SAFE WORKING LOAD & SAFETY FACTOR

Safe working load formula

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).

Section 28(1)(c) of the Factories Act, prescribes as under:

“The maximum safe working load (SWL) shall be plainly marked on every hoist or lift, and no-load greater than such load shall be carried thereon”

Section 29(1)(b) of the same Act, prescribes as under:

“No lifting machine and no chain, rope or lifting tackle shall, except for the purpose of the test, be loaded beyond the safe working load which shall be plainly marked and duly entered in the prescribed register, and where this is not practicable, a table showing the safe working loads of every kind and size of lifting machine or chain, rope or lifting tackle in use shall be displayed in prominent positions on the premises”

Rule 60 of the Gujarat Factories Rules 1963 Safe working load require that:

  1. The safe working load has to be specified by a competent person after testing and examination.
  2. Where the safe working load may be varied by the raising or lowering of the jib, a table indicating the SWL at the corresponding indication of the jib or corresponding radii of the load shall be attached with the jib-crane.
  3. A table showing the SWL (Safe Working Load) of every kind and size of chain, rope or lifting tackle in use, and in case of multiple slings, the SWL at different angles of the legs, shall be posted in the storeroom.

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 of different types of the sling are given in the picture below:

Safe Working Load of different types of the sling are given in the table below:
WORKLOAD LIMIT OF SLING

Place of marking of Safe Working Load (SWL) of different equipment is shown in the table below:

Type of EquipmentPlace of Marking of SWL
Lifting eye boltSide of the eye. Size and type of thread also be marked.
ShacklesOn one leg SWL and on another leg identification mark and symbol are marked.
Chain SlingOn terminal fitting (ring) or on a separate idle link.
Multiple legged slingsSWL at 90° angle is marked on the upper terminal ring.
Wire ropesA metal sleeve or tag with marking is attached.
ChainsMarking for quality of material on every 20th link or 3 feet apart
HooksSWL 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 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 ItemFactor of Safety
1.Mild Steel Hook5
2.High Tensile Steel Hook5
3.Wire Rope Sling6
4.Endless Wire Rope Sling8
5.C Hook5
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
10Hook (General Purpose 25-100 Ton)As per IS: 7847
11.Chain Grade: 304
12. Chain Grade: 404
13. Chain Grade: 634
14. Chain Grade: 804
15.Chain Sling4
16.Eye Hook for Chain Sling4
17.Synthetic Fiber Rope
Diameter
12 mm.12
14-17 mm.10
18-23 mm.8
24-29 mm.7
40 and above6
Polyester Belt7 (For Belt)
4 (For Lifting Belt)

Proof Test

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.

Load testing of lifting sling, Proof testing of lifting equipment
Proof testing

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:

GradeTest load in Tons
3012 (d)2
4016 (d)2
6024 (d)2
8032 (d)2

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:

ItemProof 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 blocks4 SWL
Multiple sheave pulley blocks
Up to 20 Ton2 SWL
Up to 40 TonSWL + 20 Ton
Over 40 Ton1.5 SWL
Electric Hoist Cranes (Mobile & EOT)
Up to 20 Ton1.25 SWL
20 to 50 TonSWL + 5Ton
Over 50 Ton1.10 SWL
Endless slings4 SWL of single part
Derricks1.25 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.

Also, read this:

Safety in hydra crane

Safety in rigging and erection operations

Mechanical handling

Manual handling

EOT crane safety

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