Mounting a pull pull arm to a Hitec 7955 servo wheel
06-16-2019, 03:39 PM
#1
Mounting a pull pull arm to a Hitec 7955 servo wheel
I'm converting a GP 25% Yak 54 from dual rudder servos to one pull pull servo. No one makes a 4" double arm with the correct offset, either of the aluminum cnc splined variety or the the aluminum type that bolts on to a servo wheel. Offset must be 7/16" -1/2" with crossed cables and location of rudder horns.
I was thinking to fab my own arm out of 1/8" 6061 aluminum and mount it to a plastic servo wheel that came with my 7955 servo using 4ea. 4-40 bolts but before I do I need to know; will the plastic splines hold up to very aggressive aerobatics?
I was thinking to fab my own arm out of 1/8" 6061 aluminum and mount it to a plastic servo wheel that came with my 7955 servo using 4ea. 4-40 bolts but before I do I need to know; will the plastic splines hold up to very aggressive aerobatics?
Last edited by Glowgeek; 06-16-2019 at 04:00 PM.
06-16-2019, 05:18 PM
#3
Rudder horns are 3-3/4" hole to hole. Having the servo arm holes spaced at the same distance gives a 55° control surface deflection with the greatest servo resolution. Torque ratio between servo and control horns remains at 1:1.
06-16-2019, 05:39 PM
#6
Sounds like you have a grip on the geometry, that was my concern. Back to the splines, the HD black Hitec servo wheel may work for what you have in mind. The way I would do it is with some .062 aluminum or CF but do a double arm having one on the bottom of the wheel and one above and put a ball link between the two. This would load the servo output shaft more evenly so that the load is not concentrated on the upper half of the splines.
06-16-2019, 06:12 PM
#7
A good thought Speedracer and something I had considered doing as well. Thing is, with the ball links centered between two thinner servo arms the crossed rudder cables would rub each other. I have seen where others have mounted one ball link to the top of the servo arm and the other to the bottom to alleviate rubbing. Not sure I like that idea but it is a dual bearing Ti geared heavy duty servo. May be ok on a 25% Yak.
Last edited by Glowgeek; 06-16-2019 at 06:16 PM.
06-16-2019, 06:31 PM
#8
I did pretty much the same thing on the tiller arm of my 3M Carf Extra. To eliminate the body of the ball links from hitting the arms I just spaced them apart enough. Whatever it is you do just make sure the servo arm can't flex. As far as cables rubbing, One reason why I got away from steel cables and went with nylon coated Kevlar. Having the coated Kevlar cables rub is not an issue. I however do all my pull pull setups without crossed cables these days.
Last edited by speedracerntrixie; 06-16-2019 at 06:33 PM.
06-19-2019, 06:36 AM
#10
Moderator
I think you'd be better off mounting your arm to a metal servo arm, which is readily available.
That said, is there not a pull/pull arm available that you could modify? Drilling and tapping a couple of holes is less trouble that you're talking about going to here. If not, how about starting with a double arm and mounting an extension between them that gives you the offset you need? A HD clevis could make the connection then.
That said, is there not a pull/pull arm available that you could modify? Drilling and tapping a couple of holes is less trouble that you're talking about going to here. If not, how about starting with a double arm and mounting an extension between them that gives you the offset you need? A HD clevis could make the connection then.
06-19-2019, 10:00 AM
#11
I agree with Jester that the best solution would be an aluminum arm. There would be no doubt going that direction. I have however used Nelson servo arms that use the stock plastic servo wheels on a 33% Laser without issues. However that airplane was set up with IMAC rates. It's the 3D rates that casts the doubts.
06-20-2019, 04:28 AM
#12
Thanks, more to consider. After a good deal of head scratching I realized I may have answered my own question in post #3 . No one seems to have spline stripping problems using the factory supplied arms provided with Hitec 7955 servos when set up at a 1:1 ratio. For instance using a 1.50" servo arm with a 1.50" control horn the ratio is 1:1. So if I'm using a 3.75" servo arm with 3.75" spaced rudder horns the ratio is still 1:1. The splines should not see any greater torque when set up this way.
Perhaps the question I should have asked is whether or not a single factory supplied Hitec 7955 servo arm can withstand the rudder forces of a 16 lb. 25% Yak 54 during aggresive 3D flying.
Perhaps the question I should have asked is whether or not a single factory supplied Hitec 7955 servo arm can withstand the rudder forces of a 16 lb. 25% Yak 54 during aggresive 3D flying.
06-20-2019, 04:39 AM
#13
Moderator
I think with the side load of the pull pull and the fast load changes of 3D flight, you'd be asking a lot of the plastic servo wheel. There's no doubt it can handle the torque, but the other forces at work could be a problem.
06-20-2019, 05:11 AM
#14
Thanks Jester. So if the splines are not a concern then that only leaves the molded junction between the wheel surface and spline hub as a possible point of contention? Perhaps even the possibilty of cracking the spline hub itself when put under transient off axis loads? That makes some sense to me if that were the case.
The arm I intend to make will be fabricated using 3/16"" thick 6061 aluminum. The center will be the same diameter as the wheel and secured with 4ea. 4-40 screws. The rudder cables come off the arm at an angle very close to 90° to the servo shaft so there shouldn't be much in the way of off axis loads. That said, I was intending to mount one ball link on top of the arm and the other on the bottom to keep crossed cables from rubbing. That will definitely impart off axis loads on the wheel to spline hub junction.
As you said, modifying a pre-splined cnc aluminum arm is probably the safest solution. SWB makes some nice ones.
The arm I intend to make will be fabricated using 3/16"" thick 6061 aluminum. The center will be the same diameter as the wheel and secured with 4ea. 4-40 screws. The rudder cables come off the arm at an angle very close to 90° to the servo shaft so there shouldn't be much in the way of off axis loads. That said, I was intending to mount one ball link on top of the arm and the other on the bottom to keep crossed cables from rubbing. That will definitely impart off axis loads on the wheel to spline hub junction.
As you said, modifying a pre-splined cnc aluminum arm is probably the safest solution. SWB makes some nice ones.
07-01-2019, 04:00 AM
#15
When I make a pull/pull setup I make a pulley of whatever diameter I choose then run a pushrod to that pulley from the servo. This takes all of the side load off the servo shaft from the wire tension. You can make them as tight as banjo strings if you want. I use 1 wire that threads through the pulley and wraps around it. I make mine from 3 pieces of plwood (1 center and 2 flanges) then glue them together but for something of this size maybe aluminum would be better. You'll also need to have a shaft to run it on.
carl
carl
