Page 393 equipment and instruments 4641 



and which can be reset as desired. It is geared to the shaft of the sheave by bevel gears 

 and is positive acting. The older type of indicator is merely a pointer moving around 

 a dial and is connected to the shaft by a worm gear. Any side play of the wheel in the 

 yoke decreases the accuracy of soundings measured with this type of indicator. 



Only one size of sheave is used and its circumference is such that when piano wire 

 is used one revolution will measure exactly 2 feet, or Yz fathom. When stranded wire 

 is used on this sheave each reading will be in error by about 0.5 percent because of 

 the larger diameter of the stranded wire. The readings are too low by 0.48 percent, 

 or about 34}^ inches in 100 fathoms. All soundings measured with stranded wire should 

 be corrected by the sheave factor. For most other purposes an error of this magnitude 

 will be negligible, but a correction can be made where necessar3^ 



In sounding, the wire must be in contact with the bottom of the groove in the 

 sheave for at least one complete turn to prevent slipping. With the usual arrangement 

 there is from Iji to Iji turns of wu'e on the sheave. 



A registering sheave in good condition should never be used solely as a fair-lead. 

 Cheaper devices should be used for this purpose, but if not available, a registering sheave 

 that has been discarded as a measuring device because of wear or inaccuracy may be 

 employed. 



4641. Registering Sheave Calibration 



A registering sheave must be calibrated before it is used for deep soundings or any 

 other pm-pose requiring much accuracy, and the calibration should be verified several 

 times during the season, if it is used extensively. The sheave may be calibrated most 

 accurately by running the wire over it for an accurately measured distance along a 

 wharf or other level space. A spring balance should be used to apply a tension to the 

 wire during the test equal to the tension on it while sounding. 



Another, but less accurate, method is to measure the circumference directly by 

 stretching several turns of wire around the sheave, marking it, and measuring the length 

 of the wire when laid out straight. The marks on the turns of wire must be parallel to 

 the axis of the sheave. This is similar to one of the methods described in 4467 for cali- 

 brating the taut-wire apparatus, and the same precautions should be observed. When 

 this method is used to calibrate a sheave with the pointer-and-dial type of revolution 

 counter, a verification of the counter is also required. The sheave should be marked 

 and turned a definite number of revolutions and the counter read for different numbers 

 of revolutions. The measured circumference multiplied by the sheave revolutions will 

 give the actual distances represented by the counter readings. 



Correction factors, known as sheave factors, should be computed for piano wire and 

 stranded wire, if both are likely to be used. With the type of sheave now used, the 

 correction for piano wu'e is usually negligible, and the correction for stranded wire will 

 closely approximate plus 0.48 fathom per 100 fathoms (see 464). 



The sheave factors should be reported in the pertinent Descriptive Reports and 

 special reports. 



4642. Sheave Wear 



The accuracy of a registering sheave is impaired when it becomes loose in its bear- 

 ings, or is scored by the wire. After a sheave has been in use for some time it should be 

 tested more frequently and be no longer used as a measuring device when proved un- 

 reliable. Worn sheaves may be used as fair-leads or returned to the Washington Office. 

 A sheave should be oiled frequently and properly, and should always be handled care- 



