Power transmission by belts
- A belt drive is a combination of two pulleys on each shaft with an endless belt wrapped around them having some initial tension.
- Power is transmitted from the driver pulley to the belt and from the belt to the driven pulley with the help of friction.
- Friction between belt and pulley surface limits the maximum power that can be transmitted. Once the limiting value is exceeded, slipping of the belt occurs.
- Rectangular cross-section belts are known as flat belts while V-belts have trapezoidal cross-sections.
- Flat belts are used to transmit a moderate amount of power between shafts less than 6m apart.
- V-belts are used along with pulleys having a similar cross-section as that of the belt. To further increase the power transmission capacity, multiple V-belt systems (having more than one V belt & grooves in the system) is used.
(i) There are two types of drives—rigid and flexible.
(ii) Gear drives are called rigid or non-flexible drives where there is direct contact between the driving and driven shafts through the gears.
While inflexible drives, there is an intermediate link such as belt, rope or chain between the driving and driven shafts. Due to the link being flexible, the drives are termed as the ‘flexible’ drives.
(iii) Thus, Belt drives are flexible drives.
Mechanical power transmission system (M.P.T.S.)
Advantages and disadvantages of Belt drives:
Advantages: Belt drives offer the following advantages compared with other types of drives:
(i) They can be used for power transmission between the axes of driving and driven shafts having considerable distances between them.
(ii) The belt drive operation is smooth and silent.
(iii) Simple design and low initial cost.
(i) Have large dimensions and thus occupying space.
(ii) Due to belt slip, the velocity ratio is not constant.
(iii) They impose heavy loads on shafts and bearings.
CLASSIFICATION OF THE BELTS
(i) Flat belts
TYPES OF FLAT BELT DRIVE:
(i) Open belt drive (O.B.D.):
OBD ⇒ Direction of rotation are the same ⇒ as internal gear
(ii) Cross belt drive (C.B.D.):
CBD ⇒ Direction of rotation is opposite = like external gear
(iii) Compound belt drive:
Compound ⇒ To obtain higher speed reduction (compound gear train).
Open belt drive (O.B.D.)
Let D1 = Diameter of the driven pulley
D2 = Diameter of the driven pulley or follower pulley
C = Centre distance between two parallel shafts
V1 = Linear velocity of the driven pulley
V = Linear velocity of the belt
V2 = Linear velocity of driven pulley,
⇒ This drive is not suitable for the smaller centre distance it should be medium.
C = less ⇒β↑ ⇒θ1 ↓
(T1/T2)= e μθ1 ⇒T1 ↓ ⇒ So power transmission ↓
Due to centrifugal force:
Total tension in tight side = T1 + TC
Total tension in slack side = T2 + TC
T1 + TC ≤ Tmax
Length of the belt:
Comparison between open belt drive & cross belt drive
If slip and belt thickness is taking into consideration.
Initial tension (To):
- Initial tension is the tension develops in the belt when it is in the stationary cond.
- It is provided in the belt by taking a length of the beltless than the actual required length.
- In presence of initial tension, the power transmission capacity of the belt drive increases hence it is useful.
Centrifugal Tension (Tc):
Centrifugal tension is the additional tension that develops in the belt in presence of centrifugal force acting on the belt.
Tmax = T1 + Tc
CONDITION FOR MAXIMUM POWER TRANSMISSION (Pmax):
(ii) T1 = 2Tc
V-belt is used to transmit power between two parallel shafts which are at a smaller centre distances and rotating in the same direction.
Cross section = Trapezoidal cross section
The ratio of belt tensions:
(i) air compressor
(ii) Automobile radiator fans
(iii) M/c tools like a shaper, milling M/c etc.
Creep Of Belt: It is a slight relative motion between the belt and the pulley.
While moving from the tight to the loose side over the pulley, the belt element is transferred from the zone of higher tension to the zone of lower tension.
Effect of creep:
(i) Creep results in the less angular velocity of the driven pulley than that calculated by considering the ratio of diameters of pulleys.
(ii) The efficiency reduction by 1 to 2% due to creep.
- The knuckle joint is used to connected two rods whose axes either coincide or intersect and lie in one plane.
- The knuckle joint is used to transmit axial tensile force. The constructions of this joint permit limited angular movement between rods, about the axis of the pin.
Applications of knuckle joints are as follows:
- Joints between the tie bars in roof trusses.
- Joints between the links of a suspension bridge.
- Joints in value mechanism of a reciprocating engine.
- The fulcrum for the levers.
- Joints between the links of a bicycle chain.
Advantages of Knuckle joints:
- The joint is simple to design and manufacture.
- Few parts in the knuckle joint reduce cost and improve reliability.
- The assembly or dismantling of the parts of a knuckle joint is quick and simple.
- It is used to connect two co-axial rods subjected to either axial tensile' or axial compressive force.
- It is not suitable to connect rotating shafts and transmitting torque.
Applications of cotter joint are as follows: Its applications include:
(i) Piston rod and crosshead joint in a steam engine.
(ii) slide spindle and the fork of the valve mechanism joint
(iii) the piston rod and the tail or pump rod joint.
The reasons for providing taper are as follows:
(i) The cotter insertion in the slot through the socket and the spigot provides the wedge action due to being taper. Thus, it takes care of the tightness of the joint in operating conditions and prevents loosening of the parts.
(ii) For easy removal of the cotter and dismantle the joint.
These are mechanical components used for connecting two rotating shafts and transmitting torque from one shaft to the other shaft.
Example: the connection between the electric motor output shaft and the input shaft of a hydraulic pump.
Types of Shafts Couplings
Perfectly aligned shafts are connected using the couplings. Couplings are simple and inexpensive.
Rigid Couplings are of the following types:
- Sleeve or Muff Coupling
- Clamp or Split-muff or Compression Coupling
- Flange Coupling
- Flexible couplings are used to connect two shafts having lateral or angular misalignment. Flexible elements provided in flexible coupling absorb shocks and vibrations.
Flexible Couplings are of the following types:
- Bushed pin type Coupling
- Oldham Coupling
- Universal Coupling
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