3.3 GEAR DRIVE

3.3.1 GEAR DRIVES

Common types of gears used in industrial gear drives include spur, helical, or double-helical, bevel, spiral bevel, hypoid, zerol, worm, and internal gears. Spur gears transmit power between parallel shafts without end thrust or axial displacement. They are commonly used on drives of moderate speeds such as marine auxiliary equipment, hoisting equipment, mill drives, and kiln drives. Simplicity of manufacture, absence of end thrust, and general economy of maintenance recommend the use of spur gearing wherever practicable.

Helical gear teeth are cut on a helix (oblique) angle across the gear-wheel face. Mating helical gears permit several teeth to be in mesh at the same time. This increases load-carrying capacity, ensures transmission of constant velocity, and reduces noise and vibration. Helical gears produce end thrust along the axis of rotation, which must be accommodated by thrust bearings.

Diagram 3.3.1 

Basic Types of Gear Drives

3.3.2 BASIC GEAR DRIVES

Gear drives are used to transmit power between a prime mover and driven machinery. In addition to the simple transmission of power, gear drives usually change or modify the power being transmitted by: 

(1) Reducing speed and increasing output torque 

(2) Increasing speed

(3) Changing the direction of shaft rotation 

 (4) Changing the angle of shaft operation. Gear drives are generally considered packaged units,                  manufactured in accordance with accepted and advertised specifications, to be used for a wide            range of power-transmission applications . 

3.3.3 EPICYCLIC GEAR DRIVES

                In an epicyclic gear drive, power is transmitted between prime mover and driven machinery through multiple paths. The term epicyclic designates a family of designs in which one or more gears move around the circumference of meshing, coaxial gears, which may be fixed or rotating about their own axis. Individual gears within an epicyclic drive may be spur, helical, or double-helical.  

  

Diagram 3.3.3 

Because of the multiple power paths, an epicyclic gear drive will normally provide the smallest drive for a given load-carrying capacity. Other advantages include high efficiency, low inertia for a given duty, high stiffness, and a high torque/power capability.

3.3.4 INSTALL GEAR DRIVES CAREFULLY

• When installing gear-drive units, be sure that they are well supported, accurately aligned, and securely anchored to prevent misalignment of gears or shafts. Consult the installation and maintenance instructions furnished by the manufacturer.
• Good-quality mechanical couplings suitable for the application should be used to couple the shafts of driving and driven units. Slight angular or linear shaft misalignments may be accommodated by using flexible-type mechanical couplings. 
• In some cases, torsional stresses at starting or during momentary overloads can be compensated for by use of certain types of flexible mechanical couplings.
• Proper loading of gear-drive units is essential for a long and trouble-free service life. Assuming that gear drives are properly rated for the particular applications and are properly installed, it is important that they should not be subjected to extreme or sustained overloads.
•  Torque limit switches are available as optional equipment on the gear drives of some manufacturers. Where the possibility of overloading or machinery jamming (which might produce an overload on the drive unit) is present, it is wise to insist on torque-limiting devices.
• Gear-drive housings are usually designed for proper heat dissipation under normal operating conditions. Do not allow units to operate where oil temperatures exceed those recommended by the manufacturer. Where surrounding atmospheric conditions might reduce normal heat dissipation, consult the drive manufacturer for his recommendations.

3.3.5  GEAR DRIVES LUBRICATION

• Lubricating oils for use with enclosed gears and gear units should be high-grade, high-quality, well refined, straight mineral petroleum oils. They must not be corrosive to gears or ball or roller bearings. They must be neutral in reaction. They should have good defoaming properties. No grit or abrasives should be present.
• For high operating temperatures, good resistance to oxidation is needed. For low temperatures, an oil having a low pour point to meet the lower temperature expected is needed. When the operating temperature varies over a wide range, an oil having a high viscosity index is desirable.
• On many types of gear-drive units, pressure fittings are supplied for the application of grease to bearings that are shielded from the oil.
• The lubricant should not be corrosive to gears or to ball or roller bearings; must be neutral in reaction; should have no grit, abrasive, or fillers present; should not precipitate sediment; should not separate at temperatures up to 300°F; and should have moisture-resisting characteristics. The lubricant must have good resistance to oxidation.
• Every precaution should be taken to prevent any foreign matter from entering the gear case. Sludge is caused by dust, dirt, moisture, and chemical fumes. These are the biggest enemies of proper and adequate lubrication in gear-drive units.

3.3.5 TROUBLESHOOTING GEARS

GEAR-TOOTH WEAR AND FAILURE

       Classification A: Surface Deterioration

1. Wear

2. Plastic flow

3. Scoring

4. Surface fatigue

5. Miscellaneous tooth-surface deteriorations

Classification B: Tooth Breakage

6. Fatigue

7. Heavy wear

8. Overload

9. Cracking