Secrets of Machine Design Nathan Delson. Learning Machine Design Includes: Looking at existing...
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Transcript of Secrets of Machine Design Nathan Delson. Learning Machine Design Includes: Looking at existing...
Secrets of Machine Design
Nathan Delson
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.
Lecture Topics
Structures Bearings
Exact Constraint Design
Free Body Diagram Analysis Power Transmission Axiomatic Design approach to Robust Design Redesign
Structures
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
Moments and Cantilever Loads are Often the Culprit
Cantilever deflection much larger than pure tension
Angle of cantilever can have magnifying effect
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
Bearings
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.
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”
Linear Slide Design
Large distance between bearings is critical!
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?
Examples of Exact Constraints
Examples of Exact Constraints
Examples of Over Constrained Designs
No clearance hole
Alignment of more than 2 bearings (if no flexible coupling is present)
Bearings Solutions: Rule of Thumb is Two Bearings Per Shaft
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
Rolling Element Bearings
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.
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.
Exact Constraint Design Also Applies To Structures
Problem: Due to tolerance build up, Copy Machine Baffle Sides buckle in when assembled
First Solution: Reinforce Sides
Problem: Now baffle buckles
Second Solution: Also Reinforce Baffle
Problem: Excessive stress => time to call consultantWhat Exact Constraint Solution is there?
Free Body Diagrams
What Did Theoretical Friction Analysis
Teach Us?
Power Transmissions
So Many Neat Transmissions:Each One Has Depth
Gears Check out Harmonic and Cycloidal drives
Timing Belts Flat Belts Cable Drives Friction Drives
Axiomatic Design approach to Robust Design
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
Mechanical Issues Affecting Control
Gear Backlash Back drivable vs non-back drivable motors Driving large inertias System stiffness