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 Large distance between bearings is critical! Design GuidesDesign 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 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
How Ball Bearings Are Made machine rolls the ball between two very heavy hardened steel plates called rill plates A 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 consultant What Exact Constraint Solution is there?