The bearings can be one of the two types, a plain bearing (sliding contact) or an anti-friction bearing (rolling bearing), depending on the design parameters of the machine element, each of the two types of bearings, plain and anti-friction, is available for design with linear motion, radial loads and axial loads.

Bearings may be classified into three general classes
Guide or flat bearings, which support linear motion in machine tables and slides.

Thrust bearings, which support rotational motion in machine elements that have axial loads i.e., the load is applied along the central axis of the rotating shaft

Radial bearings, which support rotational motion in shafts with radial loads i.e., the load is applied along the radius of the rotating shaft.

Anti Friction or Roller Element bearings

Anti – friction bearings or roller – element bearings, as they are often called, use a rolling element (ball or roller) between the loaded surfaces.
Anti-friction bearings are divided into two categories,
a) ball bearings
b) Roller bearings.

Ball bearings have five general types:
Guide, Radial, Thrust, Self – aligning and Angular contact.

Roller bearings have four general types: Cylindrical, Thrust, Spherical and Taper.

Roller and Ball bearing types
Guide bearing: The ball guide bearing is used for linear motion where very low co-efficient of friction and extreme smoothness in operation are desired.
Radial bearing: The first radial bearing is the single – row, deep – groove ball bearing, most widely used anti – friction bearing. Second radial bearing is the cylindrical roller bearing is capable of carrying larger radial loads at moderate speeds than those carries by radial ball bearings using the same size bearing.
Thrust bearing: First the ball thrust bearing is designed for axial (thrust) loads only – no radial loads. Second spherical roller thrust bearing is capable of very heavy axial loads as well as moderate radial loads.
Angular contact ball bearing: The shoulders in this provides for thrust (in one direction only) that is larger than the single row, deep radial ball bearing can handle.
Taper roller bearing: A pair of taper roller bearing is capable of handling both very large axial and radial loads.

A compound motor is a combination of shunt and series motor i.e., a series field winding, wound with heavy copper conductor on top of the shunt field winding. The series field winding is connected in series with the armature. So that its mmf will be proportional to the armature current and in the same direction as the shunt field mmf

Typical compound motors designed for industrial application obtain approximately 50% of their mmf from the series field wen operating at rated load.

There are two types of compound motors connection,

If the connection to the series and shunt winding is in such a way that their respective mmfs are additive is called cumulative compound motor.

If the series field is reversed with respect to the shunt field, its mmf will subtract from the shunt field mmf, causing the net flux to decrease with increasing load, resulting in excessive speed, which is differential compound motor.

Stabilized - shunt motor
Compound motors, whose series field are designed to provide just enough mmf to nullify the equivalent demagnetizing mmf of armature reaction and provide a very slight speed droop, are called stabilized – shunt motors. The series field winding of such machines generally have one – half to one and half turns / pole and depending on the application, provide approximately 3 to 10 percent of the total field mmf at rated load. The speed of stabilized – shunt motors is fairly constant, with only a slight droop in speed with increasing load. Stabilized – shunt motors are used in applications that require a fairly constant speed and a moderate starting torque.
Reversing the direction of rotation of compound or stabilized – shunt motors is accomplished by reversing the armature branch or reversing both the series field and the shunt field.

Torque in DC
The direction of the developed torque may be determined from an end view of the conductors and magnet poles. The direction of flux due to the known direction of current was determined by the right – hand rule and the direction of the mechanical force on each conductor, due to the interaction of the magnetic fields, was determined by the flux bunching effect.

BP = Flux density in air gap produced by shunt field poles (tesla)
IA = Armature current (ampere)
KM = Constant
Constant KM depends on the design of the motor and include the number of turns, effective length of armature conductors, number of poles, type of internal circuitry and units used.
The torque developed by a DC motor is proportional to the flux density in the air gap and the current in the armature.

Torque in AC
The torque developed by AC motors has two components: A Reluctance – torque component and a magnet – torque component. The reluctance – torque component is due to the normal characteristic of magnetic materials in a magnetic field to align themselves so that the reluctance of the magnetic circuit is minimum

The magnet – torque component is due to the magnetic attraction between the field poles (magnets) on the rotor and the corresponding opposite poles of the rotating stator flux.
It is also justified for salient – pole motors operating from 50 percent rated load to above 100 percent rated load, with power factors ranging from unity to leading, the reluctance torque for such loads is significantly smaller than the magnet torque.

Direct current (DC) generators are machines that convert mechanical energy into electrical energy. This conversion of energy is based on the principle of the production of dynamically induced electromotive force.

Types of DC Machines
Homo polar machines: These types of machines are used where low voltage and high currents are required e.g., Faraday’s disc dynamo
Hetero polar machines: The DC machines that are commonly used fall under this category.

The type of generator used in welding is homo polar machine. Differential compound generator also belongs to homo polar machines. The generator is so designed that it delivers a voltage high enough to start the arc and reduce the voltage as required to maintain the arc during the welding.
Unlike AC, DC flows continuously in one direction from a negative charge to positive charge. Although DC flows only one way, you can manipulate it to flow in the appropriate direction, which is its polarity.

DC welding is preferred when using high welding speeds and when welding is out – of – position.

Fatigue refers to the stress caused by cyclic loading of components.  The effect of Fatigue is particularly pronounced in components which are stiff and thick. 

One of the chief causes of fatigue is the frequent  starting and stopping of the boiler.  When the boiler is started and stopped, the temperature gradient causes the expansion and contraction of the components. Fatigue can also result in cracks in welds.   Some of the areas particularly vulnerable to fatigue are the pipe to drum connection, fitting to tube, the welding in the support attachments, etc.