What are dryer gears and pinions, and why do you need to know about them? They play a critical role in the proper operation of your industrial dryer. This article introduces you to everything you need to know about dryer gears and pinions -- from types to wear to alignment to maintenance tips -- and how to keep them in tip-top shape so your dryer runs smoothly.

1. Compare Helical and Spur Gears

Helical gears are preferred over spur gears for large equipment because they require a smaller face width for the same horsepower rating.

On a spur gear, only one tooth is in contact with the surface at one time. In the case of a properly designed helical gear, overlapping occurs, and there is always a portion of tooth contact at the pitch line. In other words, before the engaged tooth loses contact, the next tooth is engaged. A term for this factor is the face contact ratio, which must be greater than one.

A helical gear also is smaller in face width, with less weight and less manufacturing cost than a spur gear when design conditions warrant its use. Another factor to consider is the thrust load applied to the pinion pillow blocks due to the fact that the teeth are cut at an angle (helix angle).

Many processors do not choose to employ helical gears because they consider them less forgiving and have to be kept in alignment more often. This is not really the case. A spur gear is more applicable on lower horsepower drives because, in order to maintain the proper contact ratio, the helix angle gets too large, increasing the gear cutting cost and thrust load.

2. Understand Gear Rating Factors

The American Gear Manufacturers Association (AGMA) established the AGMA 6004 rating system in the early 1980s to define the engineering standards for designing gears and pinions. These standards consider factors such as rotations per minute (rpm), face contact, tooth geometry, load distribution and alignment, hardness, dynamic factors, machining quality, allowable contact stress and allowable bending stress. When choosing a gear, you must familiarize yourself with these standards and ask the right questions.

3. Learn About Undercutting of a Gear

Undercutting results in removal of a portion of the involute profile below the pitch line. Undercutting greatly reduces overlapping and tooth strength and constitutes a problem. Ignoring the fact that 20° full depth involute pinions with less than 18 teeth will exhibit undercutting in the cutting process is a mistake.

Undercutting can be avoided in several ways:

  • By increasing the pinion’s outside diameter.
  • By increasing the addendum and decreasing the outside diameter of the gear.
  • By decreasing the addendum the same amount as the pinion.

The action does not change the gears center distance, pitch diameters or whole depth, but does modify the tooth thickness.

4. Consider Tooth Geometry

In gear and pinion tooth geometry, it is important to understand pressure and tooth angles. A 20° pressure angle, which is most common, should not be combined with a 25° angle tooth, or the gear will not work properly. Many will find that mismatching these measurements may sometimes result in a satisfactory operation, but in the end will result in failure.

5. Remember Gear Lubrication

AGMA provides clear guidelines for the lubrication of gears. Once the lubricant is placed on the gear, the diluents evaporate, leaving the dense lubricate intact. If a gear is kept clean, aligned and lubricated properly, it should last 20 years or more, as explained in AGMA Standard 9005 E02. The selection of the proper lubrication is a function of operating temperature, ambient temperature, pitch line velocity and type of application.

6. Learn Critical Terms

Three critical terms to master are radial runout, backlash and root clearance.

The axial runout occurs when the gear wobbles back and forth because the gear side face is not perpendicular to the axis of rotation of the shell. This error will reduce the face contact surface between the pinion and gear teeth. Radial runout occurs when the gear is not concentric with the center of rotation of the shell. Radial runout results in a sliding motion between the gear and pinion rather than rolling and, if excessive, can cause the tip of tooth to root interference.

Backlash is the error in motion that occurs when a gear changes its direction. It is recommended that a gear is initially set up with the help of a feeler gauge to make sure the backlash is the same on both sides. The use of a “Prussian Blue” or other means to check the contact pattern is also recommended because, on worn pinions or gears, setting the backlash equally on both sides may not result in a good contact.

Root clearance is a critical item and is defined as the distance from the tip of the pinion tooth to the root of the gear tooth when they are in mesh. Most gearsets have the pitch lines scribed on the side face. The pinion pitch diameter must always be outside the gear pitch diameter. Most dryer open gearing is cut with no backlash because the center distance is adjustable.

On a new gearset you can set the root clearance by setting the backlash to the correct value. However, on a worn gearset, backlash cannot be used to set the root clearance. Checking the position of the pitch line and an actual measurement of root clearance at several points around the gear is necessary in order to take into account radial runout.

Another consideration is gear thermal expansion. Heat will be transferred from the dryer shell to the gear; thus, the gear will expand into the fixed position of the pinion, reducing the root clearance and backlash. Because you are setting the pinion in the cold condition, the root clearance must be increased to accommodate this factor. If you have an operating unit, it is advised that periodic measurements of gear temperatures be taken. Insufficient root clearance can result in extremely costly tooth failure.

7. Understand Dual Pinion Drives

In high production applications, dual pinion drives are used to nearly double the horsepower rating of the gearset for the same face width. The downside to employing two electric motors is that it is difficult to have matching speed load curves. This will result in two motors fighting each other. Even variable-speed drives (VFDs) pose problems when running together long term. The use of two hydraulic drives is recommended, as hydraulic motors set at the same operating pressure will result in equal loads to each pinion.

8. Think About Gear Mounts

Achieving successful gear mounts depends on the temperature of the dryer. Usually, in high heat, a leaf spring mount is used, where springs allow for expansion and transmit the load. With this type of mounting, gears do not get as hot as the other surfaces and are stressed less due to differential thermal expansion. Flange mounts are not recommended. Lug mounts with slotted holes are applicable for low temperature applications. Leaf spring mounts can be used on any application.

9. Remember Gear Inspections

Regular shutdown and inspection of the gear sets should be a priority. It is also suggested that the gear be tested prior to installation. While the unit is not running or the gear is just being installed, ultrasonic testing, mag-particle inspection, eddy current testing and various other ways to detect possible cracks can be employed. What many don’t expect is a crack in the root of the gear tooth that can create sudden catastrophic failure. Shutting down production after a serious gear break and waiting for the replacement is a time-consuming and cost-prohibitive process.

10. Perform Thermal Imaging

Because you can’t fully inspect a dryer while the unit is in production, a heat gun should be used to measure the temperature and look for refractory failures. If hot spots are found and corrective action is not taken, it is likely that a shell replacement and unit alignment will be needed. A relatively uniform temperature is the desired result. This should be performed at least once a week and logged. Recording bearing temperatures, tire temperature and the shell temperature near the tire, temperature across the pinion face and tire creep should be done weekly. Plants with a strong training record show better results on their units.