How to Build a Device for Measuring Torque on Electric Motors

Conventionally, motors are tested with dynamometers -- devices that measure the horsepower of cars. Dynamometers, however, measure torque in a roundabout way by measuring force exerted on a heavy-duty treadmill by interpreting hydraulic resistance. For electric motors, the additional steps of the complex hydraulics and treadmill aren't necessary. Torque can be measured directly at an electric motor's drive shaft. An efficient way to do it is to make a device that connects an economical, electronic torque-load cell that can easily fasten to a variety of different-size drive shafts. You can adapt a replacement chuck for a drill for this purpose.

Things You'll Need

  • Plasma cutter
  • MIG welder
  • Replacement drill chuck
  • Electronic rotational load/torque sensor
  • 1/4-inch steel plate
  • Drill press
  • Tap and die
  • Cotter pin


    • 1

      Cut three squares of steel plate 10 inches by 10 inches and cut one of the squares in half diagonally with a plasma cutter.

    • 2

      Drill four quarter-inch holes in one of the square plates. Position each hole 1 1/2 inches diagonally from a corner of the plate.

    • 3

      Drill a five-eighths-inch hole in the center of the second square plate.

    • 4

      Weld the two squares together at a right angle like a bookend.

    • 5

      Weld the two triangular plates from the square that was cut in half at the edges of the bookend for support.

    • 6

      Bolt the bookend to a work bench using four lag bolts or through-bolt it with nuts, bolts and lock washers.

    • 7

      Press a bearing into the half-inch hole. While the outside diameter of the bearing must fit snugly in the five-eighths-inch hole, the inside diameter must fit the drive axle of the replacement drill chuck. Chuck drive axles often have a hex shape so they can engage a drill motor without spinning. You can machine the axle round. You can also make a bearing adapter by fitting a socket to the drive axle, then grinding the back of the socket off with a grinder. Then choose a bearing with an inside diameter that matches the outside diameter of the socket.

    • 8

      Slide a washer over the drill's drive axle, then slide the drive axle through the bearing so the chuck is opposite the braced side.

    • 9

      Slide a second washer over the drive axle on the other side of the plate, then make a dot on the axle with a fine-tipped pen, snug to the washer. Using a sharp, small-diamater, steel-cutting drill bit, drill a hole through the axle on the mark. Temporarily remove the drill chuck to make the hole, if need be.

    • 10

      Place a cotter pin through the hole and bend the edges over. Trim them if they are excessive or sharp. The assembly will now be riding on a bearing and locked in place with washers and a cotter pin.

    • 11

      Mount an electronic rotary load sensor/torque sensor on the steel plate. You can secure it with additional mounting holes. Shim it to a height so the axis of the sensor is perfectly aligned with the axis of the chuck. Use steel plate and/or sheets for shims.

    • 12

      Couple the chuck axle with the load sensor shaft with a steel-tube coupling. Slide the coupling over each shaft. Drill a small hole at the edge of each coupling and tap it with a tap and die set, then thread a set screw into the hole. You may need to drill and tap all the way through the shafts or even make spot welds at the ends of the shafts, depending on how much force you'll be measuring.

    • 13

      Connect the load sensor to your laptop, if it has a PC interface, rather than its own LCD readout. The chuck can now easily and quickly grasp a wide range of drive-shaft diameters for all kinds of electric motors and bench-test them for torque very quickly and conveniently.

Tips & Warnings

  • Electronic load sensors can be very expensive, but the expense is generally in conjunction with extremely high accuracy or other features that probably aren't important for quickly bench-testing the power of electric motors.
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  • Photo Credit Comstock/Comstock/Getty Images

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