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 |  موضوع: كتيب بعنوان Bolted Joint Design - Input Data الأربعاء 01 سبتمبر 2021, 12:13 am | |
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Bolted Joint Design - Input Data
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FACTORS THAT AFFECT THE WORKING LOADS ON BOLTS
• Sequence=procedure: The procedure with which a group of bolts are tightened can
a�ect �nal results substantially. Procedure includes such things as the sequence with
which they’re tightened, whether they’re tightened with a single pass at the �nal
torque, or in several passes at steadily increasing torques, etc.
• Residual preloads: The preloads le� in the bolts a�er embedment and elastic interactions.
• External loads: External loads add to or subtract from the tension in the bolts, and
therefore from the clamping force on the joint. Such loads must be predicted and
accounted for when the joint is designed and when the ‘‘correct’’ preload is chosen.
External loads are created by such things as pressure in the pipeline or engine, snow on
the roof, inertia, earthquakes, the weight of other portions of the structure, etc.
• Service conditions: Severe environments can a�ect operating conditions in the joint
and bolts. This is especially true of operating temperatures. These can create di�erential
expansion or contraction, which can signi�cantly alter bolt tensions and clamping
force. Corrosion can cause change as well. Contained pressure will a�ect clamping
forces.
• Long-term relaxation: There are some long-term relaxation e�ects that must also be
considered: relaxation caused by corrosion, or stress relaxation or creep, or vibration.
And again, we want correct bolt loads for the life of the joint, not just for a while.
• The quality of parts: We won’t get correct preload, or satisfactory performance
from the joint, unless the parts are the right size, are hardened properly, and are in
good condition. This factor can’t be handled separately; it gets in the act by a�ecting
the others. If the bolts are so�, for example, we won’t get the expected preload
for a given torque, and relaxation will be worse. If joint members are warped or
misaligned, it may take an abnormal amount of tension in the bolts (created by an
abnormal amount of preload) to create the necessary clamping force between joint
members.
HOW MUCH PRELOAD?
We always want the maximum possible preload, but in choosing this, we must consider:
• Strength of the bolt and of the joint members under static and dynamic loads
• Accuracy with which we expect to tighten the bolts
• Importance of the joint, i.e., the factor of safety required
• Operating environment the joint will experience in use (temperature, corrosive fluids, seismic shock, etc.)
• Operating or working loads which will be placed on the joint in use
PROBLEMS CREATED BY INCORRECT PRELOAD
• Static failure of the fastener: If you apply too much preload, the threads will strip.
• Static failure of joint members: Excessive preload can also crush or gall or warp or
fracture joint members such as castings and flanges.
• Vibration loosening of the nut: No amount of preload can �ght extreme transverse
vibration, but in most applications, proper preload can eliminate vibration loosening
of the nut.
• Fatigue failure of the bolt: Most bolts that fail in use do so in fatigue. Higher preload
does increase the mean stress in a fastener, and therefore threatens to shorten fatigue
life. But higher preload also reduces the load excursions seen by the bolt. The net e�ect
is that higher preload almost always improves fatigue life.
• Stress corrosion cracking: Stress corrosion cracking (SCC), like fatigue, can cause a
bolt to break. Stresses in the bolt, created primarily by preload, will encourage SCC if
they’re above a certain threshold level.
• Joint separation: Proper preload prevents joint separation; this means that it reduces or
prevents such things as leaks in a fluid pipeline or blow-by in an engine. The latter, of
course, means that proper preload allows the engine to produce more horsepower.
• Joint slip: Many joints are subjected to shear loads at right angles to the axis of the
bolt. Many such joints rely for their strength on the friction forces developed between
joint members, forces created by the clamping force exerted by the bolt on the joint.
Again, therefore, it is preload that determines joint integrity. If preload is inadequate,
the joint will slip, which can mean misalignment, cramping, fretting, or bolt shear.
• Excessive weight: If we could always count on correct preload, we could use fewer and
smaller fasteners, and o�en smaller joint members. This can have a signi�cant e�ect
on the weight of our products.
• Excessive cost: The cost of many products is proportional to the number of assembly
operations. Correct preload means fewer fasteners and lower manufacturing costs—as
well as lower warranty and liability costs.
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