4a. SHEAVES AND GROOVES BACKGROUND

Traction And Arc Of Contact Basically, the main factor behind the success of any installation is the amount of traction created by the ropes in combination with the sheave. Simply put, "traction" is the adhesive friction of a body on a surface over which it moves (e.g. the rope on a sheave). The larger the amount of traction attained, the more efficiently a load may be moved. In elevator installations the amount of friction created is largely dependent upon the overall volume of sheave surface actually contacted by the rope's surface. This degree of contact of the rope around the sheave's inner groove surface is what we refer to as "Arc of Contact." Certainly the simplest, most efficient roping arrangment would be one where the drive sheave is positioned directly above a car and where the hoist ropes go up from the car, pass around the drive sheave, and then course down to the counterweight below (as seen in the 'Ideal Configuration' diagram above). Indeed, we have found that some installations still do use this basic design and incorporate large drive sheaves to do the job efficiently. Such designs offer the ideal degree of 'Arc of Contact' ( for the ropes, which creates excellent traction, while at the same time putting the ropes through as few bends as possible. Unfortunately such direct roping arrangements are becoming less frequent due to the use of close, multiple sheave placements. New system designs have led to a reduction in the rope's Arc of Contact with the sheave. To counteract this designers often utilize a wide range of undercut groove designs to regain, or to augment, the amount of traction lost.

Groove Types And Their Impact Upon Rope Life Expectancy

Arc Of Contact (ideal)

Today there are basically three rope groove designs used in modern elevator installations: Round (also known as U-Groove), Undercut U-Groove and V-Groove. Round Groove/U-Groove Due to its simplicity in manufacture, the Round shape (also known as the U-Groove) was the first to be used in traction sheaves for elevators. Its design proved to be ideal for early elevator installations that were used in relatively undemanding environments and required the use of large sheaves Round/ and large diameters ropes. U-Groove This type of design cradles the rope somewhat, creating lower specific pressures in the contact area between rope and groove. This can help hoist ropes attain longer life expectancies. The larger D/d (D/d: is the relationship betweent "d"—the wire rope diameter—to "D"—the diameter of the sheave) ratio makes bending easier, provides a large area of contact between rope and sheave and reduces operating stresses. Another advantage is a lower degree of noise, which is especially noticeable at high speeds. However this design provides less traction than either Undercut U or V-Groove designs. Today if Round Groove/U-Groove sheaves are used used with high-speed elevators (greater than 393.7 ft/min or 2.0 m/s), multiple wrap roping arrangements are often used. Undercut U-Groove An Undercut U-Groove offers distinct advantages in traction, as compared to simple U-Grooves, and distributes its loads more favorably (creating lower groove pressures on the ropes) than V-Grooves. The Undercut Undercut U-Groove was designed to help U-Groove designers cope with the problems inherent in the use of smaller sheaves, and smaller sheave diameters in modern installations. Undercut U-Grooves are commonly used in double-wrap roping arrangements and for installation speeds of less than or equal to 393.7 ft/min or 2.0 m/s. To compensate for the loss of traction created by modern installation designs, an undercut (preferably under 90° and not greater than 106°) has been placed within the groove. In effect the undercut serves to help grip the rope. But this also creates concentrated pinch points on the rope and

Machine Sheave

To Car

To CWT

Ideal Configuration

Arc of Contact

Machine Sheave

Deflector Sheave

To Car To CWT

Single Wrap Configuration Brugg Lifting_0513

Elevator design has changed greatly in the last few decades. This can be attributed not only to the creation of taller buildings and the need to transport their populations more quickly and efficiently, but changing societal and fiscal demands that require them to run nearly continuously. Couple these factors to the need by architects and engineers to use floor space more efficiently (and profitably) and you can understand the rise of installation designs that require the use of smaller, less bulky (and therefore less robust) components and stronger, more exotic composite hoist ropes (as opposed to old fashioned sisal ropes) which use Parallel and Point Contact designs. It seems like that this 'demassification' trend will only continue in the future. Indeed the entire elevator industry is moving towards creating 'tight systems'—which feature smaller diameter drive sheaves, closer sheave placements, and lower safety factors in dynamic rope loads.

4a. SHEAVES AND GROOVES BACKGROUND

greater specific pressures within the sheave itself—which can lead to accelerated wear of the sheave, wire fatigue and hoist rope breakage. Indeed such wear can be very significant as analyses performed using Brugg RLP (Rope Life Prediction an online application based on the works of Dr. Klaus Feyrer of the University of Stuttgart) reveal that an undercut of 75°can reduce rope life by 60%, while an undercut of 105°reduce rope life by as much as 93.4%. Clearly, the increase in traction provided by undercuts are balanced by a significant trade-off in reduced rope life. Drive Sheave with Undercut U-Grooves for four ropes

V-Groove/Undercut V-Groove Both V and Undercut V-Groove (also known as the Progressive V-Groove) designs offer even greater amounts of traction to installation designers. However this design also creates higher specific pressures within the groove, and stresses and distorts the hoist rope. Naturally this negatively impacts rope life expectancy. The angle for a

γ

D

V-Groove

Undercut V-Groove

Drive Sheave with Undercut V-Grooves for four ropes

D

F

32°- 40°

F

In contrast to the Round or U-Groove, the V-Groove's degree of undercut dramatically increases specific pressures placed on the hoist rope and creates specific areas of contact that can be identified by pronounced crown wear on the surface of outer wires.

V-Groove is usually 30° but can go as high as 38°, while an Undercut-V may range between 32-45°(measured as the included angle of both sides of the groove). V-Grooves are often used for installations of speeds up to 157.5 ft/m and higher (or less than or equal to 0.8 m/s), while Undercut V-Grooves are more commonly used with slower speed elevators carrying heavier loads. Using Brugg RLP (Rope Life Prediction), studies show that an Undercut V-Groove featuring an undercut of 35°can shorten rope life by as much as 94.6%, while a 45°undercut can decrease rope life by 75%.

The Positioning of the Secondary Sheave Has A Critical Impact on Sheave Wear and Rope Life Modern elevator designers are constantly working to create installations that use floorspace more efficiently. These compact designs frequently utilize less massive components, more challenging groove profiles, and often demand the close placement of secondary, deflector sheaves and primary sheave. This necessitates the implementation of complex roping arrangements and the use of multiple sets of sheaves. Such conditions often compromise rope performance dramatically by increasing the number of rope bending cycles and creating improper fleet angles. This in turn leads to greater system operating stresses, which can negatively impact sheave surface wear, rope performance and hoist rope life expectancy. Bending Cycles Many professionals fail to appreciate that hoist ropes are not single static pieces of wire but are carefully crafted machines bearing a surprising number of moving parts. Each rope is made up of strands of wire. These are composed of bundled collections of smaller wires that are helically wound together. Each rope must work in harmony with the surrounding equipment. To illustrate the complexity of the matter, consider that a single 8 x 19 Seale hoist rope is basically composed of 152 parts (8 strands, 19 wires per strand). All these parts are composed of soft steel that must be strong, relatively lightweight and flexible — which poses a real engineering challenge. In addition, bear in mind that hoist ropes are constantly in motion; bending over a sheave (frequently multiple sheaves), adjusting to stresses and then straightening out again. And this process is carried on often for millions of cycles. It should come as no surprise that this exponential rise in bending cycles has resulted in a corresponding increase in elevator breakdowns and decrease in rope life expectancy. To address this fact some manufacturers have developed new rope designs and are using advanced, innovative materials in rope construction. However there

4a. SHEAVES AND GROOVES BACKGROUND

Fleet Angles

Fleet Angles induce torsion into hoist ropes, causing them to roll into sheave grooves.

This drawing details the natural progression of groove wear after a new rope is installed on a new sheave. Over time the rope and groove wear together. This results in a loss of rope diameter and the simultaneous retreat of the rope within the groove channel (creating both a deeper and narrower groove channel than). Should the corrupted groove host a new rope—without the groove being regrooved, or the entire sheave being replaced—a mismatch between rope diameter and groove shape/diameter will occur. This will be marked by evidence of early crown wear, wire breaks on the rope, shortened rope life, and poor system performance overall.

Technically speaking, the angle between the rope axis and the radial plane of the pulley is called the 'fleet angle.' Simply put, a fleet angle is a measure used to describe the angle of a rope as exits from one sheave (the main traction sheave) to connect with another sheave (the deflector or secondary sheave). This angle must be carefully monitored and remain relatively shallow for an installation to work properly. Too wide an approach angle and the hoist rope will rub up against the side of the side of the sheave groove (the flanges), leading to increased wear on one plane of the rope, resulting in exaggerated rope wear and premature rope death. For instance a 4° fleet angle can reduce rope life by as much as 33%. Close placement of secondary sheaves and the drive sheave results in high fleet angles, which induces rope torsion and cause the rope to roll into sheave grooves. Incorrect alignment of sheaves present serious problems for rope life expectancy and sheave performance. Close sheave configurations, or the failure to thoroughly consider the impact of high fleet angles during the design and installation process results in

RdnRdRd n n

A. A. A. New Groove Groove New Groove NewNew Groove with New Rope New New RopeRope with with Newwith Rope

excessive bending cycles for the ropes and higher overall system operating stresses on all components.

Sheave Groove Wear versus Rope Wear: a relationship that tends to wear on each. One quickly finds that discussing sheave groove wear without simultaneously addressing the topic of rope wear is a fruitless endevour. Indeed it can be said that no other pairing of components has such such a profound impact upon the wellfare and performance of the other. As detailed previously, the type of sheave groove used has a dramatic impact on the amount of specific pressures and the kind of stresses placed upon the rope itself. Modern aggressive groove angles (featuring dramatic undercuts) coupled with roping arrangements featuring reverse bends, and close placements of secondary sheaves, create unforgiving conditions where rope cross sections actually deform under load conditions. How this occurs is detailed in the various enclosed illustrations we have provided. However the key point to understand is that once a new rope has been placed into a grooove, stresses naturally wear down the diameter of the rope, forcing it to settle more deeply into the groove. Simultaneously, as the rope withdraws into the groove, the sheave groove surface is worn as well. Consequently should a new rope be placed into the worn groove (even if the new rope matches the older rope in every detail), the more corrupted groove will accomodate the rope poorly and lead to shortened rope life expectancy. This is a natural sequence of events. And, despite what some professionals choose to believe, this continuous cycle of rope and groove wear, and premature rope death, has absolutely nothing to do with modern rope construction, rope design, or the type of materials used in rope composition today.

RdRd Rdw ww

B. B. B.

Worn Worn Groove Groove Worn Groove Worn Groove with Worn Rope with Worn Worn RopeRope with with Worn Rope

Areas of increased wear where hoist rope crown wear will be evident.

RdrRdRd r r

C. C. C.

New New Rope Rope (Replacement) (Replacement) NewRope Rope (Replacement) New (Replacement) in Worn in Worn Groove Groove inWorn Worn Groove in Groove

Brugg Lifting_0513

is a practical limit as to how far one can go to create a rope that can totally withstand increased bending cycles. For while it is theorectically possible to construct a nearly indestructible rope, such a rope would be far too unwieldy to install, expensive to maintain, and far too rigid for practical use as a flexible support medium. Indeed such a rope would be nearly useless to all, as it is only through a careful examination of rope wear that one can determine the existence and cause of even larger problems with surrounding machinery. Clearly, our industry's preference in utilizing installation designs that place secondary sheaves within close proximity of the drive sheave, and the steady rise in public elevator usage (due to societal and economic factors), can only mean that problems due to increased bend cycles will only increase.

4a. SHEAVES AND GROOVES BACKGROUND

Rope and Groove Wear Progression

Round/U-Groove

Undercut U-Groove

Undercut V-Groove

Road Deformation Effects (No Load vs. Under Load)

Round/U-Groove

Certainly manufacturers have managed to counteract these effects somewhat by creating new compositonal materials, construction processes and structural designs in hoist rope. However these measures have also created issues of their own, such as increased rope weight, not to mention the cost of more advanced ropes (and the attention care that must be taken to install and maintain them properly). No matter how much attention manufacturers pay to rope design, they are helpless should maintenance professionals fail to adequately and routinely lubricate and tension hoist ropes. Other than a basic quantity of lubricant applied in the factory, and general suggestions on tensioning, the matter is largely out of their hands. And the impact of tensioning and lubrication is not insubstatial. For they can, (either by themselves or in combination with other factors) severely impact rope life, and drastically affect With Brugg GDC sheave groove integrity as one can gauge well. Such conditions create groove depth variances, as well a multiplicative effect, where as calculate altered one factor impacts another pitch diameters and creates consequences far and rope traveling more damaging that it could distances. by itself. Indeed, such basic matters such as lubrication and tensioning can create conditions where a rope can actually file into a groove, slicing through the hardened outer casing of the sheave. If this situation is left unaddressed (with either the sheave being regrooved, or the sheave being totally replaced) the widening discrepency between sheave groove diameter and rope diameter will lead rope slippage, rope failure, loss of system peformance and decreased rope and sheave life.

Variable sheave groove depths impact rope life. Undercut U-Groove

As stated previously, the movement of the industry towards MRL elevator designs (and the need to compact elevator machinery) has created

installations that have led to the use of smaller sheaves, which create higher radial pressures on hoist ropes. Should maintenance proove less than adequate (e.g. performing infrequent lubrication or poor tensioning), or those responsible for maintaining an installation be forced to take shortcuts (through not monitoring sheave wear or replacing worn sheaves), one will detect evidence of rope crown wear and this will lead to shortened rope life. However this is not a sign that the rope is inadequate for the job or that the diminished rope diameter is the sign of an inherent constructional flaw. Rope will naturally stetch and become smaller in diameter over time due to the impact of both friction and load stresses. Indeed, they are made to do this. Instead, if one were to measure the amount of variation in groove depths of the drive sheave, one would quickly see that this equates into a difference in pitch diameter between sheave groove and rope as well. A difference in pitch diameter (the contact point between rope and groove) is more than just an interesting discrepency. Indeed, any difference in groove height will result in a difference in how far each rope will individually travel. And since ropes are constructed to be used as a cohesive group, working in unison with each other, this hastens rope wear and impacts the sheave groove surface. A rope that has situated itself into a lower groove carries less load in comparison to other ropes and will slide in the groove to equalize the length. As this action is repeated thousands of times rope life is dramatically impacted. This wearing action, caused by pitch diameter discrepencies, is why simply replacing the ropes (without addressing the underlying cause of failure) results in reduced rope life in subsequent generations of replacement ropes—no matter how closely one tries to match future rope choices to the original rope's specifications. This is dramatically illustrated in the chart labeled "Impact of Worn Groove On Estimated Hoist Rope LIfe". That this situation is even a problem in our industry today is due to a confluence of factors: sheave and rope manufacturers have not fully

The first drawings detail the long-term effects of sheave groove and rope interaction, while the lower images detail the natural impact of loads placed upon ropes. The development of an ovoid cross section can be addressed through various remedies, the most efficient being the substitution of high-performance hoist ropes for Sisal Core ropes. Undercut V-Groove

4a. SHEAVES AND GROOVES BACKGROUND

PD

PD

PD

Visual A

Visual B

Visual A details both the exterior view and a cross section of a single groove of an elevator sheave. The Pitch Diameter (PD) is the contact point between rope and sheave groove. Visual B illustrates how a small variation in groove depth equates into a difference in Pitch Diameter. Despite the fact that that both of the ropes are moving at the same speed, the discrepency

Impact Of Worn Groove On Estimated Hoist Rope Life 18 16 14 12 10 * Estimated Bending Cycles 8 Measured 6 In Millions 4 2 0

Bending Cycles*

cooperated together to find solutions for the problem; a failure by rank and file members to invest the time and effort necessary to throw aside old industry maxims and keep informed; when confronted by the expensive cost of changing a sheave versus replacing a rope, short term financial concerns take precedence.

If the original metal casting for the sheave is made correctly (and this means meticulously controling the chemistry and cooling rate of the drive sheave blank when the casting is poured), and the rope is properly matched, intstalled and maintained, then the possibility of a rope proving to be too hard becomes less than negiligible.

The myth of our times — Elevator ropes are just too hard for today’s sheaves. 1st

2nd

3rd

4th

Subsequent sets of ropes

Visual C Visual C reveals how successive generations of ropes placed on sheaves with unequal groove depths can have life expectancies impacted in an inversely proportional manner. No matter how closely one tries to match the original specifications of rope initially placed upon a sheave, this pattern is inevitable. And though replacing the initial rope with a high performance rope can delay the occurance of subsequent reropings somewhat, the replaced rope will never match the longevity of the first set of ropes used.

Over the decades some have encouraged the idea that hoist ropes (featuring outer strand wires in tensile strengths in MPa of 1180, 1370, 1570 and 1770) are too hard for sheaves. This is not only unkind to sheave manufacturers, as it encourages the view that they are unresponsive to new challenges in elevator design, but it fosters a view that rope manufacturers are blithely creating ropes that stress strength above performance—neither case is true. The idea that today's ropes are simply too hard for modern sheaves is categorically untrue. As we have shown, there are factors that have far greater impact upon sheave and rope life, than some fictional clash of the original metal casting of the sheave versus the hardness of outer strand wires.

Brugg Lifting_0513

original

in groove depth means that the ropes do not travel equal distances. Although the individual difference in distances traveled by the ropes may seem insignificant, over time this condition will lead to excessive rope wear, a loss of system performance and continued sheave/groove degradation, which further exacerbates the problem.