Wednesday, January 9, 2008

The electrical & electronic control systems:

The electrical system:

Electricity powers most of the devices in the car. The starting motor requires electricity to crank the engine, the ignition system requires electricity to deliver spark inside the cylinders. The fuel injectioin system, lights, horns, radio, air conditioner and other accessories require electricity to operate. The battery supplies electricity while the engine is off and for cranking the engine. After the engine starts, the alternator recharges the battery and supplies power for the electrical load.

The electronic control systems:

Many devices and systems on the car require a control system for safe and proper operation. A basic control system has three parts. These are inputs, the control unit and the outputs. The inputs are switches and sensors; they provide information to electronic control modules (ECM). Then the ECM signals the output devices or actuators to take the required action. The ECM has a self-diagnostic capability. This means a memory stores information about faults or malfunctions that have occurred and perhaps disappeared. When recalled from the memory this stored information helps the technician to diagnose and repair the vehicle.

Types of Lathe:

Lathes of various designs and constructions have been developed to suit the various conditions of metal machining. But all of them employ the same fundamental principle of operation and perform the same function.

Generally used Lathes are:

  • Speed Lathe
  • Engine Lathe
  • Bench Lathe
  • Tool Room Lathe
  • Capstan and Turret Lathe
  • Special Purpose Lathe
  • Automatic Lathe

The Speed Lathe:

The speed lathe, in construction and operation, is the simplest of all kinds of Lathe. It consists of a bed, a headstock, a tail stock and a tool post mounted on an adjustable slide. There is no feed box, lead screw or conventional type of carriage. The tool is mounted on the adjustable slide and is fed into work purely by hand control. This characteristic of lathe enables the designer to give high spindle speeds which usually range from 1200 to 3600 rpm. As tool is controlled by hand, the depth of cut and thickness of chip is very small.

The headstock construction is very simple and only two or three spindle speeds are available. Light cuts and high speeds necessitate the use of this type of machine where cutting force is minimum such as in wood-working, spinning, centering, polishing. Etc. The “Speed Lathe” has been so named because of the very high speed of the Head stock spindle.

The Engine Lathe or Centre Lathe:

This Lathe is most important member of the lathe family and is the most widely used. The term “Engine” is associated with the lathe owing to the fact that early lathes were driven by steam engines. Similar to the speed lathe, the engine lathe has got all the basic parts for example bed, head stock and tail stock. But the head stock of engine lathe is much more robust in construction and it contains additional mechanism for driving the lathe spindles at different speeds. Unlike speed lathe, engine lathe can feed the cutting tool both in cross and longitudinal direction with reference to the lathe axis with help of carriage, feed rod and lead screw. With these additional features an Engine lathe has proved to be a versatile machine adapted for every type of lathe work.

Engine lathes are classified according to the various deigns of the head stock and methods of transmitting power to the machine. A lathe that receives its power from an overhead line shaft is a belt driven lathe and is equipped with a speed cone and one or more back gears to get a wide range of spindle speeds. A lathe that receives its power from an individual motor integral with machine is called a motor driven lathe. A geared head lathe gets its power from a constant speed motor and all speed changes are obtained by shifting various gears located in headstock. It has no cone pulley.

The Bench Lathe:

This is a small lathe usually mounted on a bench. It has practically all the parts of an engine lathe and it performs almost all the operations, its only difference being in the size. This is used for small precision work.

The Tool Room Lathe:

The Tool room Lathe having features similar to Engine lathe is much more accurately built and has a wide range of spindle speeds ranging from a very low to a quite high speed up to 2500rpm. This is equipped besides other things, with a chuck, taper turning attachment, draw in collet arrangement, thread chasing dial, relieving attachment, steady and follower rest, pump for coolant etc. This lathe is mainly used for precision work on tools, dies, gauges and in machining work where accuracy is needed. The machine is costlier than an Engine lathe of same size.

The Capstan and Turret Lathe:

These Lathes are development of the Engine lathe and are used for Production work. The distinguishing feature of this lathe is that the tail stock of Engine lathe is replaced with a hexagonal turret, on the face of which multiple tools can be fitted and fed into the work in proper sequence. The advantage is that several different types of operations can be done on a work piece without re-setting of work or tools, and a number of identical parts can be produced in minimum time.

Special Purpose Lathe:

As the name implies, they are used for special purposes and for jobs which cannot be accommodated and conveniently machined on a standard lathe. The wheel lathe is made finishing the journals and turning the tread on railroad car and locomotive wheels. The gap bed lathe, in which a section of the bed adjacent to head stock is recoverable, is used to swing extra large diameter pieces. The T-Lathe, a new member of the lathe family is intended for machining of rotors for jet engines. The axis of the lathe bed is at right angles to the axis of head stock spindle in the form of T. The duplicating lathe is one for duplicating the shape of a flat or round template on to the work piece. Mechanical, air and hydraulic devices are all used to co-ordinate the movements of the tool to reproduce accurately the shape of the template. The Missile Lathe, which has a very large swing for accommodating long missile component of very large diameter, is the most modern and latest in Lathe design.

Automatic Lathe:

These are high speed, heavy duty, mass production lathes with complete automatic control. Once the tools are set and the machine is started it performs automatically all the operations to finish the job. The changing of tools speeds and feeds are also done automatically. After the job is complete, the machine will continue to repeat the cycles producing the identical parts even without attention of the operator. An operator will simply look after general maintenance of the machine and cutting tool, load up a bar stock and remove finished products from time to time.

The Lathe

The Lathe


The Lathe is one of the oldest machine tools and came into existence from the early tree lathe which was then a novel device for rotating and machining a piece of work held between two adjacent trees. A rope wound around the work with its one end attached to a flexible branch of a tree and the other end being pulled by a man caused the job to rotate intermittently. Hand tools were then used. With its further development a strip of wood called “lath” was used to support the rope and that is how the machine came to be known as Lathe. This device continued to be developed through centuries and in the year 1797 Henry Maudslay, an Englishman, designed the first screw cutting lathe which is the forerunner of the present day high speed heavy duty lathe, a machine tool which has practically given shape to our present day civilization by building machines and industries.

Function of Lathe:

The main function of lathe is to remove a metal from a piece of work to give it the required shape and size. This is accomplished by holding the work securely and rigidly on the machine and then turning it against cutting tool which will remove metal from the work in the form of chips. To cut the material properly the tool should be harder than the material of work piece, should be rigidly held on the machine and should be fed or progressed in a definite way relative to the work.


Refrigeration cycle of Window Air Conditioner

Preventive Maintenance of Air-conditioner :

Periodic cleaning of the air-conditioner is necessary to prevent breakdown and excessive wear of the unit. A soft cloth or vacuum cleaner can be used to clean the components. The grills, condenser and evaporator coils, fans and dampers should be freed from obstructions. The air seals in the window and cabinet should be checked to see that they have not loosened or deteriorated. Such cleaning and periodic checking give troublefree service in most of the cases.

Air-conditioning Cassettes :

The term “Cassette” came into use in 1875 to describe a flat box used for transporting photographic plates. In more recent times, we have used this term for hi-fi equipments as videos, computers, compact cameras and so on. From 1980 onwards, the ceiling cassette became terminilogy in air-conditioning industries. The cassette air-conditioner has made a bigger impact in Heating, Ventilating and Air-conditioning trade during last 10 years than any other single product in its history. The first ceiling cassette was brought in the market in 1984. By 1985, other two companies Daikin and Hitachi from Japan entered injto this market, type of air-conditioning cassette which is of drawing board size and sufficient to cool a single room. The sale of air-conditioning cassettes in U.K. was of 11 million dollars in 1987 which went to 18 million dollars in 1988.

Cassette system has rapidly established themselves as an economical alternative to other packages or central plant with a better return on capital investment. This is because, cassettes give higher flexibility for retail operations which are frequently on the move or subject to periodic re planning. The popular concept “Shop Within a Shop” has often concentrated high lighting and occupancy loads in relatively small areas with the need for additional spot cooling, conveniently provided by Cassette unit.

Hi-tech electronics provide further refinements in terms of temperature and operating efficiency. This latest unit claims 30% more output on heating, 20% on cooling and 20% less power consumption and continuous capacity control. The next generation of units will need to be more compact, less distraught and overcome the occasional problems of condensate disposal.

A Hitachi Air conditioning cassette creates a musical sound by distributing four way air and requires hardly 28cm space with a virtually flush face side diffuser. Micro computer controls makes the cassette easy to operate and you can choose cooling only, heat pump only and heat pump models with supplementary electric heating element to give the security of additional heat on extra cold days. The Hitachi air conditioner cassette is quiet, light weight, easy to install and economical also to operate.

Installation of Room Conditioner :

Since the window air conditioner uses an air-cooled condenser to cool the hot refrigerant, the cooler the condenser, the more efficient is its operation. If the window unit is exposed to the sun’s rays the heat of radiation and warm air can diminish the efficiency of the exposed condenser and reduce the capacity of the unit. It is thus advisable to install the window unit for a minimum of sun exposure. The order of preferential exposure for optimum performance is : north, east, south and lastly west, of course, the shape of the room relative to air circulation patterns, location of the windows and the location preferred by the occupants often overrule the above consideration. In installing the conditioner, good air pattern in the room should be planned to establish comfortable condition satisfactory to the occupants. The following points should be kept in mind during the installation of conditioner.

1) Whenever the unit is exposed to the sun, a shade should be provided such as an awning.

2) It should be carefully seen that louvers of the outer cabinet are not blocked up or restricted by a brick block or by the wall itself.

3) The air-conditioner should slope toward the outside about 1.5 cm/m. This ensures the flow of condensate water from the evaporator back to the singer ring on the condenser fan blade or to the drain hose.

The Thermostat control


The pressure actuated type thermostat uses a bulb containing gas which expands or contracts with variation in room temperature. This expands or contracts a bellow which, in turn, opens or closes an electrical contact and the compressor operation is automatically controlled to maintain a constant temperature. Thus, if the room air returned to the unit becomes cooler than the thermostat setting, the gas in the bulb contracts, causing the bellows to contract. The electrical circuit opens and the compressor stops. When the room air temperature goes above the thermostat setting, the gas in the bulb expands, the bellow expands, the electrical contracts close and the compressor starts. In this way, the room air-conditioner maintains the temperature desired. The room occupants need adjust the thermostat control to a new position when they wish to change the desired, temperature.

A second type of room thermostat is a bi-metal temperature running device in place of the gas filled sensing tube. If two metals are joined together in a bimetallic strip and then heated, the varying rates of expansion of the metals cause them to bend. This bending in response to heat or cold is used to close or open an electrical contact. This contact is wired in series with compressor relay and its controls compressor operation in the same manner as does the gas-bulb thermostat described above.

Electric Controls :

The electric controls are placed either on the top, side or front of the unit for convenience and style. Most units have two main electrical controls (1) a master control that starts and stops the fans and compressor, the occupant uses this control to choose the operation desired (cool, exhaust, heat or ventilate) and to select fan speed, (2) a thermostat control that adjusts the thermostat to cooler or warmer side.

The master control.

The master control is often called the selector switch or cooling and fan control. It controls the compressor motor, condenser fan motor and evaporator fan motor. When the control switch is in its COOL position, all motors operate and the complete unit functions as an air-conditioning unit. When the control switch is in the VENTILATE position, the evaporator fan works as ventilating fan. In the EXHAUST position, the condenser-blower motor operates drawing stale air from the room as does an exhaust. The damper control must be at its similar position to allow the air to flow either out or in.

The Thermostat control.

The thermostat element in room air-conditioners is usually located in the returned air stream near the filter. The thermostat control is wired in the series with the starting relay that controls the compressor motor. Thus both the thermostat control and master control govern compressor operation.

Air-distribution System :

The evaporator fan used in the room unit is of the centrifugal or propeller type and is surrounded by acoustical insulating material that reduces operating noise. The evaporator fan pulls in fresh outside air through louvers and a ventilation damper. Room air enters at the room end of the air-conditioner through return louvers in the cabinet. The fresh air and room air mix and are drawn through the evaporator cooling coil. The cooled air is then supplied to the room through the discharge grills in the front of the cabinet. A one-ton unit circulates air at 40 m3/min in the room. If 25% fresh air and 75% recirculated room air are used then one ton unit results in 1.3 air changes per hour for a room of 40 m3 volume.

Air should be filtered as soon as possible after it enters the air-conditioner. In the window unit, the air filter is located on the air entering side of the cooling coil. Either permanent or replaceable filters are used. The permanent filter is a metallic mat or plastic foam type. It is coated with odourless mineral oil that helps in catching dust particles. It may be cleaned as needed by washing in soap flakes. Replaceable filters are made of spun glass or similar materials and are also coated with oil or other suitable adhesive. They may be easily removed from the unit and replaced by a new filter.

The fresh air intake to the room conditioner is regulated by a dampered door inside the cabinet. Most units operate with from 0 to 25% fresh air, depending upon the damper-control knob adjustment within the ventilate position. On hot days, the proportion of fresh air intake should be decreased, because the more fresh hot air, the greater the cooling load on the unit.

To exhaust room air, the fan control is set to EXHAUST position. The condenser blower starts operating when this is done. The exhaust damper musrt also be open to permit room air through to the outside and therefore the damper control is also set to its exhaust position. With these controls are in this setting, the room air-conditioner acts as an exhaust fan. In this way, smoke and odours are removed from the room by the condenser fan which draws air through the damper and exhausts it through the louvers in the rear of the unit.

The air system on the outside includes in the condenser fan and dissipates the heat taken from the cooled area to the outside atmosphere. The air system within the cooled area circulates room air over the cooling unit. These two air systems are so arranged that the cooling coils are located on the room side of the window and condensing units on the outside of the window. It is important that no intermingling of hot condenser air and cooled room air takes place. A partition inside the air conditioner separates the room air from the condenser air. The rubber seals and panels at the window prevent exchanges of inside and outside air. Tight installation at the window is of utmost important.

Refrigeration Cycle :

The compression refrigeration cycle which is described earlier is also used in air-conditioner. The basic components are compressor, condenser, capillary and evaporator in addition to filters, accumulators, motors, fan and controls. High pressure, high temperature refrigerant gas coming out of compressor is condensed in the condenser using outside air as coolant. The liquid refrigerant is further passed through capillary to reduce the pressure and then it is passed through the evaporator. The air from the room is passed over the evaporator where it is cooled to require temperature and then discharged in the room itself. The evaporator refrigerant vapour coming out at lower pressure from the evaporator is compressed in the compressor and cycle is repeated. The arrangement of the refrigeration system using a sealed unit in air-conditioner is shown in figure..

The window-unit compressor is of the hermetic type. This kind of unit has the compressor, its motor, and all moving parts operating within a sealed gas tight housing which is welded and closed.

The condenser coil is a continuous coil of copper tubing to which aluminium fins are bonded to increase the heat transfer surface area. A propeller type condenser fan is located immediately in front of the condenser coil. This fan cools the condenser coil and also exhausts air from the air conditioned space when the exhaust air damper is opened. Moisture condensed on the surface of the evaporator coil during dehumidification of the air, is also exhausted by the condenser fan.

The copper capillary tube is 40 cm long with an inside diameter of 0.75 mm. The dimensions are based upon the desired pressure difference between evaporator and condenser.

The evaporator is located at the front end of the room air-conditioner. It is also made of copper tubing and has aluminium fins bonded at right angles to the tubing to increase the heat transfer area. For one kW unit, the evaporator and condenser coils have a total surface area of 7.5 and 14 m2 respectively. This larger surface area for condenser coil is necessary because, in addition to the heat which entered through the evaporator, heat resulting from the compressor action also enters the refrigerant and this total must be discharged.

Introduction To Room air-conditioner:

  • The window unit is the simplest example of the refrigeration cycle used in a packaged air-conditioner. In the unit, cooling, filtering and air-distribution systems are combined in a compact package which has obtained worldwide popularity.
  • The manufacture and use of packaged air-conditioners is one of the big steps in the development of air-conditioning. Mass produced compact air-conditioners in completely self-contained packaged units not only lowered the equipment cost but also made installation, servicing and operation simple and inexpensive . In this way, packaged units have brought air-conditioning to widespread public use and made available to the public in the quickest, most convenient and least expensive way. These packed units work very satisfactorily as long as the air-conditioner is properly selected, installed and serviced.
  • This article details the operation, installation and servicing of packed type room air-conditioner. Room air-conditioners include packaged window units, wall units and portable units. The components, specifications and operation of window and wall units are similar. The position of the components and control in the packaged window and wall unit lies in the installation only.