Airfield Models - Hardware for Flying Model Aircraft

About Pushrod Clevises for Model Airplanes

June 13, 2009



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Airfield Models (http://www.airfieldmodels.com/)Nylon and Metal Clevises for Model Aircraft

(Not pronounced,"cleeeevis")

Clevises have two arms that surround a control horn or servo arm and have a pin that connects the clevis to the horn.  The pin acts as a pivot in the horn.

The pin can be molded to one arm, welded to one arm or be a separate piece such as a cotter pin.

Clevises with permanently attached pins are connected to a horn by gently spreading the clevis arms apart until the pin can slide over the horn and into the hole it engages.

The second arm of the clevis will close over the pin or must be snapped shut depending on the clevis type.  A locking device should be used to hold help hold the clevis shut so that it can not disengage.

A clevis having a separate pin does not need to be spread apart to place on a horn.  Generally clevises using separate pins are for use with larger aircraft.

Clevises should be used only when the pushrod is very close to being in the same rotational plane as the horn.

One advantage to clevises over almost every other type of connector is that they center the pushrod on the horn.  Most other connectors, such as bends and ball links require the pushrod to be offset from the horn.  The farther the offset the more leverage the pushrod has over the horn which can cause the horn to twist.  Usually it's nothing to be concerned about but something to keep in mind.

Clevises come in a variety of sizes and materials.  Some clevises are threaded on and others must be soldered in place. The idea being that some connections need to be adjustable and others do not.

There are three materials that I have seen clevises made from nylon, steel and styrene.  Do not use styrene clevises.  Nylon and steel clevises are very reliable, but they can fail in inappropriate applications, through other mis-use or by wearing to the point of failure.

I have never had a clevis fail unless you count me breaking it when putting it on or taking it off.  If a clevis fails it is usually in one of the following ways:

  • Weld failure of pin on metal clevis.Opens in flight and disconnects from the horn.  This is very avoidable using methods to be addressed.
  • Broken Pivot Pin.  I've only seen pins break loose from metal clevises.  The clevis shown to the right came with an ARF.  The pin fell out the first time the clevis was opened.
  • Arm breaks.  I've broken arms on metal clevises by spreading them too far.  I've never seen an arm break on a nylon clevis.
  • Threads strip due to vibration resulting in the clevis sliding on the rod and in some cases sliding off the rod.  This happens with threaded metal clevises that are not secured with a locking nut.  Locking nuts should always be used with threaded metal clevises.  A nylon clevis can't unthread itself or slide off unless the rod it threads onto is too small or possibly if the clevis shank was threaded with a tap.

Metal and nylon clevises are equals.  Metal clevises aren't always a better choice than nylon and vice-versa.

 
 

Nylon Clevises

Nylon is a very strong material.  All nylon clevises are self-threading which is a good thing.  Any given size threaded rod can vary significantly between manufacturers.  If you thread the clevis onto the actual threaded rod it will be used on it will be a good fit, it will not slide off, it won't unthread itself and it doesn't conduct electricity.

Do not even think about gluing a slip-fit nylon clevis to a pushrod.  Ok, I brought it up and now you've thought about it.  That is because I've seen it done.  Don't ask...

Threaded metal clevises lose on all the above points.  You get what you get and it fits the rod however it fits sometimes good and sometimes not good.  Metal clevises can destroy the soft threads on the rod from vibration through the airframe over time.  The looser the fit, the worse the problem.

Some pushrod types can conduct electricity and radio waves.  This can be a problem if it is not insulated before it gets close enough to the servo to feed back into the radio system.  If a pushrod or pull-pull system is not conductive then it doesn't matter what type of clevis is used from conductivity stand-point.

However, if the pushrod is conductive, then I use some type of connector that is not conductive to help insulate the radio.  That usually means a nylon clevis.

The two drawbacks to nylon are its flexibility and wear properties.  The pin in a nylon clevis can flex and add more play in the system.  This isn't usually a problem if the appropriate size clevis is used and if it is fully closed and locked.

A worse problem is that the pin can wear through and eventually break.  I've had planes that had many seasons of flight on them that had clevises with the pin nearly worn through.  I don't make it a habit to open clevises and inspect the pin, so this sort of thing could lead to clevis failure followed by destruction of the model.

Nylon clevis pins wear fastest when they are inserted through metal, such as a throttle arm, or an abrasive material, such as fiberglass-filled nylon arms or horns.  They wear slowest when inserted through a smooth, non-abrasive bearing surface such as another piece of nylon.

I'm not trying to frighten you away from using nylon clevises.  I use them all the time and trust them.  But the pins should be checked occasionally for wear and the clevis should be replaced as necessary.

 
 

Threaded Metal Clevises

Metal clevises are longer wearing and usually stronger than nylon clevises.  They are also more rigid.  For small planes, there is not enough force on a clevis for rigidity to be an issue.  Aircraft that have significant force on the clevis such as planes intended for extreme aerobatics or larger aircraft should use metal clevises.

Always use a jam-nut to prevent a metal clevis from vibrating and destroying the threads on the rod or in the clevis.  Vibration can loosen the jam nut so be sure to check during your pre-flight inspection.

Non-hardening Loctite is a good idea for metal clevises.  Do not use Loctite on plastic clevises.  The bottle states that Loctite attacks some plastics.

If both ends of the pushrod use threaded metal clevises (or another threaded metal connector) then the pushrod can rotate such that it threads more into one of the connectors and out of the other connector.  Goodbye plane!  Always set up one end of the pushrod so that it can't unthread itself.  Use a nylon clevis, ball link, bend or other connector that doesn't thread on and off easily.

Metal Solder Clevis and Threaded Clevis with Locking Devices.

Two metal clevises.

Top: A Solder Clevis.

Bottom: A threaded clevis.

Note that each has a locking device.

The top clevis uses a short piece of fuel line to ensure the clevis can not open.

The lower clevis is made by Sullivan and includes a lock clip.  A separate locking clip is shown alone at the bottom of the photo.

 
 

Metal Solder Clevises

Solder clevises are a favorite of mine.  They are very reliable and easy to install.

I plan to include instructions for soldering clevises hopefully not long from now.

 
 

Locking Clevises Shut

Clevises by DubroNo matter what type clevis you use, you should always use something to lock it so it can't open and disconnect.  Most commonly a small piece of fuel tubing is slid over the arms after the clevis is closed.

The tubing should be confined the area on the arms between the shank and the horn.  The idea being that it slightly compresses the arms and can not slide off.  Tubing that is too wide can slide off the arms and back onto the shank.

A piece of fuel tube about 1/8" to 3/16" wide is plenty.  Fuel tubing clevis locks do not last forever and should be replaced usually at least once during a flying season.

Manufacturers are now making clevises with built-in locking devices.  I trust the ones I have seen so far, but at some point I am sure some cheesy manufacturer will make an equally cheesy locking device that does not work properly.

 
 

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L-Bends and Z-Bends for Model Airplane Pushrods
Ball Links for Model Aircraft

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