Gears and Types of Gears

Gears are toothed supporters which are used to transmit the motion or power in between the two shafts. The power transmission between them takes place by using meshing without any slip. The gear drives are also known as the positive power drives.

In the gears the smaller gear is known as the pinion and the larger gear is known as gear immaterial, which is also known as driving gear.

In case of the pinion is known as the driver, here it acts as a step down drive so that the torque increases and the input speed will decreases. In the other case the gear is the driver so it results in the step up drive, where the torque decreases and the output speed increases.


Types of gears:

  • Spur gear
  • Helical gear
  • Double helical gear
  • Internal gear
  • Rack and pinion
  • Straight bevel gear
  • Spiral bevel gear
  • Hypoid bevel gear
  • Worm gear
  • Spiral gear

Spur gear:

Spur gear

In the spur gear to the axis the teeth are parallel. These types of gears are used to transmit the power in between the two parallel shafts. Coming to the construction it is the simplest model to make construction and also easy to manufacture. The cost of the spur gear is less compare to others. Mainly these types of gears are used in the high speeds, where the application of the loads is also very high. The spur gears are used in all type of trains. The velocity ratio is also in wide range. We can observe these types of applications in clocks, motor cycles, house hold gadgets, railways and automobiles.


Spur rack and pinion:

Spur rack and pinion

The spur rack is the special case of the spur gear. It is made up of the infinite diameter so that the pitch surface is plane. The spur rack and pinion converts the rotary motion into the translatory motion or translatory motion into the rotary motion. In this gear the lathe in which the rack is transmitted a motion is created to the saddle.

Helical gear

Helical gear

Helical gear is also called as helical spur gear. For parallel shaft drives helical gears are used. In these gears the teeth are inclined to the axis. As the gears are in parallel and the teeth are longer when compare with the spur gears. The helical gears have the capability of carrying high loads. When compared to the spur gears the contact ratio will high. They work smoother and fast when compare to the other gears. High accuracy can be obtained. Mainly they are recommended for the high loads and also high speeds. These types of gears are used in the gear boxes automotive. Compared to the spur gear the efficiency will be lower in the helical gear. The shaft helical angle also introduces axial thrust.

Double helical gear

Double helical gear

The double helical gear is also known as the herring bone gear. It is used for the transmission of the power. The power transmission takes place in between the two parallel shafts. Based upon the non-gap or gap the manufacturing of the opposing helical teeth gear is done. Due to the opposite direction of the gear the shaft moves freely from any of the axial force. In this gear the load capacity must be high and it is difficult to manufacture the helical gear. Compare to the single helical gear double gear is more costly. We can see the application very limited. Due to high capacity reduction drives of the crushers and cement mills take place.

Internal gear

Internal gear

The internal gears are used for the power transmission in between the two parallel shafts. In the internal gears the annular wheels have the teeth on internal boundary. So due to this the drive is very compact. The annular gear and the meshing pinion run in the same direction. The accuracy of this gear is fair. These gears are used for the high speeds and high loads. We can also see the high reduction ratio application. The applications are seen in the automobile automatic transmission, cement mills reduction gear boxes and wind mills step up drives. Due to the design of this gear it is not recommended for the precision meshing, inspection limitations and fabrication. When there is a requirement of the internal features these are used.

Rack and pinion

Rack and pinion

Rack is the segment of a gear of infinite diameter. The design of the tooth is spur or helical. Mainly these types of gears are used in the conversion of the rotary motion into the translatory motion or translatory motion into the rotary motion.

Straight bevel gear

Straight bevel gear

In between the intersecting shafts the straight bevel gears are used for the power transmission. These gears are operated at high loads and the high speeds. The accuracy of the gear is impartial to good. Mainly these gears are suitable for the high velocity ratios and also for right angle meshes to any other angles. Particularly for low ratios the selection must be right angle drive. It is difficult for formation and fabrication, as it is limited to attain precision. On the train they are located at less critical meshes. We can observe such applications in the automotive differentials.

Spiral bevel gear

Spiral bevel gear


The spiral bevel gears are used for the power transmission between the intersecting shafts. In this gears the spiral tooth are in more contact ratio. Compared to the straight bevel gears they work very smoother and also they must have higher load capacity. Compares to the straight bevel gear the efficiency is slightly lower. Mainly the spiral bevel gears are used in the automobiles.

Hypoid bevel gear

Hypoid bevel gear

These gears are used for the right angle drive. In this the angle does not exist. This permits the lowering of the pinion axis in the automobiles, to avoid hump inside the automobile drive line power transmission takes place. Lubrication is required to reduce the wear and friction during the time of the intersection. Compares to the other two types of gears the efficiency must be lower. These types of gears are used in the automobile drive line power transmission.

Worm gear

Worm gear

Worm gear consists of a worm. The worm gear is similar to the screw gear. Mainly these gears are used for the right angle skew shafts. Without any shock application, the engagement occurs in the gear. Due to the sliding action in the operating system, it results to produce the frictional heat. The possible reduction ratio ranges between ‘8 to 400’.

Spiral gears:

Spiral gears

The spiral gears are also known as crossed helical gears. The spiral gears are at the high helix angle and the power transmission takes place between the two non- intersecting non parallel shafts. Initially it is observed to have the point contact and due to the considerable velocities the final gear must have a line contact. These gears are used for the low speed applications and light loads like sewing machine, instruments etc., and the accuracy are poor. These gears are used in the textile machine industries.

Crown gear:

Crown gear

The crown gear is a special gear from the bevel gear. The teeth are projected at the right angle to the plane of the wheel. The teeth orientation looks like points on the crown. The crown gear meshes the other bevel gear very accurately and sometimes it meshes up with the spur gear. We can see that the crown gears are meshed up with the mechanical clocks.

Non circular gear:

non circular gear

The non – circular gear are designed for the purpose of the special cases. It improves the transmit power to another engaged member with minimum noise and wear. The efficiency must be maximized. The objective of the non-circular gear varies in the ratio; plus the displacement of the axle also is observed etc. We can see the applications in the instruments like Continuous Variable Transmission, Potentiometers and Textile machines.

Epicyclic gear:

In the Epicyclic gear one or more axes of the gear move.

Sun and planet gear:

Sun and planet gear

The sun and planet gear are used in the steam engines. These are used to convert the reciprocating motion into the rotary motion. By using this gear there is an advantage that the flywheel speed increases.

Harmonic gear:

Harmonic gear

The harmonic gears are used in the robotics, aerospace and industrial motion control. In this gear we can observe the high gear ratio and compactness.

Magnetic gear:

Magnetic gear

Complete cogs related to every gear component of magnetic gears work as a standard magnet with regular alteration along the magnetic poles on mating surface. Gear elements are overlapped with a backlash capability similar to the some other mechanical gearings. Even though it cannot use as much pressure as a traditional gear, such gears acts by not touching and are immune to wear; these are less noisy and may slide without making any damage to them. This does happen in configurations, which are not likely for the gears that touch physically. These can work with a non-metallic barrier by entirely sorting out the drive forces by the load. The coupled magnet can transfer the forces to a completely sealed envelope without the help of a radial shaft seal that may leak.

Classification of the gear:

Gears are classified according to the peripheral velocity of the gear

  • Low velocity gear – (0 -3) m/s
  • Medium velocity gear – (3-5) m/s
  • Large velocity gear – greater than 15m/s

Terminology used in the gears

Active profile:

Active profile is known as the tooth gear that comes into the contact with the mating tooth along with the line of action.


Pitch circle:

The pitch circle must be denoted by “p”. It is an imaginary circle drawn and it passes through the pitch point. Circle must be perpendicular to the axis. In the predefined position of the gear the pressure angle, helix angle and circular tooth thickness of the gear are defined.

Pitch point:

It is a point where the two pitch circles of the mating gear touch each other.

Rotational frequency:

It is denoted by “n”. They measured in the rotation over time. The units are revolution per minute. (RPM)

Angular frequency:

The angular velocity must be measured in radians per second.

1 RPM = Π/30 radian/ sec

Angular pitch:

Angular pitch is the angle traverse by the circular pitch

Number of teeth:

It is denoted by “N”. An integral number of teeth that is present on the outer peripheral.


When the two gears are intersection in those two gears the smaller one is known as pinion.

Path of contact:

It is a Path monitored by the point of contact among two meshing gear teeth.

Pressure line:

Pressure line is also known as line of action. In the pressure line the forces between the two meshing gear teeth is directed. Due to the engagement of pair teeth we can see the change of action from one moment to other moment because of the engagement. Along with the same line the tooth to tooth force is always directed. The line of action must be constant. Coming to the involute the contact must be in a straight line.


The axis is to be at the center line of the shaft.

Pitch diameter:

The pitch diameter is denoted by “d”. The gear pitch diameter is a predefined diametral position where the pressure angle, circular tooth thickness and helix angle are defined. The pitch diameter is a basic dimension which cannot be measured. By using this other measurements are to be made. The valve of the pitch diameter is based on the no. of teeth.

It can be calculated as:

d=\frac{Nm_n}{\cos \psi}

The units are must be in metric units.


Module is also known as modules and is denoted by “m”. With the help of the irrational factors it is not possible to calculate the circular pitch. By using a scaling factor the regular value must be replaced. So the modules or module of the wheel can be defined as

m = p/Π


“m” is known as the module

“p” is known as circular pitch

Operating pitch diameter:

It is denoted by d_w . Diameters determined from the no. of teeth and the gear must be operating at the center distance.


Center distance:

Center distance is known from the center of the gear, where the radial distance to the center of the mating gear.

Bore diameter:

Bore diameter is known as the hole diameter in a gear

Circular thickness:

On the pitch circle tooth thickness of the gear is known as circular thickness. It is denoted by “t”.

t = 0.5∙Pn

P= circular pitch

Pitch surface:

The two tangent imaginary friction wheels surface have the same angular velocities and the same axes as individuals of a pair of real gears in mesh.

Pressure angle:

It is denoted by \theta . The pressure angle is also known as angle of obliquity. The pressure angle must be in between the gear wheel tangent and the tooth face. For involute gears the pressure angle must be constant.

Outer diameter:

The outer diameter must be denoted by D_0 . This diameter is to be uniform from the most of the tooth.

Root diameter:

The root diameter must be measured from the base of the tooth.


Addendum must be denoted by “a”. It is defined as the radial space from the pitch surface to the outer tip of the tooth.


Spur gear – a = 1 / Pd

Helical gears – a = 1/ Pnd

Pd = diametral pitch

Pnd = normal diametral pitch


Dedendum is denoted by “b”. It is defined as the radial space from the depth of the tooth to the pitch surface.

b = (D – root diameter) / 2

Whole depth:

The whole depth is denoted by  h_t . The whole depth is defined as the distance from the highest of the tooth to the root and it must be equal to the addition of addendum and dedendum or addition of working depth and clearance.


Clearance is defined as the distance between the root disc of a gear and the dedendum circle of its mating gear.

C= 0.157/ Pd

Pd = diametral pitch

Working depth:

Depth of arrangement of two gears, i.e., known as the total operating addendums.

Diametral Pitch:

The diametral pitch is denoted by “DP”. The diametral pitch is defined as the ratio of the no. of teeth to the pitch diameter. They are measured in teeth per centimeter or teeth per inch.

DP = N/d = Π / P

Base circle:

In the involute gear, the tooth profile must be involute of the base circle. Compare with the pitch circle base circle is smaller.

Base pitch:

Base pitch is denoted by  p_b . Base pitch is defined as the circular pitch in the plane of rotation at the base circle.

p_b=\cos \phi.\pi/Pd


Φ = pressure angle

Pd = diametral pitch


Interference is known as the contact between the teeth apart from the projected parts of their surface.

Helical angle:

Helical angle is an angle between the gear axis and the tangent to the helix. For spur gear it is zero for the limiting case.

Normal circular pitch:

In the normal circular pitch the plane must be normal to the teeth.

Transverse circular pitch:

Transverse circular pitch is known as in a transverse plane it is a circular pitch

Pt = Pn/\cos \psi

Pt = Π / Ptd

Pn = normal circular pitch

 \psi = helix angle

Ptd = Transverse Diametral Pitch

In the equation t indicates normal


Lead is an axial development of a helical gear for completion of one turn, as teeth of helical gear and thread of a cylinder worm.

L = Π ∙D / tan  \psi

D = pitch Diameter

 \psi= Helix angle

Linear Pitch:

It is denoted by P. Liner pitch is defined as the distance from one point on a thread to the parallel point on the adjacent thread. They are measured parallel to the axis. When we consider a single thread at that case linear pitch and the lead are same.

Lead angle:

Lead angle must be represented by “λ”. Lead angle is defined as the angle between the plane perpendicular to the axis and the tangent to the helix.

Tooth contact nomenclature

Point of contact:

Point of contact is defined as the any point at which two tooth outlines connect each other.

Line of contact:

Line of contact is defined as the curve or line alongside which two tooth exteriors are tangent for each other.

Path of action:

Path of action is defined as, where the throughout the phase of arrangement the locus of successive interaction points among a pair of gear teeth. For conjugate gear teeth is the action permitted through the pitch point. In the plane of rotation the surface action is trace.

Line of action:

“Line of action is the portion of the common tangent to the base circle. It connects between the mating involute teeth takes place”.

Arc of action:

Arc of action must be denoted by Qt. “the arc of the pitch circle by which a tooth outline moves from the starting to the ending of the contact with a mating profile” is known as arc of action.

Arc of Approach:

Arc of approach is denoted by Qa. “In the arc of approach the arc of the pitch circle, through which a tooth travels for the first time and makes contact with the mating tooth up to it is in interaction at the pitch point” is known as arc of approach.

Arc of recess:

The arc of recess must be denoted by Qr. In the arc of recess the arc of the pitch circle by which a tooth outline moves from contact at the pitch point until contact ends”.

Contact ratio:

Contact ratio must be denoted by  or ε. It is defined as the ratio of arc of action to the circular pitch. During the contact period one needs to measure the average no. of teeth. So the teeth comes in contact to the mating gear.

Transverse contact ratio:

The transverse contact ratio must be denoted by  m_p or \ep_\alpha . “The transverse ratio is defined as, the ratio of angle of action to the angular pitch”. For involute gears it is a straight line found as the ratio of length of action to the base pitch.

Face contact ratio:

The face contact ratio must be denoted by m_F or \ep_\beta . “The face contact ratio in the axial plane or the ratio of the face thickness to the axial pitch” is known a face contact ratio.

Total contact ratio:

“The total contact ratio is known as the sum of the face contact ratio and transverse contact ratio”.



Modified contact ratio

The modified contact ratio must be denoted by m_0 . ‘The modified contact ratio of the bevel gears is the square root of the addition of the squares of the transverse and face contact ratios’.


Limit diameter:

The gear diameter at which intersects at the maximum addendum circle of the mating gear is known as limit diameter. This is also mentioned as start of contact, start of active profile, end of active profile, or end of contact.

Start of active profile:

It is represented as ‘SAP’. The start of active profile is also known as intersection of the boundary diameter and the involute profile.

Face Advance:

Face advance can be defined as, ‘The distance on the pitch circle through which a helical tooth moves from the starting position at which the contact initiates at one end of the tooth curve to the final position at the other end, across the face width when the contact ceases’.

FA=b \times \tan \beta

Tooth thickness nomenclature

Circular thickness:

The circular thickness is known as the length of the arc between the two sides of the tooth gear on the identified datum circle.

Transverse circular thickness:

Transverse circular thickness is known as the circular thickness in the transverse plane.

Normal circular thickness:

Normal circular thickness is denoted as ‘tn’. Normal circular thickness is a circular thickness in a plane normal to the helix angle.

tn = tt ∙\cos \psi

tn = 0.5 Pn

tt = Transverse circular tooth thickness

 \psi= helix angle

Pn = normal circular pitch

Axial thickness:

“In the worm gear or helical gear the tooth thickness must be in an axial cross section at the regular pitch diameter” this is known as axial thickness.

Base circle thickness:

“In involute teeth on the base circle the length of the arc between the two involute curves forming the profile of a tooth” is known as base circle thickness.

 Normal chordal thickness:

In the plane normal to the pitch helix the length of the chord that subtends a circular thickness arc is known as normal chordal thickness

Chordal height:

The chordal height is also known as chordal addendum and it is denoted by ‘ac’. The chordal addendum is defined as the radial distance from the circular tooth thickness chord to the topmost of the tooth.

Profile shift:

Movement of the simple rack datum line from the locus of the cylinder made by non-dimensional by separating the normal module is known as profile shift. Mainly it is used to identify the thickness of the tooth, regularly for the zero backlashes.

Rack shaft:

Movement of the total datum line from the locus of the cylinder, made non-dimensional by dividing the normal module. Mainly it is used to specify the thickness of the tooth.

Measurement over pins:

Measurement of the distance occupied over a pin located in a tooth space and a position surface. The position surface may be the locus axis of the gear. These pins are used to find out the thickness of the tooth.

Span measurement:

In a normal plane the measurement of the distance through several teeth, the measuring device has similar measuring surfaces. Here the measuring surfaces must be in contact on unmodified portion of the involute. Along with the line tangent to the base cylinder the measurement needs to be done. This measurement is used to determine the thickness of the tooth.

Modified addendum teeth:

Teeth on a standard designed center distance, one or both have the short or long addendum is known as Modified addendum teeth.

Full depth teeth:

In this the working depth must be equal to 2000 and it is divided by the normal diametral pitch, it is known as full depth teeth.

Stub teeth:

In this the working depth must be less than 2000 and it is divided by the normal diametral pitch, it is known as stub teeth

Equal Addendum Teeth:

It is where the two engaging gears the teeth are in equal addendums

Short and long addendum teeth:

It is the teeth in which the addenda of the two engaging gears are unequal.

Back lash:

We can observe the backlash when there is an error in the motion that occurred during the change of gear direction. Due to the gap between the drag face of the driving tooth and behind the driven gear the leading face of the tooth the backlash needs to exist. Before the transfer of the force to the new direction the gap is to be closed. The backlash can be used to measure the size of the gap.

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