Mech 1200 Final Review
Mech 1200 • Some types of maintenance techniques: • Preventative maintenance: Performing scheduled and unscheduled tasks on an equipment for optimization and preventing failure. • Predictive maintenance: condition monitoring of an equipment in comparison to a preset standard or baseline. • Proactive maintenance
Mech 1200 Examples of Predictive maintenance • Vibration analysis: Analysis of the oscillatory motion in a cyclic manner. • Oil Analysis: Investigation of the physical properties and presence of contaminants in oil samples. • Thermography: Analysis of the temperature of an equipment at different operating conditions.
Mech 1200 Lockout Requirements • Use a positive means to keep the energy-isolating device in the safe position. • The lockout device must be able to withstand the environment it is exposed to, such as excessive heat, radiation, freezing..etc. • The lockout and tagout devices must be standardized in the facility in size and/or colorand/or shape. • The lockout device must be strong enough to prevent removal without excessive force. • Notification of employees before the application and after removal of the lockout/tagout. • Only the employee who applied it can remove it.
Mech 1200 TagoutRequirements • Must contain a clear warning. For example: Danger. • Must state clearly that moving the energy-isolating device from the safe mode is not allowed. • Must be placed in the place a lock device would be placed, or if not possible, as close as safely possible in a position obvious to anyone who attempts to operate the device. • Must be able to withstand the environment. • Must be standardized in the facility. • Must have a nonreusable type of attachment. For more Lockout/Tagout requirements, please read pages 27 and 28 of the text.
MECH1200 Topics: • Introduction to Chapter 3: Mechanical Power Transmission Fundamentals • Energy • Force • Inertia • Acceleration • Friction • Work
MECH1200 • Usually mechanical systems consist of the following elements: • A prime mover: such as en electric motor or an internal combustion engine. • Linking components: such as shafts, gears, belts, joints..etc. • Driven components such as wheels.
MECH1200 Newton’s Laws of Motion Newton’s First Law (Law of Inertia): A body in rest will stay in rest and a body in motion will stay in motion unless an external force is acted upon it. Newton’s Second Law (conservation of momentum): The net force acting on an object is equal to the rate of change of its momentum. Forcenet = mass × acceleration Newton’s Third Law (action-reaction law): For every action there is a reaction equal in magnitude and opposite in direction. FA = - FB
MECH1200 Force Force: causes an object that has a mass to change velocity. • A vector quantity: It has direction and magnitutde. • Some common units of force: Newton (N) Pound force (lbF) = 4.45 N
MECH1200 Inertia • Inertia: the resistance of an object to any change in its motion state. • Example 1: Object A (1 kg) and object B (10 kg) are each acted upon by a force of 1 N to the right. which would move at a higher acceleration? A 1kg 1 N B 10 kg 1 N Object B has a larger inertia, thus more resistance to change in its rest state, so it has a lower acceleration.
MECH1200 Angular Speed • Angular speed: a measurement to describe an object rotating about an axis. • Units of measurement: • RPM • Rad/s Surface velocity (ft/min) = RPM× π × diameter (ft) = RPM ×0.262 × diameter (in inches) Example 4: what is the surface velocity of a 20 in diameter rotor that rotates at 1500 rpm? Solution: = 1500 × 0.262 × 20 = 7860 ft/min
Viscosity • Viscosity: A measure of the fluid’s resistance to flow. • The best lubricant is that which has the lowest viscosity possible to maintain a film that separates the metal parts. • Viscosity Index (VI): a measure of the change in viscosity with temperature. As VI increases, the less is the change of the lubricant’s viscosity with temperature. • Kinematic viscosity: is the measure of inertial force to viscous force. • Units of kinematic viscosity are: • m2/s (the SI unit) • the Stoke (1 m2/s = 10,000 stokes ) • Saybolt Universal Seconds (SUS) Source: Wikimedia Commons.
Gear Drives MECH1200 The pitch diameter is the diameter of the pitch circle and is used to calculate the speed and torque to the driven shaft. The pitch circle is only significant in determining the pitch diameter.
Gear Drives MECH1200 Pressure Angle: The angle at which power is passed from the tooth of one gear to the tooth of the other. Commonly pressure angle is 20o Backlash: the space between the non-driving sides of the adjacent teeth of two meshing gears. Backlash prevents gear binding and wearing. Root Clearance: the radial distance between the top of the tooth of the pinion and the bottom of the tooth of the gear. Root Clearance
Gear Drives MECH1200 Diametral Pitch - The ratio of the number of teeth on the gear to the pitch diameter It indicates the relative size of the teeth on the gear. Two gears must have the same diametral pitch in order to mesh Determine if gears of different diameters or different numbers of teeth have the same size teeth and can mesh properly Common sizes are 3 - 48 DP PD –Diametral Pitch – Number of Teeth - Pitch circle diameter (inches) Gear Module (metric) =
Gear Drives Idler Gears MECH1200 • Idler gears are inserted between input and output gears • They serve to reverse the direction of rotation of the output gear. • Idler gears DO NOT affect the gear ratio between the input and output gears. Idler Gears
Gear Types 1. Spur Gears 2. Helical Gears 3. Double Helical Gears 4. Herringbone Gears 5. Bevel Gears 6. Worm Gears
MECH1200 Chain Drive • 3 Basic Parts: • Chain • Driver Sprocket • Driven Sprocket The Relative number of teeth between the driven sprocket and the driver sprocket determines the speed and torque of the driven shaft Ratio of the teeth can be selected to increase or decrease speed or torque to the driven shaft
MECH1200 Four common types of chains
MECH1200 Belt Drive Terminology Driven Sheave (Pulley) Arc of Contact Driver Sheave (Pulley) Belt Tight side T1 Center Distance Slack side T2 Source: Drawing is in the public domain and was taken from Wikimedia Commons, Wikimedia Foundation
MECH1200 Belt Terminology Effective Tension (Te): Te = T1 – T2 Total Tension: Ttotal = T1 + T2 Power: Drive Ratio (also called Pulley Ratio):
MECH1200 Types of V-Belts Fractional Horsepower V-belts: used for light duty applications below 7.5 Hp. Classical V-belts: standard v-belts, used in heavy duty applications. Notches may be added to the belt to reduce bending stress. How to identify classical V-belts? • Cross sections are designated as A, B, C, D, and E. • Belt length measured on the circumference is designated numbers following the letter prefix • Notched V-belts are identified by an “X”
MECH1200 Synchronous Belts • Also called Timing Belts, positive drive belts, and gear belts. • Slip is not allowed in synchronous belt drives • If jamming occurs the V-belt will slip, but the synchronous belt will shear off.
MECH1200 Belt Sheaves
MECH1200 Introduction • Electric Machinery: - Electric Motors: convert electrical energy into mechanical energy. - Electric Generators: convert mechanical energy to electric energy. • The Electric machine is composed of: Stator Rotor Image is in the public domain. Source: Wikimedia Commons, Wikimedia Foundation
MECH1200 Principles of Operation of Electric Motors • Faraday’s Law is the principle of operation of electric machines. • There are three methods by which electric motors work: • Having a stationary magnetic field in the stator and an alternating magnetic field in the rotor. - This method is employed in all DC motors except for the Brushless DC • Having a rotating magnetic field in the stator and a stationary magnetic field in the rotor. - This method is employed in synchronous motors, Stepper and Brushless DC Motors • Having a rotating magnetic field in the stator and an induced magnetic field in the rotor. - This method is employed in induction motors.
MECH1200 Types of Electric Machines Electric Machines DC Machines AC Machines Special Motors Permanent Magnet DC (PMDC) Motor Stepper motor Induction Machine Synchronous Machine Shunt Motor 3Ph Wound Rotor Brushless DC motor Separately Excited motor 3Ph Squirrel Cage Hysteresis motor Series Motor Reluctance motor Capacitor Start Compounded Motor Universal motor Split Phase Capacitor Run –Capacitor Start
MECH1200 Construction of DC Machines • Field Windings: In DC machines, this is the winding in the stator. Note: Permanent magnet dc motors do not have field winding but rather a permanent magnet. • Armature Windings: The windings in the rotor . • Commutator segments: the method to convert the dc current to an alternating current in the rotor of the dc motor is called commutation. They are connected to the rotor windings, and stay in contact with the brushes. • Brushes: pieces made of carbon or graphite and pushed against a spring to maintain electrical contact with the commutator segments. • Interpole windings: A third set of windings are mounted on the stator, and connected in series with the rotor to reduce the sparking between brushes and the commutator.
MECH1200 • Name the type of each of the motors A, B, and C shown in the figure below: A: Shunt motor B: Series motor C: Compound motor
Review: Principle of Operation of DC and AC Synchronous Machines MECH 1200 DC Machines AC Synchronous Machines Stator magnetic field: Stationary Rotor magnetic field: Alternating Stator magnetic field: Rotating Rotor magnetic field: Fixed (stationary w.r.t. rotor) N N S N N N N S S S N N S S S S N S
MECH 1200 Construction of AC Machines • Induction motor: • Stator: a rotating magnetic field exists in the stator windings. The stator windings are connected to a three phase power supply (except for single phase motors). • Rotor: An induced magnetic field exists in the rotor of a squirrel cage induction motor. • Synchronous motor: • Stator: a rotating magnetic field from a three phase power supply. • Rotor: A fixed magnetic field in the rotor from a dc power supply. Therefore, it needs brushes, but no commutation as in dc motors. Rather, slip rings that stay in contact with the brushes are used. Note: Wound rotor induction motors require slip rings as in synchronous motors.
MECH 1200 Terms • Synchronous speed: the speed of rotation of the magnetic field in the stator. • Mechanical speed: this is the actual speed of the rotor • Slip: the ratio of the difference between synchronous and mechanical speeds to the synchronous speed. • Example: a) what is the speed of rotation of the magnetic field in a 3-phase induction motor that has 2 poles? b) What is the speed of the motor if the slip is 5% ? • Solution: synchronous speed = 120×60 / 2 = 3600 RPM Motor speed = (1-0.05) × 3600 = 3420 RPM
Induction Motor Speed and Torque Relationship MECH 1200 • Locked rotor torque: The starting torque of the motor. • Full load torque: the torque supplied by the motor at rated speed, voltage and frequency. Breakdown torque Torque Locked rotor torque Full load torque Speed (ω)
MECH 1200 Other Special Purpose Motors • Stepper Motor: A special type of synchronous motors, that moves in steps every time a pulse is sent from its control unit. One type of stepper motors has a permanent magnet in its rotor. • Reluctance Motor: A hybrid of the induction and synchronous motors, operates at synchronous speed, with a rotor of salient poles. • Hysteresis Motor: a self starting synchronous motor, torque is generated through hysteresis. • Brushless DC motor: A combination of a permanent magnet stepper motor with a position sensor and a control unit that gives the motor the pulses to work. It was designed to run off DC power when brushed motors are unacceptable such as in artificial heart applications or when radio frequency interference is unacceptable, or at low pressure environment such as at high altitudes.
MECH 1200 Couplings • Shaft couplings: Sleeves that provide a connection between shafts rotating at high speeds to transmit torque and power. • Three types of couplings: - Rigid coupling - Flexible coupling - Universal joints
MECH 1200 SEALS • Purpose: • Keep the oil in and the dirt out • Separate fluids or cavities • Withstand different pressures
MECH 1200 GASKET • A pliable material that is shaped and placed between two mating machined parts to form a seal. • Purpose of a Gasket • To confine; gas, oil, water, vacuum or exhaust • Seal out foreign objects such as dirt& water • Must withstand heat, cold, pressure, erosion, corrosion, moisture and oil
MECH 1200 Stuffing Box • An assembly which is used to house a gland seal. • It is used to prevent leakage of fluid, such as water or steam, between sliding or turning parts of machine elements.
MECH 1200 Bearings Purpose of bearings: • Support loads • Reduce friction The PV factor: • PV factor refers to the Pressure and Velocity ratings of the bearing. • Pressure is the load on the bearing in pounds divided by the projected area in square inches. • Velocity is the surface velocity of the shaft.
MECH 1200 Some types of Bearings Plain (Journal) Bearings Antifriction (rolling) Bearings Solid Deep Groove Ball Bearing Split Angular Contact Ball Bearings Self Aligning Ball Bearing Pure Thrust Ball Bearing Cylindrical Roller Bearing Spherical Roller Bearing Tapered Roller Bearing Needle Roller Bearing
MECH 1200 Clutches • Clutch – Go (or limit torque) • Brake – Stop (or slow down) • Backstop – Go only one direction • Primary function: Stop, slow, or prevent reversal of load in a mechanical system