The design of a winch for any purpose or the selection of the right underwater laser or light is not a trivial exercise. This FAQ is intended to explain and to help our customers make an informed choice about options available to them.
Although there are many parameters which may be measured on a winch, the four which are most commonly desired are the line out, line speed, line departure angle and the line force. Although some of these can be measured at the winch, in virtually all cases it is much simpler to monitor these factors on the overboarding sheave block rather than the winch.
Line speed and line out are complementary parameters which are typically measured using one sensor system. As the diameter of the cable on the drum is constantly changing if there is more than one wrap, it is effectively impossible to accurately gauge these parameters using a rotation sensor on the drum axle. This leads to the need to monitor the cable as it leaves the drum; the only position on the winch where this can be done easily or simply is at the level-wind roller assembly (if fitted). This requires the addition of two wheels to this assembly: a counter wheel (with a running diameter preferably equal to or greater than the minimum bend diameter of the cable) and a tensioning roller wheel to make certain there is positive contact on the counter wheel by the cable (if the cable slips on the wheel the count will be inaccurate). As the bend diameters of many cables are extremely large, it simply makes more sense to monitor these parameters on the sheave block.
Line departure angle must be monitored on the sheave block as the sheave block is the base point from which this parameter is measured.
Line force can be monitored on the winch by adding sensors to one of several places, but the calculations to relate these measured parameters are complicated and variable. For marine winches (line output angle varies) the best method is to add a running line tensiometer that follows the angle of the line. For borehole winches it is much simpler to add a load scale to the sheave block coupling that directly measures the force on the line rather than trying to interpret secondary force characteristics. Load scales (also known as crane scales) are available from several sources, including McMaster-Carr and Cooper Instruments.
There are several types of cable which may be employed on winches with the type varying by with the application. Each type has characteristics which require different winch design features.
The simplest type of cable is fiber rope. This can be nylon or polypropylene common rope or Kevlar (Aramid) and Dyneema high-strength ropes. Rope has the benefit of being very light compared to metallic cables and if the correct rope is chosen it can be a superior product for non-conductor applications. It does have the drawback that it is susceptible to fraying and some types of rope are susceptible to stretching. Ropes typically require a drum core/sheave wheel diameter in the 1" to 4" range. Typically drum cores for this type of cable are selected to maximize line speed, rather than depending on any minimum size required by the cable.
Similar to fiber rope is wire rope. The most common type used on winches is known as Aircraft Cable and utilizes a 7x19 strand configuration. Wire rope is available in galvanised steel and stainless steel, coated or uncoated. Some winch designs require a non-rotating wire rope which is much more expensive and difficult to procure, especially in stainless steel; this is because the natural twist in wire ropes will tend to cause the payload to rotate as it descends/ascends. Aircraft Cable is common to many manufacturers; McMaster-Carr carries a decent selection and the tools and fittings for working with the cable. Aircraft cable typically requires a minimum drum core/sheave wheel diameter in the 4" to 6" range.
Rubber-coated electrical signal cable is a common non-structural cable used on some very lightweight payload applications. Typically this cable would only be used for a 100 meters or less with a payload in the range of 1-20 pounds. In North America this cable is typically referred to as SJOW or SJOOW cable, and is the standard cable used on subsea pigtail connectors. Rubber-coated cables typically require a drum core/sheave wheel diameter in the 6" to 8" range.
Soft-tow cable is a subsea and/or mining cable which utilizes Kevlar reinforcement strands within a soft plastic (normally polyurethane) outer jacket. The signal cables do not stretch because the Kevlar strand bears all of the force. While this type of cable is relatively expensive it provides a durable and strong cable with a long expected lifespan. This cable can be procured from many suppliers including Falmat, South Bay and Teledyne Storm Cable. Soft-tow cables typically require a drum core in the 8" to 12" range.
Armored cable is similar to soft-tow except that it uses an armored metal sheath instead of kevlar strands surrounded by a soft plastic. Typically armored cable requires a relatively large drum core (and thus a large drum) so it is usually only fitted to our larger winches. Rochester Cable is possibly the best known supplier of this product. Armored cables typically require drum core/sheave wheel diameters in the 12" to 18" range.
Fairing is a feature that can be added to many cables. While this feaure reduces line strum (an important feature for many passive acoustic recording systems) it makes the handling of the cable much more difficult and the cable itself becomes much more expensive. Faired cable cannot be used with a level-winding system as the fairings will almost certainly entangle the level wind rollers.
Often when a particular setup is not cost-effective to manufacture with the expense of specialty cable, an operator will use paired cable, where the load is borne by a fiber or wire rope strength member and the signals are transferred with an inexpensive rubber-jecket cable. Some low-cost equipment manufacturers use a similar method where a load-bearing cable-sheath is used around rubber-jacketed signal cables. In some cases the conductor and cable and the rope are tied together with zip-ties, electrical tape or some other bonding method; in others the wire rope has snap-clips bonded along its length and the conductor cable is snapped in as it is deployed. In all of these cases the cable is more difficult to handle on a winch and operational speed of the winch will be limited.
Balloon and Aerostat deployment
Nominally similar to vertical profiling, balloon winches are typically fast and over-powered to cope with gusts of wind and sudden high winds. They almost always involve a Kevlar or Dyneema rope and almost always use a pulley set some distance from the winch, often with a rotating secondary pulley system to allow for line angles which can traverse a large conical volume.
The towing of side scan or other sonars is one of the most common uses of instrumentation winches. Towing adds several new dimensions for consideration: The drag of the cable through the water is cumulative with the speed of the line being pulled in, making line drag a real issue, as is the increased drag from the payload. Towing winches are exposed to a much higher applied load when the drum is stationary as well, as the towing forces when under full speed towing may be much greater than the winch is rated to nominally pull in. For this reason a motor brake is highly recommended; Chicago cable clamps, Kellums cable grips, pin locks or other cable tie-off mechanisms are acceptable alternates.
Borehole operations are typically limited by the maximum recommended speed in boreholes, typically 10 m/min. Anything faster than this runs too high a risk of tool/instrument break-off for most operators to consider operating at higher speeds. Loads are typically miniscule and vastly out-weighed by cable weight and drag within the borehole.
In vertical profiling the most important components of the load are the payload weight and the drag of the payload, although heavy cable weights can also add to this. Vertical profiling winches (typically CTD profilers) are designed to operate when the vessel is nominally stationary and, ideally, anchored. The instruments are dropped through the water column and then recovered, with water sampling bottles often adding to the recovery weight. Some vertical profilers are used to haul bongo nets; these applications have a very large payload drag characteristic.
The load that is being hauled by the winch is not a simple calculation. In general it can be considered to be the sum of four forces: the weight of the payload, the drag (or buoyancy) of the payload, the weight (or buoyancy) of the cable and the drag (or buoyancy) of the cable through the medium. Momentary peak loads are also induced by boat motion (heave), which may cause the winch to slow down and speed up with the heave motion, or cause the winch to stop in extreme heave conditions.
Electrical slip ring styles and types
Electrical slip rings all follow the principle of transferring electricity from rotating signal cables to stationary signal cables. In general, three major types exist: open-contact, closed-contact and mercury-contact.
Open-contact slip rings are generally used only with legacy machinery and rarely with winches as they are very sensitive to foreign debris contamination. A good way to think of these is that to visualize a record turntable and arm, operating in the air. Electrically these are the noisiest types of slip rings. In most cases the only application where these are used on winches is with legacy-compatible military systems.
Closed-contact slip rings use principles similar to the brushes used to transmit electricity to the coils of electric motors. They are generally (but not always) sealed to some extent, IP51 being common, although IP65 and even underwater-sealed models do exist. IP68 and IP65 type versions are most common on winches. Closed-contact slip rings can include linear, 90-degree and through-shaft variants. These slip rings are average for electrical noise, although high-tolerance, low noise models do exist.
Mercury-contact slip rings use mercury as the contacting fluid in the rotation. This provides a far superior (almost noiseless) electrical contact. The drawback is that many nations, airlines and states/provinces do not allow the presence of mercury in products and the use of such products may be illegal in some jurisdictions. It is almost impossible to ship products legally by air if they include mercury-contact slip rings, and in many statesin the U.S.A., the slip rings cannot even be transported across state lines.
Note that fiber-optic slip rings (and combined electro-optical slip rings) are also available; these work on the same principles but use mirrors and crystals to rotate the signals instead of brushes and generally (but not always) are much larger and much more expensive. Fiber-optic slip rings can be manufactured using single-mode or multi-mode fibers, in industrial, commercial and MIL-Spec specifications.
Rotating drums without slip rings
If the signal only needs to be measured when the drum is in a stopped condition, a much less expensive option is often employed. This involves bringing the terminated end of the cable on the first layer of the drum out through the side flange, clamping it externally and providing some sort of cable connector at that point. When the drum is stopped the read-out can be plugged in and unplugged again before the winch is restarted. Starting the winch with the cable plugged in can lead to catastrophic damage to the read-out cable, however, so this method is not recommended for beginners, low-skill-level deckhands or for most rental or commercial applications. Typically this method is only used by geophysicists operating in conditions where slip rings are too likely to be damaged by temperature or weather or by operators with very small budgets.
One very unusual variant to this is to mount a set of batteries to power the instrument inside the drum core along with a WiFi transmitter that plugs into the signal from the instrument. Rather than bringing a connector through the side an antenna is mounted on the side of the drum along with a connector for recharging of the batteries when necessary. This has significant drawbacks resulting from WiFi transmission inteference and , battery life and overcoming the inertia of the batteries.
Linearly-constructed winches are very wide, as the motor and gearbox and any other transmission components are built in-line. Some very small linear winches can be built with the transmission components inside the drum core, but this makes for very difficult maintenance of the transmission (typically the entire winch needs to be disassembled).
Vertically-constructed winches mount the transmission under the drum with drip protection between the transmission and the drum. Although this design does raise the height (and center of gravity) of the winch significantly and makes the transmission more difficult to access, the small footprint is very beneficial in restricted space locations like small craft and box trailers and the raised drum can be very useful in small craft where the geometry would otherwise make mounting the winch difficult. For example, a vertically-constructed winch makes it easier to deploy a cable over the gunwale or transom, as the cable leaves the drum at a height comparbale to that of the gunwale or transom.
Perpendicular-Horizontal constructed winches place the drive shaft parallel to the drum shaft but with the motor and most of the transmission to the side of the drum. The drum can be lowered until the flanges are only a few inches from the deck. These winches tend to be wide and stubby and work well when a wide mounting location is available.
Parallel-horizontal constructed winches place the main drive shaft of the winch behind and parallel to the drum shaft, with the motor and gearbox in the same area and the drum flanges only clearing the deck by a few inches. This construction method keeps the transmission components out of the path of moisture dripping from the incoming cable and away from the danger zone of the winch and provides a footprint that is relatively long and narrow. This footprint means they work well in truck-mount, trailer-mount ad portable applications and in small craft with long back decks. It is possible to construct a winch with the transmission components in front of the drum, but this leads to the need for extra weatherproofing of the transmission components and yields little if any benefit in most geometries (the notable exception being when the rear of the winch would be inaccessible due to ship geometries).
Why might I need a motor brake?
Motor brakes are placed inline between the motor and the gearbox and are solenoid-controlled. When the motor is turning the brake is disengaged; when the motor is stopped the brake engages. The torque the brake can hold is multiplied by the total gearbox and chain reduction of the transmission, factoring in efficiencies. Motor brakes are highly recommended when towing instruments as they are far better at preventing run-out of the cable than back-drive of worm gears and manual disc brakes.
Information on manual brakes and pin locks
Manual brakes (including pin locks) are hand-operated mechanisms to slow or stop the rotation of the drum, and to control the rotation of a drum which is free-wheeling. They generally consist of a simple disc brake mechanism, although drum brakes and band brakes have also been used. Pin locks are pins which are inserted into a pin lock guide block through a hole in a disc brake, thereby locking the drum and preventing rotation at all. The pins used can be solid steel for permanent (storage) locking or shear pins calculated to release at a certain overload condition using the principles of double-shear conditions.
What is an Overload Clutch?
Overload clutches use disc-brake like structures to release the drum from the drive shaft when an overload condition is reached. The primary usage for overload clutches is to prevent the hauling of loads that would exceed some rating of the winch, which could be some component strength, the motor horsepower or the lifting-load rating of a small craft. These clutches generally replace the drive sprocket on a chain drive in a transmission.
What is a Free-wheeling clutch?
Free-wheeling clutches (also known as dog clutches) are often used on winches where the drum needs to be decoupled from the motor and gearbox to allow the drum to rotate freely. This is common when the ability to deploy the cable quickly overrides the requirement of controlling that deployment by the drive. Nominally this free-wheeling can be called free-fall, but through many mediums the terminal velocity of the payload may exceed the maximum friction-based speed of the free-wheeling drum. In this case, if the free-fall speed of the payload is important, the winch should be designed to power out the line faster than this free-fall speed, rather than depending on free-wheeling. In all other cases (grab samplers, profiling instruments, etc.) the free-wheeling capability is normally sufficient. This ability is also very useful when doing maintenance on the winch.
Some information on Chain Transmissions
Chain transmissions are often used to transfer drive power from the main drive shaft to the drum shaft. Chains are designated by their number (which is related to their pitch) and the number of parallel strands in the chain. A chain drive generally combines a small drive sprocket and a large driven sprocket for speed reduction, or vice versa for speed increase. Winches generally require a speed reduction. Although smaller sprockets are available it is generally good design practice to limit the smallest sprocket to no less than 15 teeth, although geometric relationships (clearances and drive shaft diameter) may mean other tooth numbers are the minimum or maximum allowed for a winch design. In general, as the drive shaft speed gets slower and/or the load gets heavier, the chain size needs to be increased, although larger chain sizes mean fewer sprocket choices available for a given geometry.
Gearboxes in winches
Winches typically use one of two types of gearbox: right angle worm drive and inline drive.
Right angle worm drives take the input from a motor and output to a drive shaft, turning the output shaft 90 degrees from the input shaft. A worm gear mechanism inside the gearbox provides the needed reduction. Worm gear drives are available in ratios of 5, 10, 15, 20, 25, 30, 40, 50 and 60:1 from most gearbox manufacturers in a variety of sizes. As the reduction ratio increases, the maximum allowable input horsepower decreases; as the size of the gearbox increases, the maximum allowable horsepower for a particular size increases. Worm-drive systems have an additional benefit: they are very difficult to drive in reverse, especially at ratios in excess of 30:1, and as such act as natural brakes preventing the drum from turning when the motor and brake are not engaged. This back-drive force can be used to hold the drum in position for light loads but should not be relied upon to prevent run-out at ratios less than 30:1.
Inline gearboxes use planetary, helical, spur or harmonic gearing to provide the reduction, with the output shaft parallel to the input shaft. Except for harmonic gearing (which is limited to lower horsepowers) these gearboxes do not provide any significant back-drive resistance. They do provide even higher reduction ratios than worm-drives and their geometry can be beneficial in some design configurations.
Gearboxes multiply the torque transmitted through the gearbox by the gearbox ratio multiplied by the gearbox efficiency. As the speed is reduced the torque is increased.
What control systems are available?
Low-Voltage DC motors are generally controlled with PWM controls. Few if any special features accompany these controllers. High-Voltage DC motors are operated with SCR or SCR Regenerative controllers, with the latter employing a regenerative drive component. Regenerative drives are usually not available above 2 HP due to cooling issues. Vector-Drive AC controllers are specially-designed for that application and are complicated and relatively expensive; on the other hand they often offer much more comprehensive monitoring of the motor conditions and are generally directly compatible with computer control systems. DC motor controls usually only have computer control as an add-on option, typically as an analog voltage-following control board.
What motor choices are available?
In general there are five types of motors available that are suitable for use with winches. They are Low-Voltage DC, High-Voltage DC, Specialty DC and Vector-Drive AC.
Low-Voltage DC motors operate from 12, 24, 36 or 48 VDC and are permanent magnet brushed designs. The power is generally supplied from battery banks or transformers and they are generally controlled with simple variable-speed DC PWM controllers. They are generally available in horsepowers from 1/16 HP to 1 HP; larger horsepowers are uncommon as the amperage required to drive them would require very large diameter cables and cause excessive risk of overheating.
High-Voltage DC motors operate at 90 or 180 VDC, supplied respectively by 110 VAC or 220 VAC single-phase sources. They are controlled with either SCR or SCR Regenerative variable-speed controllers. These motors are available from 1/4 HP to 3 HP with some uncommon 5 HP models available. The 110 VAC (90 VDC) motors are available up to 1 HP while the 220 VAC (180 VDC) motors are available for the full range.
Specialty DC motors are available with high horsepower ratings that use uncommon DC voltages such as 96 VDC. These motors are very powerful for their size but operate at dangerously high voltages and require active cooling of their components such that they cannot easily be sealed for use in inclement conditions. They are typically restricted to applications such as electric forklifts and golf carts where large air volumes can be weatherproofed to protect them from the elements and provide cooling of the motor, often with auxiliary cooling fans.
Vector-Drive AC motors are the next step up from DC motors. Closed-Loop Vector-Drive AC motors drive directly from three-phase 220 VAC or greater and use sophisticated computer controls and feedback sensors to provide a powerful torque throughout their complete RPM range; standard AC motors do not have a constant torque curve and do not operate well at low RPM. Open-loop Vector-Drive solutions are also available with even more sophisticated computer algorithms to simulate Closed-Loop performance without the sensors, but these systems generally require more maintenance of the electronics and do not quite reach the low-RPM performance of the Closed-Loop systems. Vector-Drive motors are available from 1/2 HP through 50 HP; note that an application requiring X horsepower requires a Vector-Drive motor of 1.2X HP. Similar to Vector-Drive is Direct Torque Control, which uses cheaper and simpler motors controlled with sophisticated computerised electronic controls.
With the construction of special transmissions it is also possible to gang multiple motors to one drive shaft, effectively multiplying the available horsepower, but requiring significantly heavier transmission components and also multiplying the number or maintenance and potential failure points.
How to achieve a smooth spooling of the cable?
Smooth spooling of the cable is very difficult to achieve in real-world applications. The smooth spooling of a cable is dependent on four primary factors: the tension of the cable, the stickiness of the cable, the fair lead and the first wrap condition.
The geometry of the cable needed for smooth spooling is related to the fair lead of the setup. The fair lead is the distance between the axle of the drum and the axle of the first sheave block the cable passes over after leaving the drum. If the fair lead is within the ideal range the cable should theoretically spool smoothly. The ideal range is determined by the fleet angle, as shown below:
The fleet angle should be between 1.5 and 2.0 degrees for smooth spooling. In practice this means that for every inch of drum face width the sheave block axle should be 14 to 19 inches away from the drum axle (for every cm of width it should be 14 to 19 cm away). With the typical 12" face widths manufactured by A.G.O., the fair lead should be 14 to 19 feet.
Smooth spooling can also be affected by the initial wrap of the cable on the drum. With a plain cylindrical core the first wrap follows an imperfect helical path and the wrap jump at the end of the first wrap is difficult to control. If the person installing the first wrap is able to spend the time to make the wrap as perfect as possible, there should be a space between each wrap of approximately 8-10% of the cable diameter to allow for smooth spooling; the first layer cable wraps should not be touching each other. As well, while this first layer will wrap smoothly if care is taken, it should be realized that as the helix of the second wrap is opposite that of the layer below it, the second wrap may not lie smoothly either, although the smoothness of the wrap below it does play a significant role in this.
There are two common methods of resolving this wrapping issue. The first is to put a helical groove in the drum core that matches the specific cable being used. This allows the cable to be wound onto the core quickly and efficiently but is difficult to manufacture, requires a heavier and more expensive core material and prevents the use of a core access panel. The advantages of this helical groove are negligible on drums that store multiple wraps unless the cable is regularly unwrapped to the point where the first layer is partially unwrapped.
The second and by far superior method is a proprietary patented method known as a LeBus core. This specially machined core has parallel (rather than helical) grooves with two change-over points on each groove. This allows the cable to wrap evenly every single time and provides a perfect wrap (with a correct fairlead) on every layer. The LeBus core, however, is a very expensive and difficult part to manufacture and is generally only available with drum cores from LeBus themselves. These cores are typically only used with large cranes or similar winches.
When it is not possible to provide an adequate fair lead four options are available, all of which perform the same basic function: forcing the cable to one side so that the fleet angle remains in the optimum range. All level-wind systems require the cable to be under at least a minimal level of tension for the level-wind to operate. Slack cables in general do not provide smooth winding and level winds will not help if the cable is slack.
The simplest and least expensive method is a manual level-wind. This system consists of a pair of rollers attached to a traveller mounted on a set of bars parallel to the drum axle with a push-pull rod attached to the traveller. As the cable is wound in the operator manually pushes or pulls the cable into a position where the fairlead is small and the cable winds smoothly. While this is simple and inexpensive to construct, it is laborious and imperfect and in some regions may violate health & safety regulations. It is impractical for many heavier loads. Although many "skilled" or "experienced" people will encourage the use of a loose hand-held control rod with a forked end to accomplish the same task, this method is stongly discouraged as it presents a very high risk of injury or damage to the cable or winch. These systems typically cost somewhere in the range of 10-20% of the cost of a winch.
The next most common method is a direct-drive level wind mechanism. This mechanism powers the traveller and rollers from the drive axle rather than relying on manual power. The gearing for this drive is designed specifically for a single winch and cable combination and is designed to lay an optimum wrap at first layer of cable. As the length of cable on each wrap varies the level wind mechanisam will not work smoothly with every wrap, but it will cause the imperfect wraps to be spread to the full width of the drum. Typically these mechanisms should not be used if there are more than a five or six wraps on the drum as the wraps beyond this may become significantly uneven, to the extent that after the tenth wrap or more the level-wind may be acting in opposition to the ideal wrap. These systems typically cost somewhere in the range of 40-60% of the cost of a winch.
The next level of complication is a computer-controlled powered level wind. These systems use a combination of sensors and algorithms to determine which wrap they are level-winding and vary the drive speed of the traveller accordingly. These systems provide a perfect wrap throughout the depth of the drum with short fair leads but the cost of such a system can double or even triple the cost of a winch.
Another solution is a Fleet Angle Compensator, a mechanism perfected by the LeBus company. This system uses an oscillating carrier shaft with a sliding sheave and is completely autmoatic and unpowered. It is, however, difficult and expensive to construct and requires an undershot drum to operate (most drums are constructed in an overshot configuration). The Fleet Angle Compensator system does have the additional benefit that the inherent sheave wheel can be instrumented to act as a wire counter.
For very simple winches with cable diameters of at least 1/4", a sprung-roller level wind mechanism can be used such that the roller presses the cable down and helps stop the cable from running over itself. On single-layer drums this system works perfectly; as more layers are added the system becomes less effective as the spring needs to be weaker with each layer and thus is less able to resist cable climb.
How do you tie off the cable to the drum?
Drums may be constructed in several manners to allow the cable to be tied off and/or accessed.
The first method is a radial flange-slot. A curved slot is cut into the flange right at the surface of the drum core and following the arc of the core. The cable end is fed through this slot and is clamped off to the exterior surface of the drum flange. The primary difficulty with this method is that the winch has to be designed for the cable and cable clamp on the exterior of the drum such that the cable clears all other components between the flange and the frame; this can lead to a section of unsupported drum axle between the flange and the frame which is not always optimal, especially with high loads. As well, if the cable is a signal cable the only way to adapt the cable to a slip ring is to twist the wire significantly and feed it through a slot cut into the hollow axle, or the use of a hollow-center slip ring, both of which require a large space between the flange and the frame to clear these components.
The second method is a core slot with a core access panel. In this case an angled hole or slot is cut into the core so that the cable enters the core near the core-flange junction on one side of the core. The cable is clamped off inside the drum core and, if the cable is a conductor-cable, the end of the cable is fed through a slot in the side of the hollow axle inside the drum. Access to make these internal clamp locations is via an access panel which can be in the flange if the core is large enough (typically 18" or greater) or in the core (for core sizes smaller than 18").
The third type is similar to the core slot method but the portion of the flange on the core slot side that is inside the drum core diameter is recessed into the core far enough that the cable can be accessed and clamped off from outside the drum without resorting to an access panel. While logistically simpler, this drum construction method is much more laborious to manufacture.
What Drum Styles are available?
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.
How is Drum Capacity calculated?
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.
What are the basic elements of a winch?
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.