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MMU-307 DESIGN OF MACHINE ELEMENTS

Introduction to Mechanical Engineering Design

Asst. Prof. Özgür ÜNVER October 15th, 2019

Prerequisites and Future

Mechanical DesignMechanical design is complex and requires many skills.

Approach: subdivide the parts into a series of simple tasks and solve!!

• Analytical solutions,

• Numerical solutions,

• Standards,

• Cost,

• Safety,

• Survival of a design (stress and strength).

• Uncertainty (factor of safety)

• Dimensions and tolerances,

• Units

Example: http://www.youtube.com/watch?v=xpCZB7mkBIc

Designed Product Feature

•manufacturable,

• functional,

• reliable,

•Competitive (optimized),

• safe,

•marketable.

Designed product has to be;

Decision Making

Design is an innovative and highly iterative process.

It is also a decision-making process (optimization).

Decisions sometimes have to be made with

• too little information,

• occasionally with just the right amount of information,

• with an excess of partially contradictory information.

DesignerEngineers have to;

• communicate effectively (research + talk),

• work with people of many disciplines

Engineers should be

• Creative (knowledge+experience+perception),

• communicative,

• problem-solver

Engineering tools

• mathematics,

• statistics,

• computers,

• graphics,

• languages.

Example

A simple journal bearing involves;

• fluid flow,

• heat transfer,

• friction,

• energy transport,

• material selection,

• Thermomechanical treatments,

• statistical descriptions,

• etc..Being a designer or a user are different things!First define what you are!

Phases and Interactions of the Design Process

Design Considerations

Design Tools and Resources

Inexpensive computers

Robust computer software packages

design

analysis

simulation

Technical information,

basic science/engineering (textbooks, papers)

characteristics of specific off-the-shelf components (brochures or catalogs)

Engineers have a great variety of tools and resources available to assist in the solution of design problems such as;

Standards

A standard is a set of specifications for parts, materials, or processes intended to achieve;

• uniformity,

• efficiency,

• specified quality.

One of the important purposes of a standard is to place a limit on the number of items in the specifications so as to provide a reasonable inventory of tooling, sizes, shapes, and varieties.

Standards

EconomicsStandard Sizes

Large Tolerances

Breakeven Points

Stress and Strength

The survival of many products depends on how the designer adjusts the maximum stresses.

The designer must allow the maximum stress to be less than the strength by a sufficient margin so that despite the uncertainties, failure is rare.

Strength is a property of a material or of a mechanical element. The strength of an element depends on the choice, the treatment, and the processing of the material.

Stress is a state property at a specific point within a body, which is a function of load, geometry, temperature, and manufacturing processing.

Uncertainty

Factor of Safety

Significant Digits

• Usually, but not always, three or four significant figures are necessary for engineering accuracy.

• 706 3.14, and 0.00219 have three significant digits.

• For three significant figures: 91.6 × 103.

• For four significant figures: 91.60 × 103.

• Computers and calculators display calculations too many significant figures.

• If you use too many significant digits that means you are sure of all the calculations to the tiny bit portions which is not correct in many case!

Tolerances and Fits

For more info: http://www.mitcalc.com/doc/tolerances/help/en/tolerancestxt.htm

Tolerances and Fits

Nominal Size: The size we use in speaking of an element.

i.e. M10 bolt, 10 cm in diameter pipe, etc. However this may not be the real dimension!

Limits:The stated maximum and minimum dimensions.

i.e. This shaft should not be greater than 10 cm in diameter!

Tolerance: The difference between the two limits (upper and lower).

Bilateral Tolerance: The variation in both directions from the basic dimension. i.e. 1.005 ± 0.002. The two parts of the tolerance need not be equal.

Unilateral Tolerance: The basic dimension is taken as one of the limits, and variation is permitted in only one direction,

i.e.

Clearance Fit Interference Fit

Types of Fits

Types of FitsClearance: If the internal member is smaller than the external member.

Interference: The opposite of clearance, for mating cylindrical parts in which the internal member is larger than the external member.

http://www.youtube.com/watch?v=DY_CrP4R8jI

http://www.youtube.com/watch?v=7-HqrwMd1fs

• Lower the transmitted torque, the greater the probability of using a clearance fit between coupling and shaft.

• Higher the transmitted torque usually requires an interference fit. • Further, as the power and torque requirements become very high, it

is more common to find that keyless fits are required.

InternationalTolerance

Grade Numbers

(IT)

Fits for Holes and Shafts

Clearance Fit Transition FitInterference

Fit

Hole Basis System (HBS)

Shaft Basis System (SBS)

Examples of Fit

Tolerance Analysis

• Tolerances are necessary to attain manufacturingcontrol. The amount of tolerance to be given on a partdepends on:• The function of the product and allowance desired on

components,

• Process of manufacture,

• Cost of production and assembly (If the limit of tolerancesdecreases, cost of component increases)

• If the limit of tolerances decreases, requirement ofmore sophisticated machinery and requirement ofhighly skilled person increases. Therefore, there is abalance between cost and quality.

Example of Tolerance and Fit