## Posted tagged ‘horse power’

### Frequently Asked Interview Questions for Mechanical Engineers!!

September 18, 2011### Vertical screw conveyors

September 8, 2011

A vertical screw conveyor conveys material upward in a vertical path. It requires less space than some other types of elevating conveyors. Vertical screw conveyor can handle most of the bulk materials provided there is no large lump. The maximum height is usually limited to 30m.

A vertical screw conveyor consists of a screw rotating in a vertical casing. The top bearing for the screw shaft must be designed to stand against radial and thrust loads. A suitable inlet port at the lower end and a discharge port at the upper end of the casing are provided. Feeding a vertical screw conveyor deserves careful consideration. Most materials are fed to the vertical conveyor by a straight or offset horizontal feeder conveyor. The ideal operation of a vertical screw conveyor is to have a controlled and uniform volume of material feeding.

Uneven feeding and start stop operation may adversely affect the performance of the vertical screw conveyor in terms of speed, capacity and horse power.

**Average capacities and speeds of vertical conveyor**

Nominal diameter of screw in mm | Capacities in m^{3}/hr |
Speed of screw |

150 | 10 | Up to 400 RPM |

250 | 35 | 300 RPM |

300 | 75 | 250 RPM |

400 | 170 | 200 RPM |

Vertical screw conveyors or some special design of vertical screw conveyor finds wide application in ship unloading.

Practical experience with these conveyors has shown that the resistance factor for vertical conveyors is higher than those of the horizontal conveyors. Resistance factor λ may be taken as 5.5 to 7.5 for grains. 6.5 to 8.3 for salt.

The driving power of the loaded screw conveyor is given by:

**P = P _{H} + P_{N} + P_{st}**

Where,

P_{H} = Power necessary for the progress of the material

P_{N} = Driving power of the screw conveyor at no load

P_{st }= Power requirement for the inclination of the conveyor

**Power necessary for the progress of the material P _{H}:**

For a length L of the screw conveyor (feeder), the power PH in kilo watts is the product of the mass flow rate of the material by the length L and an artificial friction coefficient λ, also called the progress resistance coefficient.

**P _{H} = I_{m}.L. λ.g / 3600 (kilowatt)**

**= I _{m}.L. λ / 367 (kilowatt)**

Where,

I_{m} = Mass flow rate in t/hr

λ = Progress resistance coefficient

Each material has its own coefficient λ. It is generally of the order of 2 to 4. For materials like rock salt etc, the mean value of λ is 2.5. For gypsum, lumpy or dry fine clay, foundry sand, cement, ash, lime, large grain ordinary sand, the mean value of λ is 4.0.

In this connection it should be noted that the sliding of the material particles against each other gives rise to internal friction. Other resistance due to grading or shape of the output discharge pattern contributes to the resistance factor. That is why the parameter λ is always higher than that due to pure friction.

**Drive power of the screw conveyor at no load, P _{N}:**

This power requirement is very low and is proportional to the nominal diameter and length of the screw.

**P _{N} = D.L / 20 (Kilowatt)**

Where,

D = Nominal diameter of screw in meter

L = Length of screw conveyor in meter

**Power due to inclination: P _{st}**

This power requirement will be the product of the mass flow rate by the height H and the acceleration due to gravity g.

**P _{st} = I_{m}.H.g / 3600**

**= I _{m}.H / 367**

H should be taken positive for ascending screws and will be negative for descending screws.

**Total power requirement:**

The total power requirement is the sum total of the above items

**P = (I _{m} (λ.L + H) / 367) + (D.L /20) (Kilowatt)**

### PULLEY

August 23, 2011**Pulley:**

The diameters of standard pulleys are: 200, 250, 315, 400, 500, 630, 800, 1000, 1250, 1400 and 1600 mm. pulley may be straight faced or crowned. The crown serves to keep the belt centered. The height of the crown is usually 0.5% of the pulley width, but not less than 4 mm. The pulley diameter D_{p} depends on the number of plies of belt and may be also be determined from the formula:

**D _{p} > K.i** (mm)

Where

K = a factor depending on the number of plies (125 to 150)

i = no of plies

The compound value should be rounded off to the nearest standard size. While selecting the pulley diameter it should be ascertained that the diameter selected is larger than the minimum diameter of pulley for the particular belt selected.

The drive pulley may be lagged by rubber coating whenever necessary, to increase the coefficient of friction. The lagging thickness shall vary between 6 to 12 mm. The hardness of rubber lagging of the pulley shall be less than that of the cover rubber of the running belt.

**Pulley types:**

Pulleys are manufactured in a wide range of sizes, consisting of a continuous rim and two end discs fitted with hubs. In most of the conveyor pulleys intermediate stiffening discs are welded inside the rim. Other pulleys are self cleaning wing types which are used as the tail, take-up, or snub pulley where material tends to build up on the pulley face. Magnetic types of pulleys are used to remove tramp iron from the material being conveyed.

**Typical welded steel pulley-Drum conveyor pulley**

**Spun end curve crown pulley**

**Spiral drum conveyor pulley**

**Welded steel pulley with diamond grooved lagging**

**Welded steel pulley with grooved Lagging**

**Spiral Wing Conveyor pulley**

**Power calculation for the drive unit:**

The horse power required at the drive of a belt conveyor is derived from the following formula:

H.P = T_{e} . V

Where

T

_{e}is the effective tension in the belt in NV = velocity of the belt in m/s

The required effective tension T_{e} on the driving pulley of a belt conveyor is obtained by adding up all the resistances.