SWL Of A Crane Whether Indicates The Capacity Of The Crane Or Hoist

Rope? If It Indicates The Crane Capacity According To The Boom Angle

And Working Radius Then What About The Hoist Rope? How To Determine

The Lifting Capacity Of The Hoist Rope In A Crane? Looking Forward A

Answer In Detail.

Thanks In Advance

KESAVA PILLAI -  Member Since: Dec 2009

Dear Vijay, I may be able to help you. But before I answer I need your views on

the following: "If you can lift 50 Kg, is it the capacity of your body or of only your

hands?" Regards, Kesava Pillai

VVIJI75 -  Member Since: Nov 2010

Dear Sir,

Thanks for your reply. I got your point. The crane boom alone can't withstand the

load and at the same time the hoist rope alone too. Apart from the integral

strength of the crane, there should be a calculation or selection criteria for the

hoist rope. I would like to learn the same. Pls guide me.

KESAVA PILLAI -  Member Since: Dec 2009

Dear Vijay,

1. Learn about wire rope construction – wire rope lays, regular lay and lang lay,

wire rope lay length, wire rope sizing grades, strand classification etc. 

2. Learn about breaking strength of wire rope: 

a) Diameter squared multiplied by the breaking strength of a one inch fibre or

wire core rope. i.e. D² x B.S. of 1 inch wire rope.

D² x 42 fibre core

D² x 45 wire core.

6 x 19 x 1 inch fibre core wire rope has a breaking strength of 42 tons.

6 x 19 x 1 inch wire core wire rope has a breaking strength of 45 tons. 

3. Breaking strength examples: 

1. ½ inch wire rope (fibre rope core): D² x 42= Breaking strength.

½ x ½ x 42 = 42/4 = 10.5 tons.

2. ½ inch wire rope (wire rope core): D² x 45= Breaking strength.

½ x ½ x 45 = 42/4 = 11.25 tons.

(All breaking strength formulas are based on a diameter of one inch and in a

tonnage ratio).

4. Safe Working Load: Most hoisting jobs use of a safe working load based on a

5:1 safety factor of wire trope breaking strength. This safety factor should go high

if there is possibility of injury or death due to breakage. For example elevators

are based on 20:1 safety factor.

Safe working load = breaking strength divided by safety factor.

Example: 1 inch I.W.R.C wire rope has a breaking strength of 45 Tons

S.W.L. = 45/5 = 9 Tons

5. Rule of thumb for S.W.L : D² x 8 = (S.W.L) in tons.(this is based on safety

factor of 5)

Example: safe working load of ½ inch wire rope = ½ x ½ x 8 = 2 tons

(These formulas cannot be used in metric systems).

Mobile cranes have minimum wire rope safety factors of 2.5 to 1, 3 to 1 3.5 to 1

depending upon location of and/or usage.

Having come so far now we have to go deeper in understanding selection criteria

for a crane hoisting rope.

1.Wire rope drums: At least three full wraps should remain on a drum in all

service conditions. Some areas the requirement may be five.

2.Wire rope blocks: These blocks are much lighter than the hook blocks. They

are equipped with cheek straps.

3.Mechanical Advantage ; The mechanical advantage of a machine is the

amount by which the machine multiplies the force applied to it in order to lift or

move a load.Here the machine is the pulley or a combination of pulleys forming a

block and tackle system

4.Calculate the mechanical advantage of a block system ; Count the number of

lines supporting the load, with the exemption of the lead line when it comes down

over the top block. The lead line pulling down is not counted for mechanical

advantage. If the lead line comes up to the winch from the travelling block it will

be counted as a supporting line and included in the mechanical advantage. 

5.The amount of wire rope needed for the system is determined by multiplying

the number of parts of line by travel distance of the load plus enough wire rope to

go to the winch and have at least five full wraps on the winch drum. 

There are charts available to calculate parts of line and if parts of line is taken to

calculate the maximum load.

Hope you get some insight from this note and It is too long to explain all these

here.

Regards,

Kesava Pillai

VVIJI75 -  Member Since: Nov 2010

Dear Sir, Thank you very much for your brief explanation. I have got clear ideas

regarding my queries and i will try to get further details from the net.