1. Belt Drives

2. Definition
A belt drive is used to transmit rotational movement from one shaft to another.
Belts rely on frictional force and transmit power by contact between the
belt and the driving and driven pulley (pulley also known as a sheave)
A number of different types of belt are available, the most common are
either flat or of the vee type.
Pulleys between belts are normally of different diameters so that either
an increase or decrease in speed can be achieved.
The ability of belts to transmit power depends on:
The tension holding the belt to the pulley
Friction between belt and pulleys
Arc of contact (wrap) between belt and pulleys
Speed of the belt

3. Advantages Belts themselves require no lubrication
They are generally quieter than other types of drive
Flat belts particularly can be used where extremely long
centre distances make other types of drive impracticable
Modern belts can run at extremely high speeds
Single belt drives will accept a measure of misalignment
Compared with other methods of power drive systems they
are economical and cheap

4. Disadvantages
Belts are prone to slipping, especially after use when they
tend to stretch
They are easily damaged especially by heat, oil and grease
Apart from stepped or toothed belts, they cannot be used
where exact timing or speed is required

5. How Pulley Sizes Affect Power Output
For full power on the belt, the pulley ratio should be 3 to 1 or less. Higher ratios (shown) lessen the arc of contact, causing slippage and loss of power.

6. Flat Belts
The oldest form of belt for power drives could probably be traced back over many centuries.
If we were to do this we would almost certainly find that they would have been Flat or Round Belts. Not much has changed.
While the use of flat belts has declined over the years they have been continuously developed and improved and are still used to advantage for many types of applications.

7. Flat Belts
Pulleys for flat belts are usually crowned. This is to keep the belt central on the pulley whilst it is rotating.

8. Advantages of Flat Belts
The main advantages of Flat Belts are:
simplicity
low cost
simple maintenance
resistance to dust
will run in a crossed over state to provide drive between shafts at right angles
can be caused to move along a pulley to allow free wheeling or stopping
of the drive system
Flat Belts can be run at speeds up to 30 m/min.
At lower speeds large Flat belts can be used as conveyor belts where
the belt itself performs the work.

9. Disadvantages of Flat Belts
The main disadvantages of the use of Flat Belts are:
they often require the use of larger diameter pulleys than other types of belt
they tend to be less flexible
are more prone to slipping under frictional drive
would require more space than say a vee belt to provide similar power output

10. Applications of Flat Belts
A flat belt may be twisted to give a drive through a right angle or a reversal of The direction or rotation.
90° CROSSED BELT DRIVES
CROSSED BELT TO GIVE
REVERSAL OF DRIVE

11. Vee Belts There are two types of belt in use:
standard industrial vee belt made from rubber and rayon cord, and
wedge belts which use neoprene and terylene cords giving greater strength.
Both types are available in a series of standard sections covering a large
power range.
The wedge belt is narrower and has a deeper section and has greater
power capacity than the standard Vee belt. They also give a more
compact drive.

12. HOW VEE BELTS GRIP
Due to the wedging action of their angled sides, V-belts pull well. The
section going around the pulley tends to bulge as it bends, hugging the
flanges tightly

13. LINKED VEE BELTS
Vee linked belts are complementary to the standard endless vee belts. They are useful where other types of drive belts cannot easily be installed. Fitting together as separate linked units, a complete drive system is easily made up to suit any motor drive installation.

14. TIMING BELTS
A timing belt has transverse teeth on the inside surface which mate with grooves on the pulleys. The belt has steel wire reinforcement which enables it to transmit about three times the power at three times the speed of a conventional belt. There is no slip and exact speed ratios are maintained.

15. VEE RIBBED BELTS
Vee ribbed belts are an adaption of the ordinary Vee belt. It combines the strength and simplicity of the flat belt with the positive tracking of the Vee belt. The ribbed Vee belt also eliminates the matching problem inherent in multi-Vee belt drives. As you will see from the drawing the pulleys (sheaves) have matching grooves to those in the belt. The Vee Ribbed belt performs a similar function to the Multi-Vee belt drives.

16. BELT TENSIONING MECHANISMS
So far we have learned that all belt drive systems rely on the friction between the sheave (pulley), and the belt itself. The correct amount of friction between belt and pulley is achieved by careful tensioning of the belt against the pulleys on which it is to run.

17. IDLERS
In most successful belt drives an idler is not necessary because either the Driver or the driven sheave will be adjustable to allow correct tensioning of the belt.
Where the sheaves are not adjustable, a belt tightener with an idler pulley is used. The idler may be a straight-face pulley running on the outside of the belts or a grooved or straight faced one running on the inside of the belt.
A spring loaded or weighted idler pulley is sometimes used. It should always be on the slack side of the system.

18. IDLERS

19. BELT TENSION
Too little tension will cause slippage or slip and grab causing the belt to break. Too much tension will cause the belt to overheat and excessive stretching with possible damage to the pulleys. REMEMBER: VEE BELTS SHOULD RIDE ON THE SIDES OF THE PULLEY, NOT ON THE BOTTOM OF THE PULLEY GROOVE. Tension on a new belt should be watched carefully during its running in period. Belt tension is often checked by measuring the deflection on one side whilst applying a measured load with a spring scale. Manufacturers recommend a given force for a deflection of 16mm per metre of span (distance between pulley centres).

20. BELT TENSION

21. BELT ALIGNMENT
Belt drives require good alignment of the pulleys to ensure the longest possible life of the belt and correct and safe running of the system.
Whilst the alignments shown below are highly exaggerated, it does indicate the most common cause of shortened belt life.
Pulleys are generally out of alignment because the shafts on which they are mounted are not parallel.
The diagrams overleaf show the most common types of misalignment, all caused by shafts being out of parallel.

22. BELT ALIGNMENT

23. SHEAVES NOT ALIGNED DAMAGE THE V-BELT
Possibly the best way to check alignment is to use a straight edge
between the side of the pulleys.
The straight edge should touch the
pulleys at the four arrows, as
shown on the diagram below.
Belts will tolerate misalignment
better than other Power Drives.
However, there is a limit and we
should be as accurate as possible

24. https://www.youtube.com/watch?v=O4h-Me5lSOY var speed pulley