1) Belt Conveyor (Production Line)
Previous Answer
This
type is available in two common forms: flat belts for pallets, parts or even
certain types of bulk materials and troughed belts for bulk materials.
Materials are placed on belt surface and travel along the moving pathway. The
belt is made into a continuous loop so that half of its length can be used for
delivering materials and the other half is the return run (usually empty).
The
belt is supported by a frame that has rollers or other supports spaced every
few feet. At the each end of the conveyor ( where the belt loops back) are
driver rolls (pulleys) that power the belt.
(Computer
Aided Design And Manufacturing, Lalit Narayan Et Al., p.529)
New/Better Answer
Belt
conveyors are most suitable for handling loose bulk materials like soil, earth,
gravel, sand and crushed materials. Being continuous in operation, conveyors do
not suffer from cycle-time constraints and idle time in operation. They are
idle only when they are not in operation or when there is a breakdown or when
there is no feed on the running conveyor.
The
general construction of a belt conveyor consists of a continuous belt running
over two terminal pulleys, one of which is driven by a motor, called the head
pulley, and the other, called the tail pulley, rotated by the belt friction.
The material to be transported is fed at one end, normally the tail end and
transported by the travelling belt, and discharged at the motor-driven pulley
or the head pulley. The whole conveyor unit can be mobile, that is, mounted on
wheels for easy transportation from one place to other as required.
Belt
conveyors, when they are used for bulk materials, are troughed depending on the
troughing angle of the idler sets between the two terminal pulleys. The idler
sets, arranged at a predetermined spacing, support the load carrying belt and
prevent it from sagging and stretching between the two terminal pulleys. The
troughing angle increases the cross-sectional area of the material being
carried on the belt and, hence, the capacity of the belt for unit time. Belts
come in the standard widths of 250, 300, 400, 500, 630, 800, 1,000 and 1,200
mm, and so on. They are made of layers of cotton or synthetic material called
plies embedded in rubber. The troughing angle of idler sets is normally 20
degrees which is the angle of the axes of the side rolls to the horizontal.
Belt conveyors are specified by their carrying capacity, for example, 50 tonnes
of gravel per hour, the belt width in mm, and the troughing angle in degrees.
(Kamaraju Ramakrishna, Essentials of Project Management, p.268)
2) Photodiodes (Type of Photodetectors)
Previous Answer
A photodiode consists of a back-biased p-njunction which, under dark
conditions, behaves as a normal back-biased diode. With these conditions the
only current flowing through the diode will be the leakage current (typically 1
μA).
Absorbed light will generate electron/hole pairs, and the current through
the diode will increase to a typical value of 100 μA. A photodiode has the
response that it can be considered as a constant current device with the
current determined by the light intensity.
The current/intensity relationship is quite linear and the response is
fast; typically 0.2 μs but devices as fast as 1 ns are available. In general,
photodiodes are the smallest optical sensor which, in conjunction with their
high speed, makes them well suited for fibre optic data transmission and
similar applications. Typical operating wavelengths are 8000Å to 11000Å
(silicon) and 13000Å to 20000Å (germanium).
The relatively low level current can easily be converted to a high-level
voltage using a DC amplifier. The light-dependent diode current flows through R
to give an output voltage IR which is directly related to light intensity.
(Parr E.A., Industrial Control Handbook, pg.192, Kayra Ermutlu)
New/Better Answer
Photodiodes
arc semiconductor light sensors that generate a current or voltage when the p-n
junction in the semiconductor is illuminated by light. The term photodiode
usually refers to sensors used to detect the intensity of light. Photodiodes
have no internal gain but can operate at much higher light levels than other
light detectors. In contrast Avalanche Photodiodes (APD) do have internal gain.
The materials used to realize the photodiodes are:
Silicon:
It is now the most widely used material for photodiodes. The wavelength range
is about 200 — 1100 nm with a peak near 850 nm, at which the spectral
responsivity is up to 0.65 A/W and the quantum efficiency is close to 100%
Germanium:
The wavelength range of these junction diodes extends further into NIR to about
2 pm. The responsivity wavelength (1.4 pm) is 0.66 A/W, which corresponds to a
quantum efficiency of about 82%.
Other
materials used are InGaAs, InAsCdTe, GaASP. Often these detectors are labeled
according their structure: p-n, p-i-n. The terminology of these detectors has
undergone several changes and it is ambiguous due to the ability of the
junction detector to serve as photovoltaic or as photoconductive device. In
photovoltaic mode no bias is applied, and under irradiation the photodiode
generates a voltage of a certain polarity that may drive a current through an
external circuit. In the photoconductive mode, an external bias of a polarity
opposite to that of the unbiased mode is applied. Consequently, the current
also flows in the direction opposite to that of the unbiased mode. The signal
appears as voltage drop across the load resistor RI. Following Palmer's
suggestion (1980), photovoltaic mode corresponds to unbiased sensor, while
photoconductive correspond to biased sensor.
(Giancarlo C. Righini,Antonella Tajani,Antonello Cutolo, An introduction to optoelectronic sensors,p.501)
3) Ultrasonic Transducer (Non-Destructive Testing)
Previous Answer
The mechanical
construction of a typical ultrasonic transducer used in contact testing is
shown in Fig. 1.7. A very thin (app. 100µm thick) piezoelectric crystal is
plated on both faces; it is attached through a small electrical network
contained in the transducer housing to the external BNC or microdot of the
transducer. Since the crystal is very fragile, a ceramic wear plate protects
the front face of the crystal, as shown. The back face of the crystal is attached
to a layer of epoxy loaded with tungsten particles. This backing acts as a
highly attenuating medium that controls the shape and duration of the pulse.
There are actually two
types of contact transducers. They are distinguished by the types of motion generated
in the crystal when excited by a voltage pulse and the corresponding types of
motion subsequently present in the ultrasound beam launched from the transducer
into the part. Figure 1.8(a) shows a contact P-wave transducer with the crystal
excited in a mode that causes its thickness to expand and contract normal to
the surface, thereby producing a wave with similar motions that is called a
P-(pressure) wave. Figure 1.8(b) in contrast shows S-wave transducer with the
crystal excited in a shearing type of motion, thereby producing an S-wave
(shear) wave.
(Lester W. Schmerr, Fundamentals
of ultrasonic nondestructive evaluation: a modeling approach, p.6)
New/Better Answer
Ultrasonic
waves can be generated and detected in a number of ways. The one which is most commonly
used in NUT is described here. Quartz and some other crystals have a lattice
structure such that if a plate is cut out of the crystal with a certain
orientation with respect to the crystallographic axes, and subjected to an
electric field in the right direction, it will change its dimensions: it will
contract or expand according to the polarity of the field. Conversely, when a
similar deformation of the plate is brought about by an external mechanical
force, electric charges appear on its opposite surfaces. This phenomenon is
known as piezoelectric effect. The materials which exhibit this property are
known as piezoelectric materials.
Among
the various naturally occurring piezoelectric materials, quartz is the most
important one, because it combines reasonably good piezoelectric properties
with excellent mechanical and dielectric strength and stability. X-cut quartz
plate is used for generating and receiving longitudinal waves. \'-cut plate is
used for generating transverse and surface waves in solids. Quartz transducers
can be operated at high temperatures up to 773K. A multitude of materials
exhibiting piezoelectric properties are now available, each material having
characteristics which suit to particular applications. Besides naturally
occurring crystals like quartz, chemical compounds, such as lithium sulphate,
lead niobate etc., and specially produced polycrystalline ceramics such as
Barium titanate and lead zirconate titanate (PZT) are used for ultrasonic flaw detection.
These transducer materials are mechanically less resistant. Lithium sulphate is
the most sensitive but barium titanate is the best transmitter. Because of its
higher acoustic impedance, the matching of barium titanate is always
unsatisfactory and its sensitivity cannot be fully exploited. Lead metaniobate
and lithium sulphate are far superior in this respect. Again because of their
low acoustic impedance and high intrinsic internal damping, they are most
suited to produce short pulses as is required in pulse-echo technique.
The
transducers (piezoelectric crystals) cannot be used on their own, but have to
be mounted as suitable probes. The role of the probe is to protect the operator
from electric shock, to protect the transducer from mechanical damage, and to
make the transducer more suitable for the job. Various types of probes are made for different applications. Normal beam transducers are used for
testing by using waves at normal incidence. For under-water testing, the probe,
especially the cable, must be waterproof. For good performance, the transducer
impedance should be matched to that of the water. For very short range
operation, a twin probe is needed with separate transmitter and receiver probes
built into one housing and acoustically isolated from each another. There is an
acoustic delay rod, also called as stand-off, in front of both.
(Baldev Raj,T. Jayakumar,M. Thavasimuthu, Practical non-destructive testing, p.82)
4) Scleroscope (Hardness Test Method)
Previous Answer
The
Scleroscope test has the distinction of being the first commercially available
metallurgical hardness tester produced in the United States. The instrument
continues to be used extensively in selected applications.
The
test consists of dropping a diamond hammer, which falls inside a galss tube
under the force of its own weight from a fixed height, onto the test specimen
and reading the rebound travel on a graduated scale. The height of the fall is
250 mm. The hammer is a little less than 6 mm in diameter, 19 mm long, and
weights about 2 g. Te shape of the diamond is slightly spherica and blunt with
a diameter of approximately 0.5 mm.
(Hardness
testing, ASM International, p.91)
New/Better Answer
The
Scleroscope hardness test is essentially a dynamic indentation test wherein a
diamond-tipped hammer is dropped from a fixed height onto the surface of the
material being tested. The height of rebound of the hammer is a measure of the
hardness of the material. The Seleroscope scale consists of units that are
determined by dividing the average rebound of the hammer front a quenched (to
maxi-mum hardness) and untempered water-hardening tool steel into 100 units.
The scale is continued above 100 to permit testing of in having hardness
greater than that of fully hardened tool steel. Scleroscope hard-ness testing
can be conducted rapidly, and some testing instruments are portable so that
they can be used for testing large work pieces that would be difficult to bring
to the tester.
(Jon L. Dossett,Howard E. Boyer, Practical heat treating, p.36)
5) Discontinuous Chip (Metal Machining)
Previous Answer
When
relatively brittle materials (e.g. cast irons) are machined at low cutting
speeds, the chips often form into separate segments ( sometimes the segments
are loosely attached). This tends to impart an irregular texture to the
machined surface. High tool-chip friction and large feed and depth of cut
promote the formation of this chip type.
(Fundamentals
of Modern Manufacturing: Materials, Processes, and Systems, Mikell
P. Groover, p.491)
New/Better Answer
The
chips are small individual segments which may adhere loosely to each other. The
chips are produced as the tool advances in the direction of the feed, due to
plastic deformation of the material ahead of the tool nose and in the vicinity
of the cutting edge. The reason for generation of such chips is that as the
material gets ahead, due to advancement of the tool it ruptures intermittently,
thus producing segmented or discontinuous chips (refer to Fig. 5.6).
Conditions
favouring discontinuous chip formation
(i)
Brittle
and non-ductile work materials such as cast iron, brass castings, etc.
(ii)
Small
or negative rake angle
(iii)
Low
cutting speed
(iv)
Dry
cutting (cutting without application of cutting fluid)
(v)
Large
chip thickness, i.e., large depth of cut and high feed rate.
Characteristics
(i)
Easy
handling and disposing off due to its size
(ii)
Good
degree of surface finish as they do not interfere with the work surface
(iii)
More
tool life
(iv)
Less
power consumption.
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