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HOME MOVIES FOR JULY
PACE 279
• Cross-section diagram showing hydraulic timer in detail. Bucket in dumping position operates cable link to starting button. After water is emptied, bucket returns to normal position to be filled again with water for the subsequent operation. Size of openings in top of bucket controls length of interval.
figures 3 and 4. The bucket and counterweight are so designed that the maximum moment of the counterweight equals slightly less than the moment of the bucket and water contained therein at the filling position, and shifts in location as the bucket dumps, so that the minimum moment of the counterweight at the end of the stroke is slightly in excess of the empty bucket's moment in the dumping position. If the camera release is connected direct at one quarter the length of the bucket, from the pivot, the weight of the water contained in the bucket should be slightly in excess of half the force found necessary to operate the camera release. To obtain operation of the bucket through 45 degrees of arc, the following formulas indicate that the weight of the bucket must not exceed the total weight of the water contained:
Let W=weight of water in bucket B = weight of bucket L = length of bucket a = angle between filling and dumping
positions S — weight of counter weight R = distance of counter weight from pivot f = pivot friction
Then:
f + SR
BL
WL
cos a -| 1 cos a
(See Fig. 3, bucket filled with water ready to dump)
and:
SR +
BL
+ f
(See Fig. 4, bucket empty, ready to return to fill position)
Substituting, we have: (disregarding frictional torque f)
BL BL cos a WL cos a = +
W
which reduces to
SLOT FOR
FIG 2
then W — B where angle a = 45 c or W = 3B where angle a = 60 c
Where the angle and weight ratio do not fit the above formulas exactly, the counterweight can be located to satisfy the balance requirements as follows: Find the maximum and minimum moments and divide by the total pounds of the counterweight to be used ; draw a scale diagram of the timer similar to figure 5 ; erect perpendiculars from the base at distances required for the maximum and minimum moments, then locate a sector of operation about the pivot point by trial and error where the sector angle cuts the perpendiculars at
• Diagrams below show arc move/nent of bucket and counterweight and give formulas for calculating weight and size of bucket for the necessary result.
an equal radius from the pivot point. (The weight of the pivot block does not enter into the calculation if the pivot point axis passes through its center of gravity.)
Cameras adaptable for use with this timer should not have a release mechanism that requires more than a four pound force through a half-inch moment. This requires a container holding about two pounds or about 32 ounces. Tin cans are the easiest procured material for the purpose. A tall tomato or fruit juice can of 46 ounce capacity will give more than the required capacity. Cut two holes 1800 apart in the top at the rim, leaving about 70% of the can • Continued on Page 292
FIG. 5
FIG. 4
FIG. 3