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I needed a small linear actuator (a mechanism for creating motion in a straight line) for a project and was finding only oversized or cost-prohibitive solutions. Industrial automation actuators have plenty of muscle but rarely come in small packages. Hobby linear actuators come with a smaller footprint but can be costly. I’ve often wondered about the load capacity of the screw and nut contained in a standard lip balm tube, so I collected all the lip balm tubes I could find and started testing.

One stood out over all the others and could exert over 10 pounds of thrust with a 1.25″ stroke: the mighty Blistex tube. No other lip balm screw-and-nut assembly comes close to carrying the same load as a Blistex tube. Add a magnet to the shaft and a couple of reed switches to the case, and you’ve got an actuator with limit switches at each end!

Tools

  • Drill and drill bits: 3/8″ Forstner, 3/8″ brad point, 3/16″
  • Utility knife
  • Tubing cutter or hacksaw
  • File or sandpaper
  • Screwdriver, flat-blade

Materials

  • Clevis mounts, rear and front — Make these from ABS plastic, wood, or any material you’re comfortable working with. See dimensions in Step 7.
  • Hobby servo, full rotation — SparkFun Electronics #ROB-10189, sparkfun.com
  • Blistex lip balm tube — The actuator’s drive screw, body tube, drive nut, and cap are all made from it.
  • Proximity sensors, magnetic, NPN, 3-pin (2) — Jameco Electronics #513244, jameco.com
  • Magnet, 1/8″ square — SparkFun #COM-08644
  • Aluminum tube, 3/8″ OD × 3″ — McMaster-Carr #9056K643, mcmaster.com. The shaft can be any nonmagnetic 3/8″ OD tube of it least 1/4″ ID — even plastic.
  • Threaded insert, #8-32 — McMaster #92105A660
  • Threaded shaft, #8-32 × 3/4″ — Cut this from any #8-32 bolt.
  • Cable ties, small
  • Plastic epoxy — J-B Weld works well.

Downloads

 

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Steps

Step #1: Prepare the Blistex tube.

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  • First you’ll need to remove the lip balm. Put the lip balm tube in the refrigerator for 10 minutes. This will solidify the contents and make it easier and cleaner to remove.
  • Then, extend the lip balm all the way and twist the solid block out of the lead screw nut.
  • The label will peel off cleanly if you do it slowly.

Step #2:

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  • Pry the base off the Blistex tube. The leadscrew and base are molded together and will come off as one part. A barbed snap feature holds the base in place. Try not to dent the end of the tube when prying off the base. Eventualy you’ll glue this bottom surface of the tube to the servo.
  • Clean all the components thoroughly. Getting all the lip balm off the components is critical for good adhesive bonding.
  • Open up the bottom and cut the notch. The bottom of the body tube has a base plate with a small hole; drill it out or cut it out with a utility knife.
  • The notch is sized to accommodate a feature on the servo and can be cut with a utility knife. Mine was about 1/5" wide and 1/20" deep; cut yours to fit your servo. (If a stepper, DC, or other motor is being used, this step can likely be skipped.)

Step #3:

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  • Drill the cap. To prevent the main shaft from binding in the cap, it’s critical to drill this hole as close to the center of the cap as possible. To find the center, you can use a center-marking jig or just use the molding mark in the center of the cap; it’s visible when the cap is viewed from a low angle. Use a fine-tip marker to mark this molding point as your center point for drilling.
  • You must use a 3/8" Forstner or brad point bit because a standard twist drill will catch immediately and ruin the cap. Proceed with a slow bit speed and try not to collapse the cap by clamping it too tightly. If necessary, you can use a wooden dowel, fit to the inside of the cap, to back up the cap when you’re clamping it

Step #4: Build the main shaft.

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  • Cut a 2-3/4" length of the 3/8" aluminum tubing. A tubing cutter will provide a square, burr-free cut, but a hacksaw will also work fine.
  • Drill a 3/16"-diameter hole 1/2" from one end. This hole is for mounting the sensor magnet.
  • Using epoxy, glue the 1/8" square magnet into the hole in the main shaft. Ideally, the magnet can be wedged into the hole and secured with epoxy. If the magnet slips through the hole, use a dowel or rod in the main shaft to support it while gluing. The magnet should extend into the 51/16" gap between the main shaft and the body tube, so be sure it protrudes slightly above the surface of the shaft when glued.

Step #5:

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  • Epoxy the main shaft into the drive nut. Shaving off the tabs on the drive nut may allow it to slide with less friction. Make sure no glue seeps into the threads of the drive nut. To ensure that the drive nut is square on the end of the shaft, you can use the body tube and cap as a jig to hold the shaft and drive nut in place while the glue sets.
  • Test fit the threaded insert. If the fit is tight, lightly sand or file the inside of the main shaft tube. Epoxy the threaded insert into the end of the main shaft, making sure that no glue gets into the inside threads and that the end of the threaded insert is flush with the end of the main shaft.

Step #6: Assemble the actuator.

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  • Use a utility knife to cut the drive screw off the base. Then trim the end of the drive screw down to create a post that fits into the servo hub.
  • Epoxy the drive screw to the servo. Or, if you’d like the option to replace the drive screw, cut the head off one of the attachment screws that came with the servo. Drill a 1/16" hole in the end of the drive screw, then glue the attachment screw into this hole. You can then screw the drive screw into the servo.

Step #7:

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  • Epoxy the body tube to the servo. Use the completed main shaft to ensure that the body tube is glued in place concentric to the drive screw. To accomplish this you’ll need to have the servo powered up. With the body tube and epoxy in place, but not yet set, power the servo to pull the main shaft down into the body tube about 3/4 of the way.
  • Epoxy the limit sensors in place. The front proximity sensor is glued to the cap, 1/8" down from the top. The rear sensor is 5/8" from the bottom of the tube. Lightly sanding the tube first will help the epoxy hold.
  • Before actually gluing the sensors, do a dry run. Hook up the circuit shown below and run the main shaft in and out using the servo, holding the sensors in place with tape. Mark the axial position where the sensors make contact with the magnet. There are anti-rotation ribs between the drive nut and the body tube, but the main shaft can be completely removed and repositioned to put the magnet where you want it. Positioning the magnet and sensors along the short side of the servo allows the servo mounting holes to be used for routing the sensor wires.

Step #8:

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  • Fabricate the front and back clevis mounts out of ABS plastic or wood. The ones shown were 3D-printed in black ABS, and are roughly 3/4" square with a 1/4"-diameter hole. You may want to specifically design yours for the application you have in mind. The rear clevis is glued directly to the back of the servo.
  • The front clevis has a threaded shaft to allow fine adjustments. Use a 9/64" drill bit to drill a hole in the center of the clevis end through to the existing center hole. Thread a 3/4"-long #8-32 threaded shaft into this hole, up to the center hole, and glue it in place.
  • Finally, thread the front clevis into the main shaft, and your linear actuator is complete!

Step #9: Put it to work.

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  • To get the most out of your new powerhouse you’ll need a microcontroller to power the servo and take input from the limit sensors. The limit sensors work by changing state when the magnet is in close proximity. Normally the input to the microcontroller will be +5V. When the magnet passes by the sensor the input will be pulled down to ground.
  • The alignment of the magnets to the sensor is a bit precise and may take repositioning of the main shaft in the body tube during the initial setup. To help set up the sensors and test the completed actuator for different uses I made up an Arduino test platform, with one button to extend the actuator and one to retract it. My Arduino program does not allow you to overextend or over-retract; you can find a copy of it in the intro.
  • To test the full-force capability, I set up a scale that showed the actuator pushing at 5.75kg, or over 12lbs! Two projects I’ve implemented using this actuator are an automatic vent pipe cap (see photo) and a "spurting" action on a Halloween prop.
  • With over 10lbs of force and a 1-1/4" stroke, the mighty lip balm linear actuator packs a punch, is cost effective, and may be just what you need for your next project.

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