SHOCK
LOAD – A force that results from the rapid application of
a force (such as impacting and/or jerking) or rapid movement of a static
load. A shock load significantly adds to the static load.
DESIGN (SAFETY) FACTOR – An industry term denoting
a product’s theoretical reserve capability; usually computed by dividing
the catalog Ultimate Load by the Working Load Limit. Generally expressed
for blocks as a ratio of 4 to 1.
TACKLE BLOCK – An assembly consisting of a
sheave(s), side plates, and generally an end fitting (hook, shackle, etc.)
that is used for lifting, lowering, or applying tension. SHEAVE
/ SHEAVE BEARING ASSEMBLY – Purchased by O.E.M. or
end user to be used in their block or lifting system design.
Fittings, including hooks,
overhaul balls, shackles, links, etc., may become worn and disfigured
with use, corrosion, and abuse resulting in nicks, gouges, worn threads
and bearings, and sharp corners which produce additional stress conditions
and reduce system load capacity.
Grinding is the recommended
procedure to restore smooth surfaces. The maximum allowance for
reduction of a product’s original dimension due to wear or repair
before removal from service is:
1. Any single direction
- No more than 10% of original dimension;
2. Two directions - No more than 5% of each dimension. |
For detailed instructions
on specific products, see the application and warning information for
that product. Any greater reduction may necessitate a reduced Working
Load Limit.
Any crack or deformation
in a fitting is sufficient cause to withdraw the product from service.
Some of the blocks shown
in Crosby Group literature are named for their intended use and selection
is routine. A few examples include the “Double Rig Trawl Block”
used in the fishing industry, the “Well Loggers Block” used
in the oil drilling industry, and the “Cargo Hoisting Block”
used in the freighter boat industry. Others are more generally classified
and have a variety of uses. They include snatch blocks, regular
wood blocks, standard steel blocks, etc. For example, snatch blocks
allow the line to be attached by opening up the block instead of threading
the line through the block. This feature eliminates the use of rope
guards and allows various line entrance and exit angles to change direction
of the load. These angles determine the load on the block and/or
the block fitting. (See “Loads on Blocks.”) Snatch
blocks are intended for infrequent and intermittent use with low line
speeds.
A tackle block sheave assembly is one element of a system used to lift
or drag a load. There are other elements in the system including
the prime mover (hoist, winch, hand), supporting structure, power available,
etc. All of these elements can influence the type of tackle block
required. When selecting a block or sheave for the system in your
specific application, you should consider the other elements as well as
the features of the blocks and sheaves shown in Crosby Group literature. |
To
select a tackle block to fit your requirements, consider the following points:
1. Are
there regulations which could affect your choice of blocks, such
as federal or state OSHA, elevator safety, mine safety, maritime,
insurance, etc.?
2. What is the weight of
the load, including any dynamics of impacts that add to load value?
You must know this to determine the minimum required Working Load
Limit value of the block or load on sheave.
3. How many parts of line
are required? This can be determined given the load to be
lifted and the line pull you have available. As an alternative,
you could calculate the line pull required with a given number of
parts of line and a given load weight. (See “How to Figure
Line Parts,” page 296).
4. What is the size of
line to be used? Multiply the available line pull by the desired
safety factor for wire rope to determine the minimum catalog wire
rope breaking strength; consult a wire rope catalog for the corresponding
grade and diameter of wire rope to match. You should also consider
fatigue factors that affect wire rope life. (See “Sheave Size
& Wire Rope Strength” page 295).
5. What is the speed of
the line? This will help you determine the type of sheave
bearing necessary. There are several choices of bearings suitable
for different applications, including:
Common (Plain) Bore for very
low line speeds and very infrequent use (high bearing friction).
Self Lubricating Bronze Bushings
for slow line speeds and infrequent use (moderate bearing friction).
Bronze Bushing with pressure
lubrication for slow line speeds and more frequent use at greater
loads (moderate bearing friction).
Anti-friction Bearings
for faster line speeds and more frequent use at greater loads (minimum
bearing friction).
6. What type of fitting
is required for your application? The selection may depend
on whether the block will be traveling or stationary. Your
choices include single or multiple hooks with or without throat
latches and shackles, which are the most secured load attachment.
You should also decide whether the fitting should be fixed, swivel
or swivel with lock. If it is a swivel fitting, then a selection
of thrust bearing may be necessary. There are plain fittings
with no bearings for positioning at no load, bronze bushed fittings
for infrequent and moderate load swiveling, and anti-friction bearing
equipped fittings for frequent load swiveling.
7. How will the block be
reeved and does it require a dead end becket? (See “The Reeving
of Tackle Blocks”, page 294).
8. If the block is to be
a traveling block, what weight is required to overhaul the line?
(See “How to Determine Overhaul Weights”, page 296).
9. What is the fleet angle
of the wire line? Line entrance and exit angles should be
no more than 1-1/2o.
10. How will the block be maintained? Do
conditions in your application require special maintenance considerations?
(See “Tackle and Sheave Block Maintenance,” page 292
and “Fitting Maintenance," this page).
11. Reference current edition of "Wire Rope Users Manual"
for addtional sheave design and maintenance information. |
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