What is the safe working load (SWL)?
Safe Working Load (SWL) is the limiting safety factor to lift and carry any load safely. It must be clearly marked on any lifting device (hoist, lifts, lifting machines, and lifting tackles).
Marking System of Safe Working Load
Lifting equipment should have a tally plate indicating the Safe Working Load. The tally plate also indicates the identification number which can be mentioned in the test certificate held by the user. It should also indicate the date of the last inspection.
Safe Working Load ( Or WLL) of different types of sling are given in the picture below:
Where is the WLL mark placed on the body of lifting appliances
Place of marking of Safe Working Load (SWL) of different equipment is shown in the table below:
|Type of Equipment||Place of Marking of SWL|
|Lifting eye bolt||Side of the eye. Size and type of thread also be marked.|
|Shackles||On one leg SWL and on another leg identification mark and symbol are marked.|
|Chain Sling||On terminal fitting (ring) or on a separate idle link.|
|Multiple legged slings||SWL at 90° angle is marked on the upper terminal ring.|
|Wire ropes||A metal sleeve or tag with marking is attached.|
|Chains||Marking for quality of material on every 20th link or 3 feet apart|
|Hooks||SWL on one side of the non-vital part and on the reverse side the identification mark.|
Where marking on the body part is not possible it should be shown by tagging
Calculation of Safe Working Load (SWL):
For wire rope, FS is 6 for general purpose and 7 for heavy industry. A thumb rule formula is
SWL = (Wire rope dia) 2 × 8 kgs
Example – For 12 mm dia wire rope
SWL = 12 × 12 × 8 = 1152kgs
For the sling, SWL depends on the length of the sling leg (L) and the headroom between the hook and the load (H). Then for bridge hitch
For single hitch
SWL = SWL of single verticle hitch × H/L × 2
For double basket hitch
SWL = SWL of single verticle hitch × H/L × 4
For hoist, lift, cranes, etc., Safe working Load recommended by the manufacturer.
Safe Working Load (SWL) of any mobile crane depends on the operator’s skill, condition of the ground, boom length, the radius of rotation while lifting the load, the inclination of the boom to the verticle and outrigger blocked or free.
Safe Working Load is generally tabulated in the load chart of the crane. Sometimes, it is de-rated(decreased) due to defect in welding, bend in angle, bracing, etc. and condition of clutch, brake, etc. Modern cranes give a digital display of SWL, angle indicator, boom limit switch and alarm for exceeding load.
The factor of safety / Safety factor
The factor of the safety (Safety Factor) of fiber ropes varies from 6 to 12 mm depending on conditions of use. fiber rope less than 12 mm dia should not be used for a sling or apart of a lifting appliance. Their factor of safety (FS) varies with diameter. The factor of Safety for the hook, wire rope sling, chain, fiber rope, and belt are given in the table below:
|No.||Lifting Item||Factor of Safety|
|1.||Mild Steel Hook||5|
|2.||High Tensile Steel Hook||5|
|3.||Wire Rope Sling||6|
|4.||Endless Wire Rope Sling||8|
|6.||Shank Hook (General Purpose)||5|
|7.||C Hook with Eye (0-25 Ton)||4|
|8.||Point Hook with Shank (63-160 Ton)||5|
|9.||Hook (General Purpose 1-25 Ton)||4|
|10||Hook (General Purpose 25-100 Ton)||As per IS: 7847|
|11.||Chain Grade: 30||4|
|12.||Chain Grade: 40||4|
|13.||Chain Grade: 63||4|
|14.||Chain Grade: 80||4|
|16.||Eye Hook for Chain Sling||4|
|17.||Synthetic Fiber Rope|
|40 and above||6|
|Polyester Belt||7 (For Belt)
4 (For Lifting Belt)
Proof testing is the application of a load greater than the SWL (Safe Working Load) to detect defective workmanship, faulty weld or other inherent weaknesses. It is not a means to assess the SWL which should only be done by calculations and checked where necessary by suitable tests on samples.
The proof test is required as a part of ‘thorough examination’ u/r 60(1) of GFR and no lifting machine or tackle should be used for the first time without this proof test.
In general, the proof load applied to chains, rings, hooks, shackles, and similar gear is twice the SWL. It should be just under the yield stress for the material.
The standard proof load for mild steel or wrought iron short link chain is as under:
|Grade||Test load in Tons|
Where ‘d’ is the diameter of the material from which the chain is made.
All items used in lifting machinery should be subjected to proof test loads as under:
|Item||Proof Test Load|
|Chain, ring, hook, shackles, swivel, sling, individual components of the hoist, wire rope, chain, pulleys, hooks, eye bolts, pins, axles, bearings, turnbuckles & ringing screws.||2 SWL|
|Single sheave pulley blocks||4 SWL|
|Multiple sheave pully blocks|
|Up to 20 Ton||2 SWL|
|Up to 40 Ton||SWL + 20 Ton|
|Over 40 Ton||1.5 SWL|
|Electric Hoist Cranes (Mobile & EOT)|
|Up to 20 Ton||1.25 SWL|
|20 to 50 Ton||SWL + 5Ton|
|Over 50 Ton||1.10 SWL|
|Endless slings||4 SWL of single part|
|Derricks||1.25 SWL static & 1 SWL for all full movement|
After the above proof test, the parts are to be examined thoroughly by a competent person for signs of cracks, fatigue, deformation, permanent stretch, etc.