Test: Toothed Gearing - 3 - Mechanical Engineering MCQ

In case of involute toothed gear base circle is the circle above which profile of gear is involute and below the base circle straight line exists. Hence it is the base circle.

Rack is defined as a gear of infinite diameter.

20° stub involute system have following features:

• Shorter addendum and shorter dedendum
• Smaller interference as compared to full depth involute
• Drive is more compact
• Stronger than full depth involute teeth
• Lower production cost as less metal must be cut away

The main draw back of this system is that the contact ratio is reduced due to shorter addendum.

Two important properties of an involute
(i) A normal to an involute is a tangent to the basic circle
(ii) The radius of curvature of an involute is equal to the length of the tangent to the base circle.

For a cycloidal tooth profile pressure angle is zero at the pitch point and becomes maximum during the engagement and disengagement.

In standard
Addendum = 1 module

Contract ratio of a pair of spur pinion and gear is usually more than 1.
Here appropriate answer is (c).

Clearance is the amount by which the dedendum of a gear exceeds the addendum of its mating tooth.

Backlash is defined as the amount by which the width of tooth space exceeds the thickness of the engagement tooth measured along the pitch circle.

Module  = D/T
where
D = Pitch circle diameter
T = Number of teeth

In involute profile of gear teeth pressure angle is constant throughout the engagement of teeth. This results in smooth running of the gear.

Module: It is the ratio of the pitch circle diameter to the number of teeth. It is usually denoted by m. Mathematically.
Module, m = D/T
Addendum: The addendum is the radial distance between pitch and the addendum circles. Addendum indicates the height of tooth above the pitch circle.
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 next tooth . It is usually denoted by p_c.
Mathematically,
Circular pitch
p_c = πD/T
where D = Diameter of the pitch circle,
and T = Number of teeth on the wheel

Interference in involute gear can be avoided by increasing the centre distance. Interference can also be eliminated by increasing the pressure angle.

Pitch Circle: The pitch circle is the curve of intersection of the pitch surface of revolution and the plane of rotation. It is an imaginary circle that rolls without slipping with the pitch circle of a mating gear. The pitch circles of a pair of mating gears are tangent to each other.

Helical gears typically provide a higher contact ratio than spur gears. This is due to the additional axial contact ratio resulting from their angled teeth, keeping more teeth in contact at the mesh point for a longer duration.

To have a gear drive between two intersecting shafts bevel gears are used.