Belt and chain driveBelt and chain drive

INTRODUCTIONINTRODUCTION

1.1.A belt is a looped strip of flexible material, used to mechanically link A belt is a looped strip of flexible material, used to mechanically link two or more rotating shafts. two or more rotating shafts. 2.2.They may be used as a source of motion, to efficiently transmit They may be used as a source of motion, to efficiently transmit power, or to track relative movement. Belts are looped over pulleys. power, or to track relative movement. Belts are looped over pulleys. 3.3.In a two pulley system, the belt can either drive the pulleys in the In a two pulley system, the belt can either drive the pulleys in the same direction, or the belt may be crossed, so that the direction of same direction, or the belt may be crossed, so that the direction of the shafts is opposite. the shafts is opposite.

Advantages of belt driveAdvantages of belt drive•They are simple. They are economical.•Overload and jam protection are provided. •Noise and vibration are damped out. Machinery life is prolonged because load fluctuations are cushioned (shock-absorbed). •They are lubrication-free. They require only low maintenance. •They are highly efficient (90–98%, usually 95%). Some misalignment is tolerable. •They are very economical when shafts are separated by large distances.

FLAT BELT

TYPES OF BELTS

V- BELT

TIMING BELT

ROUND BELT

Open belt drive

Cross belt drive

OPEN BELT DRIVE WITH IDLER PULLEY

MULTIPLE BELT DRIVE:

QUARTER TURN BELT DRIVE

COMPOUND BELT DRIVE:

Desirable properties of materials used for belt

• High coefficient of friction• High flexibility• Durability• High strength

Materials • Leather• Fabric• Rubber• Polyester or polyamide

Velocity ratio of belt

• it is the ratio between the velocities of the driver and the follower or driven.

d1 = Diameter of the driver, d2 = Diameter of the follower N1 = Speed of the driver in r.p.m., and N2 = Speed of the follower in r.p.m.

Length of the belt that passes over the driver and the follower in one minute will be l1 = πd1 N1 and l2 = π d2 n2, respectively.

Since, the lengths are equal πd1 N1 = π d2 N2

Where, N2/N1 is the velocity ratio. If we take the thickness of belt into account then above expression can be written as N2 / N1 = (d1 + t) / (d2 + t)

Velocity of belt drive considering slip• The motion of belts and shafts assuming a firm frictional

grip between the belts and the shafts. But sometimes, the frictional grip becomes insufficient. This may cause some forward motion of the driver without carrying the belt with it. This may also cause some forward motion of the belt without carrying the driven pulley with it. This is called slip of the belt and is generally expressed as a percentage.

• The result of the belt slipping is to reduce the velocity ratio of the system. As the slipping of the belt is a common phenomenon, thus the belt should never be used where a definite velocity ratio is of importance (as in the case of hour, minute and second arms in a watch).

Let s1 % = Slip between the driver and the belt, and s2 % = Slip between the belt and the follower. If thickness of belt is considered N2/N1= (d1+t)/[d2+t(1-s/100)]

In a flat belt pulley, the rim surface is given a convex shape by increasing the thickness of a rim at the center. This increased thickness is called crown and the process is known as crowning of pulley.

OBJECTIVES OF CROWNING OF PULLEYi)In flat belt drive, if the two shaft are not exactly parallel, there is tendency of belt to come off from the pulley in running condition. The crowning prevents the coming off of the belt from the pulley.ii)The crowning helps to keep the belt near the mid plane of the pulley in running conditions.

CROWNING OF PULLEY

The angle of contact

The wedge angle of the belt (and groove). Because you will be selecting a standard belt you are not able to change this angle which is usually 34° or 38° depending upon the size of the belt and the size of the pulleys. The coefficient of friction.You have little control over this because it is determined by the belt material and the pulley material (and finish).

The pulley diameters.The larger the diameters, the greater the torque and power. This is simply because for a given belt tension force, the larger the pulley, the larger the torque (torque = force x radius) and hence the greater the power for a given speed.

SELECTION OF FLAT BELTS FROM MANUFACTURING CATALOUGE

The size of the belt.The larger the belt section, the greater the tension that can be carried by the belt and the greater the torque and power.

The number of belts.Belt drives with a single belt are the most common but belt drives are often used with 2 to 6 belts in parallel on multi-grooved pulleys.

Initial belt tensions.The higher the initial tension in the belts, the greater the torque and power that can be transmitted. At rest, when no power is being transmitted, the belt tensions are equal on both sides of the pulleys.

Selection of v-belt from manufacturer catalogue

1. The data is available :-a. power to be transmittedb. Input and output speed 2. Select the service factor or load correction factor for the given application3. Select type of v-belt cross section4. calculate the power5. select the standard pulley6. calculate pitch length of the belt7. Select nearest standard pitch length of the belt from manufacturer

catalogue and calculate the exact center distance

Selection of v-belt from manufacturer catalogue

8. Find the power rating of the selected v- belt from manufacturer catalogue 9.Calculate arc of contact for the smaller pulley and find arc of contact correction factor10.Find belt length correction factor11.Calculate the modified power rating of the v-belt12.Find the number of belt required

Creep In Belt• When the belt moves over the pulleys to transmit the power, the

tension in the right side is more than the tension in the slack side.• As the belt material is elastic, it elongates more on the tight side

than on the slack side, resulting in unequal stretching on two sides of the drive.

• Therefore the length of the belt received by the driving pulley is more than the length of the belt that moves off the driving pulley.

• On the other hand length of the belt received by the driven pulley is less than the length of the belt that moves off the driven pulley. Hence, the belt must creep forward slightly relative to the driven pulley rim.

Creep In Belt

Creep In Belt• This motion of the belt relative to drive and driven pulley due

to unequal stretching of two sides of the drive is known as creep.

• The effect of creep back on the driving pulley is to slow down the speed of the belt with respect to driving pulley and the effect of creep forward on the driven pulley is to slow down the speed of the driven pulley with respect to the belt.

• It is important to note that, the creep which is due to the elastic property of the belt is totally different than the conventional slip, which is due to insufficient frictional grip between the belt and pulley.

Method of belt tensioning

• In order to transmit the power, the belt must be provided with the sufficient initial tension. The power transmitting capacity of the belt drive also depends upon the initial tension.

• Some of the method use to adjust the belt tensions are as follows:-

1. Manual adjustment2. Weighted idler pulley

• The angular-velocity ratio is not necessarily constant or The angular-velocity ratio is not necessarily constant or equal to the ratio of pulley diameters, because of belt equal to the ratio of pulley diameters, because of belt slip and stretch. slip and stretch.

• Heat buildup occurs. Speed is limited to usually 7000 Heat buildup occurs. Speed is limited to usually 7000 feet per minute (35 meters per second). Power feet per minute (35 meters per second). Power transmission is limited to 370 kilowatts (500 transmission is limited to 370 kilowatts (500 horsepower). horsepower).

• Operating temperatures are usually restricted to –31 Operating temperatures are usually restricted to –31 to 185°F (–35 to 85°C). to 185°F (–35 to 85°C).

• Some adjustment of center distance or use of an idler Some adjustment of center distance or use of an idler pulley is necessary for wear and stretch compensation. pulley is necessary for wear and stretch compensation.

• A means of disassembly must be provided to install A means of disassembly must be provided to install endless belts.endless belts.

Disadvantages of belt drive

Introduction to chain drive• Chain drive is a way of transmitting mechanical

power from one place to another. It is often used to convey power to the wheels of a vehicle, particularly bicycles and motorcycles. It is also used in a wide variety of machines besides vehicles.

• Most often, the power is conveyed by a roller chain known as the drive chain or transmission chain, passing over a sprocket gear, with the teeth of the gear meshing with the holes in the links of the chain.

Advantages of chain drive • As no slip takes place during chain drive, hence perfect velocity ratio is

obtained.• Since the chains are made of metal, therefore they occupy less space in

width than a belt orrope drive.

• It may be used for both long as well as short distances.• It gives a high transmission efficiency (unto 98 percent).• It gives less load on the shafts.• It has the ability to transmit motion to several shafts by one chain only.• It transmits more power than belts.• It permits high speed ratio of 8 to 10 in one step.• It can be operated under adverse temperature and atmospheric

conditions.

Limitations of chain drive

• The production cost of chains is relatively high.

• The chain drive needs accurate mounting and careful maintenance, particularly lubricationand slack adjustment.

• The chain drive has velocity fluctuations especially when unduly stretched.

Types of power transmission chain

• Roller chain – Large reduction ratio (usually up to 1:7)– Chains can be used with long shaft center distances (normally up to

4m). By changing the number of links it is possible to freely adjust the shaft center distance.

– The chain can be used on both sides and drive multiple shafts at the same time.

– Easy installation and replacement (easy to cut and connect chains). – If the distance between shafts is short, it can be used vertically. – The sprocket diameter of a chain drive could be smaller than a belt

pulley while transmitting the same torque. – Transmitting power by meshing many teeth results in less wear of

sprocket teeth than gears. – Chain drives have greater shock absorption than gear drives.

Roller chain drive

• Inverted tooth drive chains– Inverted tooth drive chain are the optimal

solution for requirements exceeding those met by other chains (e. g. roller chains). They allow for low-backlash drives with exact positioning. They are also highly effective, inured to vibration and have minimal noise emission.

Inverted tooth chain drive

Lubrication of chain • It is very important for the effective and

durable functioning of chains.• The chains are lubricated by light or medium

mineral oils like SAE30.SAE40,SAE50• Methods of lubrication

1. Manual2. Oil bath3. Oil stream

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