The Design Process is an approach for breaking down a large project into manageable chunks. Architects, engineers, scientists, and other thinkers use the design process to solve a variety of problems.
What does Design Involve?
4 C's of design are as follows:
The design problem involves:
Mechanical power transmission system (M.P.T.S.):
Classification of the belts: Depending upon the shape of the cross-section, belts are classified as:
(i). Flat belts have a narrow rectangular cross-section.
(ii). Thin and wider Rectangular cross section belts are preferred.
(iii). The velocity ratio for flat belt is up to 4:1.
There are two types of flat belts:
(i). Leather belt:
- made of the best quality leather obtained from either side of the backbone of a steer.
- They are oak-tanned and mineral, or chrome tanned.
- The main advantage of leather belt is the high coefficient of friction and consequently, high power transmitting capacity.
(ii). Fabric rubber belt:
- The fabric rubber belts are made from several layers of canvas or cotton-duck impregnated with rubber.
- The rubber protects the fabric against damage and increases the coefficient of friction.
V Belts: V-belts have a trapezoidal cross-section.
For V-belts the velocity ratio is up to 7:1.
- The efficiency of flat belt drive is more than V-belt drive but occupy more space due to large dimensions.
- Flat belts are easy to design and thus are less expensive.
- V-belt drive can operate in any position, even when the belt is vertical while flat-belt drives are horizontal and not vertical.
- Flat belts are used for the long centre distance while V-belts have short centre distance, which results in compact construction.
- Round belts can operate satisfactorily over pulleys in several different planes. They are suitable for 90° twist, reverse bends or serpentine drives. They can be stretched over the pulley and snapped into the groove very easily.
Flat Belts: Flat belts are used in belt conveyors, baking machinery, brick and clay machinery, crushers, sawmills, textile machinery, line shafts and bucket elevators.
V belts: V-belts are very popular where an electric motor is used as the prime mover to drive compressors, pumps, fans, positive displacement pumps, blowers and machine tools. They are also popular in automobiles to drive accessories on petrol or diesel engines.
Round Belts: Round belts are limited to light duties. They are used in dishwasher drives, sewing machines, vacuum cleaners and light textile machinery.
Types of flat belt drive and Geometrical relationships:
(i). Open belt drive (O.B.D.): Direction of rotation are same ⇒ like internal gear
(ii). Cross belt drive (C.B.D.): Direction of rotation are opposite ⇒ like external gear
(iii). Compound belt drive: To obtain higher speed reduction (compound gear train).
(iv). Fast & loose pulley belt drive: To obtain same as clutch (intermittent service).
Open belt drive (O.B.D.):
θ1 = Angle of contact /lap/ wrap of the belt at driven pulley
θ1 = π - 2β
θ2 = Angle of contact /Lop / wrap of the belt at driven pulley
θ2 = π + 2β
θ1 + θ2 = 2 π radian
Tension ratio on tight and slack side:
T1 = Tension on tight side
T2 = tension on slack side
Power transmission (P) Capacity:
P = (T1 – T2 )×V
Where V is the velocity.
Cross Belt drive:
Velocity ratio (V.R.):
Assuming No slip: V1 = V = V2 i.e., by neglecting slip & belt thickness effect.
If slip and belt thickness is taking into consideration. Then:
Where S = % age of slip in a belt hive or % age of total slip.
S = S1 + S2
Where S1 = % age of slip between driven pulley & belt
S2 = % age of slip between driven pulley surface & belt.
Initial Tension (To) and Centrifugal tension (Tc):
Condition for maximum power transmission (Pmax):
Creep of belt: There is a peculiar phenomenon in the belt drive, which is called ‘creep’. Creep is a slight relative motion of the belt as it passes over the pulley.
Chain drive: A chain can be defined as a series of links connected by pin joints. The sprocket is a toothed wheel with a special profile for the teeth.
(i). Used for long as well as short centre distances. They are particularly suitable for medium centre distance.
(ii). Chain does not slip and to that extent, chain drive is a positive drive.
(iv). High efficiency of chain drive 96% to 98%.
Pitch circle diameter (D): the diameter of an imaginary circle that passes through the centres of link pins as the chain is wrapped on the sprocket.
Where Z is the number of teeth on the sprocket.
Velocity ratio (i) in chain drive is given by:
Where n1, n2 speeds of rotation of driving and driven shafts (rpm)
z1, z2 = number of teeth on driving and driven sprockets.
Chain length: It is always written in terms of number of links:
L = Ln × p
L = length of chain (mm)
Ln = number of links in the chain
Springs: A spring is defined as an elastic machine element, which deflects under the action of the load and returns to its original shape when the load is removed.
Terminology of Helical Spring:
axial load = W
mean coil diameter: D
diameter of spring wire = d
number of active coils = N
spring index: C = D/d For circular wires
L = length of spring wire
G = modulus of rigidity
SPRING DEFLECTION (δ):
WAHL’S FACTOR: In order to take into account, the effect of direct shear and change in coil curvature a stress factor is defined, which is known as Wahl’s factor.
CONNECTION OF SPRINGS:
Series Connection: If two springs of different stiffness are joined end on and carry a common load W, they are said to be connected in series and the combined stiffness and deflection are given by the following equation.
Springs in Parallel:
- The term ‘transmission shaft’ usually is a rotating machine element, circular in cross section, which supports transmission elements like gears, pulleys and sprockets and transmits power.
- Shafts are given specific names in typical applications, although all applications involve transmission of power, motion and torque.
(i). Shafts subjected to axial tensile force (P):
(ii). Shafts subjected to pure bending moment (Mb):
Bending equation is given by:
(iii). Shafts subjected to pure Torsion(T):
Torsion equation is given by:
Ip = Polar moment inertia of shaft
G = Modulus of rigidity
For Hollow circular shaft:
(i). Series connection:
Here in this case the equilibrium of the shaft requires that the torque ‘T be the same throughout the shaft.
For two shafts in series:
T1 = T2 = T
Angle of twist in series connection is cumulative. i.e.
θAC = θAB + θBC
(ii). Parallel Connection:
For parallel connection of shaft
Torque is cumulative:
T = T1 + T2
And, θ1 = θ2
For equal lengths, L1 = L2 (as is normally the case for parallel shafts)
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