Gearing up for productivity
How does gear selection contribute to productivity in today's motion-centric automation environment?
Mitch • Cone Drive: Two basic criteria must be considered when choosing a speed reducer — thermal horsepower and mechanical service factor. If a speed reducer is to operate for greater than 20 minutes continuously or have a 50% duty cycle, then the reducer's thermal hp must be taken into consideration. The thermal hp rating is generally lower than the mechanical hp rating, as is its ability to dissipate heat. The thermal hp rating of the reducer must be less than the input hp applied to the reducer.
Mechanical service factor is an indication of the reducer's durability and strength. The service factor is the number derived when the mechanical rating (either input hp, or input or output torque) is divided by the applied load. Manufacturers offer recommended service factors for different applications. For example, if a 7.50 hp motor is used to drive a speed reducer, and the application demands a service factor of 1.25, then the mechanical rating of the reducer must be at least 7.50 × 1.25 or 9.38 hp.
Chuck • Baldor: The gear ratio, output torque, and service factor of the gear reducer must be matched to the production equipment's speed and torque requirements in order to maximize productivity. It's equally important to select gear reducers that minimize maintenance and are quick to install. Look for gear reducers that are factory filled with synthetic lubricant and do not require oil changes. This will minimize installation time and greatly reduce or even eliminate changes.
Brian • QTC: In order to increase productivity, you need to reduce downtime of your automation systems, which can be caused by either periodic maintenance or catastrophic failure. In order to minimize downtime, it's important to analyze all of the system requirements before selecting gears for your application. A simple change in gear material can change the lubrication requirements or increase the time between maintenance periods, thereby reducing downtime and increasing productivity.
Another way to increase uptime is to use standard gears whenever possible. Selecting a stock gear from a gear manufacturer usually allows the end user to replace a gear within 24 hours. Custom gears are rarely available off-the-shelf and maintenance cycles must be scheduled weeks in advance. A catastrophic failure of a custom gear can result in weeks of downtime or the need to pay a premium to rush production of a replacement piece.
Rich • SEW: Gears are not some standalone component on a machine. They cannot be considered small mechanical islands, but rather integral elements of a comprehensive motion control system. In today's automation-centric world, gears, motors, and drives work together to control torque, power, and accurate movement. Long gone are the days Long gone are the days of gear suppliers saying, “We sell torque and speed.” You don't need torque and speed; you need motion. Well coordinated, well executed, precise motion. Motion in the right place at the right time, every time. That's productivity in gearing.
The number one task when selecting gears is to define what criteria are most important to your application. Is it torque and speed? Accuracy? Dynamics? Long life? Purchase price? How will the gear unit be maintained? What's the environment? Some machines must be able to adapt to new processes and products over their lifetimes. Will any of these criteria change over the life of this machine?
Answers to these questions will help you define your optimal gear unit. Will it be in a cool, dry environment and run nonstop, lightly loaded eight hours a day? Most gear units can handle that with little effort and at a low price. But what about a food-processing environment that's cool most of the day and then cleaned with steaming hot, high pressure, caustic chemical blasts at the end of each shift? Is this a conveyor gear unit in a plant with impeccable preventive maintenance processes, or is it stuck deep inside the dark reaches of a machine that will never be given a second look until it's dead? Each of these rearranges the factors you must consider.
Maintenance is key. Gear units have some parts that wear and some that don't. Hopefully the finish and the housing don't. The same goes for the gears themselves. If your gear unit is properly sized, mounted, and maintained, high quality helical or helical bevel gears should have infinite life. Bearings and seals may or may not, since these are often wear items. Oil is certainly a wear item; that's its job. So, if this unit won't get a lot of maintenance attention, consider heavy-duty bearings, better heavy-duty bearings, better quality seals, and synthetic oil. And if you can only afford one — go for the oil upgrade. Synthetic lasts at least twice as long as standard mineral oil, has a wider operating temperature range, and runs much cooler. Reducing the temperature of the oil in the gear unit keeps efficiency higher and keeps all the other components at their best.
Sizing and specifying
What's your best advice on specifying, sizing, and applying gears where productivity is the main goal?
Mitch • Cone Drive:
Ensure the thermal hp of the reducer is greater than the input power to avoid overheating, seal failure, lubrication breakdown, and premature wear.
A minimum service factor of 1.5 should be used in applications when a startup or stopping load is equal to or greater than 200% of the reducer's rating.
On high inertial loads, careful consideration must be given to the ratio of a worm gear to avoid damage during deceleration.
Low or zero backlash reducers are necessary to maintain stability in servo applications requiring bidirectional motion.
Always use lubricants specified by the supplier; some lubricants have additives that may damage gearing or seals or have improper viscosity for a given application.
Chuck • Baldor: Select an appropriate service factor to avoid shutting down production due to premature gear reducer failure. A small service factor will reduce upfront costs, but will lower productivity through unexpected failures.
Follow the manufacturer's recommendations for approved mounting positions. Avoid positions that completely flood the input seal with a head of oil. Flooded input seals can cause premature leaks and lead to catastrophic failures if the leaks are not quickly caught.
When using a variable frequency drive, select the reducer gear ratio to provide the desired output speed using the base speed of the motor. Slow the motor from base speed, rather than increase the motor speed over base speed, when changing speed to match production requirements.
Select washdown-duty gear reducers for food duty applications. Many users try to reduce upfront costs by purchasing standard reducers, only to lose time and production later with unscheduled maintenance due to corrosion or water ingress. Look for special output shaft seals, such as harsh duty seals. Some harsh duty seals are designed specifically for washdown applications and keep water from entering the reducer, which can cause catastrophic failure and lost production time.
Eliminate chains and belts on the reducer's output shaft by using hollow shaft reducers. This minimizes failures and makes a more compact arrangement, leaving more space for production equipment. Look for gear reducers with special hollow output shaft bushings. These bushings dramatically reduce fretting corrosion, which makes it easier to remove reducers during machine maintenance.
Brian • QTC: When designing a gear for maximum uptime, it's vital to consider all of the parameters that will impact the gear's success. These include physical constraints, operating speed, applied torque loads, lubricant type, and minimum life. Most people choose a gear for their application based on the physical parameters of pitch, number of teeth, face width, and bore size. Then they consider the material for which the gear will withstand the maximum applied torque. Often overlooked is the fact that the maximum bending strength torque capacity of a pinion gear is usually stronger than the maximum surface durability torque capacity. By using the bending strength torque instead of the surface durability torque as the guiding factor in their design, the end user often needs to replace the pinion prematurely, which leads to reduced productivity.
Rich • SEW: Here are a few tips to get the most out of your gears:
Use synthetic oil. It lasts twice as long. It reduces friction 25% or more, so it runs cooler.
Use shaft mounted gear units instead of attaching with chains and belts. By eliminating chains and belts, they virtually eliminate overhung load. Losing the belts and chains also means less guarding, which means much better access to the units for oil changes and other service.
Use gearmotors instead of separate motors and gear units with C-face inputs. This ensures critical alignment between the motor and the gear unit, and that means longer bearing life. They're also generally smaller and less expensive than C-face components.
Calculate your true needs with gear sizing and selection software. “Rule of thumb” and estimating results in over sizing, which upon replacement, is usually over sized again. Talk about loss of efficiency! Do it right; pay only for what you need.
Even though they are not directly part of selecting gears, apply variable frequency drives whenever possible. Hard mechanical starts and stops are shock loads to the gear unit, and this introduces a lot of unnecessary wear. VFDs soften and control starting and stopping. You may even be able to use a smaller gear unit due to the benefits to the bearings and gearing through reduced shock loading.
Think power transmission system rather than gear, motor, and control components. Work with a supplier that has expertise in all three to avoid a lot of finger pointing when things don't work.
Gearing disaster zone
What's the worst that can happen if a gear is not specified or installed correctly?
Chuck • Baldor: One of the worst things that can happen to a production facility is premature gear reducer failure. This can cause a shutdown of production equipment, which could affect an entire facility.
Brian • QTC: The worst thing that can happen is that the entire assembly fails. A spur gear that has its teeth stripped off can cause a million dollar piece of equipment to suddenly become idle. When a gear is improperly installed, the machine in which it is operating will continue to work for a period of time. As the old saying goes, “If you can't find ‘em, grind ‘em.” This is what happens until the teeth are worn to a point were the machine fails.
If you could create the ideal gear, what would it look like?
Chuck • Baldor: The ideal gear reducer might incorporate high power density, industry-specific features such as washdown packages, high overall efficiency by packaging with high efficiency motors, and low maintenance requirements.
Brian • QTC: An ideal gear from an end user point of view would have a variable tooth form and bore size. It would also be self-aligning, lubricant free, and would have zero backlash without any binding issues.
Baldor Electric Co.
City Park, NY 11040