BASIC INSPECTION CRITERIA FOR WIRE ROPE SLINGS
The goal of a sling inspection is to evaluate the remaining strength in a sling, which has been used previously, to determine if it is suitable for continued use.
Specific inspection intervals and procedures are required by local safety regulations and by ASTM B30.9. The responsibility for having the inspection done is placed upon the SLING USER.
As a starting point, the same work practices which apply to all "working" wire ropes also apply to wire rope slings. Therefore, a good working knowledge of wire rope design and construction will not only be useful but essential in conducting a wire rope sling inspection.
Since a wire rope is a rather complex "machine", no precise rules can be given to determine exactly when a wire rope sling should be replaced. There are many variables, and all must be considered.
1. Broken Wires: For 6-strand wire rope slings, 10 randomly distributed broken wires in one rope lay, or five broken wires in one strand of one rope lay. For multi part slings these same rules apply to each of the component ropes.
2. Metal Loss: Wear or scraping of one-third the original diameter of outside individual wires. This is quite difficult to determine on slings and you require some experience to perform this evaluation.
3. Distortion: Kinking, crushing, birdcaging, or other damage which distorts the rope structure. The main thing to look for are wires or strands that are pushed out of their original position in the rope. Slight bends in a rope where wires or strands are relatively in their original position would not be considered serious damage. But good judgement is indicated.
4. Heat Damage: Any metallic discoloration caused by exposure to heat.
5. Bad End Attachments: Cracked, bent or broken end fittings caused by abuse, wear or accident.
6. Bent Hooks: No more than 10% over the normal throat openings, measured to the narrowest point, or twisting is permissible. Replace hooks if latches do not close against the tip of the hook. Replace any missing- or damaged latches.
7. Corrosion: Severe corrosion of the rope or end attachments which has caused pitting or binding of wires should be cause for replacement. Light rusting usually does not affect the strength of a sling.
8. Pulled Eye Splices: Any evidence that eye splices have slipped, tucked strands have moved, or pressed sleeves show serious damage may be sufficient cause to reject a sling.
9. Mechanical Damages: One of the most common causes of damage is the kink which results from pulling the sling body through the loop (choker hitching), thus causing wires or strands to be deformed and pushed out of their original position. This unbalances the sling, reducing its strength.
10. Disposition: The best inspection program available is of no value if slings which are worn out and have been retired are not disposed of properly. Retired slings should be tagged DO NOT USE to prevent any further usage. The sling should be destroyed as soon as possible by cutting the eye and fittings from the rope.
Basic Factors Concerning the Use of Wire Rope Slings
1. RATED CAPACITY (Rated Load, WLL) of a wire rope sling is based upon the Nominal Breaking Strength of the wire rope used in the sling, AND FACTORS which affect the overall strength of a sling. These factors include ATTACHMENT or SPLICING EFFICIENCY, the number of parts of rope in the sling, type of hitch (see below), DIAMETER AROUND WHICH THE BODY OF THE SLING IS BENT, and the diameter of pin (or hook) over which the eye of the sling is rigged.
2. RATED CAPACITY of a sling is different for each of the three basic methods of rigging (see below)
3. WARNING: A hand tucked (hand spliced) eye sling can unlay (unravel) and FAIL if the sling is allowed to rotate during use.
4. NEVER "SHOCK LOAD" a sling. There is no practical way to estimate the actual force applied by shock loading. The rated capacity of a wire rope sling can easily be exceeded by a sudden application of force, and damage can occur to the sling. The sudden release of a load can also damage a sling.
5. The BODY of a wire rope sling should be protected with corner protectors, blocking or padding against damage by sharp edges or corners of a load being lifted. Sharp bends that distort the sling body damage the wire rope and reduce its strength.
6. ANY ANGLE other than vertical at which the sling is rigged, increases the loading (tension) on the sling.
7. A sling should be given a VISUAL INSPECTION BEFORE EACH LIFT OR USAGE to determine if it is capable of safely making the intended lift.
8. Whenever a sling is found to be deficient, the eyes must be cut, or other end attachments or fittings removed to prevent further use, and the sling body discarded.
9. A SLING EYE should never be used over a hook or pin with a body diameter larger than the natural width of the eye. NEVER FORCE AN EYE ONTO A HOOK. The eye should always be used on a hook or pin with AT LEAST THE DIAMETER OF THE ROPE.
Every Lift uses 1 of 3 Basic Hitches
VERTICAL, or straight, attachment is simply using a sling to connect a lifting hook or other device to a load. Full rated load of the sling may be used, but never exceeded. A tagline should be used on such a lift to prevent rotation which can damage the sling. A sling with a hand-tucked splice may unlay and fail if the sling is allowed to rotate.
CHOKER hitches reduce lifting capacity of a sling, since this method of rigging affects the ability of the wire rope components to adjust during the lift, places angular loading on the body of the sling, and creates a small diameter bend in the sling at the choke point.
BASKET hitches distribute the load equally between the two legs of a sling, within limitations imposed by the angles at which legs are rigged to the load. (See discussion of sling angles)
Frequency of Inspections
Both ASME B.30.9 and most Provincial Regulations require that wire
rope slings receive two types of inspections:
- PRIOR TO USE visual inspection, and additional inspections where severe conditions warrant.
- Daily inspections are intended to detect serious damage or
deterioration which would weaken the sling. Look for obvious things,
such as broken wires, kinks, crushing, broken attachments, severe
Additional inspections must be carried out by a designated person
who must have good knowledge of wire rope.
The frequency of these regular inspections should be based on:
- frequency of sling use
- severity of service conditions
- nature of lifts
- prior experience based on service life of slings used in similar
An inspection should look for:
- Broken wires.
- Kinks or distortions of the sling body.
- Condition of eyes and splices, and any attachment hardware.
- Reduction in diameter of the rope.
- Any damage.
INSPECTION CRITERIA FOR WIRE ROPE
- Capacity information is missing or illegible;
- End attachments, including hooks, are cracked, deformed or obviously worn;
- Hook throat opening is increased more than 15%;
- Hook is twisted out of plane by more than 10%.
THE DAMAGE: Broken Wires
WHAT TO LOOK FOR: The individual wires that make up the strands in a wire rope can break for various reasons including fatigue and overload. Wire rope slings must be taken out of service when you find 10 or more broken wires in one rope lay or 5 or more broken wires in one strand of one rope lay.
TO PREVENT: Avoid pulling rope across edges or protrusions.
THE DAMAGE: Wear
WHAT TO LOOK FOR: Flat areas on the individual wires. When wires have lost one third or more of their original diameter, the sling must be taken out of service.
TO PREVENT: Do not drag sling on the ground and do not drag loads over slings. Pad high wear areas.
THE DAMAGE: Corrosion / Heat Damage
WHAT TO LOOK FOR: Absence of lubrication and discoloration of rope.
TO PREVENT: Hang slings for storage away from moisture. Do not use wire core slings above 400° F or fiber core slings above 180° F.
THE DAMAGE: Kinking, Bird Caging
WHAT TO LOOK FOR: Bent strands of wire or strands standing out from their regular position in the body of the sling.
TO PREVENT: Protect rope from sharp edges of load by pads or other means. Do not shock load slings.
THE DAMAGE: Crushing
WHAT TO LOOK FOR: A section of rope that is flattened, where the cross section is no longer round.
TO PREVENT: Never allow loads to be set on top of slings.
Note: Alberta OH&S now requires wire rope slings to have permanently affixed and legible identification markings.