Page 19 - Pipeline Rules of Thumb Handbook

6 Pipeline Rules of Thumb Handbook

Find advantages of block and tackle, taking into account pull out friction

The efficiency of various sheaves differs. For one with 1 - .917 4
roller bearings the efficiency has been estimated at 96.2%. MA = .917 = .917
1 - .917
For plain bearing sheaves a commonly used figure is 91.7%.
The following formula will give close results: 4
Ê Á 1 - . 707 ˆ ˜ = . 917 Ê . 293ˆ
W 1 - E n Ë 1 - . 917 ¯ Ë . 083 ¯
MA = = E
w 1 - E
MA = 3.25
where MA = Mechanical Advantage
W = Total weight to be lifted by the assembly
If the load weighed 3,250 pounds, what pull would be
w = Maximum line pull at the hoist
n required on the lead line?
= Number of working parts in the tackle
E = Efficiency of individual sheaves
W
It is assumed that the line leaving the upper block goes = MA
w
directly to the hoist without additional direction change
(requiring a snatch block).
,
3 250
.
= 325
Example. Find the Mechanical Advantage of a 4-part w
block and tackle using upper and lower blocks having journal
bearings, which have an efficiency of 91.7%. w = 1,000 pounds.

Safe loads for wire rope