Machine Design - 2 - Mechanical Engineering MCQ

Important factors to be considered:

(1) Dimensional limitation: Allowable space for bearings is generally limited.

(2) Applied loads: The characteristic, magnitudes & direction of loads is important.

(3) Rotational speed: It depends upon bearing type, size, tolerance, cage type, load lubrication & cooling condition,

(4) Bearing tolerances

(5) Rigidity

(6) Misalignment of inner & outer races

(7) Noise & torque levels

(8) Installation of disassembly.

Pitch is the distance between two adjacent rows of rivet parallel to the direction of application of force. The minimum pitch will be 2.5 times the gross diameter.

Minimum pitch should not be less than 2.5 times the nominal diameter of the rivet.

As a thumb rule pitch equal to 3 times the nominal diameter of the rivet is adopted.

Maximum pitch shall not exceed 32 times the thickness of the thinner outside plate or 300 mm whichever is less.

In a single row deep grove ball bearing, caves are needed to ensure that the balls do not cluster at one point (i.e. to maintain the balls at a fixed distance apart) and maintain proper relative angular motion.

The rim of the pulley of a flat belt drive is slightly crowned to prevent the slipping off the belt from the pulley.

Lead=pitch×noofstart=4×2=8mm

The sunk keys are provided half in the keyway of the shaft and half in the keyway of the hub or boss of the pulley.

A key attached to one member of a pair and which permits relative axial movement is known as feather key. It is a special type of parallel key which transmits a turning moment and permits axial movement.

A flat saddle key is a taper key which fits in a keyway in the hub and is flat on the shaft. It is likely to slip round the shaft under load. Therefore, it is used for comparatively light loads.

A hollow saddle key is a taper key which fits in a keyway in the hub and the bottom of the key is shaped to fit the curved surface of the shaft. Since hollow saddle keys hold on by friction, therefore these are suitable for light loads. It is usually used as a temporary fastening in fixing and setting eccentrics, cams etc.

For Acme threads

Thread height = 1/2 pitch

Thread angle or included angle = 29°

They are generally used in screw-cutting lathes

A dynamometer is a device used for measuring the torque and brake power required to operate a driven machine. It has a device to measure the frictional resistance. 

Following are the two types of dynamometers, used for measuring the brake power of an engine.

1. Absorption dynamometers: The entire energy or power produced by the engine is absorbed by the friction resistances of the brake and is transformed into heat, during the process of measurement.

Example: Prony brake dynamometer, Rope brake dynamometer, Hydraulic dynamometer 

2. Transmission dynamometers: The energy is not wasted in friction but is used for doing work. The energy or power produced by the engine is transmitted through the dynamometer to some other machines where the power developed is suitably measured.

Example: Epicyclic-train dynamometer,  Belt transmission dynamometer, Torsion dynamometer.

A key is a machine member employed at the interface of a pair of mating male and female circular cross-sectioned members to prevent relative angular motion between these mating members. The key fits into mating grooves in the shaft and mating member called the keyway and transmit torque by shear across the key.

Barth key is a modification of rectangular key which has two beveled surfaces. The beveled surface ensures that the key will fit tightly. This key is usually under compression rather than shear.

Ball screw is also called as ball bearing screw or recirculating ballscrew. It is a mechanical linear actuator that translates rotational motion to linear motion with little friction. It consists of a screw spindle, a nut, balls and integrated ball return mechanism.

Ball screws are used in aircraft and missiles to move control surfaces, and in automobile power steering to translate rotary motion from an electric motor to axial motion of the steering rack. They are also used in machine tools, robots and precision assembly equipment.

Screw thread is a continuous helical groove of specified cross-section produced on the external or internal surface. A screw thread formed on a cylinder is known as straight or parallel screw thread, while screw thread formed on a cone or frustum is known as the tapered screw thread.

Crest: Crest of the thread is the top most surface joining the two sides.

Root: Root of the thread is the bottom of the groove between the two flanks.

Pitch: Pitch of a thread is the distance measured parallel to the axis from a point on a thread to the corresponding points on adjacent thread forms in the same axial plane and on the same side of the axis.

Major diameter: It is an imaginary largest diameter of the thread which would touch the crests of an internal or external thread. It is also called crest diameter.

Minor diameter: It is an imaginary smallest diameter of the thread which would touch the roots of an external thread.

Pitch diameter: It is a theoretical diameter between the major and minor diameter of screw threads.

Saddle key is a key that fits in the key way of the hub only. In this case there is no keyway provided on the shaft and friction between shaft, key and hub prevents relative motion between the shaft and the hub and power is transmitted by means of friction only.

For Uniform pressure theory

For Uniform wear theory the mean radius of friction surface is given as

Pitch: This is the distance between two centres of the consecutive rivets in a single row.

Back Pitch: This is the shortest distance between two successive rows in a multiple riveted joint.

Diagonal pitch: This is the distance between the centers of rivets in adjacent rows of zigzag riveted joint.

Margin or marginal pitch: This is the distance between the centre of the rivet hole to the nearest edge of the plate.

The longitudinal joint in a boiler shell is usually Butt joint with two cover plates this joint is more efficient than lap joint. It is also stiffer and helps to maintain circularity of the shell.

A Hook’s joint, commonly known as a universal joint is used to connect two non-parallel and intersecting shafts. It is also used for shafts with angular misalignment. A common application of this joint is in an automobile where it is used to transmit power from the gear box (of the engine) to the real axle.

Load stress factor (k)