Machinery's Handbook, 31st Edition
Ratchet Gearing 2295 Fig. c, the pawl prevents the ratchet wheel from rotating in either direction, so long as it is in engagement with the wheel. The principle of multiple-pawl ratchet gearing is illustrated at Fig. d, which shows the use of two pawls. One of these pawls is longer than the other, by an amount equal to one- half the pitch of the ratchet-wheel teeth, so that the practical effect is that of reducing the pitch one-half. By placing a number of driving pawls side by side and proportioning their lengths according to the pitch of the teeth, a very fine feed can be obtained with a ratchet wheel of comparatively coarse pitch. This method of obtaining a fine feed from relatively coarse-pitch ratchets may be prefer able to the use of single ratchets of fine pitch which, although providing the feed required, may have considerably weaker teeth. The type of ratchet gearing shown at Fig. e is sometimes employed to impart a rotary movement to the ratchet wheel for both the forward and backward motions of the lever to which the two pawls are attached. A simple form of reversing ratchet is illustrated at Fig. f. The teeth of the wheel are so shaped that either side may be used for driving by simply changing the position of the dou ble-ended pawl, as indicated by the full and dotted lines. Another form of reversible ratchet gearing for shapers is illustrated at Fig. g. The pawl, in this case, instead of being a pivoted latch, is in the form of a plunger which is free to move in the direction of its axis, but is normally held into engagement with the ratchet wheel by a small spring. When the pawl is lifted and turned one-half revolution, the driving face then engages the opposite sides of the teeth and the ratchet wheel is given an intermittent rotary motion in the opposite direction. The frictional type of ratchet gearing differs from the designs previously referred to, in that there is no positive engagement between the driving and driven members of the ratchet mechanism, the motion being transmitted by frictional resistance. One type of frictional ratchet gearing is illustrated at Fig. h. Rollers or balls are placed between the ratchet wheel and an outer ring which, when turned in one direction, causes the rollers or balls to wedge between the wheel and ring as they move up the inclined edges of the teeth. Fig. i illustrates one method of utilizing ratchet gearing for moving the driven member in a straight line, as in the case of a lifting jack. The pawl g is pivoted to the operating lever of the jack and does the lifting, whereas the pawl h holds the load while the lifting pawl g is being returned preparatory to another lifting movement. Shape of Ratchet Wheel Teeth.— When designing ratchet gearing, it is important to so shape the teeth that the pawl will remain in engagement when a load is applied. The faces of the teeth which engage the end of the pawl should be in such relation with the center of the pawl pivot that a line perpendicular to the face of the engaging tooth will pass some where between the center of the ratchet wheel and the center of the pivot about which the pawl swings. This is true if the pawl pushes the ratchet wheel, or if the ratchet wheel pushes the pawl. However, if the pawl pulls the ratchet wheel or if the ratchet wheel pulls the pawl, the perpendicular from the face of the ratchet teeth should fall outside the pawl pivot center. Ratchet teeth may be either cut by a milling cutter having the correct angle, or hobbed in a gear-hobbing machine by the use of a special hob. Pitch of Ratchet Wheel Teeth.— The pitch of ratchet wheels used for holding suspended loads may be calculated by the following formula, in which P = circular pitch, in inches (mm), measured at the outside circumference; M = turning moment acting upon the ratchet wheel shaft, in inch-pounds (N-mm); L = length of tooth face (thickness of ratchet gear), in inches (millimeters); S = safe stress (for steel, 2500 pounds per square inch or 17 MPa when subjected to shock, and 4000 pounds per square inch or 28 MPa when not subjected to shock); N = number of teeth in ratchet wheel; F = a factor the value of which is 50 for ratchet gears with 12 teeth or less, 35 for gears having from 12 to 20 teeth, and 20 for gears having over 20 teeth:
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