I will try an explanation. You can decide if it is to farfetched.
The 13,8 kg.cm comes from the marketing dept. It is probably related to their best bench mark test result. The essential of the text in this note was probably done by a technical person. It doesn’t say “the torque is 5.7kg” like you quote but “the torque exceeds 5.7kg”, which is a perfectly pragmatic interpretation of test results!
Hi Sylvain,
it is really “tiré par le cheveux”
I would appreciate reading a more consistent figure as “more then 200oz”, but even with that is still too far (+28%) from 7.26kg or 256oz :
as per 13.8kg.cm / 1.9cm drum throat radius = 7.263kg or 256.19oz
I would be afraid if the servo products, we are using, are measured with that precision !! unless the torque measuring machine are out of order !!!
Cheers
ClaudioD
No mathematical explanation apart from what you have already calculated but my interpertation is that 13.8 Kg is at the winch spindle and when you add 19 mm diameter drum reduces to 5.67 Kg.
Are you wanting to work out what Kg you will have with smaller diameter drum ?
Not really Alan,
as it is written 13.8 kg.cm means that the 13.8kg are measured with an arm length of 1cm or a drum with 2cm diameter and 1cm radius.
Since the provided drum is 4.8cm OD and 3.8cm ID, the radius is = to 1.9cm, therefore the initial torque specification of 13.8kg.cm, become : 13.8 / 1.9 = 7.26 kg that is far away by 28% from 5.67kg as reported above.
When I read 5.67, then I assume that the “radius” used was : 13.8 / 5.67 = 2.43cm instead of 1.90 and that is not mentioned anywhere !
What I intend for the “ESTEREL” is a larger drum 90mm OD and 65mm ID and one single revolution as already explained.
I got some mailing exchange with Servocity and discussing about drums. The ones they offer have little throat depth.
I wonder if Ramoser would be interested to make similar drum as in the Spectra. BTW what is the actual diameter ?
Cheers
Claudio
Ok understand, I’m in Munich next weekend sailing with Christian Ramoser, if you give me a plan I will ask him if he can make one for you, he has extensive workshop so don’t think it will be a problem but need to ask.
Sprinta winch drum is 90 mm O.S diameter with I.S of 60 mm (15 mm channel) only thing is has square mounting for RMG fitting so need to customise it to the spindle on the servo your wanting to use.
Cheers Alan
P.S oops just saw you drum specs above, are these the final dimensions ? just need to know spindle type ?
Hi Alan,
I do not know the spindle type and diameter, it may be an Hitec one, but I will use very probably the small Hitec provided horns or hubs from Servocity or making one myself with my antique little lathe machine.
http://www.servocity.com/html/servo_arms___horns.html
The dimensions may change from one servo power to another and as function of the sheet length to be used.
Actually, the above drawing is in line with the actual ESTEREL design approach, is a little larger at ID 65mm against 60mm as compared with the one used by Ramoser.
Do you have an idea of the weight ? This is an important factor !
The throat do not need to be large since only one turn is used with tiny string, let say 2 to 3mm max.
It may be separate items with the central core and flanges apart .
Ciao
ClaudioD
Hi Claudio,
I don’t know the weight but I will ask him to take it into major consideration, with one turn it does not need to be as big as the version used in the Sprinta, will let you know as soon as I have spoken with him.
Cheers Alan
Hi Alan,
this attached sketch show possible choices including the ‘Sprinta’ drum if not too heavy , taking into consideration that the actual set’s weight is :
servo 65g
gears and hub 15g
potentiometer 20g
missing the Drum for which I consider another 20g max.
for a total of 120g
Other servos considered : HS-7954SH and HS-7955TG
Certainly compared with 36kg.cm at 6volts servo arm as HS-7990TH for 134$ and 0.42" /120°, the weight will be 42g lower but with an higher consumption of energy and, in my opinion, also too fast. Easy with a fast joystick maneuver to break structural parts of the hull ! It already happen several times !!
Further with an arm of 85mm the final torque will be : 36kg.cm / 8.5cm = 4.23kg force, frictions not included and already just a little above the requirements of 4.00kg force.
Cheers
Claudio
Dear Claudio,
Allow me a second attempt at the 5,67kg mystery. And this time, no more “tiré par les cheveux”. Promised!
To be honest, I have first written to Hitec about it. Is that cheating?! Anyway, I got an answer half an hour later. Wow, good service! Unfortunately, they had no clue: “I don’t know… that data sheet is well over 15 years old.”
I pressed him to get a definition of their torque rating, knowing that there is several published values in higher-ends purpused servos (max torque, stall torque, standing torque, rating torque, etc…). He didn’t know!!! But he sent me these two “rare and old” datasheets.
HS-725BB winch servo (at 6V):
Stall torque = 14.5 kg.cm*
HS-785HB winch servo (at 6V):
Stall torque = 13.2 kg.cm
Standing torque = 10.5 kg.cm
Now, there is a lot of torque definition on the web. And finding the proper on, I mean used in physics research, doesn’t mean that Hitec is using the same.
But here is an interesting set:
Stall torque is the ability of something to move the servo.
Standing torque is the ability of the servo to move something.
For R/C sailing, what is the good of holding a position if we cannot get out of it?
I am pretty sure that the 13.8 kg.cm of your notice stands for the Stall torque. It is the label generally used by those who go to the trouble of informing us. The 10.77 kg.cm may stands for the standing torque, has used by the clever author of the notice. Seems coherent with the Stall/Standing ratio of the HS-785HB datasheet comming directly from Hitec.
Very good Sylvain,
You may have found the key !
What I can say now is that for the 785 between standing torque and stall torque there is difference of 25.1%.
If I apply the same % to the 725 with 14.5kg.cm we get : 14.5- 25.1% = 10.86kg.cm
If we divide by 1.9cm, being the drum radius, we get : 10.86 / 1.9 = 5.71kg
This value is very very close to 5.67kg that appeared on the Hitec note. Probably for the 725 the ratio was slightly different in percentage as such to get even closer to 5.67kg
Well done Sylvain ! Hitec should learn from you !!!
Cheers
ClaudioD
Hi Claudio,
No word from Ramoser and I won’t see him until next weekend … but I had some free time today and decided to have a shot at making your drum winch myself.
Took couple of hours as was cutting everything by hand and using a hand drill as per your plan, but it came out better than I expected.
Used two sheets of 2 x 245 carbon for the flanges and carbon foam sandwich for the spacer and final weight is between 10-12 grams, I think my scales maybe on the blink so you should check it when it arrives.
Will be in the post to you Monday morning.
Cheers Alan
Just Great Alan,
your workmanship is amazing and fast !!!
Thank you very much, I will prepare a set-up to carry out some torque tests with my ancient HS-725BB, it is slow but …
Very very good !!
Cheers
ClaudioD
Just a tought that may has already been taking care off by Alan. This great workmanship took me back in my earlier life involving rocketry. We had carbon fiber pulleys guiding a one inch (25mm) nylon strap. The strap once was completely cut out or sliced like a knive by the carbon flange. You have to make sure that these peripherical edges are very smooth or that the line is guided very well in the center.
I don’t think a sheer cut is a concern in your case. I am more thinking about premature wear.
Hi Sylvain,
I don’t have any machining equipment everything is done by hand, this is my lathe used for making things round and smooth the most technical thing about it is the “nail” for centering which is hammered into the table top at the required distance from edge of the table to have right diameter & then block with sandpaper that I run across edge repeatedly…
hopefully Claudio won’t be using it at rocket speeds so it should not cut sheeting lines but one thing thought about was the gap between the spacer & the flange where you don’t want the line to jam, here I bonded one flange at a time to the spacer to make sure a epoxy “seam” was formed between the two surfaces with the help of gravity.
It will help a little to guide line in the center but was more important the line does not jam.
The spacer has beading of loaded epoxy on its edging as I was using sandwiched foam and concerned the line could possibly cut through the foam, once it had hardened it was just matter of using the hand lathe to get it smooth before bonding it to the flanges.
Cheers Alan
Hi,
No jamming problems expected since the Kevlar string* diameter is 0.3mm while the throat is 10 times larger. Only 1.5 max operational turns + 1 additional turn. The servo cannot turn more the 1.32 turns including trim. Sheet length needed 270mm.
Speed expected 1.5" .
Nothing will forbid to strengthen the throat path with a 0.4mm aluminum ribbon.
Carbon spikes if any will be removed with dremmel and the covered with varnish.
What is important now is that Alan have demonstrated the feasibility and the pulley can be done within 20g even with double throat.
This is way the pulley will be used :
…
The Master actually covered with 6 coats of wax a couple more …may be !
Gel Coat and glass tissues arrived.
Temperature outside was close to 19/20° for few hours a day, but rain expected and lower temperatures as well.
Cheers
ClaudioD
Weather is changing for lamination outside, probably next Saturday, meanwhile…!!!
ClaudioD
claudio,
you have some great threads and posts on this website. i really enjoyed and learned a lot from reading through your other 123 thread.
one thing that may make deciding on a servo easier is to think in terms of work, instead of just torque and rotation.
work is simply force x distance (for linear) and also torque x radians of rotation (for rotational). Wikipedia has much more info if you are interested.
earlier in this thread, you calculated 272mm of sheet travel and 1.44kgf of sheet tension.
272 x 1.44 = 391.68 kg-mm of work.
here is a table of servos i have been putting together with torque in units of kg-mm and work in kg-mm. speed is in seconds/60 degrees.
drum_work[kg-mm] = torque[kg-mm] x (rotation[degrees] x pi/180)
| make | model | price | voltage | weight | rotation | torque | speed | stock_drum_dia | drum_work |
|---|---|---|---|---|---|---|---|---|---|
| RMG | 380EH | 261.1 | 7.2 | 175 | 3456 | 299 | 0.054 | 32 | 18035.2551 |
| RMG | 380ES | 245.5 | 7.2 | 168 | 2160 | 299 | 0.054 | 32 | 11272.0344 |
| RMG | 280EL | 238.8 | 7.2 | 134 | 2160 | 193 | 0.057 | 26 | 7275.9286 |
| RMG | 280ES | 238.3 | 7.2 | 134 | 1800 | 154 | 0.041 | 26 | 4838.0527 |
| EURGLE | 16D_22YMB | 57.55 | 7.2 | 125 | 2160 | 119.5 | 0.1 | 30 | 4505.0439 |
| EURGLE | 16D_10Y | 44.57 | 7.2 | 120 | 2160 | 119.5 | 0.13 | 30 | 4505.0439 |
| HITEC | HS-785HB | 50 | 6 | 110 | 1260 | 132 | 0.23 | 37 | 2902.8316 |
| RMG | 280EF | 241.4 | 7.2 | 134 | 1152 | 122 | 0.027 | 32 | 2452.9555 |
| HITEC | HS-7980TH | 135 | 7.4 | 76.54 | 180 | 460 | 0.17 | NA | 1445.1326 |
| HITEC | HS-M7990TH | 175 | 7.4 | 76.5 | 180 | 440 | 0.17 | NA | 1382.3008 |
| SAVOX | SV-0236MG | 110 | 7.4 | 200 | 180 | 400 | 0.17 | NA | 1256.6371 |
| FUTUBA | BLS157HV | 190 | 7.4 | 77 | 180 | 370 | 0.11 | NA | 1162.3893 |
| FUTUBA | BLS152 | 180 | 6 | 73 | 180 | 310 | 0.14 | NA | 973.8937 |
| FUTUBA | S9157 | 140 | 6 | 71 | 180 | 306 | 0.14 | NA | 961.3274 |
| HITEC | HS-7954SH | 90 | 7.4 | 65.2 | 180 | 290 | 0.12 | NA | 911.0619 |
| HITEC | HS-7955TG | 95 | 6 | 64.92 | 180 | 240 | 0.15 | NA | 753.9822 |
| HITEC | HS-815BB | 45 | 6 | 153.9 | 140 | 247 | 0.14 | NA | 603.5349 |
| HITEC | HS-765HB | 40 | 6 | 110 | 140 | 132 | 0.23 | NA | 322.5368 |
as you can see your 392 kg-mm of work can be performed by most of the above servos without messing around with the potentiometer and weight of the extra gears. most digital servos can program the amount of rotation (within limits) and the speed (if it’s too fast), this way your servos can be a stock off the shelf part and still covered by warranty.
if you wanted to use an arm instead of a drum, i think the work equation is:
arm_work = torque * 2/sin(angle/2)
the angle is in radians and the geometry with an arm only makes sense with a maximum angle of 180 degrees.
i think the above torque values are the stall torque, as mentioned in an earlier post the standing torque might be better, if available.
-scott
Claudio willing, for italian people, all news of 1.2.3.ESTEREL project , publish in
http://velarc-salerno.forumitalian.com/t63-123-esterel#1170 .-
TY Claudio
Hi Scott,
I appreciate your comment on my treads and effort in exhibiting this table, but in my opinion there are some mistakes and the most important one is the use of the rotational parameter that cannot be considered other then speed and power consumption.
Here my first comments to your table :
As you can see, is not sufficient to extract the manufacturer specifications. What it is important is just the torque value at a given voltage supply.
In my opinion, the “work” done could be used to verify eventually the energy consumption of each single servo under load.
conditions.
Just an example, you suggested that 392kg.mm is covered by most of the servos.
1st - the 392kg shall be multiplied by a safe factor that contains frictions, tolerances and a security margins. In my case I choose a factor of 2, therefore one should read 392 x 2 = 784kg.mm
2nd - the Eurgle you listed with 4505.0439 drum-work was not capable to pull back the main of 5073cm² when a gust of 16/18 kt arrived. This point is visible and commented in the published film.
Because the effective use was with 2.94 revolutions, then your formula : “drum_work[kg-mm] = torque[kg-mm] x (rotation[degrees] x pi/180)” will suggest : (119.5 x 1061° x 3.14) / 180 = 2211.7733 and not as you calculated 4505.0439, by taking all the Mfg. parameters. If further you modify the voltage supply, your figures may drop substantially !
3rd - based on above all your calculated values should be revised as function of the effective conditions of use.
With the need to get a kg force of 784, the last servo in the tables cannot be use ‘as is’ unless with the arm configuration.
The RMG are for the time being excluded because expensive.
Most of servo listed are valid with the “arm configuration” so far they can be maneuvered with 120° minimum to avoid too long arm and consequent loss of power.
There is a drawback that I do not like, and this is the rotational speed that in my opinion is often too high. Many boats has been already damaged because of that.
Of course one could educate himself to move the joystick in accordance with.
Finally the torque power of several servos listed above could be exploited with the use of a drum, but to do that there is only one way : external potentiometer.
There is also another concern is the power consumption. If I desire to contains the overall weight, I shall consider the battery pack weight and cost.
I remember another problem with arm servo, some times, under continuous pull effort they may burn out, this happen to me with the HS-815BB on the AC33 project, probably was my mistake to let my boat in the hands of somebody else !!!
So, as recalled above, not easy to come up with a solution unless you are prepared to spend a lot of money and accept the excessive weights of some dated servos.
All that of course is valid when reported to my design where the overall weight limit is the most important parameter.
The solution I have retained for the time being has the merit to be less expensive and not very heavy.
Hope one day to see a drum servo more powerful and a bit more faster then the HS-785.
Cheers
ClaudioD
I know you find the RMG expensive but you get what you pay for - I have been racing IOM with a RMG winch for years because they are reliable, fast but smooth at the same time and so flexible in their programming options. I also use them on my Mini40 (0.9m2 sail) and will be using a 380 version on the 2m once built and they can easily hold the big sail area. I hear you and understand your requirements for this boat, I can’t wait to see the results of your tests with the new carbon winch drum.