Getting the most out of gearboxes

Feb. 9, 2012
Engineers should know some gearing basics before specifying gearboxes

Authored by:
Adam Mellenkamp
Product Manager
Stober Drives Inc.
Maysville, Ky.
Edited by Stephen J. Mraz
[email protected]
Stober Drives Inc.

All factory equipment requires some degree of maintenance, monitoring, and replacement, but some need much more than others. One group of high-maintenance components are gearboxes. They are traditionally high wear items, thanks to their usual task: converting high-speed, low-torque power from electric motors into the low-speed, high-torque power needed by machinery.

Still, gearboxes remain widespread and popular because, as one of the tried and true rules of thumb in the power-transmission industry says, “Speed is cheap, torque is expensive.” And relying on motors to generate the torque required by many loads (called direct-drive technology) is usually more expensive than generating the torque with a motor/gear reducer combination. So most engineers specify gearboxes in their designs. Plus, gearboxes can eliminate other mechanical components, such as bearings, belts, chains, and pulleys, thus simplifying and bringing down the cost of motion-control applications.

Engineers should also use the most-efficient gearbox that meets their application’s needs. Such gearboxes use less energy, which saves money and the environment. It also means the motor doesn’t have to generate as much power, so designs can use smaller, less-expensive motors which don’t take up as much space. Efficiency also means less heat generation, which prolongs the life of a gear reducer as well as the oil in it. Excess heat can be also a safety issue. Hot-running gear reducers have been known to burn distracted or less-than-careful employees.

But what type of gearing is most efficient?

Comparing gearing
There are quite a few reasons to consider the various types of gearing. Each type, however, has distinct pros and cons.

Worm gearing: It’s inexpensive, especially for right-angle applications. Worm gearing is self-locking, which eliminates the need for motor brakes in some applications. Worm gearing is also quiet and relatively smooth when running in only one direction.

Worm-gear efficiency tops out at 65 to 80%. Worm gears usually experience sliding friction, which wastes energy, creates heat, and increases tooth wear, all of which shorten the life of a gearbox. And when worm gears are used in applications in which the motor reverses, backlash grows as the teeth wear over time.

Because so much heat is generated within worm-gear reducers, designers need to address pressure equalization between the inside of a reducer and the atmosphere. One method of pressure equalization is to install a breather valve that lets air in and out of the reducer. But these can also let contaminants and moisture into the reducer, causing oil breakdown and accelerated reducer failure. Another method of pressure equalization is to include a bladder inside the reducer to equalize pressure, but bladders are prone to rupture and subsequent reducer failure.

Spur gearing: It is easier and less expensive to machine than helical gearing, but tends to be more expensive than worm gearing. Spur gears are relatively efficient because the teeth have rolling, not sliding, friction, so heating is not usually an issue.

On the downside, spur gears have limited tooth engagement between mating gears. This puts more force on individual teeth, reducing the gearboxes’ torque-carrying capacity. These forces also wear down the teeth at contact points, as well as deform or deflect the teeth, which limits gear life. Limited tooth engagement also creates high torque ripple as the teeth engage and disengage, and makes for noisy gearboxes.
Harmonic gearing: This type of gearing can be used to get high gear ratios out of relatively compact gearboxes. They also demonstrate relatively low backlash.

But although high gear ratios are possible with harmonic gearing, ratios below 30:1 are not possible. And it “winds up” under torque, causing lost motion. Harmonic gears are also more susceptible to damage from shock loads than other types of gearing. In terms of efficiency, harmonic gearing is better than worm gearing, but not as good as spur and helical versions.

Helical gearing: This gearing creates rolling-gear interactions, making it the most efficient type. Helical gearing also has several teeth engaged constantly, transmitting relatively high torque, and generating little torque ripple, thanks to smooth transitions between teeth. Helical gearing can also handle both motion control and continuously-running applications.

But because helical gearing is highly efficient (typically 97%), it is not self-locking, meaning it can be easily back-driven. To prevent this, engineers must add a motor brake to hold the idle axis in place.

Durability counts, too
Durability is even more important to many U. S. factories than efficiency. Machine downtime is expensive, especially in the 24/7 operations common in industries such as food and beverage processing. Additionally, maintenance and replacement costs add up. So, how do engineers make gear reducers last as long as possible?

To maximize reducer life, it’s critical to create and maintain a contamination-free environment inside the gear reducer. This means not letting in air, water, or contaminants.

Gearbox durability also depends on lubrication that uses clean oil to keep parts from wearing. The oil must be kept from overheating, which accelerates breakdown and reduces its ability to lubricate. Excess heat also leads to brittle and prematurely worn seals, the cause of leaks. So the seals must be compatible with the lubricant and withstand exposure to any chemicals and contaminants they may be exposed to from the outside.

Another aspect of durability important in food-handling and pharmaceutical applications is the gearbox’s resistance to frequent wash downs with high-pressure, high-temperature water and chemicals. Such wash downs can compromise the paint on some gearbox housing, possibly contaminating the products being processed, as well as the gearbox itself. Second, water and cleaning solutions should drain quickly from the gearbox’s exterior surfaces. Otherwise, they can puddle, creating environments in which bacteria can thrive. This requires smooth, clean surfaces with no nooks or crannies for water to accumulate.

Housings should be made from single castings and have all features machined on one fixture. A housing made of several components usually lacks rigidity, which means the housing will flex, causing misalignments of internal components. Using several different fixtures to machine the housing also causes misalignments of internal components, as well as looser tolerances.

The best machinery has seamlessly housed gearboxes with a stainless-steel exterior. Although cast stainless-steel housings are expensive, they are required in areas where painted surfaces cannot be tolerated. Be advised that stainless steel is a poorer conductor of heat than cast iron, so it’s best not to combine inefficient worm gearing with stainless-steel housings. It increases the risks for burns and premature gearbox failure. High-efficiency helical gearing is therefore a huge advantage in stainless steel gear reducers.

Efficiency levels: How gearboxes get you there
Any time power is converted from one state to another, there are losses, usually in the form of unwanted heat and wasted energy. “Power out” is always lower than “power in.” In gear reducers, rotational power gets converted from high speed/low torque to low speed/high torque. And the heat comes from friction in the gearing, bearings, and between the seals and shaft surfaces. Oil turbulence also causes losses.

To get the most power out of a gear reducer is to first choose more-efficient gearing, which includes helical, bevel, and spur gearing. The next most important step is to always use high-grade oils which reduce friction in all internal moving parts. Next, spec high-quality bearings, whether roller, ball, or cylindrical.

But even with efficient gearing, much of the lost energy is due to friction between seals and the mating shaft surfaces. Making seal diameters as small as possible reduces running surface speed and, therefore, running friction. Making sure the shaft has the best surface finish in the seal area is also critical.

© 2012 Penton Media, Inc.

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