have addressed the issue of premature gear failure resulting in unplanned downtime:
Gear design choices early in the development process, along with preventative maintenance care, can extend the operating life of any gear drive system.
Application constraints are the single most important factor in determining the correct design, and you must consider torque shaft orientation, the transfer of power, and the environment to produce a gear drive with a long lifespan.
AGMA gear quality standards can help you determine the correct quality specs for an intended application, along with shaft and bearing design.
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Gear Drive Design FAQ