servo horns
 

I just got some used JR servos (so no manual) and they came with regular-looking 4 pointed star servo horns, and some other 'off centered' looking servo horns. I'm familiar with Futaba's off-set horns, is that what these are supposed to be? There's no marking on the horn like Futaba has. What's the idea here?

Thanks.

RE: servo horns
 

Yeah, the JR horns look bent a little bit, but they are supposed to be this way. Just make sure that the line from the screw to the holes are perpendicular to the servo.

RE: servo horns
 

Actually, you want the line from the servo screw to the pushrod connection to make a right angle with the pushrod. The line from the screw to the holes can make any angle at all to the servo when the servo is centered. The important angle is the one made with the pushrod. It needs to be 90degrees.

The 1st attachment shows one of JR's normal servo arms in the upper left.
The 2nd attachment shows a green pushrod connected at a 90degree angle to a servo arm (at location "a" in the picture). As long as you have that right angle connection from the pushrod to the servo when the servo is centered, it doesn't make any difference at all which way the servo is oriented.

RE: servo horns
 

True. I try to make both angles 90 degrees because I'm a nitpicky SOB. :)

RE: servo horns
 

Better than being just a SOB...;)

RE: servo horns
 

Not always the case. You may want differential on the ailerons, so you would not set the servo arm at 90 degrees to the pushrod. Some airplanes need differential on the elevator to size the loops (inside and outside) the same.

So the answer? It depends.

RE: servo horns
 

Actually, you want the line from the servo screw to the pushrod connection to make a right angle with the pushrod, if you wish the pushrod to be moved equally in both directions. The connection at "a" does that.

If you need differential movement, placing the servo arm so it is at an angle to the pushrod (as with the configuration as at "b" ) when the servo is centered will give a shorter pushrod movement in one direction. In the picture, for setup "b", when the servo rotates it's arm counterclockwise, the pushrod is moved far less in a straight line than when the servo moves it's arm clockwise. The straight line movement of the pushrod is what the aileron/elevator surface sees. And in the "b" setup, it sees shorter movement when the servo pulls the rod than when the servo pushes the rod toward the aileron/elevator.

Almost always, we want our pushrods to move equally when pushing as when pulling. And to do that, the angle should be 90 degrees at both the servo and the other end of the pushrod. Either end, if it's not 90 degrees, the surface won't move equally.

RE: servo horns
 

This is not entirely correct. Since the servos are slightly soft mounted, you want to align the long axis of the servo with the direction of force. If they are mounted sideways, they tend to rock back and forth more in the groment, and so the control action at the surface is less precise.

RE: servo horns
 

Absolutely true for two screw servos, but how true for 4 screw servos?

They would actually "rock" fore and aft as well, wouldn't they. And be less accurate. So you're getting less precision with any servo that has mounting that allows rocking.

BTW, I'm trying to think of any servo mounted in the last few years that had to be mounted sideways?

But it's certainly worth keeping in mind that beginners should pay attention to how tight their mounting screws are. And make sure to mount the servos that would be under heavy load so they are longwise to the load, not sideways. Of course, if you're assembling an ARF, the direction won't be your choice, but you certainly should pay attention to the proper mounting screw tightness.

RE: servo horns
 

Speaking of mounting servos....................

You know they almost all come with metal "top hat" inserts. And the majority have tiny washers while the ones that don't come with those washers have screws with "washer heads". There is a reason for all that.

Almost all servos are mounted with rubber inserts. And those top hat inserts and washers have a definite purpose.

The top hats are supposed to be inserted from the bottom of the rubber insert. Basically, the servo winds up sitting on 4 top hats. The "brim" of the top hat acts as a washer that allows the mounting screw to be tightened until it contacts the "hat" or tube of the top hat. The mounting screw is to be tightened until it presses the top hat against the wood that the servo is mounted against, or until the rubber is compressed as much as you wish. The tiny washer or large screw head is there to compress against the rubber insert.

When the screw is tightened until it contacts the top hat, the rubber insert is securely captured and held. The top hat insures that the servo isn't mounted too solidly. It's hat brim insures that the top hat isn't driven into the wood, as it would if the top hat were in the rubber insert the other way round. The top hat insures that the screw perfectly tensions the rubber insert. That is, it does if it's in the insert the right direction. And if the screw is tightened enough that the rubber is compressed.

That top hat also helps keep the mounting screws tight. If all you had was the rubber insert and a screw, the screws would have rubber pushing against them continuously, in a loosening direction. And with the vibration of the airplane, they just might loosen up until the servo wasn't mounted securely enough to keep from rocking. And you would certainly would have less precision from that servo. Is this something to worry about? No more than about the degree of accuracy you're going to lose from rocking. When you've developed enough experience that you can appreciate absolute servo accuracy, you'll also have a good feel for all these other details and their actual impact.

You can usually tighten the screw until you see the rubber insert compress. Almost everything in our hobby requires learning some technique. Getting the rubber insert compressed very slightly is one. ;) And not every technique is really very important.

RE: servo horns
 

To illustrate what Da Rock was talking about.

Ken

RE: servo horns
 

WOOPS! Mine are all in upside down then:D. They have a little give when I really tug on them, but I don't think I'm gonna have any type of failure.

RE: servo horns
 

Brett,
Unfortunately you are setting yourself up for more failure than you can possibly imagine. The setup of the servo grommets and the brass insert is designed to isolate the servo from the vibrations that are present in the airframe. If they are not set up properly those vibrations will transmit directly from the airframe to the servo. Vibration is the mortal enemy of electronics, and will kill a circuit board. It can also destroy the gear train in the servo. If your servos are setup incorrectly trust me that it's well worth the time to go back and install them properly. Otherwise you will probably be taking your plane home in a Hefty back sooner or later.

Ken

RE: servo horns
 

I guess I'll be pulling servos tonight[&o], better safe than sorry. Thanks Ken.

RE: servo horns
 

I guess the only other thing to mention is that the opening that the servo drops into needs to have a gap all around the servo.

As to the question of 2 Vs 4 screw servo mounting orientation, it is best for either to be mounted so the long axis is in the direction of force. Why bother with expensive digital servos and not give them every mechanical advantage? While most trainers have the servo locations fixed, later models that may even be built by a modeler usually leave the mounting detail to the builder.