# Is there a rule of thumb to construe a screw

When your first idea is drafted you calculate which forces/pressures and so on are working on your system. And then you break the forces down into the single screws which are built in your device.

Exemplary let’s say, that screw is loaded axially with $$8000 \ N$$ – the transverse load could be unattended.

How do you decide to take the right screw dimension? Things that should in my opinion be also considered are:

• How is the load working on the screw?
• concentrically vs. eccentrically
• static vs. dynamic
• How is the montage of the screw realized?
• Simple tightening spindle vs. tightening by angle monitoring in the plastic range
• What degree of utilization is allowed?
• What is the strength grade of the screw?
• And perhaps many more points…

The aim that I’m expecting is, something like: With these forces, this montage, this strength grade and … (simple calculation) … you have to take a screw M16x2 10.9

I’m curious to hear your experiences.

Thanks in advance for your help.

## 1 Answer

In a handbook of mechanical engineering facts you will find tables containing maximum load ratings for different diameters and types of screw fasteners, and required torque values to hold those loads.

You will also find this data on the website of a company that manufactures screw fasteners.

Most mechanical engineering design textbooks will contain a chapter on threaded fasteners which explain the connection between tensile load and required torque, thread friction, thread pitch, screw diameter, screw grade, etc.

The American Society For Testing And Materials (ASTM) specification book will contain a chapter on how screw strength is measured, and how different screws are ranked for strength.

• @ niels nielsen: Thanks for your response. But I rather expected something like: You have to roughly calculate your screw load and then pick a screw which degree of utilization is 30 %. But when the load is dynamically you must… The aim of my question was to get a calculation which is finished after 2 minutes. Here I want to do a roughly calculation only for dimensioning my device. The entire and more comprehensive approval I will do at a later stage. – tueftla Jul 9 '19 at 5:29
• @tueftla You are missing the point, that engineering is a practical subject not a theoretical one. If you want to study screws as an interesting problem in the nonlinear continuum mechanics behaviour of flexible bodies, that's OK. If you want to make something held together with screws, you look at an engineering handbook and/or a parts catalog. – alephzero Jul 9 '19 at 12:19
• @alephzero: My question has a totally practical manner… Perhaps you can mention your dimensioning-formula for a screw with a specific load. That would be very nice. In general I hoped to hear some formulas about dimensioning a screw that the forum is using. – tueftla Jul 9 '19 at 14:32
• tuefla, this is not a 2-minute exercise and is not a simple calculation. If it were, you could replace half of the world's population of mechanical engineers with a cellphone app. I would escape this fate, however, because i am a recovering ex-engineer. there's a 12-step program for that but it is notoriously ineffective. – niels nielsen Jul 9 '19 at 16:28
• @ niels nielsen: I also know a more step program to verify that your screw can endure the loads. And this program is time consuming and because of this background I put my question in the forum with the aim to get a rough dimensioning formula. This formula should give a rough estimate of the size of the screw of the first draft. But in my opinion this is not replacing the time consuming approval for your final draft. – tueftla Jul 10 '19 at 6:06