Winch Design FAQ
The design of a winch for any purpose is not a trivial exercise. This
FAQ is intended to explain many of the perceived mysteries of winch
design and help our customers make an informed choice about options
available to them.
1) What are the basic elements of a winch?
2) Winch Drum Basics
2.1) How is Drum Capacity calculated?
2.2) What Drum Styles are available?
2.3) How do you tie off the cable to the drum?
2.4) Smooth Spooling of cable
3) Winch Motor Basics
3.1) What motor choices are available?
3.2) What control systems are available?
4) Winch Transmission Basics
4.1) Gearboxes
4.2) Chain Transmissions
4.3) Free-wheeling clutches
4.4) Overload clutches
4.5) Manual brakes
4.6) Motor brakes
5) Winch Construction Styles
5.1) Parallel-Horizontal Construction
5.2) Perpendicular-Horizontal Construction
5.3) Vertical Construction
5.4) Linear Construction
6) Slip Rings and Signal Transmission
6.1) Rotating drums without slip rings
6.2) Electrical slip ring styles and types
7) Payload Characteristics
7.1) Vertical profiling
7.2) Borehole operation
7.3) Instrument towing
7.4) Balloon and Aerostat deployment
8) Cable Characteristics
9) Winch Instrumentation
1) 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.
2) Winch Drum Basics
2.1) 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
where:
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.
2.2) 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.
2.3) 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.
2.4) Smooth spooling of 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.
3) Winch Motor Basics
3.1) 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.
3.2) 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.
4) Winch Transmission Basics
4.1) Gearboxes
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.
4.2) 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.
4.3) Free-wheeling clutches
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.
4.4) Overload clutches
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.
4.5) 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.
4.6) Motor brakes
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.
5) Winch Construction Styles
5.1) Parallel-Horizontal Construction
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).
5.2) Perpendicular-Horizontal Construction
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.
5.3) Vertical Construction
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.
5.4) Linear Construction
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).
6) Slip Rings and Signal Transmission
When using a signal cable with a winch it is often necessary to
transfer those signals from the rotating drum to a stationary computer
or other readout device. This is where slip rings (also known as
rotating electrical joints) are used.
6.1) 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.
6.2) 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.
7) Load Characteristics
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.
7.1) Vertical profiling
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.
7.2) Borehole operation
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.
7.3) Instrument towing
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.
7.4) 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.
8) Cable Characteristics
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 asthe 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.
9) Winch Instrumentation
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. 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.