How to Determine the Max Torque for a Screw
You may determine the maximum, or max, torque for a screw with the mathematical formula T = K x D x P. The formula's "T" is the targeted torque for the screw measured in inch pounds. "K" in the formula is the screw's coefficient of friction, which is the estimated ratio of the force of friction between two bodies and the force pressing them together. "D" is the screw's diameter in inches, and "P" is the screw's tensile load, or pulling force, in pounds.
Find the screw's diameter, or D, by reading the information on the screw's packaging or measuring the screw. If the packaging specifies 8-32, for example, then the screw's diameter is 8 and its thread count is 32. To find the diameter by measuring, measure the screw's shank, which is between its head and threaded portion.
Determine the coefficient of friction, or K, by researching the specific screw. That method is much easier than calculating bolt preload and friction. Coefficient of friction is always an estimate. Common K factors are 0.20 for plain screws, 0.22 for zinc-plated screws and 0.10 for lubricated screws. Charts are available online and in hardware stores.
Find the screw's tensile load, or P. Tensile load is the stress at which a screw fails. Numbers most often used for tensile load are 5.8, 8.8 and 10.9, which mirrors the grade of a screw. The number before the decimal is the tensile load divided by 100. So a 5.8 screw's tensile load is 500, and its tensile yield strength is 400 because 0.8 x 500 is 400. The 5.8 screw has a tensile load of 500 and a tensile yield of 0.8. Tensile load multiplied by tensile yield load results in an ultimate strength of 400. A screw's tensile load is often evident when the screw is purchased.
Place the screw's coefficient of friction (K), diameter (D) and tensile load (P) in the equation T = K x D x P. T remains the targeted torque. Multiply K by D by P to find T. If, for example, a screw's K is 0.22, its D is 0.75 and its P is 23.046 lbs., then the equation to determine the screw's maximum torque is T = 0.22 x 0.750 x 23.046.
- Tightening values for screws can be established by tension-indicating equipment at a store when you purchase the screws.
- Use basic knowledge of diameter, grade and coefficient of friction to project how much torque you can apply to a screw. A screw with a large diameter requires more force than a screw with a smaller diameter to drive the screw into a surface. A higher grade of screw requires more force than a lower grade of screw to stretch the screw because of ample material strength. The coefficient of friction determines how much rotational force is needed for different screw textures.
- Each screw has its own strength. Experimenting with purchased screws is the best and safest way to know their strength. When building an object and calculating torque, do not overtighten screws, or the object may break or collapse. Overtightening a screw can lead to cracks, breaks and damaged products, too. Using the formula T = K x D x P results in an estimated value; satisfactory outcomes are not 100 percent guaranteed.
Bodie P. Shallenberger holds a B.A. in English and professional writing from Purdue University. He has worked with Purdue University Press and "The Exponent," Purdue's independent student-run newspaper, and served as an editor for "The Journal of Purdue Undergraduate Research."
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