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Beginners Guide To Pushrods And Connectors

For those people who are new to the hobby, the simplest things can cause a great deal of confusion. I have recently noticed that a lot of newcomers are baffled by the assortment of pushrod connectors available. So in an attempt to fill this void, I’ve put together this little article in hopes of shedding some light on the subject.


Flexible nylon pushrods are popular for their light weight and easy installation. Threaded wire ends can be used with clevises and ball links (not shown). Smooth wire ends can be bent secured with bends or EZ-connectors.
PUSHROD TYPES

First, let’s talk about the pushrods themselves. A pushrod can be as simple as a single piece of wire, or as complicated as a titanium tube with carbon fiber tips, but as this is geared toward the novice, we’ll stick to the more basic types.


Single-Wire Pushrod:
While this is the simplest of all pushrods, the problem with using the single-wire pushrod is that it is limited in length. While they work well for short runs, like from a wing-mounted servo to an aileron, if they get too long, they can create a problem. A long, single wire will still pull well, but when it pushes, the resistance of airflow over the control surface can cause the pushrod to bend, resulting in a loss of effectiveness. This may not be noticeable on small foamies or micro-fliers, but the bigger the plane, the bigger the problem, so for longer runs, the pushrod must be stiffened in some manner.


Wooden Dowel:
A common way to do this is to replace the center section of a long pushrod with a hardwood dowel, arrow shaft, et cetera. Using a drill bit similar to the diameter of the wire, drill a hole about one inch from each end of a wooden dowel. Now put a 90° bend in the wire and glue it into the hole. Wrap the end with thread and add your choice of glue, or once the wire is glued to the dowel you could slide a piece of Heat-Shrink tubing to it.


Guide Tube:
While the wooden dowel type of pushrod works very well, it is limited to a straight run (with the exception of bending the wires on the ends). If you need a slight bend in the pushrod, you could use a long wire inside a guide tube to keep the wire from bending, but the guide tube needs to be secured at both ends and, depending on its length, secured at one or more places in between.


Flexible Nylon or Cable:
If your application requires more than a slight bend, you may want to use a flexible nylon pushrod. This is a nylon tube, usually with splines on the outside, which slides through an outer plastic tube (the splines help to prevent friction through the curved areas). The hole in the inner tubing is the perfect size to screw a 2-56 thread, or glue the non-threaded end of a 2-56 pushrod wire into. If even greater bends are necessary, a flexible cable (in a plastic tube) can be used. In all cases, the outer tubing must be secured to prevent movement.


Dowel and wire pushrods can be assembled by binding with heatshrink tubing or thread and glue. Grooving the dowel from the cross hole to the end will help keep the wire from twisting under load.
Top: nylon pushrod sheaths must be glued to the structure at both ends.Bottom: dowel pushrods should be supported against flexing where they pass through bulkheads.
Wire Sizes: 
You may find some very thin pushrod wire on smaller electric or indoor planes, but for many glow planes the standard wire size is for a 2-56 thread. On large scale planes or very advanced .40 – .60-size planes, it’s not uncommon to see people use heavier, 4-40 wire, but for the beginner, 2-56 is usually all you’ll ever need.


Attaching a wire pushrod to a servo arm can take many forms. On the left we have a conventional Z-Bend, the newer Snake Bend (lower left), and an L-bend (lower right) with its nylon retaining clip. Note the trimmed arm to make room for the nylon clip.
CONNECTORS 
Now that you have a good idea of what type of pushrod you’ll be using, we come to your next set of options: How to connect the pushrod to the servo and/or control surface. Again, here you have a myriad of options, so let us first break then down into two categories: Adjustable and Non-Adjustable.


Non-Adjustable Connectors:The most widely used, non-adjustable connections are the “ZBend,” the “L-Bend” and the metal, soldered-on clevis.


The Z-Bend is a double bend in the wire that allows the wire to go through the hole in the servo arm. Once the arm is attached to the servo, the wire cannot pull out. The only disadvantage to using a Z-Bend is that it must be attached to an easily removable object, such as a servo arm, or it will be permanently trapped in place. Visit Find.FlyRC.com/041102 to learn how to make your own Z-bend pliers.


The L-Bend is a single bend which is slid into a hole and secured with a nylon clip. Unlike the Z-Bend, with this method, you can easily disconnect the pushrod by removing the clip.


The Snake Bend is something of a hybrid between the Z- and L-bends. While it has a double bend like the Z-bend, it can be twisted out of its anchor and thus is not as permanent as the Z-bend, and does not require the nylon clip of an L-bend.


Bending these connectors can be done with a conventional pair of pliers, though Zbends are best made with a dedicated tool. You can buy Z-bend pliers at your local hobby shop, or make your own in a few minutes. To find out more about making your own, log onto find.flyrc.com/041102.


A metal, solder-on clevis also allows for an easy connect/disconnect, but it should be noted that with all three of these methods, that end of the pushrod will not be able to pull through a guide tube should you be using one – so plan accordingly!


Using a Kwick-Link on a straight wire end allows you to remove a pushrod later and to easily make mechanical adjustments. I use just a small drop of blue thread lock to keep the screw from vibrating loose.
Adjustable Connectors:

At least one end of your pushrod should have an adjustable connector. Let’s face it, even if you got the length perfect, you still might need to fine tune it later. While you can use the trim buttons on your transmitter for fine tuning, they should be used for minor, in-flight trim only. Likewise, if your transmitter has End Point Adjustment, it too should be used sparingly. Another advantage of adjustable connectors is that they can be removed in case you ever need to pull a pushrod out of its guide tube.


By far the most popular type of adjustable linkage is the steel or nylon screw-on clevis. Often referred to as a “Kwik-Link,” the clevis screws on to the end of your pushrod and can be shortened or lengthened by turning it one way or the other on the threads. The clevis should be screwed on by a minimum of 6 threads, but you will want to plan them so you have a fair amount of adjustment in either direction. For a little added insurance, both types of clevis can be secured with a short piece of fuel tubing to prevent the clevis from opening in flight.


If the screw-on clevis you are using is made of metal, a lock nut should be added. This is not, as many people believe, to keep the clevis from turning, it is to provide a snug fit between the clevis and pushrod. This brings us to another topic, which is: Metal-to-Metal contact. Having a loose, metal-to-metal contact, like a metal clevis attached to a metal throttle arm, can cause radio interference as the two metals vibrate against each other. While this type of interference is not usually a problem, metal-tometal contacts should be kept tight whenever they cannot be avoided.


Both steel and nylon clevises should be sleeved with silicone tubing to prevent accidental opening. Sullivan has spring guards for some of their clevises. Note the lock nuts for the steel clevises at left. Tightening them in place helps prevent metal-to-metal interference.
EZ-Connector:

The EZ-Connector is one of the simplest methods of connecting a pushrod, but it’s not without its downfalls. The most common complaint about them is that people have had them come loose. Personally, I have used them for many years without a single failure, but I do make sure to tighten them very well. Adding a drop of thread-locking compound is not a bad idea either. They work well on just about any throttle application, but I would limit their use to 90-size planes and smaller. EZ-Connectors are secured to the servo arm or control horn with either a plastic or metal holder. Either works well, but you should avoid re-using the plastic type.

Ball links come in two basic forms. One looks like a trailer hitch socket and ball with a threaded stud (above) while the second common form (below) has a through bolt to secure the ball in place. Again, a light touch of thread lock will keep the retaining nuts secure.
Ball Links:

The final type of linkage I want to cover is Ball Links. Ball links are an excellent way to connect your control surfaces. There are two types of ball links. The first is a nylon cup that screws onto the end of the pushrod and snaps onto a metal ball, which is screwed to the servo arm or control horn. About the only disadvantage to this type of link is that they can be difficult to snap on and off. The other type is a bit more complicated. It is a metal ball that is trapped inside a nylon shroud. The ball has a hole through it which allows it to be screwed to a servo arm or control horn. This is probably one of the most secure connections available, although it can not be removed and replaced as easily as something like a clevis. The only downfall to either of the ball methods is that they can not be used on a “swinging” control horn as they will not hold the horn in a straight line and cause the horn to pivot.

Using a ball link and threaded control horn is a big no-no. As you can see, it is possible for the pushrod to twist the horn on its threaded rod, effectively changing the length of the linkage, and your model’s trim. A clevis is preferred for this use.
CONCLUSIONI hope this article helps clear up some of the mysteries for the novice. As I said earlier, they are several other ways to make your connections, but you will find that these are by far the most popular and useful. Happy flying!


Links

DU-BRO www.dubro.com (800) 848-9411
Sullivan Products www.sullivanproducts.com (410) 732-3500