Flexible Mechanical Elements

Flexible mechanical elements are defined as, "Inelastic elements to transmit power over large distances".

• Purpose of using FME's is to simplify the design of a machine and reduce the cost.
• Used as replacement for gears, shafts, bearings, other rigid power-transmission devices.
• It can absorb shock loads and vibrations.
• Positive Drive  ⇛  no slip between driver and driven component. 
• Element Life  ⇛  no infinite life and should be replaced at the first sign of deterioration (check wear, aging, loss of electricity).

Performance of Different Transmission Types

Belts or Belt Drives

Belts are used where center distance between shafts is large.

• Pulley axes must be separated by a certain minimum distance (depend upon belt type and size) to operate properly.
• Because of belt flexibility and damping capability  ⇛  reduce the transmission of unwanted shock and vibration between shafts.
• Power and torque capacities are limited by COF and interfacial pressure between belt and pulley.
• Idler/Tension Pulley  ⇛  used to avoid adjustments in center distance because of wear and stretch compensation.

Types of Belts

1. Flat Belts  ⇛  used with large center distance. Slight taper to detach belt from pulley.
2. Round Belts  ⇛  round cross-section used with grooved pulleys or sheaves.
3. V-Belts  ⇛  trapezoidal cross-section used with grooved pulleys or sheaves, for small center distance and joint-less.
4. Timing Belts  ⇛  used when perfect synchronicity is required between driving and driven shaft, with toothed wheels (or sprockets), with small center distance and joint-less.

Comparison Table of Power Transmission Devices

Belt Drives Theory

It is states that, "Belts drives or belts are used to transmit power from one shaft to another by means of pulley which rotate at same speed or at different speed".

The amount of power transmitted depends upon the following factors.

1. Velocity of belt  ⇛  2 components of velocity radial velocity and tangential (or peripheral) velocity.
2. Tension under which the belt is placed on pulleys  ⇛  higher the tension, lesser the slip. Tension can be adjusted by moving shafts away or close to each other.
3. The arc of contact between the belt and the smaller pulley  ⇛  contact area should be larger for lesser slip.
4. The surrounding conditions under which the belt is used  ⇛  in dusty environment, belt wear more quickly.

Design Consideration of Belt Drives

Aspects of design consideration are:

1. Alignment of Shafts  ⇛  if not aligned, additional load is applied on shafts and bearings.
2. Distance between driven and driven pulley  ⇛  if distance is not according to standard, additional stresses come into play.
3. Length of Belt used  ⇛  if length increases, slagging of belt results in which decreases contact area.
4. The tight side of belt should be at the bottom, so weight of slag side is in downward direction due to which contact area increases.
5. Maximum distance between shafts should not exceed 10 meters and minimum distance should be 3.5 times the diameter of larger pulley.
6. Efficient power transmission  ⇛  belt speed should be in the range of 20-22.5 m/s.

Selection Criteria of Belt Drives

1. Speed of the driving and driven shaft  ⇛  Peripheral velocities of shafts are calculated by using drive ratio. Drive ratio gives idea that both shafts are rotating at same speed or not.
2. Speed Reduction Ratio  ⇛  ratio at which shafts speed vary.
3. Power to be transmitted  ⇛  to determine belt tensioners and number of belts.
4. Center distance between the shafts  ⇛  it decides which type of belt drive should be used.
5. Positive Drive Requirement  ⇛  application require slip or not (in ICE, we use tooth or timing belt).
6. Shafts Layout  ⇛  in which configurations, shafts are placed.
7. Space Available  ⇛  space between shafts.
8. Service Conditions  ⇛  environment in which belt operates.

Classification of Belt Drives

a. On the basis of Speed/Power Transmission

1. Light Drives  ⇛  used to transmit small power at belt speed up to 10 m/s.
2. Medium Drives  ⇛  used to transmit power when belt speed is in between 10-22 m/s.
3. Heavy Drives  ⇛  used to transmit power when belt speed is above 22 m/s.

b. On the basis of Belt Cross-Section

1. Flat Belt  ⇛  rectangular cross-section, used with large center distance below than 8 m (for moderate power transmission).
2. V-Belts  ⇛  trapezoidal cross-section, used for small center distance and small space available (for greater power transmission).
3. Circular Belt or Rope  ⇛  round cross-section used with large center distance greater than 8 m (for greater power transmission).

c. On the basis of Shaft Layout

Arrangement of shafts is called Layout. Belts are divided into the following shafts on the basis of shaft layout.

1. Open Belt Dive  ⇛  used when shafts arranged parallel and rotating in the same direction.
2. Cross or Twisted Belt Drive  ⇛  used when shafts arranged are parallel and rotating in opposite direction (center distance > 20 x width of belt 'b', velocity of belt < 15 m/s).
3. Quarter-turn or Right-angle Belt Drive  ⇛  used when shafts are arranged at right angles and rotating in one direction (width of face pulley = 1.4 width of belt 'b').
4. Idler Pulley Belt Drive  ⇛  used when shafts are arranged parallel and when an open belt drive cannot be used due to small angle of contact on the smaller pulley.
5. Compound Pulley Drive  ⇛  used when power is transmitted from one shaft to another through a number of pulleys.
6. Stepped Pulley or Variable-speed Drive  ⇛  used for changing speed of driven shaft while the driving shaft runs at constant speed.
7. Fast and Loose Pulley Drive  ⇛  used when the driven shaft is to be started or stopped whenever desired without interfering with the driving shaft. If pulley is keyed to driving shaft is called Fast Pulley drive otherwise it is Loose Pulley Drive.

Length and Angle of Contact of Belt Drive

Tension Ratio for Flat Belt

Velocity Ratio for Flat Belt

Velocity Ratio for Compound Belt Drive

Slip of the Belt

When frictional grip between belt and pulley becomes insufficient, it causes forward motion of the driver without carrying the belt with it. This is called 'Slip of the Belt'.

Creep of the Belt

The belt extends when it passes through the tight side and contracts when passes through slag side. Due to these changes of length, there is a relative motion between the belt and pulley surfaces. This relative motion is called Creep.

• Creep reduces the speed of driven pulley.

V-Belts

V-belts are in V-shape trapezoid made of fabric and cord (cotton, rayon, nylon) with rubber.

• Speed range is 5-25 m/s.
• Included angle is between 30-40 degrees.
• Sheave Pulley  ⇛  Belts which run on special pulley with groove cut in it.
• Power is transmitted due to wedging action (contact surfaces are to the bottom, left side and right side of groove) between belt and V-groove in pulley.
• Clearance should be added to the bottom of belt.

Timing Belts

Drive by means of teeth rather than friction (no slippage) is done by using Timing belts.

• Rubberized fabric coated with a nylon fabric, has a steel wise within to take tension load.
• The teeth fit into grooves cut on periphery of sprockets (tooth wheel).
• Light weight, wide range of speeds (efficient at 60 m/s).
• Contact of 6 teeth is sufficient to develop full rated capacity.
• Used for no-slip condition (for constant angular speed) and synchronicity of meshing elements (like crankshaft and camshaft in ICE). 

Chain Drives

Made up of number of rigid links hinged together by pin joint to provide necessary flexibility for bending around driving and driven wheels.

• It can transmit power (up to 110 KW) over long distances (up to 8 m), with velocities (up to 25 m/s).
• It has the ability to drive a number of shafts from a single source of power.
• Chains are standardized by ANSI according the sizes and are available in single, double, triple and quadruple strands.

Types of Chain Drives

1. Hoisting and Hauling (or Crane) Chains  ⇛  used for lifting weight, operating speed is 0.25 m/s.
2. Conveyor Chains  ⇛  used for transporting luggage continuously , speed up to 2 m/s.
3. Power Transmitting (or Driving) Chains  ⇛  used with short center distance, roller chains used in bicycles, motorcycles, etc.

Some important terms are:

• Pitch of the Belt  ⇛  distance between hinged center of a link and the corresponding hinge center of the adjacent link.
• Pitch Circle Diameter of Chain Sprocket  ⇛  diameter of the circle on which the hinge centers of chains lie.
• The angle of Articulation  ⇛  angle through which the links swings as it enter contacts.
• Chordal Speed Variation  ⇛  ratio of difference in velocities of chain and smaller diameter to the peripheral chain velocity.
• Galling  ⇛  micro-welding of minut particles on the pins and bushing surfaces causing adhesive wear. Factors are: Speed of roller chain, power transmitted from driver to driven rod.

For lubrication in chain drives, click here.

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.