Kennedy Wire Rope & Sling, Co.

Crosby's "Rigging with Snatch Blocks" Calculator

Block Loading Calculator

Tackle Block Information Continued:

Example

Using a 14 in. sheave (Stock # 917191; refer to wire rope sheave section of General Catalog for dimensions) with a 4600 lb. line pull and an 80 angle between lines determine maximum allowable line speed.
BP = (4600 lb. x 1.53) / (1.50 x 1.62) = 2896 PSI

BV = 55000 / 2896 = 19 FPM ALLOWABLE

(PV Factor) (BP)

Line Speed =
19 x (12 +.75) / 1.50 = 161.5 FPM ALLOWABLE

If the application required a line speed equal to 200 FPM, then another calculation would be necessary. Trying another 14 in. sheave (stock #4104828) under the same loading conditions, the results are as follows:

BP = (4600 lbs. x 1.53) / (2.75 x 2.31) =1108 PSI
BV = 55000 / 1108 = 50 FPM

Line Speed =
50 x (12.25 + .75) / 2.75 =236 FPM ALLOWABLE

COMMON (PLAIN) BORE -
	Very slow line speed, very infrequent use, low load

ROLLER BEARING -
	Faster line speed, more frequent use, greater load. Refer to manufacturer's rating. Reference appropriate bearing manufacturer's catalog for proper bearing selection procedure.

Loads on Blocks

The Working Load Limit (WLL) for Crosby Group blocks indicates the maximum load that should be exerted on the block and its connecting fitting. This total load value may be different from the weight being lifted or pulled by a hoisting or hauling system. It is necessary to determine the total load being imposed on each block in the system to properly determine the rated capacity block to be used. A single sheave block used to change load line direction can be subjected to total loads greatly different from the weight being lifted or pulled.

The total load values varies with the angle between the incoming and departing lines to the block.
The following chart indicates the factor to be multiplied by the line pull to obtain the total load on the block.

Angle Factor Multipliers
Angle	Factor	Angle	Factor
0°	2.00	100°	1.29
10°	1.99	110°	1.15
20°	1.97	120°	1.00
30°	1.93	130°	.84
40°	1.87	135°	.76
45°	1.84	140°	.68
50°	1.81	150°	.52
60°	1.73	160°	.35
70°	1.64	170°	.17
80°	1.53	180°	.00
90°	1.41	—	—

Example A
(Calculations for determining total load value on single line system.)
A gin pole truck lifting 1,000 lbs.

There is no mechanical advantage to a single part load line system, so winch line pull is equal to 1,000 lbs. or the weight being lifted.

To determine total load on snatch block A:
A = 1,000 lbs. x 1.81 = 1,810 lbs.
(line pull) (factor 50° angle)

To determine total load on toggle block B:
B = 1,000 lbs. x .76 = 760 lbs.
(line pull) (factor 135° angle)

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Catalog provided courtesy of The Crosby Group, Inc.

	Block Warnings
	Page 293