Thursday, July 31, 2008

Balancing_Machine_Drives

Balancing machines typically employ one of following three different drive configurations 
to spin the rotating assembly: 
  • Direct end drive 
  • Wrap-around belt drive 
  • Tangential belt drive 

Direct end drive: 
The direct end drive or universal joint connection is typically used with rotors having 
large moments of inertia or high windage losses. This drive design will transmit high 
torque forces for fast acceleration and safer braking. To attach the drive shaft, the rotor ends must be prepared to accept the U-joint directly or with an adapter. With this design, the drive system becomes a part of the rotor and must be considered in the balancing accuracy of the system. Before this type of the drive can be used to accurately balance a workpiece, the U-joint assembly, itself must be balanced. The desired end result is to be able to rotate the U-joint assembly 180 degrees without any variance in the balancingmachine readout. 

Wrap-around belt drive: 
For rotors weighing below 2250 kilograms and with at least one smooth surface, a wrap- around belt drive works very well. For this drive configuration, 2250 kilograms is about the maximum rotor weights that will allow adequate torque transmission to bring the rotor up to the required balance speed. A 180-degree wrap is required to provide adequate torque transmission to rotate the rotor and keep the applied torque in the vertical plane of the balancing machine pedestal. A belt drive of this type is considerably more accurate then a direct coupled end drive in that the drive mechanism does not influence the workpiece balance. 

Tangential belt drive: 
A tangential belt drive, either under the rotor or in an over-arm configuration, is frequently used in smaller capacity, high volume production oriented balancing machines for rotors weighing less than 450 kilograms. This type of drive configuration is used to bring the rotor up to the required balancing speed and the then the drive arm is moved away from the rotor and the unbalance data is collected.

Tuesday, July 29, 2008

gas turbine

FOUR MAJOR COMPONENTS OF A GAS TURBINE ENGINE:
1. Compressor
2. Combustor
3. Turbine
4. Accessory Drive Assembly

COMPRESSOR:
->Function: Provides required air mass at the appropriate pressure to burn the required amount of fuel and to control combustion temperature
->Two types
–Axial Flow
–Centrifugal or Radial Flow


radial flow compressor

axial flow compressor






Axial flow v. Radial flow:
*Centrifugal compressors are simple, inexpensive, lightweight, and have a high pressure rise per stage
*Centrifugal compressors experience large inter-stage losses and require a large frontal area; they are typically less efficient than multistage axial compressor
*Multistage axial compressors can achieve larger compression ratios and are better suited for high-power marine applications

Uses of Compressed Air:
*PRIMARY AIR
–Typically 30% of all compressed air
–Passed directly to combustor, mixed with fuel, and burned
*SECONDARY AIR
–Approximately 70%
–Passes through holes in inner shell and mixes with combustion gases
–Two purposes
•Places an air film between the inner shell and combustion gases to prevent overheating of the inner shell
•Cools combustion gases to an acceptable inlet temperature for the turbine
*FILM AIR
–A small percentage of compressed air may be used to cool turbine blades


COMBUSTION CHAMBER:
*Function: mixes fuel and air and burns this mixture to produce hot combustion gases
*Consists of a casing, perforated inner shell, and fuel nozzles
*Arrangement
–Annular
–Can or Tubular
–Can-annular

TURBINE:
*Develops shaft rotational energy from the kinetic energy of the hot combustion gases entering through the vanes
*Usually of axial flow design
*Drives the compressor and various engine accessories
*The remaining useful energy can be used as jet thrust or shaft mechanical work

Turbine Construction:
*STATOR
–Stationary guide nozzles (vanes) discharge gas at high velocity onto the moving blades
–Attached to turbine casing
*ROTOR
–Consists of a shaft and bladed wheel (disc)
–Attached to the main power-transmitting shaft




Stator Rotor

ACCESSORY DRIVE ASSEMBLY:
*Provides the space for mounting and the motive force for driving the accessories required for the operation and control of the gas turbine engine
*May be used to drive the fuel pump, lube oil pump, etc.


Advantages:
*Weight reduction of 70% when compared to a steam plant of comparable horsepower
*Simplicity
*Reduced manning requirements with more highly automated equipment
*Quicker response time
*Faster acceleration/deceleration
*Modular replacement
*More economical

Disadvantages:
*Many parts under high stress
*High pitched noise
*Needs large quantities of air
*Large heat source

Turbine Classification

Turbines as fundamentally classified as “Impulse” or “Reaction” type by how the steam
expands through a nozzle and impact a blade. Impulse Stages are often compared with
waterwheels, reaction stages to a rotary lawn sprinkler. There are different ways to classifythe turbine based on:

A. By the action of steam:
a. Impulse
b. Reaction
c. Impulse and reaction combined
B. By the number of step reductions involved:
a. Single stage
b. Multi-stage
c. Whether there is one or more revolving vanes separated by stationary reversing vanes.
C. By the direction of steam flow:
a. Axial
b. Radial
c. Mixed
d. Tangential
e. Helical
f. Reentry
D. By the inlet steam pressure:
a. High Pressure
b. Medium pressure
c. Low pressure
E. By the final pressure
a. Condensing
b. Non-condensing
F. By the source of steam:
a. Extraction
b. Accumulator