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.