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If a wire rope leads over a sheave and on to a drum, the rope will not remain in alignment with the sheave groove. Instead, it will deviate to either side depending upon the width of the drum and its distance from the fixed sheave, often called the lead sheave. The angle between the center line through the lead sheave and the centerline of the rope leading to the drum is called the fleet angle.
Experience has shown that the best wire rope service is obtained when the maximum fleet angle is not more than 1½° for smooth drums, and 2° for grooved drums. Fleet angles of 1 ½° and 2° are the equivalents of approximately 38 feet and 29 feet, respectively, of lead for each foot of drum width either side of the center line of the lead sheave.
Based on the above information, the correct distance (DLS) a lead sheave should be located from the winch drum may be derived by using the following formulas:
DLS for 1 ½° fleet angle = DCF (in feet) x 38
DLS for 2° fleet angle = DCF (in feet) x 29
Example: For a winch with a smooth drum thus requiring a 1 ½° fleet angle:
If DCF = 20 inches (1.66 ft) then DLS = 1.66 x 38 = approximately 63 feet, the distance that the lead sheave should be positioned away from the drum.
To obtain winch performance at operating pressures other than 90 psi, select the load or speed rating required from the applicable curve and multiply that value by the factor corresponding to the operating pressure from the table.
Example: For model BU7A with 1000 lbs (455 kg) line pull, 70 psi (4.9 bar), drum half full: Determine speed.
From performance curve at 90 psi (6.3 bar): 22 fpm (6.7 m/min) x 0.72 (rope speed factor from chart above) = 16 fpm (4.9 m/min)