Wire rope sheaves on cranes are one of the critical structural components that must always maintain their structural integrity in all physical environments to ensure safe crane operation in the field. Sheave failure can lead to severe accidents.
Sheave inspection is crucial to the safe operation of your crane and a key component in reducing cost associated with replacing wire ropes. Typically, we see sheave bearing or bushes being replaced when worn and certainly these are good reasons to replace them, however, as the groove profile of a sheave changes due to normal use it will cause the wire rope to become deformed which will weaken the integrity and life span of the ropes. Always inspect the sheaves groove for excessive wear at the same time as renewing the bearings/bushes.
During normal operations the cast sheave and drum grooves are under constant pressure. The groove contours must be checked when ropes are changed as tight groove contours as a result of wear can have a dramatic effect on rope life.
A tight sheave will cause increased groove pressures at the point of contact. This is indicated when a rope is showing two parallel planes of wear and/or breakage approximately 120° to 150° apart.
A rope may eventually become “sized” to the groove and rope life will suffer. To determine if the groove is worn below recommended minimums use the correct sheave gauge to inspect the groove.
Two types of groove gauges are in general use and it is important to note which of these is being used.
The two differ by their respective percentage over nominal values from a new sheave. The groove gauge is nominal plus the full oversize percentage, or between 4 – 5% wear.
The gauge carried by most Lifting Machine Inspectors today is used for worn grooves and is manufactured as nominal plus one half the full oversize percentage, or 2.5%. This latter gauge is intended to act as a sort of “no-go” gauge.
Polyamide sheaves due to their nature are extremely difficult to obtain an accurate measurement of groove wear in the sheave wheels using conventional methods such as sheave gauges.
Correct maintenance of all the types of sheaves on which the ropes operate has an important bearing on rope life. Worn grooves, poor alignment of sheaves and worn parts may result in shock loads and excessive rope vibration that will have a deteriorating effect on the sheaves, ropes and spooling of the rope on to the hoist drum or drums.
The sheaves and bearings need to be inspected prior to each new shift using the crane manufacturers recommended check list and are generally as follows.
Frequent Inspection – visual examinations by the operator or other designated personnel using a recommended inspection check list.
Periodic Inspection – visual inspections by an appointed person making records of apparent external conditions to provide the basis for continuing evaluation. This must be recorded in the crane’s history log book.
Annual Inspection – visual/physical inspection by a competent person or Lifting Machine Inspector (LMI) which may necessitate the removal of the sheaves for closer inspection and the use of an appropriate NDT test procedure for cast or dual web sheaves. The results of the inspection shall be recorded.
HOW AND WHAT TO INSPECT
- To undertake these inspections the cranes hoist and/or derrick wire ropes must be at rest (slack) to check for excessive wear, physical defects, or damage (chips, cracks, broken flanges, flat spots in grooves, sheave walking of the hubs, rope imprints in the sheave etc.). Replace worn or damaged sheaves and possibly the hoist rope in the case of imprints in the sheaves.
- Inspect sheave to verify that they do not make contact with other sheaves or structural plates of the hook block or boom head. Repair or replace worn or damaged parts.
- Inspect the devices that retain the hoist rope within the sheave groove or flange. If damaged or missing repair or renew.
- Check the bearings for lubrication, signs of wobble and ease of rotation. Worn bearings cause vibration in the rope increasing wire rope fatigue. Replace the bearings or replace the sheaves if any signs of damage is detected.
- Examine the sheave grooves for wear and correct diameter. To check the size contour and amount of wear use the sheave gauge. Measure at three positions of every sheave wheel. The gage should contact the groove at about 120° to 150° of arc of contact between the rope and sheave groove. Note: – different countries have a different standards for sheave groove angles.
When inspecting synthetic sheaves or synthetic-lined steel sheaves, the inspector must carefully examine the rope for diameter reduction or lengthening of rope lay even if no visible damage is observed. Synthetic sheaves can greatly increase the contact area between the wire rope and sheave, by cushioning the rope. This cushioning effect may cause wire rope to wear internally (wire rope operating on steel sheaves will usually wear externally) before the damage is detected on the outer wires.
This situation places the inspector at a great disadvantage; therefore, he/she must be diligent in the detection of diameter reduction and lay lengthening to prevent catastrophic failure from internal core damage.
Polyamide sheave bearing replacement.
If a bearing has to be replaced due to unforeseen circumstances and owing to the inherently resilient and flexing of the material, some crane/sheave manufacturers recommend that the complete polyamide sheave wheel be replaced if the bearing or bushing are damaged or worn.
However, if the polyamide sheave has a factory fitted steel sleeve in the bore for a bearing then it is possible that the bearing can be replaced as per the manufactures recommended procedures. When replacing the sheave make sure you are fitting OEM parts and not a gray (pirate) part.
Remember that failure to comply with your required responsibilities may result in serious injuries or fatalities.