Secrets of Machine Design Nathan Delson. Learning Machine Design Includes: Looking at existing...

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Secrets of Machine Design Nathan Delson

Transcript of Secrets of Machine Design Nathan Delson. Learning Machine Design Includes: Looking at existing...

Page 1: Secrets of Machine Design Nathan Delson. Learning Machine Design Includes:  Looking at existing designs Take things apart  Applying Engineering Theory.

Secrets of Machine Design

Nathan Delson

Page 2: Secrets of Machine Design Nathan Delson. Learning Machine Design Includes:  Looking at existing designs Take things apart  Applying Engineering Theory.

Learning Machine Design Includes:

Looking at existing designs Take things apart

Applying Engineering Theory Doing Design Projects

But many courses and texts gloss over important areas in machine design.

Page 3: Secrets of Machine Design Nathan Delson. Learning Machine Design Includes:  Looking at existing designs Take things apart  Applying Engineering Theory.

Lecture Topics

Structures Bearings

Exact Constraint Design

Free Body Diagram Analysis Power Transmission Axiomatic Design approach to Robust Design Redesign

Page 4: Secrets of Machine Design Nathan Delson. Learning Machine Design Includes:  Looking at existing designs Take things apart  Applying Engineering Theory.

Structures

Page 5: Secrets of Machine Design Nathan Delson. Learning Machine Design Includes:  Looking at existing designs Take things apart  Applying Engineering Theory.

Secret 1: Structures Often Do Not Fail Due To Stress > Yield Stress

Before linear analysis calculates yield stress, failures can be:

Excessive deflection Angle deflections can be worse than translation if part is

holding a sensor or other critical part Vibrations. Excitations can include:

RPM of any rotating part, frequency of gear tooth engagement, control feedback.

Buckling

Page 6: Secrets of Machine Design Nathan Delson. Learning Machine Design Includes:  Looking at existing designs Take things apart  Applying Engineering Theory.

Moments and Cantilever Loads are Often the Culprit

Cantilever deflection much larger than pure tension

Angle of cantilever can have magnifying effect

                                                  

Page 7: Secrets of Machine Design Nathan Delson. Learning Machine Design Includes:  Looking at existing designs Take things apart  Applying Engineering Theory.

Structural Solutions

Symmetric support to avoid moments Long support distance when

moments are necessary When you want something

stationary, make sure it is not a mechanisms (triangles instead of rectangles)

Identify what component stretches or compresses when a load is applied

Page 8: Secrets of Machine Design Nathan Delson. Learning Machine Design Includes:  Looking at existing designs Take things apart  Applying Engineering Theory.

Bearings

Page 9: Secrets of Machine Design Nathan Delson. Learning Machine Design Includes:  Looking at existing designs Take things apart  Applying Engineering Theory.

Bearings

The role of a bearing is to allow motion in desired DOF while constraining motion in all other DOF.

Good Bearing Systems have: Low friction in the direction of motion Low wobble in constrained DOF.

Page 10: Secrets of Machine Design Nathan Delson. Learning Machine Design Includes:  Looking at existing designs Take things apart  Applying Engineering Theory.

Constraint Design

Every 6 DOF of an object needs to be explicitly constrained, if it is not a motion direction.

Constraining rotation is usually the hardest and requires 2 contacts points in the plane of rotation.

The designer should explicitly choose the contact points, rather than let the part wobble until it hits “something”

Page 11: Secrets of Machine Design Nathan Delson. Learning Machine Design Includes:  Looking at existing designs Take things apart  Applying Engineering Theory.

Linear Slide Design

Large distance between bearings is critical!

Page 12: Secrets of Machine Design Nathan Delson. Learning Machine Design Includes:  Looking at existing designs Take things apart  Applying Engineering Theory.

Secret 2 - Exact Constraint Design: Robust Bearings at Low Cost

Use the minimum necessary number of constraints

How many bearings support a shaft? What is the problem with too many

constraints? What is the problem with too few?

Page 13: Secrets of Machine Design Nathan Delson. Learning Machine Design Includes:  Looking at existing designs Take things apart  Applying Engineering Theory.

Examples of Exact Constraints

Page 14: Secrets of Machine Design Nathan Delson. Learning Machine Design Includes:  Looking at existing designs Take things apart  Applying Engineering Theory.

Examples of Exact Constraints

Page 15: Secrets of Machine Design Nathan Delson. Learning Machine Design Includes:  Looking at existing designs Take things apart  Applying Engineering Theory.

Examples of Over Constrained Designs

No clearance hole

Alignment of more than 2 bearings (if no flexible coupling is present)

Page 16: Secrets of Machine Design Nathan Delson. Learning Machine Design Includes:  Looking at existing designs Take things apart  Applying Engineering Theory.

Bearings Solutions: Rule of Thumb is Two Bearings Per Shaft

Page 17: Secrets of Machine Design Nathan Delson. Learning Machine Design Includes:  Looking at existing designs Take things apart  Applying Engineering Theory.

Exceptions to Exact Constraint Design

Pulleys can have one bearing since there is no moment (think of MAE156A turntable).

High Loads on shafts Engine crankshafts have multiple bearings which

are precision machined Parts which can be made easily in high

precision Ball bearings and shafts

Page 18: Secrets of Machine Design Nathan Delson. Learning Machine Design Includes:  Looking at existing designs Take things apart  Applying Engineering Theory.

Rolling Element Bearings

Page 19: Secrets of Machine Design Nathan Delson. Learning Machine Design Includes:  Looking at existing designs Take things apart  Applying Engineering Theory.

How Ball Bearings Are Made

machine rolls the ball between two very heavy hardened steel plates called rill platesA grade three ball has to be spherical within 3 millionths of an inch and the diameter must be accurate within 30 millionths of an inch. This

means that for a grade three quarter-inch ball, the diameter would have to be between 0.24997 and 0.25003 of an inch and the smallest diameter measured on the ball has to be within 3 millionths of the largest diameter.

Page 20: Secrets of Machine Design Nathan Delson. Learning Machine Design Includes:  Looking at existing designs Take things apart  Applying Engineering Theory.

How Precision Shafts Are Made

Centreless grinding is commonly used to produce ground bar stock and chromed bar stock. Ball bearings and other spherical products are also finished using centreless grinding methods.

Page 21: Secrets of Machine Design Nathan Delson. Learning Machine Design Includes:  Looking at existing designs Take things apart  Applying Engineering Theory.

Exact Constraint Design Also Applies To Structures

Problem: Due to tolerance build up, Copy Machine Baffle Sides buckle in when assembled

Page 22: Secrets of Machine Design Nathan Delson. Learning Machine Design Includes:  Looking at existing designs Take things apart  Applying Engineering Theory.

First Solution: Reinforce Sides

Problem: Now baffle buckles

Page 23: Secrets of Machine Design Nathan Delson. Learning Machine Design Includes:  Looking at existing designs Take things apart  Applying Engineering Theory.

Second Solution: Also Reinforce Baffle

Problem: Excessive stress => time to call consultantWhat Exact Constraint Solution is there?

Page 24: Secrets of Machine Design Nathan Delson. Learning Machine Design Includes:  Looking at existing designs Take things apart  Applying Engineering Theory.

Free Body Diagrams

Page 25: Secrets of Machine Design Nathan Delson. Learning Machine Design Includes:  Looking at existing designs Take things apart  Applying Engineering Theory.

What Did Theoretical Friction Analysis

Teach Us?

Page 26: Secrets of Machine Design Nathan Delson. Learning Machine Design Includes:  Looking at existing designs Take things apart  Applying Engineering Theory.

Power Transmissions

Page 27: Secrets of Machine Design Nathan Delson. Learning Machine Design Includes:  Looking at existing designs Take things apart  Applying Engineering Theory.

So Many Neat Transmissions:Each One Has Depth

Gears Check out Harmonic and Cycloidal drives

Timing Belts Flat Belts Cable Drives Friction Drives

Page 28: Secrets of Machine Design Nathan Delson. Learning Machine Design Includes:  Looking at existing designs Take things apart  Applying Engineering Theory.

Axiomatic Design approach to Robust Design

Page 29: Secrets of Machine Design Nathan Delson. Learning Machine Design Includes:  Looking at existing designs Take things apart  Applying Engineering Theory.

Control Considerations

Precision Over-shoot Vibration Stability Control Theory is large field

But if you identify the source of the problem, you are 80% the way to a solution

Page 30: Secrets of Machine Design Nathan Delson. Learning Machine Design Includes:  Looking at existing designs Take things apart  Applying Engineering Theory.

Mechanical Issues Affecting Control

Gear Backlash Back drivable vs non-back drivable motors Driving large inertias System stiffness