PRINCIPLES OF DAIRY MACHINE DESIGN

Module 6. Power transmission

Lesson 28
SOLVING NUMERICAL

28.1 Problem

Design a V belt drive for the following:

Drive: AC motor, operating speed is 1440 rpm and operates for over 10 hours. The equipment driven is a compressor, which runs at 900 rpm and the power transmission is 20kw

Solution:

Since it is a V belt drive, let us consider belt speed, v = 25 m/sec.

Design power

P_des = service factor(C_sev) * required power (P) = 1.3 * 20kW = 26kW

Fig. 28.1

The value 1.3 is selected from design data book for the given service condition. Now,

Hence, obvious choice for belt section is C

Now,

V= π*d_s*n / 60 X 100

Where

n=Rotational speed RPM

d_s = Diameter of small pulley

d_L = Diameter of large pulley

V = Velocity of belt

standard sizes are,

d_S=315 mm and d_L=530 mm

d_S=355 mm and d_L = 560 mm.

First combination gives the speed ratio to be 1.68

Second combination gives the speed ratio to be 1.58.

So, it is better to choose the second combination because it is very near to the given speed ratio. Therefore, selected pulley diameters are d_S=355 mm and d_L= 560 mm.

Center distance, C should be such that, d_L < C < 3(d_L + d_S )

Let us consider, C = 1500 mm, this value satisfies the above condition.

Considering an open belt drive, the belt length,

Inside length of belt = 4444 – 56 = 4388 mm

The nearest value of belt length for C-section is 4394 mm (from design data book)

Therefore, the belt designation is C: 4394/173

Power rating (kW) of one C-section belt

Equivalent small pulley diameter is,

d_ES =C_SR d_s = 355 X 1.12 = 398 mm

C_SR = 1.12 is obtained from the hand book

For the belt speed of 23 m/sec, the given power rating (KW) =12.1 KW

For the obtained belt length, the length correction factor C_V1=1.04

Determination of angle of wrap

For the angle of wrap of 3.00 radian (smaller pulley), the angle of wrap factor, C_VW is found to 0.98 for a C section belt.

Therefore, incorporating the correction factors,

2 numbers of C 4394\173 belts are required for the transmission of 20kW.