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Rigging/Signal Person Training

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Discovering the Center of Gravity:

Is this section we will discuss the importance of knowing where the Center of Gravity is in the load and how to rig to it. We will cover:

Finding the Center of Gravity by “Trial & Error;”

Inverse Proportion to Distance Formula;

We will show the importance of Rigging to the Center of Gravity;

How to determine the stress of Slings of Equal Length; and

How to determine the stress of Slings of Unequal length.

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Center of Gravity:

*The center of gravity (CG) as it pertains to rigging, is the center of the load’s weight distribution or the point in the load around which all weight is evenly distributed, no matter how it is turned or rotated.

*Question on written test: Center of gravity is:

a. In the physical center of an objectb. A point in an object around which all its weight is evenly distributedc. The tipping point of an object when it is liftedd. The balancing point of an object

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Effects of the Center of Gravity:

It might seem silly that someone would overload a donkey cart to the extent that it would lift the donkey off of the ground but this is not unheard of in the crane industry. How many times have you seen pictures of videos of cranes tipped over on their side?

In this section we will mostly be talking about the effects the center of gravity of the load has on rigging.

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Discussion:

In this picture we see an Articulating Boom Truck Crane lifting a forklift. What observations can you make about how it is rigged and where the center of gravity of the load is?

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Discussion:

1. Typically the center of gravity of forklifts is in the rear near where the counterbalance is located.2. Since the hook is not located over the center of gravity the forklift did not come up level when

lifted.3. The slings are synthetic round slings which have been doubled up and are attached to the hook by

a shackle.4. The load is being suspended nearly over the outrigger so the pressure on that outrigger will be the

greatest.5. The pad on the outrigger is very small and could sink in on softer soils or pavement.6. If the forklift weighs 8,000 lbs then the pressure on that small pad will be over twice that since it

not only supports the weight of the load but the weight of truck that is keeping the load up in the air. If the pad size is 144 square inches (or 1 square foot) then the ground pressure would be over 100 lbs per square inch.

A picture is worth a thousand words!

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Trial and Error Method of Finding the Center of Gravity:

*When a load is rigged and lifted, the center of gravity will always move under the hook. If it is

lifted abruptly, then it will swing past the CG an equal distance and then swing back and forth and so on.

This can result in dangerous load shifting and additional stress on lifting hardware and rigging. If any load tilts more than 5° after it is lifted clear of the ground it should be landed and rigged over again.

*Question on written test: If the hook is 2 feet to the left of the center of gravity when it is lifted abruptly, the load will swing

a. 2 feet to the leftb. 2 feet to the rightc. 4 feet to the rightd. 4 feet to the left

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Trial and Error Method of Finding the Center of Gravity:

*The “trial and error” method used to find the Center of Gravity is possibly the most reasonable.

1. On in initial pick, the load tilts to one side. Mark a line on the load in line with hoist line.

2. On the second pick, select slings of unequal length which will tilt the load in the opposite direction. Mark a line on the load in line with hoist line.

3. Where the two lines intersect is the Center of gravity in the horizontal (east/west)

4. Position the hook directly above the Center of Gravity and select the proper size slings.

*Question on written test: The easiest way to find the center of gravity is the “trial & error” method, picking up the load slowly to see which end is heaviest and then adjusting the slings until it comes up level.

a. Trueb. False

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Inverse Proportion to Distance Formula:

This formula will find the center of gravity of the load on a horizontal plane rather than in every direction and is good for long, wide loads where a two-legged bridle will be used to lift it. It is a simple formula that just requires that you know the weights of A & B and the distance between their center of gravities which is Z.

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Center of Gravity Exercise #1:

What is the distance of W if the distance between A (125 lbs) and B (250 lbs) is 21 feet?

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It helps to work it out one step at a time.

We don’t need a calculator to know that 2/3rd of 21’ is 14’.

And when it comes down to it, we really need a tape measure to estimate where 2/3 the distance to B is from A.

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That was way too easy! And in reality, knowing the weights of each side, one could come pretty close without doing the calculations.

Just take heavier of the two weights and divide them by the combined weight which will give you a fraction. In this example it would be 0.666 which is 2/3rd. The center of gravity will then be 2/3rd the distance from the lighter side to the heavy side.

Now we will look at a problem that is a little more complicated:

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In this exercise you will be trying to find the combined center of gravity of these three parts of the load:

A, B and C. (In this example we are looking at a top view of the load, rather than a side view.)

Hint: Find the center of gravity of parts A and B first, then using that center of gravity, find the center

of gravity between it and part C.

Before doing the calculations take a guess as to where you think the CG is and see how close you get to

the correct answer. Remember, you are looking at this object from above and not from the side.

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OK, let’s work this out together and see how you did:

First, we will determine the combined CG for parts A & B using the inverse proportion to distance formula. You already noticed that there is some information missing, didn’t you? The distances between the center of gravities. That’s ok, you don’t need it.

1. Total weight of A & B is 400 lbs + 200 lbs = 600 lbs

2. Divide Part A weight by the total weight of A & B: 400 lbs / 600 lbs = 2/3 (.66)

3.The distance between the CG’s of part A & B is 2/3rd the distance from B to A.

4. Mark that spot on a direct line between the two.

5. Now using their combined CG we will continue.

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Now using the combined CG of A & B and their combined weight we will continue.

1. Total weight of A, B & C = 400 lbs + 200 lbs + 1,200 lbs = 1,600 lbs

2. Divide A & B weight by the total weight = 600 lbs / 1,600 lbs = 1/3 (.33)

3. The distance between the combined CG’s of parts A & B and part C is 1/3rd the distance to A & B from C.

That was not hard at all! The real challenge will be in actually rigging and picking this load up, but at least we know where we are going to position the hook so it comes up level.

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Determining the Center of Gravity in the Vertical:

Determining the Center of Gravity in the vertical uses the same process as for

horizontal.

(The main reason you would need the vertical center of gravity of a load is if the pick

points (where you will attach the rigging to the load) could not be positioned above

the center of gravity, a principle we will discuss later in this training.)

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Determining the Center of Gravity in all Three Directions:

Determining the Center of Gravity in all three directions seems a little more mind boggling but just takes a little more time. Once you determine the CG in the horizontal, then calculate in the vertical. Draw a straight line between the two and the combined CG will be in the middle.

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Equal Loading of Slings:

This formula is used to determine what the stress is in each vertical sling when picking up a load where the slings are attached to a spreader bar or lifting beam.

*The key to determining the stress on the slings is knowing the total weight of the load; where its center of gravity is and the distance between the two attachment points of the slings (not the length of the load).

What we will learn is that when the center of gravity is equally spaced between the two, then the stress on the slings will be equal. But, let’s do the math anyway!

*Question on written test: The key to determining the stress in vertical slings is:

a. Knowing the weight of the loadb. Knowing where the center of gravity isc. Knowing the distance between the pick pointsd. All of the above are correct

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What is the stress that will be on each sling?

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Well, we knew that was going to be the answer.

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Unequal Loading of Slings:

When the center of gravity is not equally spaced between the two pick points the stress on the two slings will not be equal. The sling closest to the center of gravity will see the most stress.

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In this exercise we will first determine where the center of gravity of the load is. Then in Center of Gravity Exercise #4 we will determine how much stress each sling will see.

Using the inverse proportion to distance formula, where is the horizontal center of gravity of this load?

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The center of gravity of this load will be 5.25 feet from A’s center of gravity. Also, it is important to note that the combined center of gravity will always be on a straight line between the two.

I know that this may be a lot to look at (dotted lines, arrows, and figures) but it really is quite simple.

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Now that we know where the center of gravity of the load is we can focus our attentions to how much stress each sling will see.

This formula is much like the others. In order to calculate it you need to know: the weight of the load; and the distance between the center of gravity of the load and each pick point (not the end of the load).

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Piece of cake!

But, do notice that Sling #2 sees the bulk of this load. They do not share it equally, so, when it comes time to select the slings for this lift you will know that sling #2 needs a capacity of at least 6,400 lbs.

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Center of Gravity Below the Pick Points is More Stable:

*Another important rigging principle is to ensure that the pick points of a load (where the slings are attached to the load) lie above its center of gravity. When a load is lifted it’s center of gravity will always seek the lowest level below the hook. Loads where the pick points are below the center of gravity can be very unstable.

Just looking at the load on the right, our gut tells us that when we lift it, the load will swing to the right and could topple. Of course, in real life, the CG is not a yellow ball that indicates its location, but the shape of this object shows that a good deal of the load is outside of the slings, and that can’t be good!

*Question on written test: A load is more stable if the pick points are below the center of gravity rather than above them.

a. Trueb. False

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Center of Gravity Above the Pick Points is More Unstable:

This knowledge is especially important for lifting pallets, skids or the base of any object since they all have a tendency to want to topple. If you are attachied below the center of gravity you want to make sure the center of gravity is well within the triangle formed by the slings to make the pick.

It is not hard to see that the example on the right looks a lot like a swing on a swing set and only a fool would try to sit on one without holding on tight! The higher you grab on the easier it is to keep from falling off.

So, remember, if the attachments are above the center of gravity the load will be more stable.

And if the attachments are below the center of gravity the load will be more unstable.

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Hitches Used for Rigging:

Here we will be discussing the three most common hitches:

1. The Vertical Hitch which would include the single vertical, and 2, 3, & 4 leg bridles.

2. The Choker Hitch, including the single-wrap and double-wrap

3. The Basket Hitch including the true basket, reduced basket and double-wrap basket.

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Three Basic Hitches:

*The three basic hitches are the Vertical Hitch, Choker Hitch and Basket Hitch. From these there are a multitude of variations that will allow the rigger to safely control the load during a lift. Typically, the choker is about 75% of the vertical capacity and the basket is double the capacity of the vertical.

Riggers should always remember that the type of hitch that’s used to lift a load can greatly affect the rated capacity of the sling.

*Question on written test: Which of these is not one of the three basic hitches used in rigging?

a. Timber hitchb. Choker hitchc. Basket hitchd. Vertical hitch

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Capacity Tags:

Slings are required to have the tag showing the capacity of these hitches that they can be used for. The tag should also contain information regarding the manufacturer, description of the sling, material it’s made from and its size.

If the tag is illegible or missing it should be replaced or the sling taken out of service.

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Vertical Hitches:

Vertical hitches are either single-leg or multiple-leg bridles.

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Single Vertical:

The Single Vertical Hitch supports the total weight of the load on a single leg (unless they are used in tandem on a spreader bar or lifting beam), and the sling angle is 90° (sling angle is measured from the horizontal) . It has one-leg loading and poor north/south, east/west control.

The single vertical hitch should not be used for lifting loose loads, lengthy material or anything difficult to balance. It also provides very little control over the load because it can allow the load to rotate unless it is controlled by a tagline. Single vertical hitches by themselves should only be used on loads where the bulk of the weight is concentrated directly below the hook and the load is equipped with a rated eyebolt, shackle or attachment point.

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Bridle Hitches:

Two, three or four single hitches can be used together to form a bridle hitch for hoisting loads that are equipped with the rated attachment points.

Bridle hitches provide excellent load stability when: the load is distributed equally among the legs; the hook is positioned directly over the load’s center of gravity; and the load is raised level. To distribute the load equally it may be necessary to adjust the leg lengths.

Proper use of a bridle hitch requires that the stress in the slings caused by sling angles be carefully measured to ensure that the sling is not overloaded.

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2-Leg Bridle:

2-Leg Bridles are excellent for picking long-wide loads. The capacity will vary depending on the sling angle.

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Discussion:

Here is an example of a 2-bridle being used to lift this pipe.

What do you see as far as the sling angle being used, type and condition of the slings and the hitch type securing them to the pipe?

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Discussion:

1. It appears that the sling angle is more that 60˚ so the added stress to the slings and the possibility of them sliding together when lifted are minimal.

2. It looks like the slings that are attached to the hook are flat nylon slings, with the eyes attached to the hook block and swivel hooks attached to shackles.

3. The slings wrapped around the pipe are endless, round, polyester slings used in a tight, reduced basket hitch.

4. It doesn’t look like the nylon, flat slings are in very good condition.

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3-Leg Bridle:

3-Leg Bridles have excellent load control. If you are using a 3-leg bridle for a 2-leg pick then make sure the leg not being used is out of the way of the load.

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4-Leg Bridle:

*4-leg bridles have excellent load control but only have the capacity of a 3-leg bridle since not all of the slings will carry an equal share of the load.

And if the load is very rigid, you must assume that the load is being carried by only two of the legs, so it must then be “rated” the same as a two-leg bridle. The other two legs will not see as much stress and will mostly help to balance the load.

The only way to get true, 4-leg loading is to have a way (a turnbuckle, for example) to adjust the tension in each leg. This method is really not practical since it would also require a way to measure that tension. It would be easier to make the reduction and use higher capacity slings if necessary.

*Question on written test: A 4-leg bridle will have twice the capacity of a 2-leg bridle.

a. Truea. False

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Choker Hitch:

When a sling is being used in a choker hitch, there is a reduction in its rated capacity, usually around 75% of the vertical. For example: if the capacity of a sling in a vertical hitch is 12,000 lbs, then the capacity in a true choker hitch would be around 9,000 lbs. The key word here is “true.”

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Angle of Choke:

Another reduction that must be considered is due to the “angle” of the choke (not the angle of the leg of the sling).

If the load is hanging free, the normal choke angle is approximately 135 degrees. This is a “true” choker hitch and is about 75% of the capacity of the vertical hitch.

When lifting and turning a load using a choker hitch, it is not uncommon to have a severe bend at the choke. When a choker hitch is used at an angle of less than 120 degrees, you must reduce the hitch’s rated capacity as shown in the chart at right.

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Angle of Choke:

*Similarly, it is not uncommon for riggers to “cinch” the eye of the choker tight to grip the load. This practice will also reduce the capacity of the “true” choker hitch.

If you need better control of the load you may consider using a double-wrap choker instead.

According to this example, the capacity of this choker hitch would need to be reduced by 87%.

*Question on written test: Cinching down tightly on a choker hitch has no effect on the capacity of the sling.

a. Trueb. False

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Sling Hitches Exercise #1:

If this sling’s capacity in the vertical is 3,000 lbs, what would be it’s capacity in this configuration?

Trick question? Remember, the choker hitch is about 75% of a vertical hitch.

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OK, not too much of a reduction (about 300 lbs) but if this sling was part of a 2-leg bridle that had a 45˚ sling angle then the sling would see about 70% more stress than if it were at 90˚, so you can see, these reductions just keep adding up!

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Alternative to Cinching Down on the Eye of a Choker:

If you do not have slings long enough to use a double-wrap choker, but you still need to cinch down the eye to get more grip, you may consider inserting a piece of wood on top of the load and then cinching down on it. This will maintain the angle and put less stress on the eye of the sling.

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Types of Chokers:

These are the different variations of chokers that are commonly used.

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Single-Wrap Choker:

The Single Wrap Choker hitch forms a noose in the rope. It does not provide full 360° contact with the load and therefore should not be used to lift loads difficult to balance or loosely bundled. The single-wrap choker can also be doubled up to provide twice the capacity or to turn a load. (Doubling a single choker hitch is not the same as using a double choker hitch.)

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Double-Wrap Choker:

A double wrap choker is formed by wrapping the sling completely around the load and hooking it into the vertical part of the sling.

*This hitch is in full 360° contact with the load and tends to draw it tightly together.

It can be used either singly on short easily balanced loads, or in pairs on longer loads.

*Question on written test: Of these four, which hitch gives you the best control over a load?

a. Double wrap chokerb. Basketc. Verticald. Timber hitch

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Double-Choker, Eyes Up:

The double-Choker, Eyes Up, is made by putting the eyes through the bite of the sling and hanging them on the hook.

If you are going to double up the slings into a double-choker it is better not to have the eyes up since the sling won’t be able to adjust itself to get true 2-leg loading so one of the legs will be more stressed than the other, reducing its capacity.

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Double Choker, Bite Up:

The Double Choker, bite up, is a better choice than eyes up since you will get true 2-leg loading.

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Double-Choker, Bite Up:

Use a doubled-choker, bite up, to turn loads.

Place both sling eyes on top of the load pointing in the direction opposite to the direction of the turn. This sling will remain tight while the load is turning.

Never use a basket hitch to turn a load.

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Single Basket Hitch:

The Single Basket Hitch is used to support a load by attaching one end of the sling to the hook, then passing the other end under the load and attaching it to the hook. Ensure that the load does not turn or slide along the rope during a lift because both load and rope can be damaged.

Lifting a load using a basket hitch allows the two legs of the sling to function as if they were two separate slings which will give you twice the capacity of a vertical hitch as long as the two legs are 90 degrees. For this to happen it would be necessary to use a lifting beam.

The Double Basket Hitch consists of two single basket hitches passed under the load. They must be placed under the load so that it is balanced. The legs of the hitches must be kept far enough apart to provide balance but not so far apart that low angles are created and the legs pull in toward the center. The angle between the load and the sling should be approximately 60° or greater to avoid slippage. On smooth surfaces, both sides of the hitch should be snubbed against a change of contour to prevent the rope from slipping as load is applied. Otherwise use a double wrap basket hitch.

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Basket Hitch Reductions:

*If the sling capacity in the vertical was 2,000 lbs, then at 90˚ (true basket) it would be 4,000 lbs; at 60˚ it would be 3,464 lbs; at 45˚ it would be 2,828 lbs; and at 30˚ it would be 2,000 lbs.

Note: Sling angles of 60˚ or more are recommended; Sling angles less than 30˚ are highly discouraged.

*Question on written test: A true basket hitch has twice the capacity of a true vertical.

a. Trueb. False

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Using the formula below and the chart to the right, calculate what a 5,000 lb capacity vertical sling would be:

1) in a true basket2) 60˚ reduced basket3) 45˚ reduced basket4) 30˚ reduced basket

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One thing we learned is that a 30˚ reduced basket hitch has a capacity that is 50% of a True Basket Hitch. Of course, we’ve seen this before, haven’t we? This is the same formula we use to calculate sling angles for bridle hitches.

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Basket Hitches:

Here are four variations of the basket hitch: True basket, Reduced basket, Double-wrap basket and Inverted basket.

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Reduced Basket Hitch:

The basket hitch is more commonly used with both eyes attached to the hook which would be a reduced basket and the capacity would be less depending on the angle of the two legs. Like the vertical hitch the basket hitch is excellent for loads that have a pick point on top and whose mass is distributed directly under the hook.

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Double Basket Hitch:

The Double Basket hitch uses two basket hitches in a two-bridle.

Now there are two reductions to make: One reduction for the reduced basket hitch; and one reduction for the bridle hitch. The amount is reduced depends on the angle of the basket and the angle of the two legs.

Be sure to balance the load by keeping slings apart.

Prevent sling slippage by keeping the angle between the load and sling 60° or more.

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Double-Wrap Basket:

The double-wrap basket hitch is a basket hitch wrapped completely around the load and compressing it rather than merely supporting it, as does the ordinary basket hitch. The double wrap basket hitch can be used in pairs like the double basket hitch. This method is excellent for handling loose material, pipe, rod, or smooth cylindrical loads because the sling is in full 360° contact with the load and tends to draw it together.

Reduced Double-Wrap Basket:

The reduced double-wrap basket hitch is most commonly used but a reduction in capacity is in order because of the angle.

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Inverted Basket:

There are variation of use of the inverted basket.

*It’s main advantage is that it adjusts on the hook so you get equal leg loading.

The disadvantage is that it adjusts on the hook and so provides poor load control.

*Question on written test: The inverted basket’s main advantage is that it adjusts on the hook so you get equal leg loading.

a. Trueb. False

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Discussion:

From this picture, why is it rigged the way it is?

What type of slings are being used?

What hitches are being used?

What deductions need to be made?

What is the chain fall used for?

Is it ok to used the chain fall in this manner?

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Discussion:

What type of slings are being used? Answer: Two round slings and one chain fall (come-along)

What hitches are being used? Answer: Single-choker hitches

What deductions need to be made? Answer: One deduction for the sling angle and one deduction for the extreme choke angle

What is the chain fall used for? Answer: To adjust the legs of the slings over the center of gravity so the load will come up level

Is it ok to used the chain fall in this manner? Answer: As long as it is load rated and within the capacity of this pick

Why is it rigged the way it is? Answer: This pipe will probably be bolted onto another pipe and needs to be lifted level. The one area of concern is the choker hitch on the left that could slip upwards during the lift.

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