This section is from the book "Lathe Design, Construction And Operation, With Practical Examples Of The Lathe Work", by Oscar E. Perrigo. Also available from Amazon: Lathe Design: Construction And Operation.
Before starting to do any work on the lathe, it is important to become familiar with the method of working the various control levers and wheels and of locking the adjustments to prevent movement when they are set properly. The first thing to do is to try and run the spindle drive pulley cone free. This is done by making sure back gears are not in mesh and releasing bull gear clamp or locking pin that joins the large gear just back of the front spindle bearing to the pulley group. If the lathe runs freely in this position, which is called "in open belt" by some machinists, throw in the back gears and notice that spindle turns slower than the pulley group. This is done by rocking the lever F, Fig. 318-A, so the gears on back gear quill mesh with those on the spindle. The gears should not mesh deep enough to bottom. To run lathe on direct drive throw out back gears and lock the bull gear clamp. The pulley group and lathe spindle speeds are now the same. Lathe manufacturers caution the operator not to throw back gears in or out when lathe is running. To connect the reverse gear, rock the lever E, Fig. 319-B, till either gear A or B is in mesh with C, depending upon the direction of lead screw rotation desired. The same caution given about putting back gears in mesh with lathe running applies just as well in this case.