machine components 1 - wikispaces · nominal diameter number of threads per inch thread form (unc...

ME71 Introduction to Mechanical Engineering Design

Machine Components #1


Administrative Stuff •! Only teams of 3 are allowed for mini-contest •! Design reviews Thursday (sign up today on the wiki)

–! You will discuss a package of information containing your morphology chart, three design concept descriptions, a decision matrix (with a justifications for the rankings and weightings), and a final concept drawing with annotations. This is about 5-6 pages of information. Please have a hard copy or computer with slides at the review for discussion.

–! Email me a copy of this package before your design review

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Topics

•! Threaded Fasteners –! Definitions –! Types –! Analysis

•! Attaching things to shafts –! Set screws –! Pins –! Keys –! Splines

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Threaded Fasteners

•! Terminology –! Major diameter, d –! Minor diameter, dm

–! Mean diameter –! Pitch / lead, P –! Pitch Diameter, dp

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dp = d " 0.649519 Pdm = d "1.299038 P

For Unified threads


Specifying a Threaded Fastener

.250-20 UNC-2B MJ6x1 – 4h6h

Based on ASME 14.6-2001 standard

Nominal diameter

Number of threads per inch

Thread form (UNC for coarse threads, UNF for fine threads)

Thread class (1 – lose tolerance, 2 – general, 3 – close tolerance A – external, B – internal)

Metric threaded symbol

Thread form symbol

Nominal size in mm

Pitch (inverse of number of threads per mm)

Pitch diameter tolerance

Major diameter tolerance h – external, H - internal

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Typical English Sizes

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Name Classification 0-80 .060-80 UNF 2-56 .086-56 UNC 2-64 .086-64 UNF 4-40 .112-40 UNC 4-48 .112-48 UNF 6-32 .138-32 UNC 6-40 .138-40 UNF 8-32 .164-32 UNC 8-36 .164-32 UNF 10-24 .190-24 UNC 10-32 .190-32 UNF

Name Classification 1/4-20 .250-20 UNC 1/4-28 .250-28 UNF 5/16-18 .3125-18 UNC 5/16-24 .3125-24 UNF 3/8-16 .375-16 UNC 3/8-24 .375-24 UNF 7/16-14 .4375-14 UNC 7/16-20 .4375-20 UNF 1/2-13 .500-13 UNC 1/2-20 .500-20 UNF


Typical Metric Sizes

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Name Classification M1.6 MJ1.6x.35 M2 MJ2x.4 M2.5 MJ2.5x.45 M3 MJ3x.5 M4 MJ4x.7 M5 MJ5x.8 M6 MJ6x1 M8 MJ8x1.25 M10 MJ10x1.5 M12 MJ12x1.75


Types of fasteners

•! Socket head cap screw

•! Hex head

•! Philips pan head

•! Countersunk flat head –! Over constrained

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Bolt grades

•! Bolts are typically rated by tensile strength.

•! For SAE hex bolts, a marking on the head will tell you the strength grade

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Grade 2 60,000 psi

tensile strength

Grade 8 150,000 psi

tensile strength

Grade 5 105,000 psi

tensile strength


Stress Analysis

•! An unthreaded rod having a diameter equal to the mean of the pitch diameter and and the minor diameter will have the same tensile strength as the threaded rod

•! The area used for this calculation is known as the tensile stress area, As

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As ="4

dm + dp2

#

$ %

&

' (

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A Quick Example

•! ! =P/a •! A .250-28 bolt has a stress area of 0.0364 in2

•! High strength bolts have an ultimate tensile strength of 180,000 psi

•! A ! bolt can take 6,552 lbf in tension

•! Ford F-150 Truck weighs 4,693 lbf

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Thread Engagement •! The load of a fastener is taken over approximately 3

threads •! If too many threads are engaged, they will tend to

fight each other •! Generally, give enough engagement for a minimum

of 5 threads since your first and last will usually be partial threads

•! A good rule of thumb is to give yourself 1 - 2 x diameter of the bolt for thread engagement

•! Example: –! !-20 bolt has 20 threads/inch –! 5 threads = ! of threaded engagement

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Preload •! Threaded fasteners are typically tension loaded to

clamp two or more parts together •! The preload of the fastener (and the joint) is implied

via the torque of the fastener •! Torque vs preload force is a function of:

–! Thread geometry –! Fastener tensile strength –! Thread strength in the external and internal threads

•! Reducing the variability of friction in the bolted joint reduces the variability of preload in the fastener for a given torque

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Torque Table

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Washers

•! Washers help to: –! Distribute the clamping load of the fastener head

over a larger area –! Give a more consistent coefficient of friction under

the bolt head when torqueing the fastener.

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Edge Clearance

•! 2x hole diameter is a good rule of thumb •! If you are too close to the edge you can pull

through

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Bolted Joints In Friction

•! When you torque a !-20 bolt to 121 in-lb, you preload the bolt with 2864 lb of force

•! A conservative assumption for the coefficient of friction between two parts is 0.2

•! If you have a 4 bolt pattern on a 6 bolt circle diameter your bolted joint can take 6873 in-lb of torque before slipping.

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Power Screws

•! Power screws (sometimes known as lead screws) are just like threaded fasteners

•! Instead of using torque to clamp a bolted joint, they transform torque in to linear force to do work.

•! Power screws work like a wedge wrapped around an axis of rotation

18 P

Ff

N

P

Direction of axis of thread

screw

load N

Tr

Ff Tr

!


Connecting to Shafts

A seemingly simple but notoriously difficult problem

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Key Concept

•! The main thing to think about is what transmits the load from one part to another

•! Make sure to think about all loads –! Torque –! Axial load

•! Think about speed –! At high speed, unbalanced connections can cause

wobble

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Setscrews

•! Unlike bolts that depend on tension to develop clamping force, setscrews depend on compression

•! Ultimately the holding force is friction –! Since the coefficient is highly variable, this makes

this type of connection difficult to analyze. •! In dynamic applications with reversing loads,

setscrews can lose their preload if not properly torqued and staked

•! Setscrews are easy to use, but not very reliable 21


Pins

•! Pins hold two parts together based upon the shear strength of the pin

•! Pins are good when the joint must take both thrust and shear

•! Pinned joints are easy to analyze and reliable •! Pinned joints can be used as a mechanical

fuse

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•! Pinned joints are difficult to assemble and disassemble because pins are typically press fit into place shaft

pin

gear


Keys

•! Keys transmit torque in shear across the length of the key

•! Keys have all the advantages of pins, but they are easy to install in remove

•! Keys do not take any axial load

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Woodruff key Used to reduce stress concentrations in the shaft


Clamp Fit

•! Clamps depend upon friction and a the bolted connection between two parts (or a single part with a flexure) to develop the clamping force

•! Clamped connections can be easy and reliable for low torque applications

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Press (Interference) Fits

•! Two parts can be pressed together if the hole is manufactured smaller than the shaft that is installed

•! The load between these two parts is taken in friction developed by the strain of the press fit

•! Manufacturing tolerances are very tight for this type of connection –! For a 0.25 diameter shaft with a class FN2 fit

•! Shaft 0.2510 – 0.2514 •! Hole 0.2500 – 0.2506

•! Difficult to disassemble without damage to the parts

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Splines

•! Splines are essentially matching internal and external gears used to transmit torque

•! Splines are extremely reliable and efficient •! Splines are difficult to manufacture

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Retaining Rings

•! Retaining rings are used to constrain things axially in one direction on a shaft

•! Installed by machining a groove into one of the mating parts

•! Can be made external or internal

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External E-Style External Internal


Other Topics Not Covered

•! Permanent connections –! Rivets –! Adhesive –! Welding

•! Springs

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What we will cover in lecture #2

•! Power (changing from rotational to linear motion)

•! Gears •! Belts & Chains •! Rolling element bearings •! Bushings •! Shaft connections •! Touch briefly on electric motors

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References

•! Shigley, J.E. Mechanical Engineering Design, Sixth Edition, McGraw-Hill, 2001.

•! Norton, R.L. Machine Design, An Integrated Approach, Prentice-Hall, New Jersey, 1996.

•! Oberg, Erik; Jones, Franklin D.; Horton, Holbrook L.; Ryffel, Henry H. (2004). Machinery's Handbook (27th Edition) & Guide to Machinery's Handbook.. Industrial Press.

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Things for me to bring to class •! !-20 bolts •! !-28 bolts •! Nuts •! Washers •! Torque wrench •! Set screws and something attached with setscrews •! Retaining rings •! Different grade hex bolts •! Spline parts •! Keys •! Lead screw

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machine components 1 - wikispaces · nominal diameter number of threads per inch thread form (unc...

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