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BME

Unit 3

Basic Power transmission devices

Question 1) A compound gear consists of 4 gears A, B, C and D and they have 20, 30, 40 and 60 teeth respectively. A is fitted on the driver shaft, and D is fitted on the driven shaft, B and C are compound gears, B meshes with A, and C meshes with D. If A rotates at 180 rpm, find the rpm of D.?

Given:

TA= 20, TB= 30, TC= 40, TD= 60, N A=180 rpm

Find ND=?

Solution:

ND / NA = TA x TC / TB x TD

ND = NA x TA x TC / TB x TD

ND = 180 x 20 x 40 / 30 x 60

ND = 80 rpm

Question 2) two gear wheels having 80 teeth and 30 teeth mesh with each other. If the smaller gear wheel runs at 480rpm, find the speed of the larger wheel.

Given:

Larger Gear wheel T1 = 80, N1=?

Smaller Gear Wheel T2= 30, N2= 480 rpm

Velocity ratio of gear drive

N2 / N1 = T1 / T2

N1 = N2 x T2 / T1

N1 = 480 x 30 / 80

= 180 rpm

Question 3) in a simple train of gears, A has 30 teeth, B has 40 teeth, C has 60 teeth and D has 40 teeth. If A makes 36 rpm, find the rpm of the gear C and D.

Given: TA= 30, TB= 40, TC= 60, TD= 40, NA=36 rpm

Find NC=? ND=?

Solution =

NC / NA = TA / TC

NC = NA x TA / TC

= 36 x 30 / 60

= 18 rpm

ND / NA = TA / TD

ND = NA x TA / TD

= 36 x 30 / 40

= 27 rpm

Question 4) Explain Open Belt Drive?

Answer 4)

Description: Open belt drive

In these types of belt drive, the belt is employing when the two parallel shafts have to rotate in the same direction.

When the shafts are far apart, the lower side of the belt should be the tight side and the upper side must be the slack side.

This is because, when the upper side becomes the slack side, it will sag due to its own weight and thus increase the arc of contact.

In open belt drive, belt proceeds from top of one pulley to the top of other pulley without crossing.

Contact angle (or wrap angle) between the belt and pulley is comparatively small (always below 180º in smaller pulley).

LENGTH OF OPEN BELT DRIVE FORMULA:

= π (r1 + r2) + 2x + (r1 – r2)2 /x ……………. (In terms of pulley radii)

= π/2 (d1 + d2) + 2x + (d1 – d2)2/ 4x ……….… (In terms of pulley diameters)

Question 5) Explain Cross Belt Drive?

Answer 5)

Description: cross-belt-drive

In these types of belt drives, the belt is employing when two parallel shafts have to rotate in the opposite direction. At the junction where the belts cross, it rubs against itself and wears off.

To avoid excessive wear, the shafts must be placed at a maximum distance from each other and operated at very low speeds.

Contact angle between the belt and pulley is comparatively large (always above 180º in smaller pulley).

In crossed belt drive, belt proceeds from top of one pulley to the bottom of other pulley and thus crosses itself.

Here belt bends in two different planes in every rotation during its operation

Cross belt drive can be advantageously applied for horizontal, inclined and vertical positions of driving and driven shafts.

It can transmit more power as wrap angle is more.

LENGTH OF CROSS BELT DRIVE

= π (r1+r2) + 2x + (r1+r2)2 /x …………… (In terms of pulley radii)

= π/2(d1+d2) +2x + (d1 + d2)2/ 4x …………(in terms of pulley diameters)

Question 6) Define Material of belts?

Answer 6)

The Materials used for belts and ropes must be strong, flexible, and durable. It must have high coefficient of friction: like

1- Leather belts

2- Cotton belts

3- Rubber belts

4- Balata belts

1. Leather belts :

The leather may be oak tanned or mineral salt tanned. Example chrome tanned. When the thickness of the belt required is more then, two or more strips are cemented together. Leather belts require periodic cleaning.

2. Cotton belts or Fabric belts :

Fabric belts are made by folding canvas or cotton ducks in layers and stitching together. The fabric belts are cheaper and best suited in warm climates, damp atmospheres. These are mostly used in Farm machinery, belt conveyor etc.

3. Rubber belts :

Rubber belts are made of Fabric with rubber layer. Rubber belts are used in sawmills, paper mills, etc.

4. Balata Belts:

Balata belts are similar to rubber belts except that balata gum is used in place of rubber. Balata belts are acid proof and water proof and it is not affected by animal oils or alkalies. The balata belts should not be at temperatures above 40° C because at this temperature the balata begins to soften and becomes sticky. The strength of balata belts is 25 percent higher than rubber belts.

Question 7)Explain type of belts?

Answer 7)

Different types of belt are as follows

Flat Belt

V – Belt

Circular Belt

Description: Belt Drive - Types, Advantage And Disadvantage | Mecholic

1. Flat Belt :

Flat belt has a rectangular cross section. Flat belts are capable of transmitting power over long distance between pulley centres. The efficiency of this drive is around 98 % and produces less noise.

This belt is mostly used in factories and workshops, where a moderate amount of power is to be transmitted, from one pulley to another when two pulleys are not more than 8 metres apart.

Flat belts drives are of following types:

Open belt drive.

Cross belt drive.

Stepped or cone pulley or speed cone drive.

Fast and loose pulleys.

Jockey pulley drive or belt drive with idler pulley

Compound Belt Drive System

2. V – Belt:

V-Belts also used with grooved pulleys, V-belts are trapezoidal in cross section. V-belts permit large speed ratios and can transmit higher power. Multiple drives are possible.

This belt is mostly used in factories and workshops, where a moderate amount of power is to be transmitted, from one pulley to another when two pulleys are very near to each other.

3. Circular Belt or Round Belt :

Circular Belt or Round belt has a circular cross section and is used with grooved pulleys, this belt is mostly used in factories and workshops, where a great amount of power is to be transmitted, from one pulley to another when two pulleys are more than 8 metres apart.

Question 8) Write advantage and disadvantage of rope drive?

Answer 8) Advantages of the rope drive

Significant power transmission.

It can be used for long distance.

Ropes are strong and flexible.

Provides smooth and quiet operation.

It can run any direction.

Low-cost and economic.

Precise alignment of the shaft not required

Disadvantages of the rope drive

Internal failure of the rope has no sign on external, so it if often get unnoticed.

Corrosion of wire rope.

Question 9) How is rope drive different from Belt drive and Chain drive?

Answer 9)

A Rope Drive uses a rope to transmit power from one system to another, for e.g in pulleys over a well. It is different from a belt drive because it does not use a belt to transmit power. A rope drive is expected to give a smoother operation and less noise than chain drive, but can have higher power losses

Question 10) Write types of gear?

Answer 10) A gear is a kind of machine element in which teeth are cut around cylindrical or cone shaped surfaces with equal spacing. By meshing a pair of these elements, they are used to transmit rotations and forces from the driving shaft to the driven shaft. Gears can be classified by shape as involute, cycloidal and trochoidal gears. Also, they can be classified by shaft positions as parallel shaft gears, intersecting shaft gears, and non-parallel and non-intersecting shaft gears.

Type of gear:

Following are the different types of gears:

Parallel gears

Spur Gears
Helical Gears
Double Helical or Herringbone Gears.

Intersecting gears

Spiral gears
Bevel Gears

Non-intersecting and Non-Parallel gears.

Worm Gears

Rack and Pinion gears

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