LIFTING EQUIPMENT - SHEAVES, BLOCKS , DRUMS WITH SIGNIFICANCE TO FLEET ANGLE

SHEAVES  AND BLOCKS

RULES OF THUMB :
SHEAVE SIZE : ROPE DIAMETER X 20
GROOVE SIZE : MINIMUM 1.5 X ROPE DIAMETER
FSWR SITTING ON SHEAVE : 120 DEGREES OR 1/3 OF CIRCUMFERENCE

The  general cross section of  a sheave is as shown:

Materials Sheaves Are Made Of:	Types of Bushings:	Types of Web:
Cast Steel Drop Forged Steel Malleable Welded Fabricated Steel	Plain Bore Bronze Bushed Graphited Bronze Bushed Bronze Bushed ,  Groove Roller Bearing Timken Ball Bearing	Spoke Solid Relieved

HOW TO ORDER WIRE ROPE SHEAVES

QTY	A. Outside Dia.	B. Rope Size	C. Rim Width	D. Distance thru Hub	E. Bore Dia.
2	6"	3/4"	1-1/4"	1-1/2"	1-1/4"
1	8"	5/8"	2"	2"	3"
F. Tread Dia.	H. Type of Bushing		Materials Made of		Type of Web
4-3/4"	Plain Bore		Cast Steel		Spoke
6"	Bronze Bushed		Malleable Iron		Solid

Sheave Defects    

              

(i) Excessive wear in the groove of a sheave. 

(ii) Cracks or any damage in the flange of a sheave. 

(iii) Twisted/deformed or out of shape. 

(iv) Worn sheave pins, hinge pin wear. 

(v) Damaged cheek plates or cheek plate wall/partition is too close or too far from sheave. 

Blocks
A block is a frame that encloses one or more sheaves and is provided with a hook or some other means that allows attachment to cargo or to a fixed anchor point. The purpose of a block is twofold. First, it is used to change direction of a wire rope line. Second, when used in pairs, blocks increase mechanical advantage by allowing the use of multiple parts of line. Blocks range in size from several pounds capacity to hundreds of ton

A block consists of a shell (or side plates), a center pin, and an end fitting. There are a variety of end fittings such as hooks, shackles, and clevices that facilitate attachment of the block to the cargo or to a fixed anchorage. Blocks are also equipped with a becket or mouse ear whereby the end of the rope line is affixed to the block. The sheaves of the block transmit the load from the wire rope to the center pin and then to the shell straps or side plates

There are three basic types of blocks:

1. Crane Block : A crane block is required to perform long lifts under continuous service conditions. Crane blocks are characterized by multiple large diameter, long service life sheaves, and the addition of cheek plate weights to the block side frames to increase overhaul weight. Crane blocks typically are ouffitted with a swivel hook that allows the cargo to be rotated without fouling the multiple parts of reeving

2.Snatch Block :Snatch blocks refer to a group of intermittent service blocks that jerk or snatch their load over comparatively short distances. Snatch blocks are characterized by a side-opening plate that facilitates threading the wire rope through the block

3.Wire rope (construction or fixed) blocks.:Fixed blocks or construction blocks are typically used as upper blocks in multi-part reeving arrangements in derricks or material hoists. As such, they have large diameter multiple sheaves like crane blocks but the lack the additional cheek plate weights required for overhaul.

 .. 

DRUMS

Hoist drums store, spool, and transmit power to the wire rope. Hoist drums must have power to
hoist, lower, hold, immediately stop, and start functions as recommended by the manufacturer.

A hoist drum barrel is grooved to seat the first layer of the wire rope closely and uniformly. The correct way to wind wire rope on a drum will depend on the lay of the rope.

Each turn of the rope around the full circumference of the drum is called a wrap. Rope is wrapped around the drum, starting at one end flange and progressing to the other flange, which is called a layer. Drum flanges should extend beyond the fully loaded drum by a minimum of two rope’s diameter.

 The wire rope end is attached to the drum by a socketing or clamping arrangement. A minimum of three wraps must remain on the drum at any time during the hoisting operation when required rope is spooled out.

It is important to install wire rope on a smooth drum correctly in regard to maintaining a correct relationship between direction of the lay of the rope (right or left) and direction of the rotation of the drum (overwind or underwind), winding from left to right or right to left. For proper installation of the wire rope on a drum, the following measures are required:

• Make sure that the rope is properly attached to the drum

• Maintain sufficient tension on the rope as it is being wound on the drum

• Be certain that each wrap on the drum is guided as close to the proceeding wrap as possible

• Use at least two wraps of wire rope on the drum when the rope is fully unwound for any function of the crane lift

Drums should have sufficient rope capacity with proper rope size and reeving to perform all hoisting and lowering functions. In addition, all hoist drums should be provided with adequate means to ensure even spooling of the rope on the drum. Where the operator cannot see the drum or rope, drum rotation indicators should be provided for the operator’s sensing

Fleet angle
The fleet angle is defined as the largest angle of the rope between the first sheave and the drum flange, relative to the centre line of the drum

Fleet angles may cause increased wear or strain on wire ropes. With coiling onto a smooth drum, the fleet angle should be 0.5 to 2.5 degrees. If the rope is damaged by adjacent windings, the service life may be improved by using compacted or lang lay ropes.

The fleet angle on drums should likewise not exceed 2.5 degrees.
With multilayer or parallel twisted rope constructions, the angle should not exceed 1.5 degrees.

Fleet angle also ensures that the running wire rope cannot run off the flange of the sheave or drum.

The points where the rope enters the equipment at a fleet angle need special attention in the course of monitoring the wire ropes in use.
When spooling wire rope onto a drum, it is necessary for the rope to come onto the drum at a very slight angle, just enough to encourage each wrap to sit tidily next to the previous wrap, and for each layer to ride cleanly onto the layer beneath.


In fact, apart from the design of the drum itself, this angle – the fleet angle – is the most significant factor in the behaviour of a spooling system.

Put simply, optimum fleet angle means maximum rope life and smooth, safe spooling operations.

. With all types of drum, the rope is subject to a fleet angle which directly influences its behaviour and impacts on its service life.




If the fleet angle is too big, the wire will tend to pull away from the flange as the layer changes. It will want to spool towards the centre and so leave gaps. Gaps mean ragged spooling, which means (at best) excessive rope wear, or (at worst) snagging, catastrophic system failure and physical danger to all those around.

If the fleet angle is too small, the rope may not pull away from the flange soon enough. It will pile up on the flange for,  two or three wraps and then bang down with considerable force, damaging the rope and the equipment. Again, catastrophic failure and personal injury is a real threat.

The fleet angle should be between 0.25 and 1.25 degrees. This is not an absolute rule of physics; it depends on the rope construction. Nor has it been calculated mathematically. Rather, it has been learned from years of experience.

The fleet angle can be varied by moving the first sheave closer to or further away from the drum. If the sheave is too close to the drum, the fleet angle will be greater than 1.25 degrees; if it is too far away, the fleet angle will be less than 0.25 degrees.

In general the distance between sheave and drum should be at least 20 times the width of the drum. Ideally a ratio of 23:1 works very well, we have found. Thus, the larger the drum, the further away the sheave needs to be to keep the fleet angle between 0.25 and 1.25 degrees.

It is not always possible, however, to achieve the optimum fleet angle. For example, there are massive winching systems at the top of mountain cable car systems, often housed in compact machinery sheds. There is often no space to rig a sheave the requisite distance from the drum.

For just such cases two additional spooling devices are available. One is a fleet angle compensator, which is driven automatically by the rope tension. The other is a level winder that is mechanically driven. Both offer a solution to guide the cable along the drum between flanges, but each has its advantages and disadvantages.

Fleet angle compensator


The fleet angle compensator (FAC) is driven by the movement of the wire rope as it goes through the crossover sections of the drum. As the rope winds or unwinds, the FAC shaft slowly oscillates, allowing its sheave to slide back and forth across the shaft to maintain an optimum fleet angle and guide the rope smoothly onto the drum.

Certain operating conditions are necessary for the Lebus fleet angle compensator to function properly. The rope must go from the drum over the compensator sheave with a minimum contact angle of 60 degrees to a fixed point such as a fairlead or fixed sheave. To avoid excessive angles of the rope on the sheaves, the minimum distance between the fairlead (fixed sheave) and the compensator sheave must be at least six times the drum width. If spooling in multiple layers, the drum must have Lebus-style parallel grooving. For a single layer, helical (screw thread) grooving will also work. As always, there must be sufficient tension on the cable during the spooling operation. We recommend that minimum tension should be 1-2% of the wire rope’s breaking load.

There are three primary advantages of the Lebus fleet angle compensator. First, there is no mechanical connection between the drum and the compensator. Second, installation is easy and quick. And third, it is completely automatic and, after initial adjustments when the rope is first spooled onto the drum, only a minimum of maintenance is necessary.


 level winder
Level winders can be quite sophisticated pieces of kit, hydraulically or electrically driven and computer controlled. But mechanical level winders also work perfectly well and have much less to go wrong. In our experience, it is generally best to keep things simple and so we usually recommend the mechanical solution.


A mechanical level winder comprises a main shaft (the lead screw) with helical screw grooving along which the rope feeder travels. The rope feeder housing includes two vertical roller bars and one horizontal roller or, alternatively, a wire rope sheave. The lateral movement of the housing is generated by a chain drive sprocket ratio between drum and lead screw, as shown in the image. The automatic level winder fitted is designed and engineered to be compatible with the grooving on the drum. Perfect, controlled spooling is guaranteed regardless the number of layers and slight changes in wire rope size.

The level winder unit – also sometimes called level wind pay-on gear – must be installed in front of the drum in line with the first fixed sheave when using the vertical rollers to guide the wire rope.

Alternatively, a sheave can be integrated and installed within the housing frame. In this case, the system can be set up anywhere around the drum.

The level winder is engineered to be compatible with the parallel grooving on the drum. It is adjusted for the specific rope diameter, and the gear ratio is fixed (using a standard sprocket-chain connection) to match the ratio between coils of wire on the drum to the pitches on the lead screw. The result is perfect and controlled spooling, regardless of the number of layers or slight changes in wire rope size.

As before, certain operating conditions are required for the level winder to function properly. The rope must go from the drum through the vertical rollers or the level-wind sheave to a fixed point such as a fairlead or fixed sheave. To avoid excessive angles of the rope on the sheaves, the minimum distance to the fairlead or fixed sheave must be at least seven times the drum width. There must be a minimum tension of 1 to 2% of the wire rope’s breaking load when spooling more than one layer.

The advantages of level winders are that they keep the rope spooling properly even if there is slack in the line. As with the fleet angle compensator, once it is set up no more adjustment is necessary and very little maintenance is required. In case of damage to a mechanical level winder, parts are easy to replace and there is nothing electrical or hydraulic to worry about.

Oceanographic installations that spool rope up to 46 layers have demonstrated that level winders give synchronised and totally controlled spooling in the very harshest, most testing conditions.

The disadvantages of level winders is that they do require a little more space than fleet angle compensators and they are sensitive to high axial forces and shock loads.