38906083 Fastener and Screw Torque Design
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Fastener and Screw Torque Design and Equation General The importance of torque in many applications can not be overemphasized. Critical applications such as automotive engines, brakes, aircraft, and structural installations are torque design-manufacturing sensitive. Under- torque can result in unnecessary wear of nuts and fasteners as well as the parts they are securing together. When insufficient pressures are applied, uneven loads will be transmitted throughout the assembly which may result in excessive wear or premature failure due to fatigue. Over-torque can be equally damaging because failure of a nut or bolt from overstressing the fastener and secured areas. Torque requirements for particular applications should be determined and the proper torque applied. Design Considerations The first requirement in determining the amount of torque to apply is a knowledge of the desired bolt stress. This stress based on the yield strength of the bolt material. It is recommended that the induced stress not be allowed to exceed 80% of the yield strength. In the design of a fastener application which will be subject to external loading, whether static or dynamic, it will be necessary to establish bolt size and allowable stress in accordance with current engineering practice. The mathematical relationship between torque applied and the resulting tension force in the bolt has been determined to be as follows: T = Torque required (inch pounds) F = Bolt tension desired (Axial Load) (pounds). D = Nominal bolt diameter. (major dia.) EQUATION: T = .2 D F This relationship is based on the assumption that regular series nuts and bolts with rolled threads are used, acting on surfaces without lubrication.
Transcript of 38906083 Fastener and Screw Torque Design
Fastener and Screw Torque Design and Equation
General
The importance of torque in many applications can not be overemphasized. Critical applications such as automotive engines, brakes, aircraft, and structural installations are torque design-manufacturing sensitive. Under-torque can result in unnecessary wear of nuts and fasteners as well as the parts they are securing together. When insufficient pressures are applied, uneven loads will be transmitted throughout the assembly which may result in excessive wear or premature failure due to fatigue. Over-torque can be equally damaging because failure of a nut or bolt from overstressing the fastener and secured areas. Torque requirements for particular applications should be determined and the proper torque applied.
Design Considerations
The first requirement in determining the amount of torque to apply is a knowledge of the desired bolt stress. This stress based on the yield strength of the bolt material. It is recommended that the induced stressnot be allowed to exceed 80% of the yield strength. In the design of a fastener application which will be subject to external loading, whether static or dynamic, it will be necessary to establish bolt size and allowable stress in accordance with current engineering practice.
The mathematical relationship between torque applied and the resultingtension force in the bolt has been determined to be as follows:
T = Torque required (inch pounds)F = Bolt tension desired (Axial Load) (pounds).D = Nominal bolt diameter. (major dia.)
EQUATION: T = .2 D F
This relationship is based on the assumption that regular series nuts andbolts with rolled threads are used, acting on surfaces without lubrication.
Minimum Length of Thread Engagement Formula and Calculations
Screw Thread Strength Calculations FED-STD-H28/2B, 1991 And Machinery's Handbook twenty fourth Edition.
These calculations strictly apply to UN thread series however, may be used for metric series threads. In practice when the values are calculated the value for the screw shear strength is similar to the very convenient formula provided above.
Length of Thread
Where:
Le = Fastener thread engagementAt = Tensile Stress AreaKnmax = Maximum minor diameter of internal thread.Esmin = Minimum pitch diameter of external thread.n = Number of threads per inch
If material in which the female thread is tapped is significantly weaker that the screw material then J must be evaluated.
As = Shear area of the external thread (screw or fastener)An = Shear area of the internal thread ( hole )
If the value of J is greater than 1 then the length of engagement must beincreased to at least
Where:
Le = Fastener Thread Engagement
Minimum Thread Engagement Formula and Calculation ISO
The screw should fail before the thread strips. For this, it is necessary the shear area of the threaded feature be at least 2 times the tensile area. (Assuming the threaded feature is of the identical material).
Where:
Le = Thread Engagement LengthAt = Tensile Stress AreaD = Major Diameter of fastener (screw)p = 1 / Number of threads per inch (n)
Threaded Hole / Fastener Ratio
This assumes that the male and female thread materials have the same strength. If the Female Material strength is lower i.e. J as calculated below is greater than 1 then the length of engagement must be increased to prevent the female thread Stripping
Where:
J = Thread Engagement Ratio
If the value of J is greater than 1 then the length of engagement must beincreased to at least
Where:
le = Thread Engagement Length
Shear Area Internal and External Thread Formula and Calculation
Shear Area For External Thread (Screw or Fastener)
Where:
Kn max = Maximum minor diameter of internal thread.Es min = Minimum pitch diameter of external thread.Le = Fastener thread engagementn = Number of threads per inch
Shear Area For Internal Thread
Where:
En max = Maximum pitch diameter of internal thread.Ds min = Minimum major diameter of external thread.Le = Fastener thread engagementn = Number of threads per inch
Fastener / Thread Tensile Area of External Thread Formula and Calculations
Screw thread Strength Calculations
Tensile Stress Area of an external thread ( fastener, screw )
Where:
At = Tensile Stress AreaD = Major Diameter.p = 1 / Number of threads per inch (n)
Fastener / Threaded Pitch Circle Diameter Formula and Calculation
Screw thread Strength Calculations ISO 898-1:1999
Thread Pitch Circle Diameter
Where:
dp = Pitch circle diameter of threadD = Major Diameterp = 1 / Number of threads per inch (n)
Screw thread Strength Calculations ISO 898-1:1999
Thread Pitch Circle Diameter
Where:
dp = Pitch circle diameter of threadD = Major Diameterp = 1 / Number of threads per inch (n)
Fastener / Threaded Shear Area Formula and Calculation ISO
Thread Shear Area
Where:
A th = Thread shear areaLe = Thread Engagement Lengthdp = Pitch circle diameter of threadD = Major Diameterp = 1 / Number of threads per inch (n)
