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I'm working on a project where I need a DC motor that is capable of tightening a bolt with a motor. The applications will typically be automotive, therefore, the bolts will need to be tightened to torques between 20 and 250 ft-lbs. Most DC motors that I have found have a stall torque of less than 1 ft-lbs.

What specifications of a motor to I need to consider to find a motor and gearbox that would be capable of achieve these torque values?

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  • \$\begingroup\$ Voltage Rating, Current use (esp at stall torque), RPM (a gear box works by decreasing rotations per minute to increase torque)... You've mostly already figured this out. Decide on the maximum torque required, then find a combination of motor (within your power usage requirements and price range) with a gear box that multiplies the motor torque to at least 1.25 of your expected max requirements. \$\endgroup\$ – Kurt E. Clothier Oct 18 '15 at 4:11
  • \$\begingroup\$ Almost all automotive applications that need to tighten and loosen bolts at the kind of torques that you are talking about use impact technology whether that be powered electrically or pneumatically. These have the decided advantage that the unit can remain small enough that it can be held and operated with one or two hands. Your idea to use a motor and gearbox tends to be rather impractical because it will require the unit to have a long handle to hold the tool from turning whilst it is applying the high torque to the bolt. Such handle may very well limit the utility of a motor powered unit. \$\endgroup\$ – Michael Karas Oct 18 '15 at 5:45
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The short answer to your question is that you start with the specifications you need - speed, torque, etc. - and then from that subset of units, you pick the one you can power, pay for, and fit where it's needed. If the last three elements don't fall into place, revisit the initial spec to see if it makes sense, and if it can be massaged into something workable. Sometimes the thing doesn't exist yet - sometimes it can't. Play nice with mechanical engineers and physicists, and they'll tell you what's impossible so you know where to stop.

Engineering is sometimes just a highly specialized branch of applied economics.

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