me 551 - 05 structural design (rev. 1.2)

8/17/2019 ME 551 - 05 Structural Design (Rev. 1.2)

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Outline – Machine Structures• Design Requirements

• Structural Elements

– Materials

• Design Considerations –

– Beds

• Manufacturing Techniques

– Cast Iron

– Welded Steel

– Polymer Concrete Casting

– Granite-based Structures

– Carbon Fiber Composites

• Structural Damping• Finite Element Analysis

• Elimination of Static Deformations

Chapter 5 ME 551 2

Desi n Re uirements

• Structure of the machine houses (and supports the

operation of) all the vital (moving or stationary) elements of

the machine.

– s e s e e on o e mac ne. – Without a good structure, the rest of the machine will be ineffective.

– Symmetrical (and Simple) Design

– Minimum Wei ht

– High Static and Dynamic Stiffness

– High Structural Damping

– High Secular and Thermal Stability

– Independent Foundation

Chapter 5 ME 551 3


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Structural Elements• Structural elements can be

Flat Bed

– Machine Beds

• T-Bed, Slanted Bed,

• Floor Plates, etc.

– Columns

Slanted Bed

– Portals/Bridges

• Open or Enclosed Design

• Machine structures can be

categorized into three classes: – Open Frame

– Closed Frame

– -

Chapter 5 ME 551 4

O en Frame Structures1,2

• Most traditional machine

tools employ this

configuration.

– Also known as C- or G frames

• Provides eas access to the

workspace.

•

frames.

.

• Prone to Abbe offset errors.

Chapter 5 ME 551 5


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Closed Frame Structures1,2

• Most precision machine

architecture:

Deformed Frame

–

frames Axial Force

Tool

is fairly easy.Structural Loop

(“Force Flow”)

• ymme r ca s ruc ure s

quite rigid.

Workpiece

• Main actuator must be

located on the bridge.

Chapter 5 ME 551 6

Truss-t e Structures

,

are deployed:

– Cubic / Cuboid

– Tetrahedron

– Octahedron etc.

• Such (truss-type) geometries yield stable- and strong

enclosed structures that are especially suitable for parallel

mec an sms e exapo p a orms .

– High thermal stability

dimensions of the frame.

Chapter 5 ME 551 7


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Exam le - Octahedral Hexa od

• Machine is built by Ingersoll Company

• It employs an octahedral geometry to

support the hexapod “tool” platform.• The hexapod (Stewart platform

concept originally developed for flight

freedom.

–

degrees from the vertical.

• Advanced controller architecture andalgorithms make programming

possible.

Chapter 5 ME 551 8

Structural Materials

Ferrous metals Nonferrous metals Non-metals

Com osites

Cast IronSteel

Al. (Cast 201) Al. (6061-T651)

GraniteZerodur ™

Invar

Super Nilvar ™

Copper

Brass (Cu Alloy)

Polymer concrete

Portland concrete

Chapter 5 ME 551 9


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Column Structures1

With Ribs

Chapter 5 ME 551 10

Stiffness Pro erties of Columns1

Chapter 5 ME 551 11


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Bed Structures1

Chapter 5 ME 551 12

Bed Structures Cont’d

Chapter 5 ME 551 13


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Pro erties of Bed Desi ns1

Chapter 5 ME 551 14

Cast Iron Structures2

• Widely used in machine

• Stable with thermal anneal,

,

relieve

•

heat transfer

• Low cost for moderate sizes

• Integral features can be cast

in place

• Design and manufacturingrules are well-established.

Chapter 5 ME 551 15


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Welded Steel Structures2

• Often used for larger

-

• Stable with thermal anneal• ow amp ng, mprove w

shear dampers

• Integral features/parts can be

• Structures can be made from

tubes rofiles and lates.

Chapter 5 ME 551 16

Pol mer Concrete Castin 2

• Polymer concrete (PC) is a relatively new

design.

– Special polymers are mixed with specially

prepared/sized aggregate. – Epoxy-granite-, mineral-, and reactive-

same technique.

• For PC castings, the same rules for draft

allowance apply as for metal castings if

the mold is to be removed.

– ,

develop hot spots while curing even inthick, uneven sections.

Chapter 5 ME 551 17


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PCC Cont’d

• ,

internal foam cores to maximize their

stiffness-to-weight ratio.

• PC can accommodate cast in place

components such as bolt inserts,

conduit, bearing rails, hydraulic lines

etc.

• g y oa e mac ne su s ruc ures

(e.g. carriages) are made from cast

.

Chapter 5 ME 551 18

PCC Cont’d

• PC structures can have the stiffness

of cast iron structures.

– They can have much greater damping.

• PC does not diffuse heat as well ascast iron.

– Attention must be paid to the isolation of

heat sources to prevent the formation of

hot spots.

• When bolting or grouting non-PC

components to a PC bed, bimaterial

effect must be considered.

Chapter 5 ME 551 19


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Granite-Structures• Used exclusively in precision instruments and

CMMs.

– Serves as reference planes/surfaces

– Quite costly• ,

– It must be sealed off properly to avoid

absorbtion of water. Otherwise, it will distort!

• This very hard (and brittle) material is very

stable:

– 3.

– Elasticity modulus: 40 [GPa]

– Tensile strength: 16 [MPa]

– erma exp. coe c en : . × -

• Not all grades are suitable for precision

machine design.

Chapter 5 ME 551 20

Carbon Fiber Com osites3

• Fibre reinforced composites have

very g va ues o a spec c

modulus of elasticity and specific

strengt . – Mechanical properties can be tightly

contro e

– Joining process can be complicated

– Quite expensive

• The application of this new material

to this field is still in its early stages.

Chapter 5 ME 551 21


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Slanted Bed Desi ns for Lathe1

Cast iron guideway plate

on cast concrete

Adhesive Joint

Adhesive Surface

Location of Cast-ribs

Lower Section

Steel Insert for

Mounting for

Hydraulics

of Bed

Machine Foot

Transfer TaConduit

Oil Chamber

Chapter 5 ME 551 22

Turning Fixture

Structural Dam in

• Damping is needed to absorb energy from the process:

– o preven c a er an amage o e sur ace

– To absorb energy from structural modes excited by the servos

and other sources

• Damping can be obtained by internal means:

– Material damping

– Damping by micro-slip at joints

• Damping can be obtained by external means:

– Constrained layer dampers (or shear dampers)

– Vibration absorbers

–

• Velocity control loops in servo systems• Actively controlled masses attached to the structure

Chapter 5 ME 551 23


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Combined Dam in Effect6

• Major part of the damping for a machine system can be generated at

the mating surfaces (i.e. bolted joints, slides) of the various

- .

Chapter 5 ME 551 24

Shear Dam ers4

• Steel structures are known to

ave e n erna amp ng.

• One alternative method to

structure is to employ shear

dam ers.

• Visco-elastic layer damps

motion between structure and

constraining layer (from

bending or torsion) by

ss pa ng ne c energy n o

heat.

Chapter 5 ME 551 25


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A lication - Shear Dam ers4

,

considered.

– Inner tube serves as constrainin la er.

– Constraining layer is wrapped with damping material.

– Coated inner tube is inserted and gap filled with epoxy.

Chapter 5 ME 551 26

A lication Cont’d

Chapter 5 ME 551 27


14/22

Exam le - Precision Grinder 7

T-bed of the precision grinder includes

-

inserts. Square inserts also allow the

circulation of cooling fluid.

ross-sec on o e s ear

damped test beam

Chapter 7 ME 551 28

Vibration Dam in Ta es• Vibration damp(en)ing tape/foil is a

“ - ”

damping.

• Commercial roducts like 3M

434/435/436™ constitute a visco-elasticpolymer coated on a soft Aluminum

constraining layer.

• Very useful in dampening the vibrations

o me a p a es an compos e pane s.

• Somewhat sensitive to high

– Nominal operating temperature range (forthe tape) is -60 to 20oC.

Chapter 5 ME 551 29


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Vibration Absorbers1

a g n

i f i c a t i o n

f u n c t i o n

M

Chapter 5 ME 551 30

Absorber Desi ns2

TMD

Constrained-

Layer Beam Adjustable Position

Anchor Mass

Chapter 5 ME 551 31

ruc ure


16/22

Finite Element Method• FEM is an indispensible engineering analysis tool

to find a roximate solutions to technical

problems defined by partial differential equations.

• FEA Packages (ANSYS™, MARC/Mentat™,

Nastran/Patran™, Abaqus™, etc.) are routinely

utilized to design/analyze/optimize structural

.

• Large number of engineering analysis can be

conducted b FEA acka es:

– Stress/Strain (in elastic- or plastic region)

– Heat Transfer – Mechanical Vibrations

– Electromagnetic Fields (Maxwell™)

–

Chapter 5 ME 551 32

FEM Anal sis1CAD Model of the Machine “Specs” of the Simulat ion

Determination of the

simulation ob ectives

■

Abst raction for the

FE-model Creation Abstrac tion of Guides

and Drives as Springs

(deformations, stresses,

natural modes)

Determination of modeling

strategy (volume- or shell

■

Choice of element order(linear, parabolic)

■

Presentation of the Results

Definition of boundary conditions

(force, temperature) and constraints

■

List of basic stiffness

Program’s Internal Processes

■

Compilation of the Overall Model

matrices

Structure the global

stiffness matrix

Consideration of the

boundar conditions

■

■

Solution of the resulting

linear systemDerivation of stress from

deformation values

■

■

Chapter 5 ME 551 33


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FEM Anal sis - Illustration1

Chapter 5 ME 551 34

Example – Portal Frame1

Chapter 5 ME 551 35


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Vibration Anal sis - Illustration1

Chapter 5 ME 551 36

Natural Fre uencies & Modes1

Natural Frequency: 42.4 Hz Natural Frequency: 73.4 Hz Natural Frequency: 102 Hz

Chapter 5 ME 551 37


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Vibration Anal sis1 Cont’d

Chapter 5 ME 551 38

Frequency [Hz]

Elimination of Static Deformations5

• ere are ree me o s

to compensate the elastic

machine structure under

-

loads:

a. ompensa ng curve

b. Preloaded support

c. Counter-weight

Chapter 5 ME 551 39


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Com ensatin Curve1

• The bearing rails are finished (grounded) so that

they deform to the desired shape when themachine axes move.

– Grinding process is expensive but it saves structural

costs.

• When the primary weight is that of the machine

axis (not the workpiece!), this method can be

very effective.

Chapter 5 ME 551 40

Counter-wei ht S stems

• For vertical axis, the servo-motor

system needs to support the dead-

weight of that axis.

– ay nee o c oose overra e servo-system.

– Ener wasted due to IR losses of

machine.

• Dead weight can be supported by an

external system (i.e. counter weights,

hydraulic systems, floats, etc).

• or ynam c mo ons, mo or a -

screw inertia usually dominates.

Chapter 5 ME 551 41


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Counter-wei ht Cont’d• Counterweight system contributes to the

– Cogging effect, elastic effect etc.

, ,

utilized to carry the counterweight.' – ,

as it rotates producing a small cogging effect.

•

least variation in force.

– Cables are elastic com ared to chains and hence the shouldonly be used for quasi-statically (i.e slow) moving elements.

– Pulley friction and friction in the counterweight's bearings should

.

Chapter 5 ME 551 42

Some Desi n Rules2

• ,

golden rectangle (Height/Width = 1.618) in mind.

• Utilize s mmetr wherever ossible.

• Minimize the structural loop and use closed sections

whenever possible.

• Large plate sections should be stiffened with ribs and

other means to kee them vibratin like drumheads. – When needed, use active damping systems.

• Maximize thermal diffusivit of the machine and

minimize heat input.

Chapter 5 ME 551 43


22/22

Desi n Rules Cont’d• Locate the work volume at the center of mass and in plane

.

• Start at the tool tip (or workpiece) with estimates on

cuttin forces and acceleration

– Then work backward through the structural system.

– Use guesstimates for sensor, bearing, and actuator limitations to

help size structural components.

• Try to make the natural frequencies of the various

. .

together.

– .

• Use as many design tools as possible in design stage.

– Especially, Solid Geometric Modeling and FEM Packages.

Chapter 5 ME 551 44

References

1. M. Weck, C. Brecher, Werkzeugmaschinen (Band 2), Springer-

, .

2. A. H. Slocum, Precision Machine Design, SME Press, 1992.

• A. H. Slocum, ME 2.075 Course Notes, MIT, 2001.

3. L.N. L. deLacalle, A. Lamikiz, Machine Tools for High PerformanceMachining, Springer-Verlag, 2009.

. . , , , .

5. Cranfield Unit for Precision Engineering (CUPE), Precision

Engineering Course Notes, Cranfield Institute of Technology (UK),

1998.

6. Löwenfeld, K., “Zweites Forschungs und Konstruktionskolloquium

Werkzeu maschinen ” . 117 Vo el-Verla Cobur 1955.

7. E. R. Marsh, A. H. Slocum, “An integrated Approach to StructuralDamping,” Precision Engineering, vol. 18, pp. 103-109, 1996.

me 551 - 05 structural design (rev. 1.2)

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