Operation, Inspection and Maintenance of Wire Rope
Sheaves and Drums
In the course of normal operations, wire rope comes into contact with sheaves, drums, rollers and scrub boards- all of which must be maintained in good condition.
What causes wear in both groove and wire rope? Essentially, the answer derives from the fact that wire rope, when loaded, stretches much like a coil spring. When bent over a sheave, the rope’s load-induced strength causes it to rub against the groove. As a result, both groove and rope are subject to wear. Within the rope itself additional rubbing is encountered as the rope adjusts-by movement of the wires and strands- while bent around the sheave or drum. The smaller ratio of sheave diameter (D/d), the greater the adjusting movement and the more rapid the resulting wear.
The amount of wear and the speed at which it takes effect on both the wire rope and grooves of the sheave or drum are also determined by the sheave material and the radial pressure between rope and groove. Simply stated, excessive wear can be caused either by sheave or drum material that is too soft or a diameter (thread diameter) that is too small.
To determine the unit radial pressure between rope and groove use the following formula:
P=2T/Dd
Where p= Unit radial pressure in pounds per square inch
T= load on the rope in pounds
D= tread diameter of the sheave or drum in inches
d= nominal diameter of the rope in inches
The values given are typical for the materials listed; they are not precise values since these materials are made to a wide range of specifications.
In the foregoing equation, if the calculated value of ‘p’ exceeds the allowable radial pressure for the sheave or drum material, the groove will wear quite rapidly. Wear will manifest itself in the form of either an undersize or corrugated groove- either of which will contribute to accelerated wear in the rope.
Values for the allowable unit radial pressures are intended solely as a user’s guide; use of these figures should not be taken as restrictive with regard to other or new materials.
There are, for example, certain elastomers in current use, but there is sufficient data to support clear recommendations. It is best for the user to contact the elastomer or device manufacturer for specific recommendations.
Note: all verification tests of retirement criteria are based on the use of steel sheaves.
Bending Wire Rope over Sheaves and Drums
Sheaves, drums and rollers must be of a current design if optimum service is to be obtained from both the equipment and the wire rope. Because there are many different types of equipment and many different operating conditions, it is difficult to identify the one specific size of sheave or drum most economical for every application.
The rule to follow is this: the most economical design is the one that most closely accommodates the limiting factors imposed by the operating conditions and the manufacturer’s recommendations.
All wire ropes operating over sheaves and drums are subjected to cyclic bending stresses, hence the rope wires will eventually fatigue. The magnitude of these stresses depends, all other factors being constant, upon the ratio of the diameter of the sheave or drum to the diameter of the rope. Frequently, fatigue from cyclic, high magnitude bending stress is the principle reason for shortened rope service.
To illustrate, in order to bend around the sheave, the rope’s strands and wires must move relative to one another. This movement compensates for the difference in diameter between the underside and the tope side of the rope, the distance being greater along the top side of the rope, the distance being greater along the top side than it is on the underside next to the groove. Proper rope action (and service) is adversely affected if the wires cannot move to compensate for this situation. Also, there can be additional limitations to wire movement because of excessive pressure caused by a sheave groove diameter which is too small, or by lack of rope lubrication. Changing the bending direction from one sheave to another should be avoided as this reverse bending further accelerates wire fatigue.
The relationship between sheave diameter and rope diameter is a critical factor that is used to establish the rope’s fatigue resistance or relative service life. It is expressed in the tread D/d ration mentioned earlier in which D is the tread diameter of the sheave and d is the diameter of the rope. Table 9 lists suggested minimum D/d values for this ratio for various standards such as those listed in Table 10. Smaller values can affect rope life.
Inspection of Sheaves and Drums
Under normal conditions, machines receive periodic inspections and their overall condition is recorded. Such inspections usually include the drum, sheaves and any other parts that may come into contact with the wire rope and subject it to wear. As an additional precaution, rope related working parts, particularly in the areas described below should be re-inspected prior to the installation of a new wire rope.
The very first item to be checked when examining sheaves and drums is the condition of the grooves. To check the size, contour and amount of wear, a groove gage is used. The gage should contact the groove for about 150 degrees of arc.
Two types of groove gages 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 rope diameter.
For new or re-machined grooves, the groove gage is nominal plus the full oversize percentage. The gage carried by most wire rope representatives today is used for worn grooves and is made nominal plus ½ the oversize percentage.
This latter gage is intended to act as a sort of “no-no” gage. Any sheave with a groove smaller than this must be re-grooved or, in all likelihood, the existing rope will be damaged.
When the sheave is re-grooved it should be machined to the dimensions for “recommended minimum new groove” given in Table 13 on the next page. This table lists the requirements for new or re-machined grooves, giving the groove gage diameter in terms of the nominal wire rope diameter plus a percentage thereof. Similarly, the size of the “no-no” gage is given against which worn grooves are judged. Experience has clearly demonstrated that the service life of the wire rope will be materially increased by strict adherence to these standards.
If the fleet angle is large, it may be necessary to accept a smaller arc of contact at the throat, 30 degrees for example instead of 150 degrees. This is done to avoid scrubbing the rope on the flange of the sheave.
As previously noted, the groove size is evaluated on the basis of how the gauge fits the groove. Daylight under the gauge is not tolerable when using the worn groove gauge. If a full over size gauge is used, some daylight may be acceptable, but this really must be judged by relating the measurement to the actual size of the rope.
For new rope, extra caution should be observed as to its fit in the groove. Characteristically, ropes become smaller in diameter immediately after being placed in service. As a result, they would operate satisfactorily in a “worn” groove; one that was gauged OK by the “worn” groove gauge. Nonetheless, in some cases a rope may not “pull down”, and if this happens, abnormal wear may occur.
It is important to remember that a tight groove not only pinches and damages the rope but that the pinching prevents the necessary adjustment of the wires and strands on the other hand, a groove that is too large will not provide sufficient support; in this case the rope will flatten and thereby restrict the free sliding action of the wires and strands.
The size of the groove is not the only critical item to be examined closely. The condition of the groove is also an important factor. Is it smooth or corrugated? If the groove is imprinted then it must be re-machined or, if it is corrugated too deeply, it means that sheave, roller or drum must be replaced. If replacement is indicated, a larger sheave or drum should be installed. If possible or a harder material should be specified for the replacement.
Groove examination should also concern itself with how me groove is wearing. If it is worn off center, thereby forcing the rope to undercut or to rub against the flange, it then becomes necessary to correct the alignment of the reeving system, and to specify a harder material.
When checking the grooves, the bearings of the sheaves and rollers should also be examined. They should turn easily. If not, each bearing must be properly lubricated. “Wobble” in the sheave from broken or a worn bearing is not acceptable. Bad bearings will set up vibrations in the wire rope that can cause rapid deterioration unless the condition is remedied. Bad bearings also increase the force on the rope that is needed to move a give load, since friction forces will be greatly increased.
Sheaves with broken flanges may allow the rope to jump from sheave and become fouled in the machinery. When this happens, the rope is usually cut, curled and the crowns of the wires in the strands are burred. There is ample evidence to support the rule that sheaves with broken flanges must be replaced immediately.
A sheave or drum with a flat spot can induce a “whip” into the line. This whip or wave may travel until the end terminal, at which point the rope may bend severely, stops it. This condition helps to accelerate the fatigue breakage of wires. Sometimes the reeving is such that the whip or wave is arrested by a sheave or drum. In these circumstances, the whipping will cause wire breaks along the crowns of the strands. Obviously, sheaves or drums that excite vibrations of this sort must be replaced or replaced.
In addition to the items discussed, inspection should also focus on any and all conditions that could cause wear and eventual damage to the wire rope.
For example, plain face (smooth) drums can develop grooves or rope impressions that will prevent the rope from winding properly. Imprinting is greatest at the pickup point when the machine is accelerating. If this happens, the surface would be repaired by machining or replaced. This condition is particularly critical when double drums are used because a differential force will be set up that can break the drum and shear the shaft.
No matter what type of drum is in use, excessive drum wear will usually result in rapid rope deterioration. This condition will accelerate rapidly when winding in multiple layers.
Strength Loss of Wire Rope over Stationary Sheaves or Pins
Rope breaking strength is determined in a standard test where fittings are attached to the ends of the rope and the rope is pulled in a straight line.
If, however, the rope passes over a curved surface (such as a sheave or pin) its strength “is decreased”. The amount of such reduction will depend on the severity of the bend as expressed by the D/d ratio. For example, a rope bent around a pin of its own diameter will have only 0% of the strength attributed to it in the standard test. This is “50% efficiency”. Even at D/d ratios of 40, there may be a loss of up to 50%. At smaller D/d ratios, the loss in strength increases quite rapidly.
The angle of bend need not be 180 degrees, 90 degrees, or even 45 degrees’ relatively small bends can cause considerable loss.
All discussion of strength pre-supposes a gradually applied load not to exceed one inch per minute.
Fleet Angle
The achievement of even winding on a smooth faced drum is closely related: to the magnitude of the D/d ratio, the speed of rotation, load on the rope and the fleet angle. Of all these factors the one believed by experts to be perhaps the greatest influence on winding characteristics, is the fleet angle.
In an application where the wire rope runs from a fixed sheave over a floating sheave and then onto the surface of a smooth drum. The fleet angle may be defined as the included angle between two lines; one line drawn through the middle of the fixed sheave and the drum and perpendicular to the axis of the drum and a second line drawn from the flange of the drum to the base of the groove in the sheave. (The drum flange represents the farthest position to which the rope can travel across the drum.) There are left and right fleet angles, measured to the left or right of the centerline of the sheave, respectively.
It is necessary to restrict the fleet angle on installations where wire rope passes over the lead or fixed sheave and onto a drum. For optimum efficiency and service characteristics, the angle here should not exceed 1- ½ degrees for a smooth drum, or 2 degrees for a grooved drum. Fleet angles larger than these suggested limits could cause such problems as bad winding on smooth drums and the rope rubbing against the flanges of the sheave grooves. Larger angels also create situations where there is excessive crushing and abrasion of the rope on the drum. Conversely, small fleet angles less than 1- ½ degrees should also be avoided since too small an angle will cause the rope to pile up.