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Mechanical characteristics of screws
What are the mechanical characteristics of steel screws?
What is yield strength? What needs to be considered when calculating the yield strength? What do the numbers on the screws mean and what technical information can be obtained from them?
The following is a comprehensible summary.
The three most important mechanical characteristics of screw fasteners:
- Tensile strength (Rm)
- Yield strength (ReL)
- Elongation at break (A)
Tensile strength Rm in N/mm²
The tensile strength is the stress at which the undamaged screw must withstand the highest force.
–> Tensile strength = maximum tensile force : cross-section of the surface
Yield strength ReL in N/mm²
The yield strength is the stress at which the tensile force on relatively soft materials initially remains the same or drops briefly as the specimen elongates.
0.2 Yield strength Rp0.2 in N/mm²
The yield strength is no longer clearly recognizable with harder materials. This is replaced by the tension at which the permanent elongation is 0.2%.
Elongation at break (elongation A in %)
The elongation indicates the extent to which the original shaft length has increased until the screw breaks.
Strength class designation system for screws
A number combination identifies the most important features of the mechanical values of a screw.
For example, for a screw with the following characteristics and the subsequent calculation
–-> 8.8 <–
The first number indicates 1/100 of the minimum tensile strength in N/mm².
–> Tensile strength: 8 x 100 = 800 N/mm²
The second number indicates 10 times the ratio of the yield strength to the nominal tensile strength.
–> (Minimum yield strength : minimum strength) X 100
The multiplier of both numbers indicates 1/10 of the minimum yield strength in N/mm².
–> Minimum yield strength: 8 x 8 x 10 = 640 N/mm²
The choice of material depends on the requirements, such as:
- Mechanical and chemical stress
- Safety
- Service life
- Corrosive stress
- Weight
- Thermal stress
- Costs
It is important to pay attention to the appropriate strength classes and the associated mechanical values, because if the strength classes aren’t selected correctly, the screw fasteners won’t achieve the desired values and, in the worst case, will deform or even break.
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