1. Belt Drives Definition Advantages/Disadvantages
Factors Effecting Power Output
Flat Belts
Vee Belts
Toothed (Synchronous) Belts
Variable Speed Belt Drive

2. Definition of Belt Drives
Belt drives are a method of transmitting rotational motion from one shaft to another by means of a pulley mounted on the first (driving) shaft moving a belt which then drives a pulley mounted on the second (driven) shaft.
In most cases belt drives rely on friction between the belt and the pulleys they are mounted on to transmit the power, however some do transmit power by positive drive using teeth on the belt and groves on the pulleys.
Two of the most common belts that use friction are flat belts and Vee belts.
The pulleys they are attached to are commonly of different diameters so they can increase or decrease speed depending on the function.
The ability of belts to transmit power depends on the following factors
The arc of contact between the belt and the pulley
The tension holding the belt to the pulley
The amount of friction between the belt and the pulley
The speed of the pulley

3. Advantages of Belt Drives
They are generally quieter in operation than other types of drive systems such as gears and chains.
Belt drives do not require lubrication unlike other drive systems.
They can transmit power over greater distances than other systems especially flat belt drives.
Modern belt drives can run at high speeds.
Single belt drive systems can still operate efficiently where some degree of misalignment is present.
Drive belt systems are cheaper than other methods of drive systems.

4. Disadvantages of Belt Drives
Friction Belts are prone to slipping, due to stretching after use and need to be adjusted.
They are easily damaged by heat.
Friction belts slip and will need replacing if they become contaminated by oil or grease.
Apart from toothed belts, they cannot be used in applications that require accurate drive or speed such as timing drives on internal combustion engines

5. Factors Effecting Power Output
Pulley size
Ideal drive would be achieved by having both pulleys the same size this would give an arc contact on each pulley of 180º, however this is not always possible due to the need to increase or decrease speed.
To achieve full power output the pulley ration should be 3:1 or less as seen opposite
Higher ratios reduce the arc of contact on the pulley and can cause slipping and loss of power.

6. Belt Tension and Friction
Belt tension is very important for efficient belt drive.
Too little tension will cause reduced friction and slippage of the belt or slip and grab where the belt begins to slip then grabs on the pulley which produces a shock load and can cause damage or breaking of the belt.
Too much tension will result in increased friction with the belt overheating which leads to excessive stretching, this can also result in damage to the pulleys.
Belt tension is commonly measured and
set by measuring the deflection on one
side of the belt whilst applying a specified
load using a spring scale, the load and
permitted deflection is specified in
Machine/Vehicle manuals by the supplier.

7. Belt Tensioning Mechanisms
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.
In most successful belt drives an idler is not necessary because either the Driver or the Driven pulley will be adjustable to allow correct tensioning of the belt.
Where the pulleys are not adjustable, a belt tensioner with an idler pulley is used. The idler may be a straight face pulley running on the outside of the belt (fig. A) or a grooved or straight faced idler running on the inside of the belt (fig. B) a spring loaded or weighted idler pulley is sometimes used. It should always be on the slack side of the system.
Figure C shows a more suitable position for the outside idler as this create a greater contact of arc on the larger pulley.

8. Flat Belts
As the name suggests flat belts have a flat face, they are not used as extensively in todays world however they have been developed and improved and still serve to advantage for many applications.
They are useful for transmitting power over longer distances and also have the advantage of being able to operate in a crossover formation to reverse the rotation of the driven pulley, or transmit power through 90º
The pulleys used for flat belts are usually crowned this is to keep the belt centrally located on the pulley to prevent it from slipping off.

9. Advantages and Disadvantages of Flat Belts
Simple and generally low cost.
Maintenance is simple.
Resistant to dust contamination.
Due to flexibility can be used to produce drive at right angles by twisting or reverse the direction of motion by cross over of the belt.
Can operate where there is some degree of misalignment
Can run at speeds up to 30M/Min.
At lower speeds can be used as a conveyer belt to move components or items.
Disadvantages
Often require the use of larger diameter pulleys than other types.
More suited to transmitting lower loads than other belts.
More prone to slipping than Vee belts.
Require more space than Vee belts to provide similar power output.
Changing speed is not as simple as vee belts which can use multiple diameter pulleys.

10. Vee Belts
There are two types of Vee belt in common use, standard vee belts and Wedge belts.
The wedge belt is narrower and has a deeper section which gives it greater power capacity and more compact drive.
The Vee belt works on the principle of wedging action between the sides of the belt and the sides of the pulley walls when in operation, when bent round the pulley the walls of the belt bulge creating greater friction.
The belt should sit high in the grove with the top close to or level with top of grove never high above or below and never with the bottom of belt touching bottom of grove

11. Typical Vee Belt Drive Systems
Vee belt drive can be can be used in a number of configurations, being a simple single belt with one driving pulley and one driven pulley either the same size or different sizes to increase or decrease the speed.
Multiple vee belt drive which is used on machine tools such as lathes and milling machines for increased power transmission
Stepped vee belt drive to provide a range of different spindle speeds such as the system used on pillar drills

12. Cogged Vee Belts and Hexagon Double Vee Belts
Cogged Vee belts are similar in shape to Vee belts but they have a series of cut-outs/groves on the internal face.
This makes them more flexible than conventional vee belts allowing use on smaller pulleys they also generate less heat and can transmit greater loads.
Hexagon Double vee belts have the Vee profile on both inner and outer faces this enables them to be used in applications where the drives have one or more reverse bends such as on lawn mowers, garden equipment, crushers and agitators.

13. Vee Belt construction and material
Vee belts are constructed of different lavers as seen in the illustration opposite.
The outer cover can be made from neoprene rubber or other types of fabric material.
The cords can be made from polyester, nylon or in some cases wire, these cords act as reinforcement for strength and prevent excess stretching of the belt
The compression section is usually made of rubber
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14. Linked Vee Belts https://www.youtube.com/watch?v=LFu_ojA w-K8
Linked Vee Belts can also be used in place of endless vee belts.
They are used where other types of belts can not easily be installed.
They have the added advantage that they are adjustable and can be made to any size due to having individual links.
They are suited to harsh environments or applications where the belt could be exposed to oil, grease, chemicals and solvents which could cause rubber belts to deteriorate.
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15. Toothed/Synchronous Belt Drive
Toothed or synchronous drive belts are used where slipping of the belt can be detrimental to the driven components, an example of this is the timing belt on an internal combustion engine where slippage could change the valve timing resulting in the valves striking the pistons.
These type of belts provide a positive drive with teeth on the belt which engaging with teeth cut into the circumference of the pulley/sprocket.
This type of belt drive is capable of transmitting much greater loads and is also quieter than other types of belt drive.

16. Variable speed drive belts
Variable speed pulley drive systems consist of split pulleys which are spring loaded, the belt is wider than conventional belts such as vee belts.
The two halves of the pulley are either moved apart or closed up by means of an adjuster mechanism depending on the speed required.
If the two halves of the driven pulley are moved together at the same time the two halves of the driving pulley move apart decreasing speed.
If the two halves of the driven pulley are moved apart at the same time the two halves of the driving pulley move together increasing speed.
This system is typically used on milling machines such as Bridgeport's