It is the same with the pulley pattern as with most other patterns - the number of castings required and the complexity of the demands determine the method of molding. Several methods of molding a pulley, and the dependent pattern, are considered.
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.
Where a wood pattern for 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 obtained, 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 2 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 24 1/4 +1/4 = 24 1/2 inches, standard-rule measurements, or 24+1/4=24 1/4 inches, shrinkage-rule measurements. As a very smooth surface free from hole3 is required, \ 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 24 1/4+ 1/4+1/8 = 24 5/8 inches, with shrinkage rule. If the final thickness of the pulley rim is to be 3/8 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-rim pattern must be built up on a chuck, as described for the 20-inch by 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 6 1/2 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.
At least four pieces of stock about 3 inches long by 2 inches wide and | inch thick should be furnished the molder to bed in on the outside of the wooden-rim pattern at the mold parting, to permit casting lugs on the rim for clamping the casting to the faceplate while it is being finished to final dimensions, the casting being made wide enough to cut these lugs off when the lathe work is completed.
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 23 1/4 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:
In which - all dimensions being taken in inches b = the breadth of the arm at the outer end d = the outside diameter of the pulley u =the width of the rim n=the number of arms.
Thus, for a pulley 24 inches in diameter, with a rim 6 inches wide and fitted with 5 arms, the formula becomes:
= 1.53 inches, or say 1 1/2 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 1 1/2 inches wide at rim 3/4 inch thick at rim 1 7/8 inches wide at hub 1 inch thick at hub.
As a rule, all of the dimensions of the pulley should be furnished the pattern maker by the designer.