SCREWED JOINTS
A screwed joint is when two or more components are joined with screws or bolts, securing them together through pre-drilled or threaded holes. The screwed joints are widely used where the machine parts are required to be readily connected or disconnected without damaging to the machine or the fastening.
ADVANTAGES & DISADVANTAGES SCREWED JOINTS
Advantages:
1. Screwed joints are highly reliable in operation.
2. Screwed joints are convenient to assemble and disassemble.
3. A wide range of screwed joints may be adopted to various operating conditions.
4. Screws are relatively cheap to produce due to standardisation and highly efficient manufacturing processes.
Disadvantages:
The main disadvantage of the screwed joints is the stress concentration in the threaded portions which are vulnerable points under variable load conditions.
STRESSES IN SCREWED FASTENERS
Stresses in Screwed Fastening due to Static Loading:
1. Internal stresses due to screwing up forces,
2. Stresses due to external forces, and
3. Stress due to combination of stresses at (1) and (2).
Initial Stresses due to Screwing up Forces:
1. Tensile stress due to stretching of bolt,
2. Torsional shear stress caused by the frictional resistance of the threads during its tightening,
3. Shear stress across the threads,
4. Compression or crushing stress on threads,
5. Bending stress if the surfaces under the head or nut are not perfectly parallel to the bolt axis.
STANDARD DIMENSIONS OF SCREW THREADS
DESIGN PROCEDURE OF BOLT OF UNIFORM STRENGTH
WATCH THIS VIDEO FOR DESIGN PROCEDURE OF BOLT OF UNIFORM STRENGTH
Example 1. Determine the safe tensile load for a bolt of M 30, assuming a safe tensile stress of 42 MPa.
WATCH THIS VIDEO FOR SOLUTION
Example 2. Two machine parts are fastened together tightly by means of a 24 mm tap bolt. If the load tending to separate these parts is neglected, find the stress that is set up in the bolt by the initial tightening.
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