Fastener Theory - Determining the Ideal Torque for a Bolt

In summary, the conversation discusses how to determine the ideal torque for a bolt on a car using the formula T = K * D * P. The K value is the coefficient of friction, which can be found online for different finishes. The D value is the diameter of the bolt, and the P value is the yield strength of the bolt. The conversation also touches on the importance of using the correct units when calculating torque.
  • #1
YoshiMoshi
226
8
Homework Statement
Determine the ideal torque for a class 8.8 bolt that is 6 mm in diameter made of steel that is dry (not greased or wet). The bolt has 1 mm pitch threads.
Relevant Equations
T = K * D * P
T = Target tighten torque
K = Coefficient of friction
D = Bolt nominal diameter
P = Bolt's desired tensile load (generally 75 % of yield strength)
Source: http://www.zerofast.com/proper-bolt-torque
So this is for my car. If I have a bolt that I don't know what the ideal torque is supposed to be because the manufacturer of the car doesn't publish a torque, it's possible to calculate it? This can be done just give the specification of the bolt? Regardless of what the bolt is fastening? I'm not entirely sure. Is this correct?

K = 0.12
I don't know what the coefficient of friction is for my bolt, the specifications can be found here: https://www.mcmaster.com/94036A111, it has a "Black Ultra-Corrosion-Resistant-Coated Steel" finish. I'm not using a grease or an anti-seize compound, just he dry bolt. I used this as a reference https://rommet.com/remco/misc/friction.htm, which lists a "Phosphate Steel" coefficient when dry as 0.12-0.18. I should assume worse case of 0.12 to avoid over tightening?

D = 6 mm
I measured with a micrometer.

P = 69,000 psi
Yield strength for a Class 8.8 bolt is 92,000 psi (source: https://www.engineersedge.com/hex_bolt_identification.htm)
T = K * D * P
T = ( 0.12 ) * ( 6 mm ) ( 1/25.4 in/mm ) * ( 69,000 p/in^2 ) = 1,955 p/in
( 1,955 p/in )( 12 in/ft ) = 23,470 p/ft

Looks like I have done something wrong? I'm not sure what though. Also, isn't torque always in foot*pounds or inch*pounds? But the formula I'm given provides units of measurement of a unit of weight per a unit of displacement.

Thanks for any help!
 
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  • #3

1. What is the purpose of determining the ideal torque for a bolt?

The ideal torque for a bolt is determined to ensure that the bolt is tightened to the appropriate level, providing the necessary clamping force to hold the joint together. This prevents the bolt from either being too loose and causing the joint to fail, or being too tight and potentially damaging the components.

2. How is the ideal torque for a bolt calculated?

The ideal torque for a bolt is calculated using the equation T = K x D x F, where T is the torque, K is the coefficient of friction, D is the diameter of the bolt, and F is the clamping force required for the joint. This equation takes into account the material properties of the bolt and the joint, as well as the desired level of clamping force.

3. What factors can affect the ideal torque for a bolt?

There are several factors that can affect the ideal torque for a bolt, including the material and surface finish of the bolt and the joint, the lubrication used, the thread type and pitch, and the presence of any washers or other components. These factors must be taken into consideration when determining the ideal torque for a specific application.

4. How can the ideal torque for a bolt be measured or tested?

The ideal torque for a bolt can be measured or tested using a torque wrench or torque measuring device. The bolt is tightened to the calculated torque value and then checked to ensure that it meets the desired clamping force. If necessary, adjustments can be made and the process repeated until the ideal torque is achieved.

5. Are there any safety precautions to consider when determining the ideal torque for a bolt?

Yes, there are several safety precautions to consider when determining the ideal torque for a bolt. These include wearing appropriate personal protective equipment, following proper torqueing procedures, and using calibrated tools. It is also important to consult the manufacturer's specifications and guidelines for the specific bolt and joint being used.

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