Method of drawing plastic film in a tenter frame

A method of drawing plastic film with linear motors using tenter clips attached to active and passive carriages, comprising the steps of:

• attaching a synchronous and a hysteresis secondary to each active carriage;
• placing a passive carriage between each active carriage;
• attaching a hysteresis secondary to each passive carriage;
• providing first linear motor primaries along the forward sides of a pair of opposed endless loops, the first primaries being positioned adjacent the synchronous secondaries and engaging the synchronous secondaries on the active carriages to draw the film;
• and
• providing second primaries along the return sides of each loop, the second primaries being positioned adjacent the hysteresis secondaries on both the active and passive carriages and engaging the hysteresis secondaries on both the active and passive carriages to return the carriages to the forward sides of the loops.

A method of accelerating at a constant rate a plurality of carriages having two different masses traveling along an elongated path, using a linear motor, comprising the steps of:

• attaching a first hysteresis secondary to a first carriage, the secondary having a size to develop a first linear motor propulsion force proportional to the mass of the first carriage;
• attaching a second hysteresis secondary to a second carriage, the secondary having a size to develop a second linear motor propulsion force proportional to the mass of the second carriage which is different from the first carriage, the proportion being the same as that for the first carriage;
• positioning a linear motor primary along the path, located adjacent the first and second hysteresis secondaries, and;
• engaging the first and second hysteresis secondaries with an electromagnetic wave developed by the primary to develop propulsion forces, the wave traveling at a speed different from the speed of the plurality of carriages so that the result of the first force divided by the first mass is essentially the same as the result of the second force divided by the second mass, thereby accelerating all carriages at essentially the same constant rate.

A method of controlling the propulsion of a plurality of carriages guided in opposed endless loops of a tenter frame for drawing film using linear motors, the loops having forward sides opposed to each other and return sides, and the carriages having film clips attached which grip the film during drawing, comprising the steps of:

• attaching a linear motor synchronous and hysteresis secondary to each carriage that is to actively draw the film;
• attaching a linear motor hysteresis secondary to each carriage that is to passively transport the film, the passive carriages being placed between adjacent active carriages;
• positioning first linear motor primaries adjacent the synchronous secondaries of the active carriages along the forward sides of each loop, and positioning second linear motor primaries adjacent the hysteresis secondaries of the active and passive carriages along the return sides of each loop;
• arranging the first primaries in each loop to include a plurality of groups of coils with the groups of coils in the primaries of one loop sized to match opposed groups of coils in the primaries of the other loop, and with each of the opposed groups of coils being electrically joined and defining a single control zone;
• arranging the second primaries in each loop to include a plurality of groups of coils with the groups of coils in the primaries of one loop sized to match opposed groups of coils in the primaries of the other loop, and with each of the opposed groups of coils being electrically joined and defining a single control zone;
• and
• energizing the control zones in a predetermined manner to thereby engage the carriages and propel the active carriages along the forward sides of the loops in paired symmetry to draw the gripped film and propel the active and passive carriages along the return sides of the loops in substantial symmetry after the film is released by the film clips.