The silent chain is manufactured for any reasonable power capacity, and is designed for high speeds. It is therefore employed in connection with motor applications. The silent chain is an assembly of specially designed links which operate on cut tooth sprockets, generally termed pinion and gear. The silent chain drive is applicable in all channels of industrial operations. It is particularly adaptable for direct chain connection between motor and machine or motor and line, head or counter shafting. The most desirable speed for this type of chain is 1,200 to 1,5oo r.p.m. and ratios of 15 to I have proven successful. (See CHAIN MANUFACTURE.) Gear Reduction Units.—Motor speeds of soo to I,800 revo lutions per minute are too high for industrial purposes, therefore, some mechanical reduction medium must be interposed between the motor and driven machine. The modern speed reduction gear unit has made possible the elimination of a multiplicity of chains, gears, shafting and belting for ratios such as 15 to 1 and higher. It utilizes four general types of gearing, namely, the worm, the spur, the herringbone and the bevel. Reduction units using these various types are available in suitable sizes and ratios for almost any power and speed reduction required. Each type has inherent limi tations, therefore, care should be exercised based on a knowledge of the units in question before a selection is made for a particular requirement. One of the most important factors in the successful operation of a speed reduction gear whether it be the worm, spur, herringbone or bevel type, is the connection to the motor and the connection to the shaft of the driven machine or apparatus. The flexible type of coupling is imperative at these points, because it is not mechanically possible to line two shafts perfectly, each supported on its own bearings, so that both will revolve about the same axis.
The efficiency of a modern worm gear speed reducer is entirely dependent in many cases on the helix angle of the thread of the worm. The greater the helix angle (up to about 42°) the greater the efficiency; and the smaller the angle, the greater the power loss. The worm of a modern reducer is usually integral with an alloy steel shaft with threads hardened and ground. This combin ation is rigidly supported on ball or roller bearings, the rear bear ing being of the double row maximum type, capable of carrying the full thrust load of the worm in either direction as well as one half the radial load. The other half of the radial load is taken by the forward bearing. The worm gear usually consists of a chilled cast bronze rim shrunk on and pinned to a high grade cast iron centre. The housing of the reducer is cast iron, compact and sealed tight so that the oil in which the gearing operates can be perfectly maintained.
Ratios of 6o or 7o to I are common, but beyond this the effi ciency drops rapidly. To obtain higher reductions and still retain the right angle feature of the worm drive, two methods may be employed. One is to provide an auxiliary shaft supported inde pendently, on which is mounted a pinion which in turn meshes with another gear. Another more recent method is to employ a re duction unit specially designed with suitable bearings to permit the mounting of the pinion directly on the slow speed shaft. For spe
cial applications and ratios up to io,000 to I, the double reduction worm gear unit may be employed. Modern worm gear reduction units can be operated at worm speeds up to 4,000 r.p.m. and are therefore, suitable for direct connection to steam turbines.
The spur gear reduction unit has been evolved to cope with ratios as high as 500 to 1 and reduces in a straight line. The spur gear reducer can be classified into two distinct types, the planetary and the non-planetary. The planetary type is capable of giving the largest speed reduction because it consists of spur gears or idlers radially disposed about a central pinion and in turn mesh with a stationary internal gear. The planetary reducer with single reduc tion is satisfactory for ratios between 4 and 8 to I, but should not be considered for ratios of less than 3 to I. High ratios such as Too, 200, 40o and Soo to i are obtainable in the planetary type by the reductions being doubled or tripled and coupled in series in the same cast iron housing. The non-planetary type of spur gear re ducer has the advantage of giving comparatively low reductions and keeping the rotational speeds at minimum. Ratios as low as I to T and as high as 30o to I are practical with the non planetary system. This type consists usually of spur gears radially disposed about a central pinion. Each of the spur gears is keyed directly to pinions which in turn mesh with a central gear mounted upon the slow speed shaft.
The herringbone gear speed reducer does not possess the right angle feature of a worm drive nor the "shafts in line" feature of the spur gear reduction unit. It reduces in parallel. The single reduction type is manufactured up to a ratio of 8 to i. In ratios over this the reduction is obtained in two steps. There are two distinct types of the double herringbone reduction unit. One type consists of two sets of herringbone gears arranged in series. In the other type, the first set of gears is split, one set being cut right and the other left hand. They are mounted far enough apart to allow the insertion of the final drive pinion between the helical gears. This form of construction equalizes the loads of all bear ings as well as making a more compact drive. These two types are manufactured in ratios up to 6o to T.
Variable Speed Mechanisms.—Methods both electrical and mechanical have been devised for varying applied speed to appara tus. For many operations of this character, the variable speed motor is ideal, but generally, this type of motor possesses a certain definite range of speed alterations, and therefore is not suitable for close speed adjustment. Fine adjustment is possible with modern mechanical speed transformers. These devices operate in connection with constant speed motors and are most efficient. There are certain phases of industrial operations where speeds must be gradually increased or decreased, otherwise destruction of the product results. For operations such as this, the mechani cal speed transformer is imperative. (W. STA.)