mech3230 chapter 1 lecture - auburn...

D R . B E A L E

MECH3230Chapter 1 Lecture

Chapter 1 Topics and Assignments

Reading/HW for Tuesday 1/24 Sections 1.1 – 1.5 (briefly, not testable material)

Section 1.6: Systems of Units, Problems 22, 23, 24, 26, 27 (Due Tuesday)

Section 1.7: Methodology for Solving HW Problems

Section 1.8-10: Work and Energy/Power/Conservation of Energy

Reading/HW for Thursday 1/26 Problems 34, 40, 47, 55

Section 1.6: System of Units

Metric mishap caused loss of NASA orbiter

NASA's Climate Orbiter was lost September 23, 1999

(CNN) -- NASA lost a $125 million Mars orbiter because a Lockheed Martin engineering team used English units of measurement while the agency's team used the more conventional metric system for a key spacecraft operation, according to a review finding released Thursday.

The engine fired but the spacecraft came within 60 km (36 miles) of the planet -- about 100 km closer than planned and about 25 km (15 miles) beneath the level at which the it could function properly, mission members said.

Table 1.4 (p. 16)English, British, and SI Units for Length, Time, Mass, and Force

Fundamentals of Machine Component Design, 4/E by Robert C. Juvinall and Kurt M. MarshekCopyright © 2006 by John Wiley & Sons, Inc. All rights reserved.

Figure 1.3 (p. 17)Comparison of units of force (or weight) and mass. Note that the weight for each of the standard masses is valid only or the standard earth gravitational field (g = 9.81 m/s2 or g= 32.2 ft/s2).

Comments on Units

The thing that messes everybody up occurs when English Engineering (EE) units are used for calculations in formulas for calculating weight and inertia force terms. In EE: W=mg/gc inertia force term: ma/gc, where

gc = 32.174 lbm ft/(s2 lbf) = 1 BG “fixes” EE, by replacing lbm with slug In BG: W=mg inertia force term: ma In SI: W=mg inertia force term: ma You don’t have to memorize these formulas if carry the units with

you and remember: N=kg m/s2, gc = 32.2 lbm ft/(s2 lbf)=1, or lbf = 32.2 lbm ft/s2

lbf=slug ft/s2. QUESTION: What is g on the moon (approximately)? What is

gc on the moon?

Problem 1.21

Homework Problem Methodology

Known: State known values

Find: State what is to be determined

Schematic and Given Data: handsketch of system, FBD

Decisions: For design problems where you pick something, e.g. a material, size, etc.

Assumptions: list any simplifying assumptions to simplify the problem (e.g. “neglect friction”)

Analysis: show equations, calculations

Comments

Work and Power

Problem 1.31

Conservation of Energy

Punch Press Problem SP1.3

Known: 60 rpm (60 punches/minute), 1200 rpm motor, gearbox. No flywheel and link inertias small

Find: Motor PowerAlso: Punch presses are

slider-crank mechanismsSince no flywheel, the motor

must be able to supply the peak torque of 10 kNm. Without a flywheel to help, the motor must be able to supply it by itself

Sample Problem 1.3 Calculations

Sample Problem 1.4

Known: 60 rpm (60 punches/minute), 1200 rpm motor, gearbox. FLYWHEEL

Find: Motor Power A flywheel stores energy.

We should be able to pick a flywheel that can release enough energy during the punch to reach the crank torque required load, and after that build up rotational kinetic energy (i.e. the flywheel speeds up).

Sample Problems 1.4/1.5


Figure 1.9 (p. 28)Punch press flywheel proportions.

Problems 1.33/1.39

Automotive Performance Analysis

Figure 1.10: Vehicle Power Requirement (hp) vs. Speed (mph). This is the “load”. What is the major force contributor here?

Figure 1.11: Engine Output Power (hp) vs. Engine Speed (rpm). Wide open throttle.

Use Power In = Power Out. “Power In” is the Engine Output Power from Figure 1.11. “Power Out” is the Vehicle Power Requirement

Figure 1.12: Specific Fuel Consumption (lb/hp h) vs. Engine Output Power show the effect of a transmission. Where do you want to be on the curves for best gas mileage?


Figure 1.13 (p. 31)Vehicle for Sample Problem 1.4.


Figure 1.10 (p. 29)Vehicle power requirement. Typical 4000-lb sedan (level road, constant speed, no wind).

What force type is the major contributor to this?What sort of experiment(s) could you use to measure this??


Figure 1.11 (p. 30)Engine output power versus engine speed. Typical 350-in.3 V-8 engine.

Full Throttle - How would this be measured in an experiment?

Brake HorsePower and Dynamometers


Figure 1.12 (p. 30)Specific fuel consumption versus engine output power. Typical 350-in.3 V-8 engine.

Figure 1.12

Where would you operate on Figure 1.12 for best gas mileage for 1000 rpm? 2000 rpm? In general?

Where are the full throttle conditions on Figure 1.12?


Figure P1.29 (p. 35)

Problem 1.27 (Assigned)

John Stapp

Examples

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