Flexible Connectors

Light drives Medium drives Heavy drives

Power transmitted Low Medium Large

Belt speeds (V) V< 10m/s 10 < V< 22m/s V >22m/s

Applications Agricultural machines Machine tools Compressors & generators

d<<8m d<8m d>8m

Material:

High

Co-efficient

of Friction

Leather Cotton Rubber Balata

Thumb Rules

1. The arc of contact on the smaller pulley should be as large as possible, hence, the pulleys

should not be too close together.

2. The pulleys should not be too far apart, to avoid:

•the belt weighing heavily on the shafts, increasing the friction load on the bearings.

•sideward swinging of the belt, risking run out of the pulleys.

Flat versus V Belt

V Belt

Types of flat belt drives (Configuration)

Parallel shafts, rotating in same direction

A pulls the belt from one side (RQ) and delivers it to the other side (LM). Thus:

• the tension in the lower side will be more than that in the upper side belt.

• lower side is known as tight side, while the upper side belt is known as slack side.

Flexible Connectors

A pulls the belt from one side (PQ) and delivers it to the other side (LM). Thus:

• the tension in the belt RQ (tight side) > that in the belt LM (slack side).

Parallel shafts, rotating in opposite direction

To prevent the belt

from leaving the pulley

The face width of the pulley

>=

1.4 * width of the pulley

Flexible Connectors

From one, to manyTo increase the angle of contact

of the smaller pulley

Flexible Connectors

Different output speeds

by shifting stepsStart-stop

Velocity ratio of belt drive

Inverse of the

gear train velocity ratio

Rpm and

diameters

remain

Inversely

proportional

as in gear

drives

Velocity ratio of compound belt drive

Slip of the beltAssumption: Firm frictional grip between the belts and the shafts

Realism: The frictional grip may be insufficient

Effects - Some forward motion of:

• the driver pulley without carrying the belt

• the belt without carrying the driven pulley

This is called: Slip of the belt (reflecting in reduction of the Velocity ratio: Out/In)

Creep of the Belt

When the belt passes from the slack to the tight side, a portion of the belt extends,

and it contracts again when the belt passes from the tight to the slack side.

Due to these changes of length, there is a relative motion between the belt and

the pulley surfaces. This is termed as Creep, whose effect is to slightly reduce the

Speed of the driven pulley.

Length of an Open Belt Drive

Length of a Crossed Belt Drive

Length of the open & crossed belt drives

Power transmitted by the belt

Ratio of Driving Tensions for Flat Belt Drive

In terms of Coefficient of friction & Angle of contact

Resolving tensions

horizontally and

vertically

does the trick

Determination of Angle of Contact at smaller pulley

Centrifugal tensionAs the belt runs continuously, some centrifugal force is

caused, that tends to increase the tension on both, tight

as well as the slack sides. The tension caused by

centrifugal force is called centrifugal tension (Tc).

For V<10m/s, Tc is small but should be accounted for at

larger V.

How does Fc split into extra tension?

Condition for transmission of Maximum Power

When the power transmitted is maximum, 1/3rd of the maximum tension is absorbed as

Centrifugal tension

T

Initial Tension in the Belt

The motion of the belt from the driver & follower is governed by a firm grip, due to friction

between the belt and the pulleys. In order to increase this grip, the belt is tightened up. Even

when the pulleys are stationary, the belt is subjected to some tension: Initial Tension

When the driver starts rotating, it pulls the belt from one side (increasing the tension that

side) and delivers it to the other side (decreasing the tension that side).

Assuming the belt material is perfectly elastic, implying the length of the belt remains

constant, when at rest or in motion

Summary

•Velocity ratio of belt drive •Velocity ratio of compound belt drive

•Slip of the belt •Creep of the belt

•Power transmitted by the belt •Ratio of driving tensions

•Open belt drive • Crossed belt drive

Angle of Contact

Length

•Centrifugal tension •Condition for transmission of max power

Max T = T1 + Tc, where,

•Initial tension in the belt

Problem-1

A shaft which rotates at a constant speed of 160 rpm is

connected by belting to a parallel shaft 720 mm apart,

which has to run at 60, 80, and 100 rpm. The smallest

pulley on the driving shaft is 40 mm in radius. Determine

the remaining radii of the two stepped pulleys for:

(a) A crossed belt,

(b) An open belt.

Neglect belt thickness and slip.

Problem-1

Determine the remaining radii for:

(a) A crossed belt,

(b) An open belt.

Problem-1

Determine the remaining radii for:

(a) A crossed belt,

(b) An open belt.

Problem-1

Determine the remaining radii for:

(a) A crossed belt,

(b) An open belt.