Intro: 3D Printed Device to Lower Blood Pressure
Research has shown that simple hand exercises when performed a few times a week for 8 weeks can lower a person's resting systolic blood pressure by up to 10%.
These hand exercises involve using a hand grip device and squeezing at 30% of your maximum grip strength for 2 minute intervals. This interval should be repeated 4 times with a 3 min rest in between each interval to complete one session. A session should be performed at least 3 times a week for 8-12 weeks.
The problem with doing these exercises is that it requires you to monitor your grip strength to make sure you are at about 30% of your maximum grip strength. There are devices out there that will monitor and display your grip strength, but they all cost hundreds of dollars.
To avoid shelling out $600, I decided to 3D print my own device that would cheaply and effectively monitor grip strength.
Those who wish to learn how to print and assemble this device may skip to step 3
Step 1: Design
The current commercial options for monitoring grip strength use expensive pressure sensors to read a person's grip strength.
To get around using expensive sensors and electronics, I decided to design my device to use a scissor type mechanism to pull on an inexpensive luggage scale. The scale is placed between the two upper arms and is pulled when the user squeezes the bottom two handles.
The most important part of the design was making the measurements correct so that the luggage scale reads the same force that the user is squeezing. If the scale is too far or too close to the pivot point, the torques on the either side of the pivot point will not be equal. The scale applies a point force to the device so it is easy to determine the torque. The user's hand however, does not apply a point force, so I assumed that the center of pressure from the users hand will be in the center of the palm and between the middle and ring finger.
I have attached both the STL and Solidworks files for each arm.
Step 2: Prototyping
The first thing I did was whip up a quick design in Solidworks to test out my concept. I knew that the biggest potential problem with this device will be the strength of the device, so I decided to print out my first design in PLA to see how it would hold up under pressure. After printing I assembled the two plastic arms using an M8 nut, bolt, and some washers.
I printed in PLA with 15% infill so I pretty much expected the device to break at some point, I just wanted to experiment to see how much force it could withstand. It ended up snapping at 45 pounds which is pretty high considering that it is composed of nearly hollow PLA.
Also the device was pretty uncomfortable to hold so it definitely had to be redesigned to make it more ergonomic.
For the second iteration, which can be seen in the second picture, I added some ergonomic handles and placed the scale handles to a lower position that would hopefully help it withstand more force.
Step 3: Printing and Assembling
Since this device will be under a lot of force it is important to get the print settings right, or else your printed pieces might snap while in use. These are the settings I used:
Plastic: ABS - I used ABS for this project, however you could certainly use a stronger material like polycarbonate, nylon, carbon fiber, etc. Since these pieces are fairly long and narrow I used an ABS and Acetone slurry on my print bed to help adhere the prints.
Infill: 50% - I tried doing 90% infill for a practically solid piece, but ended up with horribly warped prints, even with the ABS slurry. The higher the infill here the better, but 30% worked fine for me.
Layer Height: .3mm - I'm going for strength with this print, not surface finish so .3mm is the way to go.
Support: Yes - There is a significant overhang that would be nearly impossible to bridge, so support is a necessity.
I also use an 8mm brim on my prints which definitely helps prevent the ABS parts from warping.
1 - M8 x 50mm bolt
1 - M8 nut
2 - M8 washers
1 - luggage scale https://www.amazon.com/Digital-Hanging-Fishing-Lug...
3 - small zip ties
Assembly instructions are fairly straight forward, thread the bolt through both holes on the printed pieces and attach the nut to the other end while making sure to place washers on either end. It is a good idea to sand down the areas of the parts that rub against each other to ensure that the device functions smoothly. It is also a good idea to tighten the nut by hand. Too tight and the handles will not turn properly, but too loose and they won't stay lined up.
Next wrap the strap around the handle as show in the pics above. Make sure the strap rests in the notch, this is the point at which the torques are balanced and is important as it allows the scale to provide an accurate measurement. There is a slot cut in the middle of the handle that allows you to experiment with different ways to wrap the strap around the handle. This could come in handy, especially if your strap is longer or shorter than the one I'm using.
Next cut off the metal chain connected to the other end of the handle so just the small metal ring remains. Then thread a couple zip ties through the hole and around the handle to secure it in place. It is important to note that the tightness of the zip ties will determine the resting point of the handle. the ideal resting point of the handle is such that the two grips are parallel.
Now you're all done and ready to do some exercises!
To recap on the exercises:
Using the gripping device squeeze at 30% of your maximum grip strength for 2 minutes then take a 3 minute break. Repeat this a total of four times to complete one session. Perform at least 3 sessions a week for 8 weeks.
When designing this device I did not take into account the slight elasticity of the strap, which causes the handles to travel forward. In the future I may try to find a more rigid strap to use.
Also please be careful with this device if you choose to build one. If not constructed properly with the proper materials, it can break under pressure. In the near future I plan on buying some polycarbonate filament that I hope will be much stronger than ABS.