The pulley being intended for a 1¾-inch shaft, the core prints x and y, Fig. 173, should be 1½ inches in diameter, which will give 1/8 inch of metal for boring out to fit the shaft. The two core prints (Fig. 175) should be turned separate from the hubs, and loosely attached with a pin ¾ inch in diameter, and ¾ inch long, into each half hub, so that other sizes may be used for larger or smaller shafts. The two half hubs are often made loose so that they may be changed for larger or smaller hubs as may be required for different sizes of shafts. In such cases they are attached centrally to the arms as described for the core prints.
A second method in the construction of such special pulleys is as follows:
The half rim having been glued up as described, the inside only of this half is turned to the required dimensions and draft, sand-papered, and finished, the width of the half rim being made in this case 5/16 inch less than the half of the face of the completed pattern. The arms are carefully centered and glued to this half rim; and the intervening spaces between the ends of the arms are filled in with six segments 11/16 inch in thickness, which, when glued and dry, are planed, not turned, true and even with the surface of the arms.
A layer of segments of the regular thickness is fitted and glued on over the arms, and layer after layer continued until the full width of the face of the pattern is reached, thus building the arms directly into their place in the pattern as the rim is glued up. In turning and finishing, care must be taken not to strike the arms with the tools while turning out the inside of the outer half of the rim.
This method will be found convenient for all pulleys of moderate width of face; but as the spaces between the arms cannot be turned out, great inconveniences in reaching these places will be found when the face of the pulley is twelve or more inches in width. These spaces must be dressed out by hand.
The instructions regarding the construction of the last three patterns should be studied carefully, because the general points involved enter largely into the construction of patterns of all kinds, and especially for all work having arms with circular rims.
When pulleys of standard sizes for line shafting are manufactured in quantities, a skeleton pattern consisting of hub, arms, and an independent iron rim is used. This iron rim is of moderate width but may be used for obtaining any width of face desired.
Wooden patterns complete in themselves, as that described for Fig. 173, are used for all special pulleys on machines when the required sizes and widths, as also hubs and connections, are irregular and designed only for the special machine, so that the making of pulley patterns is important in nearly all foundries and pattern shops.
Where the iron rim is to be made, the same care is necessary in the building up of the original wooden pattern. It must be remembered that before the final casting is obtaiued, two shrinkages will take place; first, the shrinkage of the original casting from which the iron ring is turned, and then the shrinkage of the casting made from this pattern. In addition to this, there must be the allowance for turning the ring both inside and out and for the turning of the outside pulley rim. Suppose the pattern is to be made for a pulley two feet in diameter. The usual allowance for a single shrinkage is made by the shrinkage rule. In this case the allowance must be doubled. Thus in the above pulley, the diameter of the wooden pattern becomes 24¼ + ¼ = 24 1/8 inches, standard rule measurements, or 24 + ¼ = 24¼ inches, shrinkage-rule measurements. As a very smooth surface, free from holes, is required, ¼ inch in diameter, or 1/8 inch all around, must be allowed for outside finish on the iron ring, and 1/8 inch for finish on the rim of the cast-iron pulley.
The outside diameter of the original wooden pattern is 24¼ + ¼ + 1/8 = 24 5/8 inches, with shrinkage rule. If the final thickness of the pulley rim is to be § inch, this, with the allowance of 1/8 inch for turning out the inside of the iron ring, makes the inside diameter of the wooden pattern 23 inches, and the thickness of the wooden rim 13/16 inch, all shrinkage-rule measurements.
This wooden-ring pattern must be built up on a chuck, as described for the 20-inch X 6-inch pulley, the segments, six in number for each layer, fitted, glued, and clamped with three hand screws to each segment until a width of 6½ inches is reached.
It is then turned to the above dimensions, without any draft, and sent to the foundry, where it may be used for obtaining an iron rim of any required width by first ramming the sand about the pattern, partly drawing it, and then ramming again to a new level.
The casting thus obtained is then turned to the dimensions called for by an ordinary pattern; that is to say, the shrinkage-rule measurements would leave it 23½ inches in diameter on the inside and 24 1/8 inches on the outside, permitting a final finishing of the outside of the rim of the pulley to a diameter of 24 inches. When this is done, two 3/8-inch holes should be drilled near one edge of the rim and diametrically opposite each other, into which hooks may be inserted for drawing the pattern. This rim should also be turned straight and without any draft.
The arms are usually made with a wooden pattern, which has a dowel-pin hole on each side at the center for attaching the hubs that are loose, the object being to change their length and diameter to suit the width of the rim and the diameter of the shaft upon which the pulley is likely to be placed.
The arms of all pulleys should be straight because of the greater strength given to the pulley as a whole, the driving and resisting power being at least one-third greater than in a pulley of the same dimensions having curved arms. Curved and shaped arms of all kinds are now used only for ornamental purposes and for very light work.
The arms should be six in number, except for very small pulleys, when five and even four are often used. The dimensions of the arms vary greatly, depending on the purpose for which the pulley is to be used, and the weight of the machinery to be driven.
For the beginner the following formula is safe to follow: b = ∛dxw/nx8, in which b = the breadth of the arm at the outer end, d = the outside diameter of the pulley, to = the width of the rim, n = the number of arms, all dimensions being taken in inches. Thus, for a pulley 24 inches in diameter with a rim 6 inches wide and fitted with 5 arms, the formula becomes.
b = ∛24x6/5x8 = ∛/3.6
Hence, b = 1.53 inches or 1½ inches.
The width of the arm should be one-fourth greater at the hub than at the rim. The thickness at the hub and rim should be one-half the width, and the section should be elliptical. The arm just calculated then becomes, 1½ inches wide at rim, 3/4 inch thick at rim, 1 7/8 inches wide at hub, 1 inch thick at hub.
For the skeleton pattern last described, the common method of constructing the pattern for the arms, is to make each arm of a separate piece of wood with the grain running in the general direction of the arm, and to fasten them together at the center with glue and a flat plate or disc, which can also be used as a rapping plate. This pattern need be parted only in the case of very large and heavy wheels. For all ordinary work it can be made in one piece and moulded as directed in connection with the hand wheel, Fig. 167.