CONCLUSION

Maintenance of gear drives involves proper selection, proper installation, proper loading of the unit, proper lubrication, and periodic inspection. Metallic gears have tremendous service life when properly used and cared for.

3.4 FRICTION DRIVE

3.4.1 TYPES OF FRICTION DRIVES (BELT AND SHEAVE)

Adjustable While in Motion (Nonenclosed)

• These are usually sold as separate, complete pulleys, designed for mounting directly on a motor shaft. These pulleys are normally spring-loaded and may have either one or both of the disk halves capable of axial movement. They are designed to drive a fixed-diameter sheave. Output speed variation at the fixed sheave is controlled by varying the center distance between the fixed and spring-loaded sheave. This usually involves a sliding-motor mounting base. 
• Pulleys with one fixed and one movable disk may be used to drive either a flat-face or a V sheave. Note that as the pitch diameter traversed by the belt on the spring-loaded sheave changes, the center line of the belt moves axially. If a flat-faced pulley is the driven element, both motor shaft and driven shaft must be parallel, and the motor should move perpendicularly to the driven shaft.
  

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

3.2 CHAIN DRIVE

3.2.1 CHAIN DRIVES

Chain drives consist of an endless series of chain links which mesh with toothed wheels, called sprockets. The sprockets are keyed to the shafts of the driving and driven mechanisms. A roller chain has two kinds of links—roller links and pin links—alternately assembled throughout the chain length. A roller link consists of two sets of hollow rollers and bushings, the bushings being press-fitted into the apertures in the roller link plates, the rollers being free to rotate on the outside of the bushing. The pin link has two pins press-fitted into the apertures of the pin-link plates. When the chain is assembled, the two pins of the pin links fit within the cylindrical bushings of the two adjacent roller links. The pins oscillate inside the bushings, while the rollers turn on the outside of the bushings. This latter action eliminates rubbing of the rollers on the sprocket teeth.

Diagram 3.2.1a

Dimensions for roller-chain identification.

3.2.2 ADVANTAGES

1. Drive efficiency: This is normally in excess of 98 percent.
2.  Uniform driven speed: Roller and silent chain drives are positive; the principle of teeth,        not tension, results in no loss in rotative speeds through slipage or creep.
3. Low bearing loads: Slack side tension is not required.
4. Larger ratios: Less wrap on driver sprocket is required, which permits a higher speed ratio in given area than can be obtained from belt drives.
5.  More power per inch of width: Strength of steel permits greater loads for any given diameter and speed.
6. Relatively unrestricted center distances: Chain can be made endless to any length within limits.
7. Ease of installation: Center distances and alignment do not require close tolerances.
8. Standardization: Industry standardization of chain and sprockets means that replacements are available from many sources.
9. Repair on the job: Repair links are available for quick emergency replacement of worn or damaged links.
10. Drive multiple shafts: Chain is one of the most convenient methods of driving several shafts from one power source.
11. Long drive life: Wear is reduced through distribution of load over a number of sprocket teeth. Normal chain wear is a slow process and therefore requires infrequent adjustment.
12. No deterioration: Adequately lubricated chains do not deteriorate with age, nor are they adversely affected by sun, oil, and grease.

3.2.3 INSTALLATION PROCEDURE

1. Align each shaft with a machinist’s level applied directly to the shafts. (Shafting with silent chain or multiple-width roller chain sprockets may be aligned by applying the level across the sprocket teeth.) Check shafts for parallelism with a feeler bar. After adjusting for parallelism, recheck the shaft levels. Repeat these adjustments until both level and alignment are satisfactory.
2. Mount sprockets on shafting, and align by checking with a straightedge along the finished sides of the sprockets. A taut wire may be used if the center distance is too long for a straightedge. If a shaft is subject to end float, block it in its running position before aligning the sprockets. Secure the sprockets against axial movement by tightening setscrews.
3. Before installing the chain, recheck the preceding adjustments and correct any that may have been disturbed. Wrap the chain around the sprockets, bringing the free ends together on one sprocket. To accomplish this, shorten shaft centers sufficiently. Connect the free ends by use of the connecting link or pins provided.
4. Readjust shaft centers to check chain tension. Chains should be installed fairly tight with only a small amount of slack. In the case of vertical drives, the chain should be kept snug and provision for adjustment of chain may be necessary.
5. New chains will loosen slightly owing to the seating of the joints as the chain is cycled over sprockets under load. After the first several weeks of operation, it is advisable to adjust the centers, if needed, particularly on long center drives. After this initial elongation, with proper care and lubrication, precision chain drives will give long service without undue elongation or wear.

Diagram 3.2.3a

Shaft alignment & Sprocket alignment

3.2.3 LUBRICATION OF PRECISION CHAIN DRIVES

         Heat

•  Proper chain-drive lubrication will serve to increase the drive life by dissipating frictional heat generated in the joint area.

         Improper Lubrication

• A lubrication-starved chain drive will show a brownish (rusty) coloration around the joints and in the roller-bushing areas when the link is disassembled and the pin is inspected. The normal highly polished surface of the pin will have deteriorated to a roughened, grooved, or galled surface, which can eventually destroy the hardened surfaces of the chain parts and increase wear until the drive is completely destroyed.

         

          Windage

• A chain drive can be running through a sump of good lubricant and still destroy itself from lack of lubrication if the speed exceeds 2500 ft/min (fpm). The chain is actually blowing the lubricant out of its path. In high-horsepower, high-speed drives, it is necessary to use pressurized streams to ensure proper lubrication of the articulating components and to dissipate the heat generated. This lubricant should be sprayed onto the inside of the chain

         Contamination

•  Lubricants should be protected from dirt and moisture. A filtering system should be utilized to remove wear particles and abrasive particles to minimize wear on the drive chain.

        Oil Viscosity

•  A good grade of lubricant should be used between the chain parts to maximize the wear life of a chain drive. Lubricants containing anti foam, anti rust, or film-strength-enhancing additives may be useful. It is essential that the lubricant reach the side plate wearing surfaces and pin bushing areas. Therefore, normally heavy oils and greases are not recommended. The lubricant should be free-flowing at the prevailing temperature.

DRIVE MECHANISM

3.0 TYPES OF DRIVE MECHANISM

• BELT DRIVE
• CHAIN DRIVE
• GEAR DRIVE
• FRICTION DRIVE

3.1 BELT DRIVE

       3.1.1 V-BELTS

                Most V-belt drives used in industrial applications fall into two categories: heavy duty (industrial) and light duty (fractional horsepower). There are primarily two types of industrial belts: the classic cross sections (A, B, C, and D), which have been used for decades, and the narrow cross sections (3V, 5V,and 8V), which are relatively new. Most of the sections are available in banded (wrapped) and molded notch (cog) constructions. The banded belt has a fabric cover which completely encloses the exterior of the belt. 

Diagram 3.1.1A

Typical classic cross-sectional V-belt molded

notch construction.

Diagram 3.1 .1B 

Typical narrow cross-sectional V-belt,banded construction. 

     3.1.2 BELTS MAINTENANCE

        There are three possible causes of pulley misalignment:

1. Driver and driven shafts are not parallel.

2. Pulleys are not properly located axially on the shaft.

3. Pulleys are tilted due to improper mounting.

.

        Belt tension may be summarized with a few simple rules:

1. The best tension for a V-belt is the lowest tension at which the belt will not slip at the                           highest load. Since synchronous belts are more sensitive to tension, use the force-                                 deflection numerical method described below and tension gauges to set tension for these                      belts.

2. When installing a new drive, set the tension, rotate the drive a few revolutions, and then                       recheck the tension. Check the tension once more after the first day of running.

3. For V-belt drives, check the belt tension periodically thereafter.

INTRODUCTION OF BELT DRIVE

Power transmission is simply defined as the transfer of energy from its place of generation or storage to a location where it does work.