Helical and Spiral Gears MCQ Quiz - Objective Question with Answer for Helical and Spiral Gears - Download Free PDF

Explanation:

Hypoid gears:

• Hypoid gears are a type of spiral bevel gear whose axes do not intersect.
• These gears are used to transmit power between shafts that are neither parallel nor intersecting.
• They are similar to spiral bevel gears but with the advantage of allowing the axes of the gears to be offset, which provides more design flexibility.

Spur gears

• Spur gears are used to connect two shafts when they are transmitting the power in a parallel direction.
• In spur gears when the teeth start getting engaged the contact between the teeth extends across the entire width on a line parallel to the axis of the rotation this results in the sudden application of the load, high impact stresses.
• It is used to transmit low power.

Bevel Gear: 

• Bevel gears are used to transmit motion between shafts at various angles to each other, thus they are nonparallel but intersecting. 
• The teeth profile may be straight or spiral.

Helical gear:

• These gears are used to connect the shafts whose axes parallel and teeth are inclined to the axis.

Explanation:

Skew Bevel Gears:

• When two straight bevel gears are mounted on shafts, which are non-parallel and non-intersecting, they are called skew bevel gears.
• They are shown in Fig. It is seen that the apex point of the pinion is offset with respect to that of the gear.

Skew bevel gears have the following characteristics:

• Skew bevel gears are mounted on non-parallel and non-intersecting shafts.
• Skew bevel gears have straight teeth.

In Skew bevel gear teeth are not curved; they are cut straight and at an angle to the shaft centerline, and the axes are non-parallel and non-intersecting.

Explanation:

Various terms used on gear terminology are:

Module (m):

• The module is a dimension that represents the size of the gear tooth.
• It is defined as the ratio of the pitch diameter to the number of teeth.
• Having the same module ensures that the teeth of one gear fit perfectly into the gaps of the other gear without any interference, ensuring a smooth transmission of motion and power.
• Additionally, having the same module means that the gears have compatible pressure angles and tooth profiles, which further contributes to efficient power transmission and minimal wear.
• It is expressed in mm.

Circular pitch

It is the distance measured on the circumference of the pitch circle from a point of one tooth to the corresponding point on the tooth. 

\({P_c} = \frac{{\pi ~ \times ~D}}{T}\)

Diametral pitch

It is the ratio of the number of teeth to the pitch circle diameter. 

\({P_d} = \frac{T}{D}\)

Pitch circle: It is the imaginary circle on which two mating gears seem to be rolling.

Addendum Circle: It is the circle drawn through the top of the teeth and is concentric with the pitch circle. It is also called the Outside circle.

Dedendum circle: It is the circle drawn through the bottom of the teeth. It is also called the root circle.

Base Circle: It is the circle from which the involute tooth profile is developed.

Addendum: It is the radial distance of a tooth from the pitch circle to the top of the tooth (or addendum circle).

Dedendum. It is the radial distance of a tooth from the pitch circle to the bottom of the tooth (or dedendum circle).

Land: The top land and bottomland are surfaces at the top of the tooth and the bottom of the tooth space respectively.

Working depth: This is the distance of the engagement of two mating teeth and is equal to the sum of the addendum of the mating teeth of the two gears. It is the radial distance from the addendum circle to the clearance circle.

Whole depth/Total depth: This is the height of a tooth. It is the radial distance between the addendum and the dedendum circles of a gear. It is equal to the sum of the addendum and dedendum.

Explanation:

Helical Gears:

• A helical gear has teeth in the form of a helix around the gear.
• Two such gears can be used to connect two parallel shafts.
• In the case of single helical gears, there is some axial thrust between the teeth which is a disadvantage.
• In order to eliminate this axial thrust, double helical gears (herringbone gears) are used.
• It is equal to two single helical gears, in which two equal and opposite thrusts are provided on each gear and the resulting axial thrust is zero.

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Explanation:

Herringbone Gear:

• Herringbone gears, also known as double helical gears, are a type of gear that consists of two mirrored helical gears that are placed side by side. The teeth on each gear are angled in opposite directions, which creates a "V" shape. This design helps to reduce noise and vibration that can occur in standard helical gears.

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• The main advantage of herringbone gears is their ability to transmit power smoothly and quietly. The opposing angles of the teeth on the two gears cancel out any axial thrust that would occur in a standard helical gear, which can result in smoother operation and less wear and tear on the gears.
• Another advantage of herringbone gears is their ability to handle higher loads than other types of gears. The design of the gear teeth provides a larger surface area for contact between the gears, which allows for greater torque transmission and higher power transfer.
• However, herringbone gears can be more difficult and expensive to manufacture than other types of gears, due to the complexity of their design. The two helical gears must be precisely aligned and positioned to ensure that the teeth mesh properly and the opposing angles cancel out the axial thrust. This requires greater manufacturing precision and accuracy, which can drive up the cost of production.

Explanation:

Gear is a machine element which, by means of progressive engagement of projections called teeth, transmits motion and power between two rotating shafts. 

Gears can be classified on the basis of the relation between axes of power transmitting shafts:

• Parallel shafts: Spur gear. rack and pinion, helical gears, herringbone gears, etc. 
• Non-parallel and Intersecting shafts: Straight bevel gear
• Non-parallel and non-intersecting shafts: Worm gears

Worm gear:

• Worm gearing is essentially a form of spiral gearing in which the shafts are usually at right angles.
• The two non-parallel and non-intersecting, but non-coplanar shafts connected by worm gears.
• The worm shaft has spiral teeth cut on the shaft and worm wheel is a special form of gear teeth cut to mesh with the worm shaft.
• These are widely used for speed reduction purpose.

Additional Information

Helical Gear: 

• In a helical gear, the teeth are cut at an angle to the axis of rotation. It is used to transmit power between two parallel shafts.

• Types of Helical gears
  • Parallel Helical gears
  • Crossed Helical gears
  • Herringbone gears
  • Double Helical gears

Spur Gear: The teeth are cut parallel to the axis of rotation. The spur gears are used to transmit power between two parallel shafts.

Oldham Coupling is used to join two shafts which have a lateral misalignment. It consists of two flanges A and B with slots and a central floating part E with two tongues T1 and T2 at right angles. The resultant of these two components of motion will accommodate lateral misalignment of the shaft as they rotate.

This coupling is used when there is some lateral misalignment, that is, axes of both the shafts are parallel, but not co-axial.

Torque is transmitted through the pressure between the slot and tongue. The pressure is the maximum at the outer periphery and zero at the centroid.

Explanation:

Skew Bevel Gears: When two straight bevel gears are mounted on shafts, which are non-parallel and non-intersecting, they are called skew bevel gears. They are shown in Fig. It is seen that the apex point of the pinion is offset with respect to that of the gear. Skew bevel gears have the following characteristics:

(ii) Skew bevel gears are mounted on non-parallel and non-intersecting shafts.

(i) Skew bevel gears have straight teeth.

In Skew bevel gear teeth are not curved; they are cut straight and at an angle to the shaft centerline, and the axes are non – parallel and non – intersecting.

Explanation:

Bevel Gear: 

• Bevel gears are used to transmit motion between shafts at various angles to each other.
• The teeth profile may be straight or spiral.

Additional Information

Spur Gear: 

• The teeth are cut parallel to the axis of rotation. 
• The spur gears are used to transmit power between two parallel shafts.

Helical Gear: 

• In a helical gear, the teeth are cut at an angle to the axis of rotation.
• It is used to transmit power between two parallel shafts.
• The end thrust is exerted by the driving and driven gears in the case of helical gears and the thrust may be eliminated by using double helical gears. These gears are called herring-bone gears.

Explanation:

Helix angle:

• Helix angle is the angle between any helix and an axial line on its right, circular cylinder or cone.
• Helix angles are selected to obtain a minimum overlap ratio and to provide good load sharing.
• Helix angles vary from 5° to 45°.
• The helix angle for Single-helical gear falls between 15° and 35°, and for double-helical gears it falls between 20° and 45°.
• The thrust generated is also a function of the helix angle.
• An increase in the helix angle increases the thrust; thus, this increase is the main reason for the lower helix angles in single-helical gearing.

Explanation:

Helical Gear: 

• In a helical gear, the teeth are cut at an angle to the axis of rotation. It is used to transmit power between two parallel shafts.

Types of Helical gears

1. Parallel Helical gears
2. Crossed Helical gears
3. Herringbone gears
4. Double Helical gears

Crossed Helical gears

• These are used for shafts, which are not parallel. their teeth may have the same or different hand of helix depending on hte angle between the axes of the shafts.
• For 90° angle between shafts, helix angle of both the gears is same as 45° and use the same hand.
• Salient features for these gears are:
  • Owing to curvature of tooth surfaces, it does not have line contact, but a point contact
  • Owing to small contact area of these gears, load capacity is low.
  • Owing to small contact area and high loads, wear is also high. 

The resultant force on helix gear i.e. F_N can be resolved as:

ϕ = pressure angle

ψ = helix angle

i) F_T i.e. Tangential force

ii) F_R i.e. Radial force

iii) F_a i.e. Axial force

• F_a induces axial stress in the shaft
• F_T and F_R are responsible for bending of shaft in two plane.
• Due to F_T tangential shear stress will be induced in the shaft.

Concept:

Velocity ratio for helical hears can be calculated by:

\(\frac{{{N_2}}}{{{N_1}}} = \frac{{{D_1}\cos {\psi _1}}}{{{D_2}\cos {\psi _2}}}\)

where D₁ and D₂ are pitch diameters and ψ₁ and ψ₂ are respective helix angles.

Calculation:

Given:

D₁ = 80 mm, D₂ = 120 mm, ψ₁ = 30°, ψ₂ =22.5°, N₁ = 1440 rpm

Velocity ratio 

\(\frac{{{N_2}}}{{{N_1}}} = \frac{{{D_1}\cos {\psi _1}}}{{{D_2}\cos {\psi _2}}} = \frac{{80 \times \cos 30^\circ }}{{120 \times \cos 22.5^\circ }} = \frac{{80 \times 0.866}}{{120 \times 0.923}} = 0.625\)

N₂ = 0.625 × N₁ = 0.625 × 1440 = 900 rpm

Explanation:

Helix angle:

• It is the angle between any helix and an axial line on its right, circular cylinder or cone.
• Helix angles are selected to obtain a minimum overlap ratio and to provide good load sharing.
• Helix angles vary from 5° to 45°.
• The helix angle for Single-helical gear falls between 12° and 20°, and for double-helical gears it falls between 20° and 45°.
• The thrust generated is also a function of the helix angle.
• An increase in the helix angle increases the thrust; thus, this increase is the main reason for the lower helix angles in single-helical gearing.

Double Helical Gear:

• A double helical gear is equivalent to a pair of helical gear secured together, one having a right-hand helix and the other has a left-hand helix. The teeth of the two rows are separated by a groove i.e. used for tool run out.
• In double helical gear the helix angle is 45°.
• Axial thrust occurs in the case of single helical gears eliminated in double helical gears. This is because the axial thrust of two rows of the teeth cancels each other.
• These gears can be run at high speeds with less noise and vibrations.

 Additional Information

Helical Gear:

• They have curved teeth, each being helical in shape.
• Two mating gear have the same helix angle but have teeth of opposite hands.
• At the beginning of the engagement, contact occurs only at the point of the leading edge of the curved teeth. As the gear rotates, the contact extends along a diagonal line across the teeth. Thus load application is gradual, low impact stresses and reduction in noise.
• Helical gears have the disadvantages of having end thrust as there is a force component along the gear axis. The bearing and mounting must be able to withstand the thrust load.

Explanation:

Herringbone Gear:

• If the left and right inclinations of a double helical gear meet at a common apex and there is no groove in between, the gear is known as herringbone gear.
• Axial thrust gets canceled due to the opposite hands and these gears can be used at high speeds with less noise.

Explanation:

Helical gear: 

• These gears are used to connect the shafts whose axes parallel and teeth are inclined to the axis.
• In helical gears due to the angle of the teeth, they create a thrust load on the gear when they mesh. 

Double helical or Herringbone gears:

• These gears are used for transmitting power between two parallel shafts.
• They have opposing helical teeth. Two axial thrusts oppose each other and nullify.
• To avoid axial thrust, two helical gears of the opposite hand can be mounted side by side, to cancel the resulting thrust forces. These are called double helical or herringbone gears.

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Spur Gear:

• They have straight teeth parallel to the axes and thus are not subjected to axial thrust due to tooth load while meshing.
• At the time of engagement, the contact extends across the entire width on a line parallel to the axes of rotation. This results in the sudden application of the load, high impact stresses, and excessive noise at high speeds.

Bevel Gear: 

• Bevel gears are used to transmit motion between shafts at various angles to each other.
• The teeth profile may be straight or spiral.