Close

Design Iterations

A project log for EVPR: Electric Variable Pitch Rotor

An electrically actuated variable pitch rotor with a wireless interface

peter-mccloudPeter McCloud 08/12/2017 at 17:210 Comments

Since the last project log, the design has undergone a few iterations. The main problem had been keeping the rotor blades attached to the hub. One failure mode was that the blade came detached from the axle and the second was that the axle detached from the hub.


The axle was fixed by added a second retaining ring so that both pillow blocks keep the axle from flying out. It also became apparent that the pillow block bearings were not adequate to take the axially loads. A thrust bearing was added at each pillow block to improve the design.


Better bonding techniques were used to attach the blades to the axles. Another test was conducted (see the video later in the log) and for the first time everything survived past the engine starting up. Then after running for about 90 seconds the blades detached from the axles.

It was decided that only using epoxy to keep the blades attached wasn't going to work and that rivets should be added. However the 1045 carbon steel axles are difficult to machine (at least with the equipment I have) due to their hardness. After some research it was decided to switch the steel axles out for 7075 aluminum which has nearly the same tensile strength, but is easier to machine and much lighter.


The latest iteration of the axle is shown above. A knurled surface was added to further improve the adhesive bonding to the axle. After the axle was placed into the blade, holes were drilled though the assembly and six solid aluminum rivets were added.


The new axles and rivets did the trick. For the first time the whole assembly survived startup, running for about 60 seconds and engine shutdown.

A few other issues have popped up as well. The screws holding the servo in place allowed some movement of the servo which wasn't desired. Torquing the screws down further wasn't an option with the plastic flanges. The solution was to use precision shoulder screws to mount the servos.

The screws are shown in the image above next to the red and black wires. They work great, but at $2.24 a piece they are pricey.

Losing rotor blades caused vibrations that damaged the quadcopter frame. Three of the gussets on the rotor arm were cracked. Below is a shot of one of the worst cracks.

Part of the problem was the concave corner that I had cut too sharply. The gussets were replaced, but a large radius was added to the corner to prevent a crack from starting.

I'd love to say that the hardware design is complete, but one last issue popped up during the last test. Inspecting the hardware after the test, one of the servo was not holding position and would make strange noises when the blade was rotated out of position. Taking about the servo it was found that one of the gears which just has a friction fit had come loose.

The culprit gear is the brass gear in the middle of the assembly above. Why did it come loose? While the gear case holds the gear in place from top, the centrifugal forces are likely pulling the gear out of the friction fit. To keep this from happening the servo should have the top pointing inwards instead of outwards so that the centrifugal force helps the gears stay in place instead of pulling them apart. 

Looking at the design it should be straight forward to make the changes. It'll have the added benefit off keeping the main servo gear attached to the spline as well. The only downside will that it will more difficult to adjust the blade pitch with the gears inside of the hub, but it'll still be doable.

Discussions