# Good estimates for torque generated while turning a dial

I'm trying to design a little device which is hand-driven and which involves turning a bevel gear pair. I want a small gear pair but I don't want it to be weak. The forces involved will be small (it's attached to a pinion along which will slide a rack < 1 kg), but of course I still need to check the safety factors involved. So here's my question:

Since it's hand-driven, what torque value can I enter for the strength-calculations? I search for "typical torque values human hand" (and variations on this), but I keep receiving maximum human turning strengths which are definitely not going to turn up with this device. The dial has a ~3 cm diameter.

• If you can't design your gearing to accommodate the maximum torque, one option would be to add a slip clutch between the knob and the gears to protect the gearing. – Ethan48 Apr 2 at 15:18

This a table of a search done on research that was conducted between June 1959 and October 1960 by the Controls Section, Engineering Psychology Branch, Behavioral Sciences Laboratory, 6570th Aerospace Medical Research Laboratories Engineering Branch, Electronics Technology Laboratory. Units are ounces-inches.

They have a variety of knob sizes and textures.

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This is the link to the article.

• Possibly the best answer on the site. Amazingly arcane yet appropriate reference. – Eric S Mar 17 at 21:24
• Instructions to the participants: "Grasp the knob with your fingertips ... and twist the knob as hard as you can." I'm searching not for maximum exertable torque, but rather how much torque would a person normally exert on a knob e.g. to moving a microscope table up and down. – Suryetto Mar 18 at 9:23
• @Suryetto, I suggest you design to max torque, or somebody will quickly break it. But a test with a torque wrench or a string & a weight could also give you what you need. – Tiger Guy Mar 19 at 17:36

You can easily measure this by dangling a weight from a string and winding that around a pulley of known diameter.

This arrangement will produce a fixed torque on the pulley, which you can adjust by changing the weight.

Now all you have to do is attach a knob to the the pulley and you can feel what a given torque is like. Adjust the weight until the torque required to turn the knob feels like a reasonably maximum to you. From there you can do a bit of math and figure out what that torque is.