In this article we will discuss about:- 1. Introduction to Gear Drives 2. Basic Theories of Gear Drives 3. Nomenclature 4. Classification 5. Gear Trains 6. Advantages and Disadvantages.
Introduction to Gear Drives:
Gear drive has become the most important and popular means of motion and power transmission system. A gear drive is a pair of meshing gears as shown in Fig. 9.18.
Gears are components of machines for the transmission of power and motion from one shaft to other separated by small distance. It is a toothed wheel, i.e., wheel with a number of teeth. It provides with projections known as teeth and in between two teeth, there is a vacant space, which is called tooth space, to accommodate the incoming tooth in rotation.
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As it appears, toothed wheels avoid the problem of slippage which is quite prominent in belt drive. Hence, these wheels produce positive drive and no slip. When one gear wheel rotates, other wheel also rotates in the opposite direction as has been shown in Fig. 9.18(b). The power is transferred effectively and no loss is observed. The velocity ratio remains same throughout the operation.
The motion of meshing gears is similar to that of a pair of two pitch cylinders which roll without slip.
Basic Theories of Gear Drives:
Let us assume that two plain wheels P and Q (Fig. 9.19) are fixed to two parallel shafts and pressed tightly in contact with each other. If wheel P rotates about its axis, other wheel Q also rotates in opposite direction due to friction. The surface of the two wheels rotates with the same speed if there is no slipping. Thus, rotary motion is transferred from one shaft to another.
It is quite apparent that with the increase of load to be transferred, the wheels will begin to slip with each other. In order to prevent slipping, grooves may be cut on the cylindrical surface of the wheel and projections of metal are added in between them alternatively. These grooves cut and the projections of profile form teeth and the wheels with these will be called toothed wheels or toothed gears as shown in Fig. 9.20.
Nomenclature of Gear Drives:
Drive gears have a wide range of unique terminology known as gear nomenclature.
i. Pitch Circle:
In every pair of gears in mesh, when teeth size becomes too small, the gears as two smooth cylinders in contact have the same speed as gears and do not slip. Such imaginary cylinders are called pitch cylinders and the circle is known as pitch circle as shown in Fig. 9.19.
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ii. Pitch Surface:
The cylindrical surface which the pitch circle represents is called the pitch surface.
iii. Pitch Point:
It is the point of contact between the pitch circles of two meshing gears, as indicated by W in Fig. 9.19.
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iv. Center Distance:
It is the distance between the centers of the pair of mating gears, as shown in Fig. 9.20.
v. Tooth Face:
The upper portion of the tooth profile which is above the pitch circle is called face of the tooth [Fig. 9.21(b)].
vi. Tooth Flank:
The lower portion of the tooth profile is called flank of the tooth [Fig. 9.21(b)].
vii. Addendum:
The radial distance from the pitch circle to the top of teeth is known as addendum [Fig. 9.21(b)].
viii. Addendum Circle:
The circle passing through the upper portion of the teeth (crest) is known as addendum circle.
ix. Dedendum:
The radial distance from the pitch circle to the bottom of teeth is known as dedendum.
x. Dedendum Circle:
It is the circle which passes through the bottom of the teeth.
xi. Crest:
It is the outer portion at the top of tooth [Fig. 9.21(b)].
xii. Tooth Depth:
It is the height of tooth denoted by h [Fig. 9.21(a)].
xiii. Tooth Thickness:
It is the thickness of the tooth denoted by t [Fig. 9.21(a)].
Toothed gears can be classified on the basis of the following conditions:
(a) Relative position of shaft
(b) Relative motion of the shaft
(c) Tooth profile/form of teeth
Important Relationship:
Length of pitch circle = Circular pitch × Number of teeth
D = tsT
where D is the pitch diameter, ts the circular pitch, and T the number of teeth.
∴ D = tsT/
The ratio ts/ is termed as module (m).
D = mT
or m = ts/
The module is the main design quantity in determining the size of teeth.
Classification of Gear Drives:
Following are the main types of gears:
(a) Spur gears
(b) Helical gears
(c) Herring bone gear drive continuous teeth (double helical gear drive).
(d) Bevel gears
(e) Worm gears
(f) Rack and pinion
(a) Spur Gear Drive:
Gear drives are most widely used in mechanical system designs. The gear transmission of power between two parallel shafts is accompanied by means of a spur gear. In this case, straight teeth parallel to the axis of shaft are formed as shown in Fig. 9.22. These are used to transfer power between two parallel and coplanar shafts.
They are used as components in gear box of automobiles, machine tools, watches, and measuring instruments. The smaller gear among the pair of meshing spur or bevel gears is usually the driving gear and is called pinion; the larger is the driven one and simply called a gear or gear wheel.
A pictorial view of spur gear wheel has been shown in Fig. 9.23(a) and another type of transmission system has been shown in Fig. 9.23(b). Another pictorial view of spur gear wheel meshing with pinion has been shown in Fig. 9.23(c).
(b) Helical Gear Drive:
The gear transmission of power between two parallel and coplanar shafts is accompanied by means of helical gear. In case of helical gear, the teeth cuts are at some inclination to the axis of the shaft. In the case of two helical gears meshing together, if one gear has a right hand helix, the other gear will have a left hand helix as shown in Figs. 9.24(a) and 9.24(b). In regards to the constructional details, they are formed as helix on the cylindrical body.
The operation of helical gear drive is smooth and noiseless. Helical gear drives are used for heavy transfer of power and high-speed power system.
In case of the helical gear transmission of power, the shaft bearing takes extra thrust. This is the disadvantage of this gearing system. However, this problem can be eliminated by using a double helical gear (herring bone gear).
(c) Herring Bone Gear Drive Continuous Teeth (Double Helical Gear Drive):
Herring bone continuous gear drive is shown in Fig. 9.25 in two different views. It is well known that in case of single helical gear drive, when gear wheel and pinion are meshing, the shaft bearing takes the extra thrust on account of the inclination of teeth cut on meshing pinion and gear.
However, the demerits of single helical gear can be eliminated by using double helical gear (herring bone gear). This is analogous to two helical gears of opposite hand placed side by side on each shaft. This secures opposite thrust reaction to balance each other.
During the operation, equal and opposite thrust reaction is produced on gear and pinion which cancel the axial thrust on shaft. This is very popular to transmit heavy load at high speed and is used in automobile industries.
Double Helical Gear Drive (Grooved):
Figure 9.26 shows a double helical grooved gear drive. It is just similar to double helical gear drive continuous, except there is a central groove made all over the circumference of the gear wheel and pinion.
The width of the wheel and pinion is divided into two parts. Both the parts act separately as gear wheel and pinion. The helical gear is cut. The left part of the gear wheel is cut with left hand helix whereas the right side of the gear wheel is cut with right hand helix (Fig. 9.27).
Similarly, the left part of pinion is cut with right hand helix and the right part with left hand helix. During the operation, equal and opposite thrust reaction is produced on gear and pinion which cancel the axial thrust on shaft. This is used to transmit heavy load in automobile and heavy equipments.
(d) Bevel Gear:
Power transmission by bevel gear drive is widely used in engineering-specific applications. It is well known that when two shafts are parallel and coplanar, spur gear drive is quite useful for the power transmission. But when two shafts axes intersect each other, bevel gears are used.
A bevel gear drive is the assembly of two bevel gears wheel. In this case, gears are always made in pairs. They are not interchangeable, i.e., if one wheel tooth gets damaged, other wheel has to be also replaced.
In bevel gear, teeth are formed on conical surface (part of frustum of cone). A typical bevel gear wheel is shown in Fig. 9.28. A portion of shaft has been separated.
Two bevel gear wheels may be used to connect at any angle. Figure 9.29(a) shows the view of bevel gear. The angle of intersection of two shafts may be less than 90°, equal to 90°, or more than 90°. Generally, two shafts making an angle of 90° is the common practice as shown in Figs. 9.29(b) and 9.29(c).
The bevel gear of this nature has been termed as right angle bevel gear. Sometimes, the intersection of angle of shaft may be different from 90°. If it is less than 90°, it is known as acute angle bevel gear. If it is more than 90°, this may be called as obtuse angle bevel gear.
(e) Worm Gears:
Worm and gear wheel drive is an assembly of screw, which is known as worm and gear wheel. A pictorial view of the combination drive is shown in Fig. 9.30. It is used to transmit motion between two shafts whose axes are crossing each other. That is, the axes of both worm and gear wheel are at right angle. It consists of work which is a screw with a trapezoidal thread and a gear wheel. They have been shown separately in Figs. 9.31(a) and 9.31(b).
In this combination, worm is the driving member which is meshing with the gear of the wheel. Such gear drives are used to obtain high speed ratio. The speed can be either increased or decreased. During the operation, power loss due to friction is more prominent. Worm and gear wheel drives are used in heavy machine tools and other equipments. For clear understanding, Figs. 9.32 and 9.33 may also be studied.
(f) Rack and Pinion:
A rack and pinion gear is used to convert rotary motion into translational motion. It consists of a rack and spur or helical gear wheel known as pinion as shown in Fig. 9.34. A rack is a long rectangular bar with a number of spur gear profile cuts which mesh with pinion. Theoretically, rack is considered as the spur gear of infinite diameter. Pinion is a driving element whereas rack becomes the driven element.
When pinion rotates either clockwise or anticlockwise, rack also moves left or right. Thus, the rotary motion of pinion is converted to the linear movement of rack. The rack and pinion combination system is very popular and used in machine tools such as lathe, drilling, milling, shaper machines, etc. The rack and pinion gear pictorial view is shown in Fig. 9.35.
Gear Trains:
Gear trains are the assembly of a number of meshing gears to transmit power from driving shafts to driven shafts. Each gear wheel may have large number of teeth which may be unequal. The gear wheel may be spur, spiral, bevel, etc.
Gear train may be of two types:
(a) Simple gear train
(b) Compound gear train
(a) Simple Gear Train:
In a simple gear train, a series of gear wheels are fitted on different shafts between driving shaft and driven shaft. The power is transmitted from the driving to the driven shaft. Figure 9.36 shows a simple gear train. Wheels (1), (2), and (3) are the three spur gear wheels.
Wheel (1) meshes with wheel (2) which further meshes with wheel (3). Let n1, n2, n3 and T1, T2, T3 are the revolutions and number of teeth of gear wheels (1), (2), and (3), respectively.
(a) Gear wheel (1) drives gear wheel (2)
(b) Gear wheel (2) drives gear wheel (3)
(b) Compound Gear Train:
When velocity ratio is very high, simple gear train is quite impossible. For such cases, compound gear train is found suitable to get the power transferred from the driving shaft to the driven shaft. In a compound gear train, the intermediate gear shaft holds one more gear wheel. In fact, the intermediate gear wheel shaft will have two wheels and both the wheels have the same speed. Figure 9.37 shows a compound gear train.
Gear (1) is the driving gear and gear (4) is the driven shaft. Gears (2) and (3) both mounted together have same speed. Gear (1) will mesh with (2).
(a) Wheel (1) drives wheel (2)
(b) Since gears (2) and (3) are mounted on the same shaft, both rotate at the same speed, that is, n2 = n3. But the number of teeth are not same, i.e., n2 = n3. But number of teeth are not same, i.e., T2 ≠ T3.
(c) Wheel (3) drives wheel (4).
Advantages and Disadvantages of Gear Drive:
Advantages:
i. It is a positive drive.
ii. Velocity ratio is constant throughout.
iii. There is no slippage.
iv. It can be operated smoothly.
v. Most reliable operation.
vi. It can transmit more power comparing to belt drive.
Disadvantages:
i. Manufacturing of teeth is complex, requires technique.
ii. Production is costly.
iii. More power loss in friction.
iv. Requires proper lubrication.
v. Maintenance cost is high.
vi. Not suitable for large center-to-center distance of shaft.