Rope drive

ROPE DRIVE

• Utility: The Rope drives are widely used when large power is to be transmitted continuously from one pulley to another, over a considerable distance. One of the main advantage of rope drives is that a number of separate drives may be taken from the one driving pulley.

• Material used: Ropes are commonly made of cotton, manila or hemp.

• Cross-section: Ropes are usually of circular cross-section or sometimes of square cross- section.

• Type of ropes: Two types of ropes are 1. Fibre ropes, and 2. Wire ropes.

• Advantages of rope drives over the belt drives:

Rope drives have practically

(i) no limit to the amount of power transmitted,

(ii) less width of the pulley face,

(iii) no need of accurate shaft alignment,

(iv) smaller losses due to slippage, and

(v) are less noisy.

1. Ratio of Driving tensions for Rope Drive

The limiting tension ratio for a rope drive is given by

where μ, θ and β have usual meanings.

Example 8.14 

A rope drive is required to transmit 230 kW from a pulley of 1 m diameter running at 450 rpm. The safe pull in each rope is 800 N and the mass of the rope is 0.4 kg per metre length. The angle of lap and the groove is 160° and 45° respectively. If μ = 0.3, find the number of ropes required.

450 rpm; Tmax

Given data: 

P = 230 kW = 230 × 103 W; d = 1 m; N = 450 rpm; Tmax = 800 N; m = 0.46 kg/m; θ = 160° = 160° × (π/180°) = 2.793 rad; 2β = 45° or β = 22.5°; μ = 0.3

Solution:

Power transmitted by one rope, P = (T – T2) v

= (544.67 – 61.014) × 23.56 = 11.395 kW

Example 8.15 

100 kW is to be transmitted by a rope drive through a 160 cm diameter 45° grooved pulley running at 200 rpm. Angle of overlap is 140° and coefficient of friction between pulley and rope is 0.25. Mass of rope is 0.7 kg/m and it can withstand a tension of 800 N. Considering centrifugal tension, find the following:

(i) Number of ropes requires, and

(ii) Initial tension in the rope.

Given data: 

Rope drive; P = 100 kW = 100 × 103 W; d = 160 cm = 1.6 m; 2β = 45° or β = 22.5°; N = 200 rpm; θ = 140° = 140° × (π/180°) = 2.443 rad; μ = 0.25; m = 0.7 kg/m; Tmax = 800 N.

Solution: 

(i) Number of ropes required:

Centrifugal tension, TC = m v2

= 0.7 × (16.76)2 = 196.63 N

Tension on tight side, T1 = Tmax - TC

or T1 = 800 - 196.63 = 603.37 N

We know that power transmitted by one rope,

(ii) Initial tension in the rope: