rmh

I may look inside one - and see what is differences may be . again, the info I have seen posted suggests the linkage setup is the contributing factor -possibley twisting loads or high vibes - without looking at the setup , I would not comment more.

lrb75

My Feedback: (12)

This sounds alot like the Hitec Karbonite gear failures from a few years ago. All new stronger plastic gear material followed by output shafts breaking off and planes crashing. Hitec provided longer screws reminded everyone not to use loctite on the screws (you don't need it on plastic and it ruins it anyway) and suggested not using the servos on planes greater than 12lbs. Haven't heard nearly as many complaints after that. Maybe Hitec sold JR the Karbonite formula for a good price but not all the fixes to make it work. The DS811 (I have one that works great) was standard nylon gears and the DS821 uses "New, longer-lasting plastic gear material" and is listed as a composite. You might be able to retro fit the DS811 gears into the DS821 if you trust nylon more that the new stuff.

JoeyCoates

I guess that I will chime in here. Go to runryder.com (a popular heli site) and do a search on DS821. You will find many, many horror stories about DS821 servo's leading to many a helicopters demise, and in the classified section you will find many a screaming deal on the DS821's! My father just bought his first helicopter in 20+ years last Thursday, a Raptor Titan 50. When buying the helicopter the salesman suggested the DS821's and the price was right so we bought them. To get the 821's he opened up a Spectrum Radio and took them from it at which time he told us that a lot of people want the radio without these particular servo's. I do not think that he was trying to pull the wool over our eyes, he just realised that it could be good two ways, he gets to sell the servo's and the radio seperatly making money off of both. Anyway, I did a search on Runryder this evening and I have broke the news to my father that he is going to have to shell out a bit more money even though his bank card is already smoking from the last visit as I am not going to watch him get discouraged as he is taking his first tentative steps back into this hobby due to faulty servo's. Just do some reading on that forum. So when we get back in town (we are at our farm right now) we are heading back to the LHS to turn in the DS821's for either some Hitec 5625mg's or Futaba 9252's depending on how his wallet is feeling. I personally think that it is pretty p!ss poor that JR is refusing to recognise that there is a problem with this servo, acting like they know nothing about the reported issues, when sooo many complaints have been called and e-mailed in. I guess that this is a case of CYA but they ought to take a lesson from what Hitec experienced and then did in 2001 with some of their early digital servo's and just deal with it and do right by the consumers. I mainly use Futaba but have occasionally used, and always respected JR. It is discouraging though to see real problems that have been reported by many users just denied and swept under the rug. Like I said, go to runryder to get some great deals on DS821's!

hrosee

They can't admit there is a problems. They would be liable for all the crashes they have caused. In stead they will quietly fix the problem and it will slowly go away........

Jack Skip

Myth Busting In Abaco


We put some heavy testing on a DS821 servo this afternoon. Not sure what we proved, except that testing can be fun. Here is what we did and some calculations on the results.

Anyone who can help in the calculations, or give us some ideas on how we could do more testing, please chime in.

We mounted the DS821 to a board 45.25� long using a single zip tie. Using the standard servo arm screw we mounted a servo arm that was 0.59� from center to the outer hole used. A loop of fishing line was tied thru the outer arm hole. A metal “S� clip was hung from this. Various fishing weights were strung on another loop and attached to the “S� clip. The servo was then moved thru its full motion. Up movements then back to center, down movements then back to center. We tried smooth stick movements and jerky stick movements. We also swung the weights to create twisting loads.

Here are the weights tested: 33oz. 49.7oz. 60.2oz. 70.8oz. 83.8oz.

Dick Hanson suggested that improper mounting could result in servo failure so we inclined the mounting board various amounts to create a twisting load. We used a clevis that was also 0.59� long to provide the twisting motion on the arm.

We ran the full test of the above weights using a 22.5 degree incline on the board. This would provide a 22.5 degree twist in the arm.

We then ran the heaviest weight using a 30 degree incline on the board. This would provide a 30 degree twist on the arm.

Then we ran the full weight at a 45 degree incline. The twisting load was strong enough to open the clevis and a clevis keeper was needed to complete the test. This provided a 45 degree twist.

At no time did the output shaft break. Under heavy loads the servo speed was dramatically reduced (not measured by seemed a second for full travel). The heaviest weight seemed to be at or very near to the maximum the servo could handle as the servo seemed to struggle with the load. The twisting loads were strong enough to bend the servo arm in towards the weights. The centering tests showed the servo arm “bouncing� around the center.

Given the servo arm was under an inch long our weights did not come close to the ratings of 72 ounces per inch. 83.8 ounces at 0.59 inches equates to about 50 ounces per inch torque.

To test the 72 ounces per inch we would have needed to test with 119.52 ounces (calculated using the reciprocal of 0.59) . We did not test this, but I don’t think the servo could have pulled this weight.

The testing done provided a constant load on the servo thru its full range of motion. This would in no way simulate sudden application of resistance to the servo. We could not think of a way to test this. I do not think dropping the weights would have been fair. And a hammer was not handy.

My conclusions.

Our testing was not like loads that would be present in a plane or helicopter, which would include sudden and sharp application of resistance or changes in direction of movement.

The DS821 torque rating seems very optimistic. The servo was struggling at about 50 ounces per inch torue.

Smooth application of resistance to the servo does not cause breakage of the output shaft. Twisting loads, smoothly applied do not cause breakage.

Sorry for the long post, but trying to pass on as much info as we can.

Jack Skip

I forgot.

The DS821 tested came from a Spectrum radio.

rmh

Some asked if 811 output gear would interchange
It will not -
as for breakage -a shock load or vibe still looks like a reason -and as I don't know which arms were used (do you? ) The question is still up for grabs . My DS821's all work just fine
I tried replacing the output gear as dia and teeth all match but the 811 gear is overall thicker in the gear area .
Non of any of my other JR gears would adapt - The 8411 gear may but the center which rives the pot is different
So - I suspect this is an entirely new gear set.

Jack Skip

Dick Hanson,


Thank you for the considerable time you must have spent trying. I was hoping for a quick swap to provide a cure.

Looks like we may have to wait for JR to make the changes and tell us.

Can anyone think of a way to similate a shock load or vibration? I want to test a longer screw and see if it is a cure.

Could an opposing servo of equal strength be used perhaps?

Jack Skip

Danny Snyder said: "There is no size model the servos are intended for, that is based on the particular models torque requirements etc. "


If the rated torque is not accurate and the servo is only able to put out about 50 ounces per inch torque, then people may be asking too much from the servo.

That may explain the two motor failures noted in Hrosee's Rapture 50 helicopter. And he said: "Horizon replaced them under warranty and suggested that they are not right for my application. I have to agree."

But the inaccurate torque ratings do not explain the large number of output shaft failures.

Jack Skip

Irb75, I did some reading on the Hitec problems and it appears similar in many ways.

This was said by "bentgear" and sums it all up "From what I have seen of the gears they are able to take a pretty good load in the air, even doing 3D, but if you bump the controlled surface pretty hard getting it in or out of storage you may have just set yourself up for failure."

bentgear

And I still stand behind that comment. Except for a few products out there, we the end users are our own worst enemy. We expect to buy the cheapest and have it perform like the best. We expect a new product designed for one thing...........to be good at all things.

Ed M.

Jack Skip

But what about when a manufacturer pumps up their product to make it look better than it is? How are we to know.

When searching for a servo I am trusting the specs have not been overstated.

I definately think that is the case here. The DS821 may be a good standard servo, but the stated specs are in the high performance range.

But fragile gear material that cannot absort shocks or vibration. I think we need a warning on that too.

Jack Skip

Dear Danny,

We tsted the DS821 on the weekend and were gettng only about 50 ounces or torque per inch.

Can you ask the JR testing crew to retest their torque numbers on the DS821?

robl45

LOL You can't get them to admit they need a longer screw in the servo, something that heli flyers have used to stop the output shaft from cracking off, but you think they are going to retest the servo? Well I'm done here, Thanks for the help JR. oops No help, you folks should be embarrased. I wouldn't show my face if I was ignoring problems like this.

Jack Skip

robl45,

I am sure they will respond. there are over 1300 views on this thread so far and they will not ignore that. JR testing crew just needs some more time to complete their work.

ira d

HMMMM could it be possible the JR servos are made with the same source of parts as
Hitec? I read in another fourm here on RCU that some JR servos had the same motor
as Hitec and were having problems.

The way I see it as long as JR has been making RC Products there is no excuse to put
an untested servo on the market or skimp on quailty just to increase profits.

Jack Skip

Dear Danny,

With almost 1,500 views now it seems other people may be interested in the results from checking the torque numbers. Any word back from the JR test team yet?

robl45

remember the beginning of the thread when Danny said they had no reports of failures? This was after people on the thread told him. Now they have like 20 reports in this thread alone. Comon JR, do something.

Jack Skip

Even after the testing we subjected the DS821 to I decided not to use these in my 3D beater. I love my Fusion too much for that risk.

Not only because I am unsure of the servo, but because JR is so unsure of the servo. If JR was willing to stand by their roduct, I am sure they would have said more by now.

I am presently club President and get asked a lot of questions on equipment. From this point forward, unless there is a dramatic change, I wll NOT recommend any JR equipment. Becasue of lack of support. If JR does not stand behind it, why should I stand there?

Unfortunately for all the JR stuff I have already recommended, I will have to lose face and explain. But no more.

monjo

My Feedback: (4)

Danny is currently out-of-the-office... per his 'sticky' posted 1/18...

"I will be out of the office for the remainder of the week and all of next week, and will return on the 29th. If you have any questions in the mean time, please feel free to contact our product support department at 877-504-0233 or send an email to "[email protected]". "

I think Danny will post when he's back on the 29th.

rmh

FWIW (for what it's worth)
If anybody else tries servo tests . be certain the power supply to the servo is constant voltage .
That is: a reading of 70 inch ounces at 4.8 volts means just that .
you start with 4.8 volts and throughout the test, the voltage remains 4.8 volts.
FWIW - you simply can NOT do this with a battery, such as we use for our models .
You need a "constant power supply". That is how the servos are rated and tested.
Here is why: the voltage of a 4.8 battery -freshly charged , is way above 4.8 volts (measure it)
Once a load is induced, -the voltage will drop as load increases .
Depending on the battery used (type size and condition)-this may go to zero.
Here is picture of a device (Whattmeter) being used to measure load and battery changes - Iwas testing batteries in this example.
The applied load (70 ounces in this case) is on a constant radius-such as a string over a 2" diameter pulley.
the distance from pivot to load is constant.
A real problem you can run into is burning up your "test" servo.
Once the servo stalls- internally the heat rise goes nuts - and the only saving possiblity is that the battery has dropped effective voltage and the total current is not enough to instantly fry the servo -in any case,
NEVER hold a servo in a stalled condition - any servo -

Carpilot

Hey Dick Hanson,

I have been reading this thread and agree with you 100%

I did a similar test and the DS821 (spektrum) servos I'm sure will deliver 70+ oz. at 4.8volts, I used my variable regulated power supply, it's a good supply, it will go from 0-12vdc and 0-2.5 amps and has amperage limiting built into it.

Also I think with that kind of torque it is possible to mess up your servo with a poorly setup linkage, that is, if your linkage puts an angle on the servo arm 1" away from the output shaft plus a high torque application guess what will happen,... to any servo.

I'm convinced these servos are another quality product getting a bad rap. I suspect these supposed failures are more installation related than anything else.
Seeing how all 5 of my airplanes use these servos now I was a little interested in this thread, but I will sleep quite well using these servos!


unsubscribing

onewasp

Jack Skip

Seems as if you take definite positions without regarding all the readily available facts i.e. Danny's sticky!

(Monjo pointed that out in his post.)

Danny posted that sticky before he left so it is not anything 'new' within the last week. Since he posted it you have made 12 posts here and obviously have not once read his message.

Why not take the time to read and consider before you take such unyielding (and indefensible) positions.

Lots of us are club Presidents but that doesn't make our opinions worth any more than the others.
You hurt your position and credibility rather than help it with such sweeping statements.

Jack Skip

I must appologise to anyone and everyone.

It appears that LockTite on the servo screws caused a deterioration of the gear material resulting in the breakage.

The servos we tested did not have LockTite. The servos that broke all had LockTite on the screws.

I did not know and was not aware that LockTite would make the gear weak.

Sorry if I got annoyed or offended anyone. My most humble appologies please.

kolarshooter

My Feedback: (1)

I thought everyone knew that.

Unsubscribing.