The purpose of this device is to use a spring to create a regenerative force to counter the forces exerted on the knee while walking down. This will reduce the grinding towards the meniscus and inflammation while walking down. This is a school project that was done by 4 UC Berkeley students.

Supplies

Step 1: Identify the Bearing and Spring Needed

To start off, we need to identify a ball bearing that is around the size of the knee so that the product is not too big or too small. My team and I made a rough estimation and we decided to get the ball bearing with the following dimensions:

Inner diameter: 45 mm

Outer diameter: 75 mm

Height: 16 mm

The ball bearing can be ordered through McMaster Carr or Amazon. The purpose of the ball bearing is to reduce the friction between the housing of the parts and the actuator holder (which will be explained later).

The spring will create a regenerative force that counters the forces exerted on the knee while walking downhill. One of the ends for the spring is locked to the housing and the other end can move freely(which will be explained later). Since we need a regenerative force to counter the forces, the force must be a specific number. This also means that we need to machine our own specific spring at a manufacturer. I won't mention too much about the math since the math will take some time to explain. The link below can give you a rough estimation about what spring you need:

https://www.thespringstore.com/torsion-spring-torque-calculator.html

Step 2: Visualizations

To visualize it on Fusion 360, try to find as many parts as possible on GrabCAD to make life easier. Fortunately, the spring and the solenoid actuator were on GrabCAD. However, The ball bearing on GrabCAD does not match the dimensions needed so that has to be CADDed out. Also, there are no Battery Holder and Velcro Strap CADs that match the design so that needed to be CADDed out as well.

Step 3: Ball Bearing CAD

Sketch the certain dimensions as follows in the image I uploaded.
Use the revolved feature to revolve everywhere other than the circle region to create your inner and outer ring.
Use the revolved feature to revolve half of the circle to create a sphere.
Use the circular pattern to create more balls within the 2 rings.
Round the corners to make it look nicer.
Apply the "Silver-Polished" appearance to make it realistic.
You may notice by now that I messed up the dimensions. To correct my mistakes, simply scale down 50% then it will work like a charm.

For the ball bearing, you may 3D print it and use it for the device. However, thermal expansion and warp may mess up the tight fit when we place it inside the housing. It is more convenient and precise to order from a manufacturer.

If you REALLY want to 3D print it, these are my recommended setups in Cura,

Layer Height: 0.1 mm

Infill Density: 10% (speed up the printing time)

Print Speed: 35 mm/s (I will take my time to print since it needs some precision)

Infill Speed: 55 mm/s (speed up the printing time)

Initial Layer Speed: 15 mm/s (to make sure it sticks to the build plate)

Support: no support is needed since there's no overhang

Build Plate Adhesion: Skirt

PS: you may need to slightly adjust the size of the CAD to give some clearance when printing to make sure the balls stay within the ring and roll when the print is done.

Step 4: Battery Holder CAD

The dimensions are based on the dimensions described on Amazon. Please refer to the link in the beginning. Dimensions here are not needed to be really specific since the CAD is just for display. However, the length, width, and height must be exactly the same as described on the Amazon page.

Sketch the square shape of the holder.
Extrude to create a rectangular block.
Round the corners (follow the image in Amazon to round corners).
Create a hole for the screws and Extrude it into the holder.
Create another hole inside the hole just created and extrude deeper.
Create a really small rectangle on 4 sides and extrude to create the edge shown in the picture (does not have to be exact same dimensions).
Create a rectangular sketch on the side that's closest to the hole(screw hole) and extrude. This is for the switch button.
Apply the "Powdered Coat - dark grey" to create that grey appearance.
Open another file and create a block (again, dimensions don't matter as long as it fits inside the slot created in the case for the switch with rounded corners and edges.
Apply the "Plastic - Texture - Regular" appearance to the block.
Drag it into the Battery holder file and position it inside the slot.
Combine the 2 parts into 1.

There you go, a replica of the Battery Holder on Amazon. The battery holder box holds the batteries for the device as a power supply. It is connected to the actuator which activates the actuator. Again, this is just for visualization. Please do not print this

Step 5: Velcro Strap

There are 2 sizes of strap needed, for strapping the thigh and the hamstring. The dimensions don't matter as long as it has the door shape displayed in the image then is good. We can scale it to fit the slots on the mounts.

Sketch a door shaped with horizontal legs.
Extrude the sketch.
Round the corners.
Apply "Fabric Black" appearance to give a semi-Velcro appearance to the strap.
When assembling all parts together, please scale to fit the slots.

The Velcro can be bought online. It can be replaced with normal straps, straps with buckles... as long as it can be used to tightly wrap around the thigh and hamstring to keep the device on the leg.

Step 6: Pin Extension

Since the pin on the Solenoid Actuator is not long enough, a pin extension is required.

Sketch 2 circles with the dimensions used in the image.
Extrude the region between the outer and inner circle to create a cylinder
Round the corners and edges.
Apply the "Silver-Polished" appearance to make it realistic.

This needs to be 3D printed. The design is a tight fit so it might be slightly hard to put the pin into the extension. Feel free to sand a bit to help combine them. Since it is the major component to help the spring to generate force when turned on. The extension must not block the rotation of the spring when turned off and block the free end of the spring when turned on So the extension might need to be reprinted or cut depending on the height of the spring.

There is no specific Cura setup. As long as the infill is more than 25% to prevent the spring breaks the extension, then is good to go. One thing that might be needed will be the build plate adhesion since this is a thin rod, a brim will help it stick to the heat bed

Step 7: Lower Leg Mount

The Lower Leg Mount is used to help mount the device to the user's leg, slots are created for the Velcro.

Sketch the plate as described in the sketch (the length of the plate depends on the user's leg).
As for the slots, the user can create more to get a better strap if they want ( 4 straps are better than 1).
Round the corners and edges.
Apply "Stainless Steel - Polished" to make it look nicer.

Although this can be 3D printed, it might be slightly time-consuming since is a flat plate. There is an alternative way to do this. It is highly recommended to laser cut the plate. Lasering cutting an Acrylic plate with a thickness of 0.125 inches will be sufficient and does the same job as the 3D printed plate.

Some recommended Cura setups:

Layer Height: 0.2 mm (speeds up the printing speed)

Infill Density: 35% (makes sure it won't break while strapped on)

Print Speed: 40 mm/s

Infill Speed: 50 mm/s (speeds up the printing speed)

Initial Layer Speed: 15 mm/s (makes sure it sticks to the bed)

Support: no

Build Plate Adhesion: Skirt

Step 8: Upper Leg Mount

This is the Upper Leg Mount. This is used for mounting to the user's upper leg.

Sketch a circle described.
Extrude the circle to create a cylinder-like object.
Sketch the shape similar to the attached image to create a slanted plate.
Extrude the sketch to create a plate and extrude the base to connect the 2 parts.
Sketch a rectangle on the far end of the sketch and extrude.
Sketch 2 smaller rectangles and extrude to create slots for the Velcro Straps.
Sketch a circle on the cylinder created before and extrude cut to create a hole.
Round the corners and edges.
Apply "Stainless Steel - Polished" appearance to make it look good.

This might be slightly difficult to print since the part is somewhat too big for the printer to do. Try your best to fit it inside the printer.

Some recommended Cura setups:

Layer Height: 0.2 mm (speeds up the printing speed)

Infill Density: 20% (makes sure it won't break while strapped on)

Print Speed: 40 mm/s

Infill Speed: 50 mm/s (speeds up the printing speed)

Initial Layer Speed: 15 mm/s (makes sure it sticks to the bed)

Support: Yes (need it for the slots) / No (print it in another orientation)

Build Plate Adhesion: Skirt

Step 9: Solenoid Actuator Holder

This is the Solenoid Actuator Holder.

Sketch the circle with the same dimension shown in the image.
Extrude the circle to create a cylinder.
Sketch another circle at the bottom of the previous cylinder created.
Extrude the circle in step 3 to create another cylinder. This should give you a T-shaped cylinder.
Create 2 circles at the bottom for. Extrude the center circle around 12 mm and the circle on the side around 1 mm into the object. This creates a slot for the pins.
Create 2 rectangles on the top of the device.
Extrude the rectangle directly on top of the side circle 29 mm deep.
Extrude the rectangle directly on top of the central circle 18 mm deep.
Round the corners and edges.
Apply "Stainless Steel - Polished" to give a better look.
Drag the Upper Leg Mount into the file and combine it with the Actuator holder(but not printed together)

This might be slightly hard to print if you only have a 1 headed extruder. I have attached a picture of me artificially creating support. You might ask why not generate the support through Cura. It is because it would be harder to remove after printing it. With my support, you can easily break them. Make sure to print it with the larger surface on the build plate.

Some recommended Cura setup:

Layer Height: 0.2 mm

Infill Density: 15% (speed up printing time)

Print Speed: 45 mm/s

Infill Speed: 50 mm/s (speeds up the printing speed)

Initial Layer Speed: 15 mm/s (makes sure it sticks to the bed)

Support: no

Build Plate Adhesion: Skirt

The smaller cylinder will be inserted into the inner wall of the ball bearing. Sanding might be needed.

PS: You may notice there's another hole at the center. That is used for a rotary encoder for further improvements. With that, we will be able to make the device autonomous. However, due to time limitations, the rotary encoder was taken out of the design.

Step 10: Housing

This is the House for the spring and all the rest of the parts.

Sketch a circle with the dimensions in the image.
Extrude the circle to create a cylinder.
Sketch another circle on top of the cylinder.
Extrude the circle to cut into the cylinder to create a cup shape object.
Create 2 circles inside the cup.
Extrude the side circle to a cylinder.
Create a plane on the surface of the cylinder created in step 6.
Sketch a circle and extrude cut into the cylinder(This is a cavity for one of the ends of the spring to go in. This is used to lock the spring from spinning freely.)
Create a circle that has a similar width of the inner circumference as the cup.
Extrude it to create an edge ( this is used to position the ball bearing better).
Extrude the central circle to create a cylinder.
Sketch a smaller circle on the cylinder created in step 12.
Extrude cut the cylinder in step 12 ( this creates a cavity for the rotary encoder).
Round all the corners and edges.
Apply "Stainless Steel Polished" appearance to get better appearances.
Drag the lower leg mount and combine it ( do not print them combined)

The cup is pretty easy to print. Make sure the lower leg mount is not printed together. Might need some sanding since is a tight fit for the bearing. The bearing can be hammered into the house.

Some recommended Cura setup:

Layer Height: 0.2 mm

Infill Density: 10% (speed up printing time)

Print Speed: 40 mm/s

Infill Speed: 45 mm/s (speeds up the printing speed)

Initial Layer Speed: 10 mm/s (makes sure it sticks to the bed)

Support: no

Build Plate Adhesion: Skirt

Step 11: Assembling

One end of the spring is inside the cavity of the cup and the other end can more freely and can be compressed. The solenoid actuator is placed inside the side cavity of the actuator holder. The rotary encoder can be placed in the middle for autonomous (which can be used as future upgrades).

The Battery Holder is held on the Upper Leg Mount with electrical wire tape.

The Battery Holder and the solenoid actuator are soldered.

The Velcro Straps are sewed to form a strap.

I used Fusion 360 to create a video to show how the assembling works, Here's the link for it:

https://www.youtube.com/watch?v=dzyCJEgJtRU

This is the Google Slides for the details for the project:

https://docs.google.com/presentation/d/1cnpprLZ7Yilb1Yt_NSfBH92oPwArX9jnnQyXzru_f4A/edit?usp=sharing

Special Thanks:

Maikel Masoud, Burkhard Lehmann, Taniel Keosseian