The driving pulleys are set at a right angle to the length of the machine, their planes of revolution being, therefore, parallel to the plane of revolution of the line or driving shaft overhead, and parallel with the lathes and other machines driven from the same line of shafting, thus taking up less floor space, while the passage ways between the different lines of machines is less obstructed.
By setting the worm driving shaft at an angle the teeth of the worm rotate in a plane at a right angle to the length of the work-table rack, and as a result the teeth of the worm have contact across the full width of the rack teeth instead of in the middle only, as is the case when the axis of a worm is at a right angle to the axis of the wheel or rack that it drives.
Furthermore, by inclining the worm shaft at an angle the teeth of the rack may be straight (and not curved to suit the curvature of the worm after the manner of worm-wheels), because the contact between the worm and rack teeth begins at one side of the rack and passes by a rolling motion to the other, after the manner and possessing the advantages of Hook’s gearing as described in the remarks made with reference to gear-wheel teeth.
By inclining the worm shaft, however, the side thrust incidental to Hook’s gearing is avoided, the pressure of contact of tooth upon tooth being in the same direction and in line with the rack motion. As the contact between the worm teeth and the rack is uniform in amount and is also continuous, a very smooth and uniform motion is imparted to the work table, and the vibration usually accompanying the action of spur-gearing is avoided.
The worm has four separate spirals or teeth, hence the table rack is moved four teeth at each worm revolution, and a quick belt motion is obtained by the employment of pulleys of large diameter.
It is desirable that the belt motion of a planing machine be as quick as the conditions will permit, because the amount of power necessary to drive the machine can thus be obtained by a narrower belt, it being obvious that since the driving power of the belt is the product of its tension and velocity the greater the velocity the less the amount of tension may be to transmit a given amount of power.
Fig. 1566.
The mechanism for shifting the belt to reverse the direction of table motion is shown in [Fig. 1566] removed from all the other mechanism.