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  • Securing gears to shafts: set screw vs key

    Discussion in 'The main mechanical design forum' started by murrdpirate, May 12, 2012.

    1. murrdpirate

      murrdpirate Member

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      Are there any rules of thumb about when to use a key and keyway rather than a set screw? I'm looking at two stepper motors that I want to add gears in order to increase torque by around 50-100:1. These are the two motors:


      • 83 oz-in holding torque, 5mm round shaft
      • 166 oz-in holding torque, 0.25in round shaft with detent

      They're from the same manufacturer, so the addition of the detent in the 166 oz-in model makes me think that it is intended to work with a set screw and I shouldn't have problems. Is that a fair assumption to make?
       
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    3. jibberjabber74

      jibberjabber74 Member

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      Set screws are generally used for light torque. keyways are used if more torque is required
       
    4. murrdpirate

      murrdpirate Member

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      Any idea how much torque is too much for set screws?
       
    5. MDR

      MDR Active Member

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      That's a good question - I should research that for the next edition of the book!

      The torques you mention are what I would consider low, and it's a safe bet that if the gear comes with a set screw attachment only, it is rated for a torque within the range of set screw attachment.

      A couple of notes about set screws - you get more holding power from a cup point since it digs in, but it will mar the shaft surface. You may want to have a flat on the shaft where the set screw hits so that you can disassemble the thing after gouging the surface.

      Make sure that the torques you are considering include impact forces....if you have fast reversals or stops in your gear train, or impulse loading, you need to figure on some surprisingly high momentary torques.
       
    6. murrdpirate

      murrdpirate Member

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      Thanks, MDR. Great point about the fast reversals and stops...hadn't even considered that.
       
    7. vic.blackall

      vic.blackall Well-Known Member

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      As you are using small diameter shafts you don't have much option as to the many types of gear fixations possible. With the torque figures mentioned a cup point grub screw fixation will be OK but I would suggest that you use two screws at 90 degrees to each other clamping onto flats on the spindle. Also make sure that you use the closest shaft/hole tolerances feasible.
       

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