22.322 Mechanical Design II

22.322 Mechanical Design II Spring 2013

Cam Project

Lecture 7 Review from Last Lecture • Sizing the cam • Base Circle (flat-faced followers) • Pitch Circle (roller followers) • Pressure Angle & Radius of Curvature • Keep minimum radius of curvature of cam pitch curve at least 2-3 times as large as radius of roller follower • Dumbbell Curl Example • Designing a Cam to maximize workout efficiency

Lecture 7 Negative Cam Design • We would like to design a cam for an exercise machine using a negative cam design:

Lecture 7 Negative Cam Design • So if the strength increases for a given angle, R must decrease since PRs is constant. Design the cam by inversion:

Lecture 7 Negative Cam Design

Lecture 7 Cam Manufacturing and Practical Considerations • Cams are usually made from medium to high carbon steels and sometimes plastics. • To make a cam, a milling machine or grinder is needed. • Usually continuous numerical control (CNC) machines are required to generate the precision needed. Common increments are 1/10, ¼, ½, and 1 degree. • Since the machine only has the x and y coordinates of the specified displacement, the machine has to interpolate the missing data.

Lecture 7 Cam Manufacturing and Practical Considerations • Manufacturing errors can occur due to: • Cutting process feed rate • Tool sharpness • Milling speed • Chatter • Milling tool deflection • Etc. • Consider a cam that has a roller follower. The following cam was milled on a high quality CNC milling machine using 1 degree linear interpolation: The actual displacement is true to the theoretical, but the acceleration has a significant amount of vibratory noise.

Lecture 7 Cam Manufacturing and Practical Considerations • Compare with the same shaped cam that has the same geometry except it was turned and ground (less noise & wear; smoother run): A ground cam is superior to a milled cam but is more costly to make (in small quantities, grinding almost doubles the cost of a cam). Automotive valve cams are ground. Made in large quantity, run at very high speed and are expected to last for a very long time with minimal maintenance.

Lecture 7 Cam Lubrication • Cam lubrication is also very important to reduce wear & heat (friction) • Could lead to early failure • Automotive cams are literally drowned in a flow of filtered and sometimes cooled engine oil. • Unless there’s a good reason not to use lubrication, a cam-follower should be provided with generous supply of clean lubricant • Camera mechanisms often run dry or else lubricant would find its way to the film

Lecture 7 Roller vs. Flat-Faced Follower • Roller follower is a better choice from a cam design standpoint simply because it accepts negative radius of curvature on the cam. • Allows more variety in the cam program • Reduced friction • Not too expensive • Flat follower can save space • Can be custom-designed • More expensive • Largest users are automobile engine makers  although many have switched to roller followers for improved friction/better fuel economy

Lecture 7

Lecture 7 Quiz • Sketch the equivalent fourbar linkage for the position of the cam and follower shown:

22.322 Mechanical Design II