On rebuilding damaged DS505s

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As usual, an askJOHN begins with a customer. This time was no different because a fellow wrote me saying . . . 'I’m wanting to send you two 505 servos that were involved in a *hard* landing. The one that I opened up has a stripped output gear, and they both have twisted splines. Please take a look and let me know if they are salvageable.' So I called to give him an RMA, and after we chatted a bit I hung up and promptly forgot about the whole thing.

Sooooo, occasionally a tech gets a servo in for repair and calls me in to see it. Usually it's goes something along the lines of, 'Jeez, John, you gotta come look, you won't believe this shit!' and well, it happened again last week so he had two servos to show me. Yup, they belonged to the fellow with the hard landing. Thing is, there are hard landings and there are HARD landings. This was clearly the latter sort.

So in this image eyeball the output shafts of this pair of DS505BLHV servos and you'll realize why he sent them in. And nope, your eyes don't deceive you, they're twisted! Both of them. They're made of 303 stainless steel. Tough stuff. Saying this takes a lot of force is not an exaggeration.

- DS505 with all steel gear trains where the outputs are twisted like pretzels

Maybe maybe you're wondering, how in the heck? Especially if, like me, you realize they weren't in a model truck (where this kind of carnage is so common we developed the BLS2 series servos with the gnarly 15T spline) but were instead, mounted within a model airplane. After all, there's not much cumulogranite for airplanes flying around in the sky, right?

So now for the rest of the story, and I promise you, this is a pretty good one . . .


First, I'd like you to meet a young man name of Cameron DeVries. He and his father John are avid aviators. And not with just any old models, but with really, really nice ones. Anyway, the protagonist aircraft in this story is one of those sweet ultra lightly built Pilot Sbach models spanning 125" and powered by a DLE170.

- Cameron DeVries, Zimmerman, MN and his gorgeous 125" wingspan Pilot Sbach

The model is equipped with DS505BLHV servos all around except for throttle where they used something they had on hand. And note; they ganged a pair of DS505 on the rudder (so about 1000oz-in). And these are the damaged servos in question, and because there's one on each side, this explains why there's an opposite twist in the output shafts.

Anyway, if you think it's strange for tail mounted servos (by definition the last servos to arrive at the scene of a crash) to be as badly damaged as these, you're not alone!

In his own words . . .

'These DS505BLHV servos were installed in the tail of a Pilot 125" Sbach, on the rudder. The throttle servo (not ProModeler) died at 25% throttle. Cam made an approach and cut the ignition on final. Now dead stick he slightly underestimated the glide ratio. Touched down long, which would have been OK except for the irrigation pipe laying across the end of the field. He knew about it and once he had it spotted he raised the nose to clear the pipe, but the low airspeed meant the tail hit the pipe just forward of the tailwheel and ripped out the tailwheel mount and pulled the rudder off its hinges. The servo arms didn't bend, and the servo mounts didn't give. Instead, the servo's gears and output shafts raised the white flag. The airplane is now mostly fixed (easy repair and we had to because it was the last 125" Pilot Sbach shipped stateside). Question is, can you salvage them, or is it time for new servos?'

. . . and there you have it, straight from the horse's mouth. The other side looks the same.

So if there was any doubt regarding how much damage was done internally, take a gander at this photo where the tiny bits on my finger are some of the teeth from the output gear, several of which are sheared off completely!

- The little bits on the fingertip are the remains of some of the gears - those are teeth!

And take special note in the above how twisted the splines of output shaft are - this is where the servo arm attaches! What makes this astonishing is this output gear is made of tough as it comes high grade 303 stainless steel. Moreover the servo damage isn't confined to just the gears.

So in this next image eyeball how we've juxtaposed a new gear shaft next to one of the damaged gear shafts - yes, it's actually bent. And bear in mind, because these shafts are also stainless steel and very short, to boot, plus there's a solid steel gear overlapping nearly all of the span between where the ends of the shafts are anchored, the forces required to tweak it like this are freaking awesome!

Such a lick is enough to easily wallow out the bore of a normal servo. And quite honestly, until we gauged it and confirmed it was OK, we wondered if it had damaged the case as well. This was a pretty good blow!

- The bend is subtle but our techs caught it right away - the gear shaft must also be replaced

This begs the question, was the case damaged where these shafts are fitted? After all, any damage there means the gears will never again mesh as designed.

That, and replacing gears is one thing, but also having to replace a case makes for an uneconomical repair. It's down to dollars and cents and replacing both is almost never prudent.

Inspection and repair

If you're new to ProModeler, we don't charge labor. Means that presuming we can repair his servos he'll only be on the hook for the replacement parts themselves and the not added cost of labor, which is often what makes a repair moot.

So the first thing one of the techs does is clean and inspect. He (or she) will gauge the case to ensure nothing is out of position (remember, if the case is also tweaked, then the servo is toast because of the cost). No, it doesn't happen often *but* we always check every time because putting new parts within a damaged case is an exercise in futility, AKA pissin' your money away.

- Cleaned and ready for reassembly, the shafts are fit into steel inserts - hardpoints

In the above photo, what's important to note is how the shafts are perfectly parallel and straight from where they're mounted. If either shaft is crooked (or even slightly loose), the case is almost certainly done. The other thing to take note of are the shafts in a ProModeler servo are not setting directly in aluminum like they are with less well built servos.

Instead, look closely how the shafts are actually fitted into tiny steel bores. We call these bits of reinforcing steel 'hardpoints'. And their purpose is just that, to reinforce the relatively soft 6061-T6 aircraft aluminum by inserting a harder material to spread the load from daily use (or an impact). After all, servos are expensive and if you wear them out quickly, you won't be happy with us. Why all servo manufacturers don't do this is beyond me but hey, it's an opportunity for us amongst customers who understand the subtleties of design touches like this. Everybody doesn't because they listen, instead, to some keyboard warrior on Facebook yammering about what servos he likes because they look snazzy from the outside.

- These tiny knurled bits are turned on a high precision Swiss lathe

Now we'll admit to making a big deal about these hardpoints. But there's a reason. You see, the forces sufficient to destroy the gears and bend the shaft can easily distort the round bore hole into something resembling an egg shape, e.g. simply wallow it out.

Thing is, once the bore in which the gear shafts are fitted aren't perfect, the position of the gear mesh will never be right again. This leads to accelerated wear and breakage (basically, installing a set of gears, regardless, is just throwing good money after bad).

Case in point, this next photo shows the upper case of the very popular Savox 2290SG alongside that of one of our previous generation DS505 servos.

- Savox 2290SG upper case next to earlier generation DS505

What should jump out at you in the above photo is how the gear shafts within the servo case on the left are placed directly within aluminum bores. That's right, top of the range and no reinforcements whatsoever!

Anyway, hobby dealers sell a ton of these servos, and if we're being honest, they're offering a pretty decent product. But in truth, we feel ours is better, in part because of how we reinforce our servos to improve the service life.

Point being, while other manufacturers yak about torque and speed (while failing to show you the guts of what you're buying), we feel they're doing you a disservice. How? By leaving you to assume something about quality. Meaning without our photos comparing and contrasting, e.g. showing you what you're really getting different with ProModeler servos, then you're buying blind (or depending on keyboard experts going by BlinkyZap on forums making recommendations). Means you're left just comparing specs, which makes it impossible to know what makes ours better for 'you' . . . fair?

So making a big deal about hardpoints is more than showing you what you really get but to make the point that without hardpoints, we believe *any* servo with an un-reinforced aluminum case would have been trashed under these circumstances.

Means, in addition to a set of gears, once you also price in a replacement case (plus labor), the repair usually becomes uneconomical (meaning the customer would have been forced to throw away his servo investment altogether). Remember, the customer had questions regarding whether it could even *be* repaired! He was right to wonder!

By the way, if you're curious why our servo (in the above photo) has bronze hardpoints instead of steel it's because it's from an earlier year. You see, before switching to steel, we were using bronze. Why the change? Steel is better in this particular application. So from the outside, the earlier servo and the present production look the same - but - inside the newer servo has been improved.

Bottom line? Just as a Porsche 911 from 3 years ago and a new one may look the same but aren't, our servos are similarly subjected to a relentless process of iteration. Means we make changes to improve performance, strength, or as in this case, durability. Interestingly, when imports make changes, it's usually to lower cost. How do we know this? It's because we buy their servos - not just once - but several times, again and again through the years (this, to compare and see what's different, if anything).


So the first step in reassembling the servo is pressing the bearing for the output gear into place. The below photo shows gear 1 (the pinion which is made of 304 stainless and hidden from view but is pressed to the motor shaft) meshed with gear two, the bull gear of the #2/3 combination gear. Note, the dark gear color is an artifact of photography.

- Pinions (small) always drive bull gears (large) - never the other way around

So gear #3 (another pinion), is pressed into gear #2 a bull. The pinion is made of very hard 304 stainless steel. The very fine pitch gear #2, which is the finest pitch within the entire gear train, is also the most expensive. If you ever wondered why some manufacturers resort to using plastic for this gear - while the rest are metal - it's really about making things cheaper, e.g. to save money.

So this is the order of assembly for the remaining gears; a smaller gear (called a pinion), engages a larger gear (called a bull). Other than the pinion pressed to the motor shaft and the output gear, the rest of the gear train consists of pinions pressed into bull gears. Together these make what are called combination gears.

- The #4/5 combination gear is called the wedding cake gear for its shape

The wedding cake gear in this ProModeler servo is made of 303 and 304 stainless. The pinion is 304 and the bull is 303. These are VERY durable materials and costly but we use them because they last a loooong time.

- The output gear #8 is assembled next as it's pressed onto the lower bearing

Assembling the output gear to the lower bearing presents a tremendous opportunity for screwing up. Sans the right tools you may cock the gear on the bearing. When that happens, I've seen modelers resort to tapping with a small hammer to finish driving it home. Thing is, if it's forced, then it'll almost to a certainty damaging the bearing's race.

Look, these things are delicate and someone unfamiliar with the tricks of the trade likely won't even realize it *but* that bearing will give up the ghost really quickly. And when (not if) this happens, most folks won't remember what they did (if they even realized it) so they'll begin bad mouthing the servo . . . when it wasn't the servo's fault!

- The #6/7 gears goes on last and gear #7, a pinion, drives the output, gear #8

So are we done? Nope, not quite.The last few steps are details that can lead to a reduced lifespan if not accelerated wear. The gearbox needs to be greased.

So here's the thing, we've gotten servos in for repair where the owner had opened his servo up for maintenance and not knowing what he was doing, had resorted to lithium grease (the whitish cream colored stuff). Problem is this is the wrong type of grease and it soon dries and becomes caked on. Hard, and then it begins to make the gears wear - as if fine sand had been introduced.

Greasing is easy if you have the right stuff and the right tools. Basically, it only takes a little and it must be applied where pinion meets bull . . . in 4 places on both sides, a total of 8 little blobs of grease.

And we don't mind sharing what we use, it's Super Lube 000 with Synclon. And FWIW, I suggest you be sitting when you price it.

- One of the final steps in reassembly is greasing - don't screw this up!

So now we're on the home stretch. Basically, the technician will inspect the o-rings and replace as needed. There are two really fine custom o-rings where the three case sections come together (you'd better believe we open the bottom and eyeball it while we have it in our hands). These are rarely damaged because they're fully captured (means surrounded by metal and not exposed - you can't say the same for other brands). Plus, there's also the one o-ring where the upper bearing is fitted to the case. It too is inspected when the bearing is replaced.

- Three custom o-rings - two for sealing the case plus one where the splined output shaft exits

The last step is to secure the case together. Unlike more cheaply made servos, instead of Phillips head screws we use Allen head bolts. And beneath the head of the ten machine screws are teeny tiny o-rings derived from Rolex watch stem seals. These are fitted to individual pockets CNC-machined into the 6061-T6 case. And just like the other three, are custom made, also.

- With six assembly bolts just for the tranny section, the end result is a very stiff assembly

Bottom line? Your ProModeler servo is a fine quality hand assembled bespoke piece of equipment. When it's damaged, return it to us for repair. There's no labor charge and having us do it for you means you're assured it's been done right.

If any questions remain, just give us a call at 407-302-3361 where we're at your service.

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