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

Structures

• What is the primary reason that a structure fails to meet its performance specification in machine design?

• Is it that the stress exceeds the yield stress?

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

Often the hardest part of Machine Design

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!Design Guides (p 223) use the same fundamentals

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

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

Power Transmision is Seperate from a Bearing System

So Many Neat Transmissions:Each One Has Depth

Gears Check out Harmonic and Cycloidal drives

Timing Belts Flat Belts Cable Drives Friction Drives

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

Springs

Good for accomiating misalignments Maintianing preload Not all springs look like springs

Belleville washers Cantilvers

Neat Mechanical Components

Harmonic Drive

Minimal backlash and high gear reduction, yet still backdrivable

Regular Clutch with Ratchet and Pawl

Roller Clutch

Roller clutch provides instantanous lockup, and low noise operation