Radiation Heat Transfer

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View Factor Orientation (or View factor or shape factor) plays an important role in radiation heat transfer. View factor is defined as, "fraction of radiation leaving surface 'i' and strike 'j' ". Summation Rule (View Factor) If there is are similar surfaces 'i' and 'j' , then: Blackbody Radiation Exchange Radiation Exchange between Opaque, Diffuse, Gray surfaces in an Enclosure 1. Opaque 2. Surfaces 3. Two surface enclosure Radiation Shield It is used to protect surfaces from radiation act like a reflective surface. References: Material from Class Lectures + Book named Fundamentals of Heat and Mass Transfer by Theodore L. Bergman + My knowledge.  Photoshoped pics  are developed.  Some pics and GIF from Google.   Videos from YouTube ( Engineering Sights ).

Brakes

Brakes

It is defined as, "a machine element by means of which artificial friction or resistance is applied to a moving machine member in order to retard (or stop) the motion of machine".
  • To engage and disengage driver shaft from driven shaft clutch is used.
  • Brakes absorbs kinetic energy and potential energy of moving objects which is dissipated as heat in surrounding (or by water flowing through brake drum).
  • Brake use friction to stop member while clutch use friction to engage or disengage driver and driven shaft.
  • Worn springs are used.

Design Consideration

  • In bearing, gears, cams  ↦  objective is to minimize the frictional force to reduce power loss and wear.
  • In case of clutches, brakes  ↦  objective is to maximize the frictional force and keep it.

Factors for Designing of Brakes

  1. Unit pressure between braking surfaces.
  2. Coefficient of friction between braking surfaces.
  3. Peripheral velocity (tangential velocity) of brake drum.
  4. Projected area of frictional surfaces.
  5. Ability of brake to dissipate heat equivalent to the energy absorbed.

Energy Absorbed by a Brake

Energy absorbed by a brake depends upon the motion of the body.

1. Pure Translation

2. Pure Rotation

3. Translation and Rotation

Material Requirements

Material used for brake lining have the following characteristics:
  1. Coefficient of Friction  ↦  it should have low coefficient of friction with minimum fading. Brake pad material have COF between 0.3-0.5.
  2. Low Wear Rate  ↦  have low wear rate to increase the life of brake lining.
  3. Heat Resistant  ↦  have high heat resistant because application of brakes produce high amount of heat.
  4. Good Heat Dissipation  ↦  have high heat dissipation capability to reduce accumulation of heat in brake lining.
  5. Thermal Expansion  ↦  have low thermal expansion to keep attached brake lining with brake pads/shoes.
  6. Mechanical Strength  ↦  have adequate strength to reduce damage when comes in contact with with rotor/drum.
  7. Affects of Moisture and Oil  ↦  should not be affected by moisture and oil because they can reduce friction between surfaces and cause brake fading.
Note:
  •  Brake Fading  ↦  The melting and boiling of binding resin which keeps brake lining attached to the brake shoe/pad due to poor material (which reduce brake performance).

Types of Brakes

Following are the types of brakes:

1. Hydraulic Brakes

Hydraulic brakes works on the phenomena of fluid friction.
  • It consists of rotor and stator that resembles the impeller and runner in a hydraulic coupling.
  • Braking power (or resistance to rotation) is created by fluid friction which depends on the speed of rotor.
  • It cannot completely stop the rotating element.

2. Electric Brakes

Electric brakes uses electric current or magnetic actuating force to stop or slow the motion of a rotating element.
  • Used in packaging and food processing machinery, servo motors and robotics, elevators, escalators.
  • They are divided into 3 main components: Hub, Armature, Field Coil.
  • Magnetic brakes  ↦  non-contact brakes and use magnetic field to actuate braking force. It is further divided into 4 types: Permanent Magnet, Electromagnetic, Eddy Current, Hysteresis Power Brakes.
  • Small gap between rotor and stator  ↦  when comes in contact, rotor stops. When there is gap, it rotates.

3. Mechanical Brakes

Mechanical brakes dissipate kinetic energy into heat. Working of Mechanical Brakes is described below:
  • Drum is attached to brake on rear wheels.
  • Rotor is attached to front wheels (brake pads).
  • Stationary friction elements (Brake Pedal, Vacuum Booster) connected to brakes by hydraulic or pneumatic means (use master cylinder having fluid from fluid reservoir placed over it).
  • Flow in brakes by means of fluid lines.
  • Force generated by brake pedal is boosted using vacuum from engine. This boosting effect causes brakes to respond even more quickly.
  • As brake pad pushed, spring moves forward in master cylinder forces fluid to pass through fluid lines.
  • The pressurized fluid reaches inside Calliper and Wheel cylinder which forces calliper to comes in contact with rotor. Due to which friction is generated which reduce speed. Hence rotor stops.
Mechanical Brakes are further divided into 2 types:
  1. Radial Brakes  ↦  The force acting on drum is in radial direction (perpendicular to the axis of rotation). Further divided into External, Internal, Block/Shoe, Bend Brakes.
  2. Axial Brakes  ↦  The force acting on drum is in axial direction (parallel to axis of rotation). Further divided into Disc (in motorcycles) or Cone Brakes.

Design of Mechanical Brakes

Design of different types of Mechanical Brakes are discussed ahead:

1. Block or Shoe Brake

It consists of a block which is pressed against a wheel (revolving wheel drum) which produce tangential force.
  • Block is made of soft material than that of wheel.
  • Used in railway train.
  • Self-Energizing Brakes  ↦  if frictional force helps externally applied force 'P' to apply brakes.
  • Self-Locking Brakes  ↦  if frictional force is great enough to apply the brake with no external force 'P'.

2. Pivoted Block Brake

  • In which pivoted block is provided to lever.
  • These brakes have longer life and greater braking torque.
  • If angle < 60-degrees  ↦  pressure between block and wheel is uniform.
  • If angle > 60-degrees  ↦  normal unit pressure to the surface of contact is less at the ends than at the center.
  • Wear is in the direction of applied force, so braking torque (total angle of contact 2Ө greater than 60-degrees) is given by:

3. Double Block or Shoe Brake

  • To overcome the problem of bending of shaft in Single block/shoe brake, we use Double Block Brake.
  • It consist of 2 blocks  ↦  fixed on opposite ends of diameter of wheel (reduce unbalanced force on the shaft).
  • Spring  ↦  set upper ends of block together.

4. Simple Band Brake

  • It consists of flexible band of leather or steel lined with friction material which embraces a part of the circumference of the drum.
  • When force P is applied, band moves upward and stop wheel.
  • If thickness is given  ↦  we use Effective Radius (includes thickness) otherwise use Radius of drum.

5. Differential Band Brake

  • In order to maximize braking force/torque, a simple band brake is modified which is called Differential Band Brake.

6. Band and Block Brake

It is the combination of block and differential band brake. It consists of blocks (made up of wood) attached to band by a adhesive, form flexible band with friction blocks. This combination is wrapped around a brake drum.
  • Ends of band are attached to lever.
  • Friction between drum and blocks provides Braking action.

7. Internally Expanding Brakes

  • It consist of two shoes (S1, S2).
  • Ferodo Material  ↦  material of brake lining.
  • For CCW  ↦  right shoe is called Trailing shoe and left shoe is called Leading shoe.

References:

  • Material from Class Lectures + Book named Mechanical Engineering Design by Shigley (8th Edition) + my knowledge. 
  • Pics and GIF from Google Images.  
  • Videos from YouTube.

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