Chain drives and sprockets

Chain drives and sprockets 58/1059 Table 58.7 Roller chain numbers by chain pitch... 

The power is provided to the drawworks by the prime movers at the master clutch (see Figure 4-9) and is transmitted to the master clutch shaft via sprockets and roller chain drives. The speed and the torque from the prime movers are controlled through the compound. The compound is a series of sprockets, roller chain drives, and clutches that allow the driller to control the power to the... 

As explained earlier, roller wear usually is not a major concern. However, wear of the teeth on a small sprocket may be a very important concern because badly worn sprocket teeth ean impose large shock loads on the chain. This is explained in more detail in chapter 15, about chain drive and conveyor maintenance. 

Many factors affect the noise level of a drive. Some of these factors include the amount and type of chain loading, the amount and quality of lubrication, the numb of sprocket teeth, the chain pitch, the fit between the chain and sprocket, chain wear, and sprocket wear. There may be, and probably are, additional factors that have not yet been clearly identified. Usually it is necessary to make a series of sound tests on a prototype or the actual drive to detomine the noise level of the drive. One or two tests are not enough to show the normal variation in noise levels between one set of chains and sprockets and the next... 

When center distance must be fixed and there is no idler or other means of slack take-up, it is very important that the drive be designed to provide the correct amoimt of chain tension. This is typically done by using a center distance that is exact for the chain length and sprockets used. In some cases, chain manufacturers may recommend that the exact center distance be increased by a small amount. This is to ensure that the chain is correctly tensioned after a brief run-in period. It is also common practice to use a chain that is wider than the minimum width required for the application. This effectively reduces the bearing stresses in the chain, reduces the wear rate, and prolongs the life of the fixed center drive. Consult the chain manufacturer for specific advice on fixed centCT drives. 

Since it is good practice to maintain a selected inventory of spare parts for drives, economy can be achieved by standardizing conveyor drives throughout the plant. For example, intermediate speed reduction by means of V belts, sheaves or chains, and sprockets can frequently permit using the same speed-reducer size for several drives. Thus, it may be necessary to keep only one repair-stock speed reducer for a number of conveyors. 

A fresh start and the true beginning of bicycles becoming a popular means of transportation can be traced to around 1886 and the efforts of John Kemp Starley and William Sutton. With equal-sized wheels, tubular steel diamond-shaped frame geometry, and a chain-and-sprocket chain drive to the rear wheel, the safety bike looked much like the modern version. 

The term chain drive denotes a combination of chain and sprockets, with the sprockets mounted on rotating shafts. 

Shaft Centers. It can be readily seen that on any chain drive the minimum center distances must be more than one-half the sum of the diameters of the two sprocket wheels. Experience has shown that best results are usually obtained when the center distance between shafts is 30 to 50 times the chain pitch. Forty times the chain pitch is about normal, and 80 times the pitch is maximum. In highly pulsating loads, 20 to 30 times the pitch is more nearly the correct center distance. Center distances of 10-12 ft are permissible with finished steel roller chains operated at moderate speeds without the use of idlers. On distances greater than this, an idler should be used to eliminate the possibility of swaying or flopping, which may cause the chain to jump the sprocket. When idlers are used, they should be placed on the slack strand of the chain. The number of teeth in the idler should be the largest possible and preferably not less than the number in the smaller sprocket of the drive. 

An adjustable idler is recommended for drives having fixed centers, particularly if the line of centers is vertical or near vertical. With such idlers, the required chain tension can be maintained for correct chain and sprocket-wheel contact. 

Unlike gear sets, there can be two distinctive tooth-mesh frequencies for a chain-drive system. Since the drive and driven sprockets do not directly mesh, the meshing frequency generated by each sprocket is visible in the vibration profile. 

This section is an overview of flexible drive chains and sprockets. It provides the basic knowledge required to install, operate, and service this means of power transmission. 

Unlike V-belts, chain drives do not rely on friction to deliver power. As a result, transmission of power is positive (i.e., no slippage) and in many ways resembles the action of gears. A chain is constmcted in such a way that it provides a connection between the teeth of the drive pinion and driven sprockets. 

Since chain drives are designed to provide positive transmission of power from the pinion to the driven sprocket, there is little loss of efficiency. When properly installed, chain drives can approach 98 per cent efficiency. However, they are somewhat limited in speed and span. 

They are designed to provide a flexible link between a power supply, such as a hydraulic cylinder, and another machine component. Leaf chains do not have any rollers or other sprocket-engaging device. They are designed with connector links located at each end of the chain that can be used to connect the drive and driven machine components. 

Before installing a new chain, carefully check all sprocket teeth. If the teeth are worn to a hooked shape, the sprockets should be replaced to assure full-capacity performance and satisfactory life from the new chain. Because chain drives are not as flexible as belt drives, caution must be exercised during installation. The following steps should be observed during installation ... 

Like all drive components, proper alignment is crucial to the smooth operation of a chain drive. If the sprockets are not parallel and in the same plane, the chain will lose contact with the sprocket teeth and drive-train failure will occur. 

Proper chain-drive lubrication increases drive life by dissipating frictional heat generated in the joint area. Heat varies according to the chain speed, horsepower transmitted, center distance, sprocket ratio, drive size, amount of lubricant, and viscosity. It generally ranges from surrounding temperature to 60 to 70°F above the ambient temperature. Normal chain drive temperatures should not exceed 180°F. 

Roller conveyors also come in a wide variety of forms and are used in wide ranging applications. Typically, a roller conveyor is driven either by a belt (placed underneath the rollers), a drive shaft (that runs along the conveyor underneath the rollers, the rollers are connected to the drive shaft with industrial-grade rubber bands), a chain-and-sprocket mechanism (a sprocket is attached to the end of each roller, the chain runs along one end of the conveyor), or an electric motor placed inside the rollers themselves. 

Type of drive can be an internal rotating shaft with three blades attached. The shaft extends outside and can have a driven pulley. It generally has a sprocket and chain drive, a gear box or a VFD motor. External girth gear drive can also be provided to the rotary screen if there are more than one concentric screens for separation of two-or three-sized particles. This will depend on length and diameter of each screen and may not always be possible. 

Speed and horsepower ratings are the prime considerations in engineering a chain drive. The ratings are normally listed for the smaller sprocket, regardless of whether it is the drive or driven member. Chain manufacturers should be consulted when special conditions such as composite duty cycles, idlers, or more than two sprockets are involved in the drive cycle. 

The drive ratio is determined by the speeds of the driving and driven shafts. Normally, the speed ratio should be limited to approximately 7 1. Properly engineered, drives with ratios up to 10 1 will perform satisfactorily. However, double-reduction drives with smaller ratios have better operating characteristics and are often more economical than large ratio, singlereduction drives. Minimum chain wrap on the small sprocket always should be at least 120°. 

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