Q about (stronger? ) servo replacement

Hexy Questions, Problems and Suggestions
#1

I see thread’s popping up about bad servo’s .
Both dead on arrival and due not being strong enough (gears who break)

the ones provided are these headsuprc.com/servlet/the-23 … Sub/Detail
they seam quite similar to the “hobbyking cheapo standard” HXT900 rcgroups.com/forums/showpost … stcount=10 )

while it’s fun to disassemble servo’s and swap gears every x time are there any (metal) geared servo’s recommended/known to work better?
stuff like hobbyking.com/hobbyking/stor … 10sec.html

I know some metal gear where tested by arcbotics but had their own issues , it would be good to know which ones where tested

#2

MG90s are not direct replacements for the SG90/92 lines of servos, they have the same overall dimensions, but the tabs on the side are higher up, making them unusable for hexy. We initially had big problems with the MG90s, as they tended to get stuck as you hit the extremes, and thus decided against shipping those.

We’re looking into why so many people are having failures with their servos. We’ve assembled a couple dozens kits with them before we started to ship, and only had a handful of failures (5,6 total out of a few hundred, which is why we decided to throw in an extra servo, just in case). We’re not sure at this point whether its people mishandling them out of improper instructions on our part, or if there are problems with the factory. We’re moving forward by testing large quantities of our servos in stock to see if we can replicate the problem.

#3

thank you Joseph,

that’s useful information.
On Hobbyking there is indication that earlier this year (sept timeframe) a big shipment of HXT900’s had a lot of dud’s, so there might be a possible issue with the producer .

it that might be good to

  • emphase in the instructions to not manually position those servo’s with some force, those gear really don’t like this. This happens easily if you screw in the horns by the way.
  • advice in the build intructions how to center , before starting the build, the servo’s.
#4

[quote=“bstag”]SG90 are plastic gears.
SG92 carbon gears
MG90 are metal gears and should be a direct replacement.

They will be heavier and and maybe other things but should work as a replacement. At around 7 per servo it should be 100 bucks or so to replace them. maybe we can find mg90 repair kits with just the gears.[/quote]

yeah, i doubt if there is really such a difference between the SG90/HXT900 and the SG92…bit skeptical about the “carbon” stuff :slight_smile:
so they should be fairly interchangeable

The main issue for non-US ct’s is the shipping time, I found SG92 gear sets for £0.35 from the uk at
giantshark.co.uk/towerpro-sg … 07833.html (the SG90 gearset costs the same there)
with shipping 10 gear sets to a EU location cost you then only about £6…

If someone knows an EU source that is cheaper (and has stock :slight_smile: ) , please update the thread.

#5

Hiya guys,

Just wanted to followup on this. I assembled my Hexy and got it running on the 1.7 firmware on my Macbook but as soon as I started sending commands over PoMoCo i saw what all the fuss on the forums regarding “shaking” was about.

After trying to adjust and calibrate the servos (not running the servos for long as I was afraid the servos where going to break) several servo gears broke. I replaced some of them but did not have enough spares. It’s kind of annoying since the product is new to have to replace servos after only a few minutes of usage.

So, two questions:
Is there a recommendation for what to do? Buy stronger servos (and in that case WHICH)?
How is the shaking issue coming? Servotor 1.7 does not seem to have resolved the issue

#6

can you elaborate a bit what exactly you did to " to adjust and calibrate the servos " ?

#7

These servos appear to be a fit/torque replacement for the TowerPro with metal gears. TS-9025MG
http://www.rctimer.com/index.php?gOo=goods_details.dwt&goodsid=548&productname=.

Some anecdotal evidence online makes them sound like reasonable servo’s for not much more than the TowerPro version. (They run about $5/ea). So about $100 should get us a full set, shipped.

I’ll place an order and see how they work out.

-Sandro

#8

[quote=“guntherv”][quote=“RonnyRoos”]
After trying to adjust and calibrate the servos (not running the servos for long as I was afraid the servos where going to break) several servo gears broke. I replaced some of them but did not have enough spares. It’s kind of annoying since the product is new to have to replace servos after only a few minutes of usage.
[/quote]

can you elaborate a bit what exactly you did to " to adjust and calibrate the servos " ?[/quote]

Sorry, it was abit of an error in my phrasing. I obviously forgot to center a few of the servos during assembly, so it was just a matter of centering the servos which needed adjustment and reattaching them to the servo horns.

/R

#9

[quote=“sandro”]These servos appear to be a fit/torque replacement for the TowerPro with metal gears. TS-9025MG
http://www.rctimer.com/index.php?gOo=goods_details.dwt&goodsid=548&productname=.

Some anecdotal evidence online makes them sound like reasonable servo’s for not much more than the TowerPro version. (They run about $5/ea). So about $100 should get us a full set, shipped.

I’ll place an order and see how they work out.

-Sandro[/quote]
cheers,
they seams also to be available as Turnigy TGY 9025MG, 4.55$ /piece without shipping @ hobbyking
http://www.hobbyking.com/hobbyking/store/__17321__Turnigy_TGY_9025MG_Metal_Gear_Servo_1_8kg_11g_0_09.html
(and on ebay)

#10

[quote=“sandro”]These servos appear to be a fit/torque replacement for the TowerPro with metal gears. TS-9025MG
http://www.rctimer.com/index.php?gOo=goods_details.dwt&goodsid=548&productname=.

Some anecdotal evidence online makes them sound like reasonable servo’s for not much more than the TowerPro version. (They run about $5/ea). So about $100 should get us a full set, shipped.

I’ll place an order and see how they work out.

-Sandro[/quote]

I’d highly recommend against this, they won’t fit. These look identical to the MG90s servos in their form factor. You can see the differnece in the picture below:

#11

After reading the servo problems I took great care not to stress the servos when I built my Hexy last night. Sadly I have 3 that are not working. I am really not game to experiment any more for fear I will lose the rest. This is a big problem. What is going on with these servos?

Chris

#12

i think there are several issues
some where killed by to much voltage and some seamed to be simply bad (DOA)

But IMHO one of the main issue is that these 9g thingy’s where made for RC, where you normally don’t “stretch” the so far.
the normal “movement” used is 90° to 120° max ( 45 to 60° on each side of "center), here they are used for 180° (90°to each side of center)

I’ve been testing my 20 servo’s with the PoMoCo interface and of the 20 there where 16 who could handle the whole 180* (without load/not mounted) without to much hassle, but 4 of them however started to “gibber” at a certain point.
All 4 of those (it s a very small sample , i know) had the “gibber” (not 100% repeatable) one a point beyond 90° ( 45° left or right from “center”)
I assume the value in PoMoCo is supposed to be the pulse width , so that’s for those 4 a pulse lower than 1.25 ms ( <1250 value in PoMoCo ) or higher than 1.75 (> 1750 value ).

The Servotor pulsing smay also not be very stable, a simple test with putting one servo on port 0 and another on port 1 and “reset” port 0 and 1 and then only move the port 1 (or 0) slider shows the other servo on port 0 (or 1) also starts to slightly “gibber”.

I did not (yet) hooked the servotor board up to an analyzer or scope to confirm if it’s really the stability of the outputted pulse that is the problem or there is something else going on. In this last test power is however not an issue, the board was hooked up to a 10 amp 5v bench power supply and the servo’s had no load.

In any case if those are servo’s are “loaded” (=mounted in hexy) and start to “gibber” then i doubt the gear will take that very kindly and wear out quickly.

I’ll see if i can do some more testing this weekend…

#13

[quote=“guntherv”]But IMHO one of the main issue is that these 9g thingy’s where made for RC, where you normally don’t “stretch” the so far.
the normal “movement” used is 90° to 120° max ( 45 to 60° on each side of "center), here they are used for 180° (90°to each side of center)[/quote]

I’ve observed jittering on some servos when the range exceeds a certain limit, but I’d like to note that while the sliders in PoMoCo allow you to exercise the “full” range of the servos, none of the pre-configured moves appear exceed the 60° mark.

I’ve reviewed some other hexapod software (notably Phoenix for Arduino and noted that the software imposes some limits to prevent the servos from “over-extending”.

I look forward to the results.

#14

I come from an RC airplane background. If you buy cheap airplanes from Hobbyking they have the same set up issues. These servos on the Hexy are hit and miss in the QC dept. You. An read more about these at Rcuniverse or Rcgroups.

The servos that come with Hexy are the cheapest you can find. This keeps the price of Hexy low.
To replace all the servos with a decent set of quality servos would cost $400 to $500. If Arcbotics speced a decent set of servos the price would at least double.

I am very happy with my Hexy and plan to replace the servos with Hitec servos over time as I can afford it

I want to say thanks to the Arcbotics dude for this kit at a very affordable price.

#15

Looks like the mechanical failure mode for the servos is pretty consistent. Two of mine have failed as mentioned in my build thread and both in the same place.

800×600

Has there been any news on a metal gearset becoming available to fit these?

#16

Bradtpt that is exactly how one of my servos went.

#17

Mine failed at the same place. The gear disintegrated rather than just stripping the teeth.

Chris

#18

I just did a spec check and the TGY-9025MG from HobbyKing appear to be identical in all external dimensions to the HXT900 which is the HobbyKing rebranded version of the TowerPro SG92R. These appear to be a different servo to the MG90S which Joe was making reference to being different in its external dimensions above.

[ul]
TGY-9025MG -------HXT900
Weight (g) 11 --------- 9
Torque (kg) 1.8 ----- 1.6
Speed 0.09 ---------- 0.12
A(mm) 29 ------------- 29
B(mm) 23 ------------- 23
C(mm) 26 ------------- 26
D(mm) 12 ------------- 12
E(mm) 32 ------------- 32
F(mm) 16 ------------- 16[/ul]

So I’ve just ordered 20, I’ll advise how good or otherwise they are when they finally arrive.

#19

Please do, though I notice the stall torque is a bit less than the SG92R.

#20

I noticed that, I’m hoping if they are using a different, lower power motor to drive the servo that I might be lucky enough to be able to swap the gearset from these metal gear servos into the SG92’s that came with the hexy kit. I also ordered a set of white plastic replacement gearsets for the HXT900’s (SG92’s). From what I have read people were complaining about HXT900’s which came with black gearsets suggesting the ones fitted with white were a better thing, although I’m not sure if that’s from a mechanical durability or electronics perspective. It will be interesting to see if there is in fact any difference in quality and reliability after I try swapping in some white gears and then try a complete servo swap.

Of course if I do this, I’ll then have enough servos for two hexy’s. Looks like I’ve already found an excuse to order it a mate!