Lift-It® Manufacturing - General Information
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Transcript of Lift-It® Manufacturing - General Information
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6/25/13 Lift-It Manufacturing - General Information
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GENERAL INFORMATION
General Information
Sling Hitches
Slings carry their loads in one of three primary sling hitches. Most slings can be used in all three sling hitches, but some
slings are designed for use in only one hitch. Slings have the largest Work Load Limit when used in a basket hitch. The
vertical hitch Work Load Limit is 50% of the basket hitch. The synthetic choker hitch Work Load Limit is a maximum of
80% of the vertical hitch Work Load Limit.
Slings must be securely attached to the load and rigged in a manner to provide for load control to prevent slipping, sliding
and/or loss of the load. A trained, qualified and knowledgeable user must determine the most appropriate method of rigging
to help ensure load control and a safe lift.
CHOKER HITCH
Sling passes through one end around the load, while the other end
is placed on the hook. Load control is limited with only one sling
rigged in a choker hitch. A choker hitch will never provide full 360
degree contact. For full contact use a Double Wrap Choke Hitch.
See Choker Hitches. The Choke Point should always be on the
sling body, not on the sling eye, fitting, base of the eye or fitting,
splice or tag.
VERTICAL HITCH BASKET HITCH
One end is on the
hook, while the
other end is
attached directly to
the load. Use a
tagline to
prevent load
rotation.
The sling cradles the
load while both eyes are
attached overhead. As
with the choker hitch,
more than one sling
may be necessary to
help ensure load
control.
SLING-TO-LOAD ANGLE
The Sling-to-Load Angle is the angle formed between a horizontal line and the sling leg or body. The Sling-to-Load Angle
has a dramatic effect on sling Work Load Limits. Slings with adequate capacity to handle the scale weight of the load have
catastrophically failed because the Sling-to-Load Angle and increased tension were not taken into account.
This principle applies in a number of conditions, including when one sling is used to lift at an angle and when a basket hitch
or multi-leg bridle sling is used. When selecting a sling, always consider the Sling-to-Load Angle and the tension that will be
applied to the sling. As the Sling-To-Load Angle decreases, the tension on the sling leg(s) increases.
SLING-TO-LOAD ANGLE
The horizontal angle
formed between the sling
leg and the top of the
load.
Illustrated right - Increased
tension is magnified by any
change from vertical to
horizontal lifting. Increased
tension is imposed on the
sling leg(s) when the legs are
used at angles less than 90.
SLING ANGLE - REDUCED WORK LOAD METHOD
For years sling users have used angles to determine sling work
load adequacy. One approach has been to determine the sling-to-
load angle and multiply the work load limit
LOSS FACTOR CHART
Search
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load angle and multiply the work load limit
by the loss factor for the specific angle. The result is the REDUCED
WORK LOAD.
1. Calculate the sling to load angle
2. Determine the corresponding loss factor
3. Multiply the work load limit by the loss factor to determine
the reduced work load.
The result is the reduced work load limit.
Single angles of less than 30 should not be used, unless
approved by a qualified person.
LOSS FACTOR CHART
Angle "A"
DegreesLoss Factor
Angle "A"
Degrees
Loss
Factor
90 1.000 55 .8192
85 .9962 50 .7660
80 .9848 45 .7071
75 .9659 40 .6428
70 .9397 35 .5736
65 .9063 30 .5000
60 .8660 25 .4226
SLING ANGLE INCREASED TENSION METHOD
Another more salient approach is to determine the INCREASED
TENSION by the angle of lift. This approach has the distinct
advantage of enabling the sling user to determine the required
sling strength requirement. The user must first determine the angle
and multiply the load weight by the tension factor for the specific
angle. The result is the INCREASED TENSION or actual loading on
the sling leg(s).
1. Calculate the sling to load angle
2. Determine the corresponding tension factor
3. Multiply the load weight by the tension factor to determine
the loading on the sling leg(s).
TENSION FACTOR CHART
ANGLE "A"
DEGREES
TENSION
FACTOR
ANGLE "A"
DEGREES
TENSION
FACTOR
90 1.000 55 1.221
85 1.004 50 1.305
80 1.015 45 1.414
75 1.035 40 1.555
70 1.064 35 1.742
65 1.104 30 2.000
60 1.155 25 2.364
Single angles of less than 30 should not be
used, unless approved by a qualified person.
CHOKER HITCH ANGLE
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CHOKER HITCH ANGLE REDUCTION CHART
Choker Hitch Angle
(Degrees)Reduction Factor
120 - 180 1.00
105 - 120 .82
90 - 105 .71
60 - 90 .58
0 - 60 .50
CENTER OF GRAVITY (CG)
It is always important to rig and control the load so that stability is achieved. Determining the location of the Center of Gravity
(CG) is vital to achieving load control. The CG is the point where the load weight is concentrated and is the balance point for
an object. The Center of Gravity when suspended will:
1. Unless restrained, the CG will move directly under the suspension point.
2. The CG will move to the lowest point possible.
For best control, attach the slings above the CG. When this is not possible keep the CG contained with three or four sling
legs or use basket or choker hitches with wraps. These measures may not guarantee load control. The user must be
assured, based upon the specific application that selected methods are suitable and comply with all applicable standards
and regulations.
Multiple factors must be taken into consideration to ensure that load control and stability are attained. A load
with a high center of gravity can rotate in certain sling hitches.
SLING TENSION - LEG LENGTH/HEADROOM
Calculating the tension imposed on slings or individual legs of a multi-part sling system will enable the sling user to
select slings with adequate Work Load Limits.
Use the following steps to calculate the tension imposed upon the individual sling legs, when you know the leg Length (L)
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Use the following steps to calculate the tension imposed upon the individual sling legs, when you know the leg Length (L)
and Headroom (H).
1) Determine the Load Factor (LAF):
Divide the leg length (L) by the headroom (H)
L H = LF
Example: 20 15 = 1.33 Load Factor (LAF)
2) Determine the Share of the Load (SOL) for the
individual sling legs:
Divide the load weight by the number of sling legs.
Load weight number of legs = Share of the Load
(SOL)
Example: 12,000 lbs 3 legs = 4,000 lbs. (SOL)
3) Multiply Load Factor by the Share of the Load to
determine Sling Tension
Load Factor x Share of the Load = Tension
LAF x SOL = Tension
Example: 1.33 x 4,000 = 5,320 lbs.
Please Note: Tension calculations are based upon:
1. Sling attachment points being equidistant from the
center of gravity
2. Sling attachment points being equidistant to each
other.
3. Sling attachment points being on the same horizontal
plane
4. Equal sling leg lengths
SLING TENSION - PROPORTIONAL SHARE OF THE LOAD
More complex calculations are required when the slings are not placed equidistantly from the center of gravity or when the
Center of Gravity is not equidistant from the sling attachment points. The PROPORTIONAL SHARE OF THE LOAD (SOL) must
be determined and multiplied by the LOAD ANGLE FACTOR (LAF) to determine SLING TENSION.
Sling tension is a function of sling length, distance between the sling attachment points and the spatial relationship
between the sling attachment points and the Center of Gravity. An inverse proportional relationship exists between Distance
and Share of the Load. If a sling is attached 25% of the distance from the Center of Gravity, that sling will carry 75% of the
load weight. Likewise, if a sling is attached 75% of the distance from the Center of Gravity, that sling will carry 25% of the
load weight.
SLING TENSION - DIFFERENT HORIZONTAL PLANES
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GENERAL INFORMATION
Slings should be rigged in a manner that provides proper load control. It is dangerous to use only one sling to lift a load
which tends to shift and slide out.
(One sling is depicted for illustrative purposes only).
Ensure that lifting devices are directly over the center of gravity. If this is
difficult to determine, it must be discovered by cautious experimentation or
calculation. Raise the load carefully. If the load is not level, lower and
correct the position of the slings until the balance point is achieved and
the load does not tilt.
ADJUSTABLE BASKET HITCH
The adjustable basket hitch allows the sling user to adjust the length of the legs to raise the
load level. Adjustable hitches are particularly useful with loads having uneven load weight
distribution resulting in an off-set center of gravity.
The Adjustable Basket Hitch Work Load Limit is identical to the regular basket hitch rating.
The rating must be adjusted for the Sling-to-Load Angle. Another effective solution is an
Adjustable Rope Sling featured on the following products:
Single Leg - Adjustable Rope Slings
Double Leg - Adjustable Rope Slings
Adjustable Rope Slings W/Top Link
Adjustable Rope Slings W/Top Link
BASKET HITCHES
Inverted basket hitches are referred to as equalizing hitches
because the sling is free to slip through the hook based
upon the load weight distribution. Be sure to employ the four
ends down, North to South, load engagement system.
Extra care should be taken when using slings in abasket hitch to balance the load to prevent slippage.
As with the choker hitch, more than one sling may be
necessary to control the load.
If practical, take a full wrap around the load to grip it
firmly; be sure when using multiple slings that they do
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not cross over each other. Wrapping the load is a
legitimate method of minimizing excessive sling length.
Other methods, such as, twisting and knotting radically
reduce sling Work Load Limits. When the load is
wrapped the sling Work Load Limit is not increased,
but load control is..
Slings skipping through hardware
components can become damaged. Balancing the load is
critical and necessary to prevent sling damage and failure
CHOKER HITCHES
The choke hitch should always be pulled tight before the lift
is made, not pulled down during the lift. A sling rigged in a
choker hitch (not double wrapped) does not make full
contact with the load. Use multiple slings to balance the
load, and wrap the load to ensure full contact. Ensure
multiple slings do not cross. Choke on opposite sides of the
load, if this action will not damage the load and maintain
load control.
For a tighter choke hitch, which provides full, 360contact with the load, take a full wrap around the load
before choking the sling. Ensure that multiple slings do
not cross. When the load is wrapped the sling Work
Load Limit does not increase, but load control does.
Please note:
One sling is depicted for illustrative purposes only.
Always use a choker hitch when turning a load. If the sling is
not rigged properly, the turning action will loosen the hitch,
resulting in load slippage. Place sling eyes on top of the
load, pointing the opposite direction of the turn. The body is
then passed under the load and through both eyes. Blocking
should be used to protect the sling and facilitate removal.
Basket hitches should not be used to turn a load. Always
downgrade the choker Work Load Limit when the angle of
choke is less than 120.
The sling should be of sufficient length to ensure that thechoke action is on the sling body, never on the sling
splice, fittings, tag, eye or at the base of the sling eye or
fitting.
ADJUSTABLE HITCH
The Adjustable Hitch allows the sling user to adjust the length of the legs to raise the load
level. Adjustable hitches are particularly useful with loads having uneven load weight
distribution resulting in an off-set center of gravity. The Work Load Limit for the Adjustable
Hitch is identical to the normal Vertical Work Load Limit. The Adjustable Hitch works
reasonably well on narrow web slings (1 and 2 in. widths) and roundslings rated at less
than 7000 Lbs. choker.
DOUBLE CHOKER HITCH
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The Double Choker Hitch if applied properly will facilitate equalization of
the loading on the sling legs over the lifting hardware. If applied
improperly, one of the legs will share a greater portion of the load and
equalization will not occur. The Double Choker Hitch Work Load Limit is
twice the regular Choker Hitch Work Load Limit.
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