Mechanical Design Handbook

Many engineering applications require things move in a linear fashion or " straight-line motion ". We can use a linear motion guide that can guide a device accurately along a straight line. Manufacturing know-how of most linear guide manufacturers has let us keep expanding the range of linear guidance. The picture shown here is an example of commercially available linear guides from THK. This Linear Ball Slide is a lightweight, compact, limited stroke linear guide unit that operates with very low sliding resistance. It excels in high-speed responsive performance due to its very small frictional factor and low inertia. In the late seventeenth century, before the development of the milling machine, it was extremely difficult to machine straight, flat surfaces. For this reason, good prismatic pairs without backlash were not easy to make. During that era, much thought was given to the problem of attaining a straight-line motion as a part of the coupler curve of a linkage hav

In [ 3-Position Synthesis with Inversion Method using Unigraphics NX4 Sketch - Part 2 ], we've shown an example to do three-position synthesis of  a four-bar linkage using inversion method in Unigraphics NX4 sketch. We can make a quick motion simulation using "animate dimension" command in Unigraphics (UG) NX4 sketch. Just draw lines as per a sketch and add one driving dimension as shown below. Then use animate dimension command to set the lower and upper limits, for this case they're minimum and maximum angles.

In previous posts , the fixed pivot points were determined from the moving pivot points. We can get result that can't be fitted in our design due to space limit. The principle of inversion can be applied to solve this problem. The first step is to find the three positions of the ground plan that correspond to the three desired coupler positions. We start with our desired positions of fixed pivot points. 1) Draw desired fixed pivots (O 2 and O 4 ) and moving pivot points. Red lines are three desired positions of links (moving pivots). 2) Draw lines to make fixed relations between the ground plane (O 2 O 4 ) and the second coupler position. 3) Transfer the ground position to the first coupler position using same relations developed in previous step as shown in dashed lines. Name new ground positions as O' 2 and O' 4 respectively. 4) Draw lines to make fixed relations between the ground plane (O 2 O 4 ) and the third coupler position (A 3 B 3 ). 5) T