Titan Tricopter Part 2: Yaw Mechanism

Thanks to everyone who read and commented on my last article! If you missed it then check out part 1 in the related articles link at the end of this article.

This time I’ll be talking about the yaw mechanism that I designed for the Titan. Again, I did a lot of looking at designs that were already out there; but I decided my requirements were strict enough that I needed to start from scratch.

The most important requirement is that this thing be robust. Like the rest of the boom hardware I’ve been designing, I want the entire yaw mech to be a self-contained module that is strapped to the boom with zip-ties. The idea is that this will break away in a hard crash rather than breaking any of the components.

Again, I am using 1/8” thick delrin to make these parts. Aura asked me why I chose delrin over something like ABS; and it’s a good question. The main reason is that I will be laser cutting as many parts as possible and delrin cuts extremely well with a laser. ABS and other more common consumer plastics tend to melt and flare up in a laser. Delrin is not used in many consumer goods because it is relatively expensive, where you may see it is in high wear applications like servo gears; delrin is extremely strong, slippery and easy to machine.

Another design consideration that I had was to make the yaw mech direct-drive from the servo. I did this mainly for strength and compactness, but I also didn’t want the possible slop that could come with a linkage.

I am using a HiTec HS82-MG servo, in retrospect I should have probably have gone digital, but I wanted to keep the cost down- plus this servo has a reputation for being a workhorse.

The system uses the same motor mount that I am using for the forward booms, so if the motor gets hit the zip-ties pop loose.

Here are the plastic parts that make up the mount, the servo plate is upside down so you can see the recesses engraved in the bottom to accommodate the zip ties that will hold the servo in place:

Drilling the screw holes for the motor mount. I use a cool trick to locate my cross holes in the plastic.; when designing the part, I make a notch in the center of the hole- the laser leaves a nice mark that guides the drill bit into the precise hole location:

This is the base of the yaw mech. The shaft is 1.5” long 1/8” diameter stainless steel. The shaft is press fit into the two uprights, when the system is installed, two zip ties go around the boom and up and over the shaft holding everything together with a known point of failure:

Here you can see that the yaw mech without the servo is just over 15 grams; not too shabby!

Here it is with the servo installed. Notice that I didn’t have to double my zip-ties to make it around my servo; that’s because I finally found mini 18lb ties that are 8” long! I got them here: http://www.alliedelec.com/search/productdetail.aspx?SKU=70113387 they are expensive for what they are, but I sure like using just one tie to get around a big part:

One of the great things about delrin is that it is self-lubricating and very wear resistant; this means that I can put the shaft directly through the hole in the plastic to use it as a pivot. There is no need for bearings because the delrin will never wear out in this low-RPM application, I just gave it a zero-tolerance fit and it is silky smooth and has absolutely no slop!

Here you can see that it will do over 90degrees of rotation- and it can do it FAST! I don’t want to spoil it too soon, but I have tested it out in the air and the yaw authority is killer!

I am super pleased with this setup. I have completed the first Titan prototype and have been running it through its paces- it is just excellent. I have crashed six times already and the worst damage has been one broken prop, the arm folding mechanism has been working wonders! I’ll have the complete build post up soon so stay tuned!