chains & ropes

8/11/2019 Chains & Ropes

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2.1 CHAINS

2.1.1 Advantages and Disadvantages of chain drive over belt and rope drives.

Advantages

1. Can be used for short as well as long centre distances.

2. A number of shafts can be driven in the same or opposite directions from a single drivingsprocket.

3. More compact drive as compared to belt and rope drives.

4. High efficiency.

5. More positive drive, i.e. less slip.

6. No fire hazards and temperature effects.

7. Can be easily replaced.

Disadvantages

1. Unsuitable for precise motion due to polygonal effect and wear of joints.

2. Require precise alignment of shafts.

3. Require proper maintenance, lubrication and slack adjustment.

4. Generates noise.

2.1.2 Types of Chains

The chains are classified as:

(a) Transmission chains.

(b) Hoisting and hauling chains.

The transmission chains are used for power transmission and hoisting and haulingchains are used for lifting of load.

The hoisting and hauling chains are of the following two types, as shown in Fig. 2.1.

1. Chain with oval links:-

The links of this chain are oval in shape. The cross-Section of the link is generallycircular.

The Joint should be perpendicular to the line of action of the load where the stress isminimum.

The joint is generally welded.

The sprockets for these chains have receptacles to receive the links.

These chains are used at low speeds such as chain hoists.

2. Chains with square links:-

The links of this chain are of square shape. Such type of chains are used in hoists, and

cranes.

Chains & Ropes


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2.1.3 Chordal (or polygonal) Effect in Chain

The chain passes around the sprocket as a series of chordal links.

This action is similar to that of a non-slipping belt wrapped around a rotating polygon.

The chordal action on a sprocket having only four teeth is shown in Fig. 2.2.

In Fig. 2.2(a), the chain link AB Is at a distance of (D/2) from the center of thesprocket wheel.

Its linear velocity is given by,

As the sprocket rotates through an angle the position of the chain link AB is shownin Fig. 2.2(b).

In this case, the link is at a distance of (D/2) cos from the center of the sprocketand its linear velocity is given by

Therefore, the linear speed of the chain is not uniform but varies fromVmaxto Vmin during every cycle of tooth engagement.

This results in a pulsating and jerky motion.

The variation in velocity is given by

In order to decrease the velocity variation, the number of teeth on the sprocket shouldbe increased.

For smooth operation at moderate and high speeds, it is considered a good practice touse a driving sprocket with at least 17 teeth.

From durability and noise considerations, the minimum number of teeth on the drivingsprocket is 19 or 21.

It is better to use odd number of teeth on the driving sprocket with an even number ofchain links for more uniform wear.

This also ensures that the same link does not come into contact with the samesprocket teeth in every rotation.

The maximum number of teeth recommended on the sprocket wheel are 100 to 120.


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2.2 ROPES

Basically there are two types of ropes: fiber ropes and metallic ropes.

Fiber ropes are made of hemp: Cotton, sisal, manila, jute, nylon, poly-propylene andcoir.

Metallic ropes are made of steel, aluminum alloys, copper, bronze and stainless steel.

Fiber ropes are generally used where flexibility is a utmost importance, low powertransmission, as in instrument devices.

Metallic ropes are generally made of steel wire with either fiber core or metallic core.-

2.2.1 Steel Wire Ropes -

A wire rope is made up of strands and a strand is made up of one or more layers ofwires.

The number of strands in a rope denotes the number of groups of wires that are laid

over the central core.


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For Example:- A 6 x 19 wire iope construction means that the rope has 6 strands andeach strand is composed of 19 (12/6/1) wires.

It means that there are 114 wires in the rope.

The tensile designation of the wire rope corresponds to the minimum tensile strengthof the wire in N/mm2.

Some of the steel wire ropes are shown in Fig. 2.3.

2.2.3 Selection of a Wire Rope-

The following points must be considered while making a choice for a wire rope:

1. Strength

2. Abrasion resistance

3. Flexibility

4. Resistance to crushing

5. Fatigue strength, and

6. Corrosion resistance.

2.2.4 Number of Bends-

When a wire rope passes over drums and pulleys, it bends.

It has been found that rope life is approximately inversely proportional to the numberof bends.

One bend means the transition of the rope either from its straight position to a bentone or from a bent position into a straight one.

Reverse bending is equal to two single bends towards the same side.

The ratio of minimum diameter of pulley, d = rope diameter) for a givennumber of bids are given in Table 2.1.

2.2.5 Stresses in the Wire Rope-

The following stresses induced in hoisting ropes:

1. Direct tensile stress due to load hoisted and the weIght of the rope.

2. Stresses due to bending of the rope over the sheave.

3. Starting stresses, and4. Stresses due to change df rope speed including stops.

1. Direct tensile stress

The direct tensile stress is given by:-


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Example 2.1 Select a suitable steel wire rope for a mine hoist carrying a load of 2tones to be lifted from a depth of 100m. A rope speed of 10 ns must be attainedin 10 seconds.

Solution.


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Example 2.2 Sketch a 6 x 19 wire rope and name its parts?

Solution.

The sketch of 6 x 19 wire rope is shown in Figure.

Example 2.3 A 6 x 19 wire rope with fiber core and tensile designation 1420 isused to raise a load. The nominal diameter of the wire rope and the sheave

diameter are 10 and 450 mm respectively. Assuming long life on thebasis of fatigue consideration. Determine the maximum load (including theweight of wire rope) that the wire rope can carry.

Example 2.5 In the context of wire rope explain the meaning of 6 x17.

Solution.6 x 17 wire rope means that it has .6 strands and there are 17 wires in each strand. Thusthere are 102 wires in total in the rope.


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Example 2.6 It is required to design a chain drive to connect at 10 kW, 1440 rpmelectric motor to centrifugal pump running at 720 rpm. The service conditionsinvolve moderate shock. select a suitable roller chain and give list of itsspecification.

Solution.

Example 2.7Select a suitable chain drive to transmit 30 kW from an electric motor running at1200 rpm to a line shaft running at 250 rpm. Motor shaft diameter is 60 mm andthe centre distance is approximately 600 mm. Service is 10 hr/day, 6 days aweek. Good lubrication condition is expected.

Solution.


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