Power Transmission:Belt Drive

POWER TRANSMISSION: BELT DRIVES.

Belt drives are used for power transmission in machine tools. There are many types of belt drives such as: Simple flat belt drives (open belt drive, crossed belt drive), compound flat belt drive, V-belt drives (heavy-duty cogged raw edge V-belts, narrow-section wrapped V-belts, classical-section wrapped V-belts and other types to suit specific standards according to the grooves of the pulleys). In this write up special mention is made about synchronous belt drives.

SYNCHRONOUS BELT DRIVES:

Synchronous (toothed) belt drives are now widely used in place of traditional, roller chain drives for many applications. Unlike a flat belt or a V-belt the toothed belt cannot slip, therefore it can be used where the rotation of input (driver) and output (driven) elements of a system must always be synchronized.

The main advantage of synchronous belt drives compared with traditional roller chain drives are:

1. Substantially lower cost.

2. Quieter running.

3. Ability to operate in environments which would be hostile to a roller chain drive.

4. No need for lubrication of the drive.

5. The elastomer material from which the belt is made tends to absorb vibrations rather than transmit them.

On the other hand, synchronous belt drives cannot transmit as much power as either chain drives or as V-belt drives.

Synchronous belt drive applications are found where their special properties can be exploited. For example:

1. Office equipment where quiet running and lack of lubrication is important.

2. Food processing machinery where conventional lubrication, necessary with a chain drive, might contaminate the foodstuffs being processed.

3. Motor vehicle camshaft drives where synchronous, troblefree, quiet, lubrication-free, smooth running is required.

4. The coupling of stepper motors and servo-motors to the feed mechanisms of computer controlled machine tools where synchronous, trouble-free, vibration-free, smooth running is required.

Construction:

The teeth and the belt top are made from highly loadable polychloroprene-based elastomer compounds. They have excellent adhesion both on the tensile member and on the facing fabric.

A durable protection of the teeth is an essential precondition for a smooth operation and a long service life. This is ensured by the application of particularly abrasion-resistant polyamide fabrics with low friction coefficients.

Synchronous belt drives call for a high degree of length stability and tensile strength. These requirements are optimally met by low-elongation tensile members of glass cord helically wound over the entire belt width. Any longitudinal off-track running can be prevented by use of special type of tensile cords arranged in pairs.

The belts are also resistant to fatigue failure, temperature change, ageing, deformation and a wide range of environmental conditions.

The belts should be designated by the following data:

Pitch length (mm): The pitch length of the belt is the overall circumference measured on the natural pitch line. The pitch length is located in the middle of the tensile member. The precise pitch length can only be ascertained on suitable measuring devices.

Tooth pitch (mm): The tooth pitch is the linear distance between two adjacent teeth along the pitch line.

Belt width (mm): The belt width and the width designation are identical.

The number of teeth is equal to pitch length divided by tooth pitch.

Normally belts are available in following tooth pitch versions: 3 mm tooth pitch, 5 mm tooth pitch, 8 mm tooth pitch and 14 mm tooth pitch. The length and width dimensions can be varied.

PLASTIC, A POLYMERIC MATERIAL.

Polymeric materials are normally known as plastics. This is a misnomer since polymeric materials rarely show plastic properties in their finished condition. Actually, many of them are elastic. But, during the moulding process by which they are formed, they are reduced to a plastic condition by heating them to a temperature above that of boiling water. It is from this that they get the generic name of plastics.

There are two main groups of polymeric materials:

1. Thermoplastics: These can be softened as often as they are reheated. They are not so rigid as the thermosetting plastic materials but tend to be tougher. Some examples of thermosetting polymers are: Phenol formaldehyde, Urea formaldehyde, Melamine formaldehyde, Casein (cross-linked with formaldehyde), Epoxides, Polyesters (unsaturated), Polyesters (alkyd resins), and Silicones etc.

2. Thermosetting Plastics (Thermosets): These undergo a chemical change during moulding and can never again be softened by reheating. This chemical change, called curing, is triggered by the temperature and pressure of the moulding process. These materials are harder and more brittle than the thermoplastic materials.

POLYMERS: Polymers are formed by combining together a large number of basic units (monomer molecules) to form long chain molecules (polymers). These polymer molecules may be one of three types as mentioned bellow:

1. Linear polymer chain: Linear polymer chains can move past each other easily, resulting in a non-rigid, flexible, thermoplastic material such as polyethylene.

2. Branched linear polymer chain: It is more difficult for branched linear chains to move past each other, and materials with molecules of this configuration are more rigid, harder and stronger. Such materials also tend to be dense since the molecule chains cannot pack so closely together. Heat energy is required to break down the side branches so that the chains can flow more easily, and this raises their melting point above that for materials with a simple linear chain.

3. Cross-linked polymer: The cross-linked molecular chain is typical of the thermosetting plastics. Thermosets are rigid and tend to be brittle once the cross-links are formed by ‘curing’ the material during the moulding process. The elastomers are an intermediate group of materials which exhibit the toughness and resilience of rubber. This is achieved by a more limited cross-linking than that of rigid thermosets.

Reinforced polymeric materials: In this group of materials synthetic, polymeric materials are used to bond together strong reinforcing materials to produce high strength composites.

Glass reinforced plastics (GRP): High strength glass fibres are bonded together using polyester or epoxide resins. The fibres may be in the form of roving (ropes), woven cloth, or chopped strand mat. Glass reinforced plastics are used for a wide range of products, including printed circuit boards for high quality electronic equipment; and boat hulls and superstructures.

Laminated plastics: Sheets of paper, cotton cloth, woolen cloth, or woven glass fibre are impregnated with an appropriate synthetic resin and then stacked between polished metal sheets in a hydraulic press. The combined heat and pressure cause the laminates to bond together and to cure. The moulded sheets, rods, tubes and other sections produced by the process have high strength and can be machined on conventional machine tools into screwed fastenings, bushes, gears etc. in a manner similar to metals.

ADDITIVES:

Plasticizers: These reduce the rigidity and brittleness of polymeric materials and improve their flow properties during moulding.

Fillers: These are bulking agents which not only reduce the cost of the moulding powder, but have a considerable influence on the properties of a moulding produced from a given polymeric material. Fillers improve the impact strength and reduce shrinkage during moulding. Typical fillers are:

Glass fibre: good electrical properties,

Wood flour, calcium carbonate: low cost, high bulk, low strength.

Aluminium Powder: expensive but high strength and wear resistant.

Shredded paper (cellulose), shredded cloth: good mechanical strength with reasonable electrical

insulation properties.

Mica granules: good heat resistance and good electrical insulation properties.

Stabilizers: These are added to prevent or reduce degradation, and include antioxidants, antiozonants and ultraviolet ray absorbents.

Colorants:. These can be subdivided into dyestuffs, organic pigments and inorganic pigments. Dyestuffs are used for transparent and translucent plastics. Pigments have greater opacity, colour stability and heat stability than dyestuffs. They are unsuitable for transparent plastics.

Antistatic agents: These provide improved surface conductivity so that static electrical charges can

Leak away, thus reducing the attraction of dust particles, the risk of electric shock and the risk of

Explosion in hazardous environment caused by the spark associated with an electrical discharge.