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A typical winch consists of a frame, a drum, a motor with controller and a transmission system. Optional components can include a level-winding mechanism and a slip ring. Other components such as davits, A-frames, sheave blocks and rotator plates are technically parts of the over-boarding (or deployment) system, although they may be built as part of a winch frame in some compact configurations.
Drum capacity is calculated by the use of a formula found in many engineering reference textbooks, in this case the Machinery's Handbook, 25th Edition:
L = (A + D) x A x B x K
L = Length of cable on the drum (Drum Capacity)
A = Depth of rope space on the drum (flange diameter - core diameter - (2 x free flange))/2
D = Core diameter
B = Drum face width
K = Cable Factor (0.2341 x diameter of cable ^-1.9533)
This formula provides the theoretical capacity of a drum if the cable is spooled on absolutely perfectly. The free flange is "extra" flange diameter to allow for uneven spooling of the cable. A typical free flange space is 1.5"; in order words, with perfect winding onto the drum there will be 1.5" of flange remaining when all of the cable is wound onto the drum. Free flange is a radial (not diametral) measurement.
The core diameter of the drum is determined by the minimum bending diameter of the cable being used. Use of a core smaller than this diameter (or, in general, smaller than 20-25x the diameter of the cable) will result in excessive wear on the cable and will cause uneven spooling to occur even with level-winding apparatus.
In practice winch drums are designed around standard flange and core diameters, typically even inch diameters for flanges and standard pipe sizes for cores, to simplify production and stock-holding requirements.
Drums are available in many different styles for many different purposes.
The simplest drums are capstan drums which are simple driven tapered rollers which can have fiber or wire rope wrapped around them and held manually in tension, whereupon the rotation of the roller (capstan) causes the line to be pulled in. These winches do not store any rope on their drums; the rope is left to be manually spooled by the operator. Capstan winches can be used with small-diameter wire rope if the capstan drum is of sufficient diameter. Some capstan winches have multiple driven rollers with guide grooves to increase hauling force. Capstan winches are often used to haul in instruments which have a surface float moored to a bottom anchor; the mooring line can be wound around the capstan after recovering the surface buoy and the anchor can then be drawn up from the seafloor.
Similar to a capstan winch is a traction winch, where the cable is "grasped" between two counter-rotating rubber wheels or bands. These winches are often used for deploying subsea burial cables and rigid or semi-rigid pipelines.
The typical drum for most instrumentation winches is a line-holding drum. This type of drum has two equally-sized side flanges and a cylindrical core rotating around a central axle. This drum will hold almost any type of rope, wire rope, cable or tether and is simple to manufacture in its most basic form. The flanges are generally left plain unless high side forces are expected or the flange diameter to core diameter ratio is greater than 3:1, whereupon external gussets and reinforcing strips may be added.
A split drum is a line-holding drum which has a third auxiliary flange located between the other two flanges. In this application the two resulting line-holding spaces on the drum are used for different cables. If the cables are identical in diameter they can theoretically be used simultaneously but in practice the uneven spooling of the cable will make the tensions different on each cable. If the cables are different diameters, and most of the time when they are the same diameter, one cable will be tied off to the drum and the other used and vice versa. The purpose of this drum style is to have two cables on one winch to save the cost of a second winch or the time of exchanging cables.
Similar to a split drum is a finned drum. These drums are designed for use with sensor arrays that have large sensor elements spaced along the length of the array which would normally interfere with cable winding and/or be crushed by the cable winding pressures. The third flange in the middle of the drum is notched with radial slots roughly 3x the diameter of the cable in 3-4 locations equally spaced around the flange, making it look like a crude windmill propeller. The center flange is then reinforced with gussets intruding in the second line-holding space. As the array is wound in, whenever an array sensor element is about to wind onto the drum the cable is manually fed through the nearest slot and the sensor element is tied off within the second line-holding space in such a way that little or no pressure is placed upon it. The cable is then fed out the next slot and cable winding is resumed until the next sensor element is encountered. These drums are time-consuming and difficult to use but can prevent major damage to expensive array elements.
Winches may also be constructed with double-drums, where two drums are located on a single axle and a double-acting clutch is used to engage one or the other drum. If a reversing gear is added these double-spooling drums can operate in opposite directions, allowing a cable to be wound on one drum as it is wound off the other, with a pulley at a distance in between. With double-acting drums the winch can handle two different cables with different gear ratios available for heavier or lighter loads (and correspondingly higher or lower line speeds). With double-spooling drums the winch can move a load along a fixed path in a similar manner to a clothesline or zip-line; these rigs are commonly used in logging and bridging. Double drums may also be constructed with the drums parallel to each other rather than on a single axle; the choice of the arrangment is generally made by examining the available mounting geometry. Single-shaft double-drums are common on ships while parallel double-drums are common on skid- or truck-mounted applications.
A very special type of drum is available for geophysics winches, known as a removeable drum. Although all winch drums can be removed in the shop, these drums can be removed and replaced in the field with minimal tools. They incorporate rotating hollow-axles within the drums and stationary removable main axles that ride in slots in the winch frame uprights. These systems are often used in remote applications where multiple cables are required for a job but it is difficult to employ or transport multiple winches. The removable drum feature also allows the winch to be transported in multiple pieces, reducing the individual components to manageable sizes and weights.
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