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Written by Terry Dunn
Choosing the proper linkage for each situation
Feature
As seen in the May 2019 issue of Model Aviation.


No matter what type of models that you fly, the connections at the ends of your pushrods are critical pieces of hardware. A poor interface with a servo or control surface could lead to premature wear, inconsistent movement, a stalled servo, flutter, or a host of other unwanted gremlins. Success hinges on choosing a proper linkage for each situation and configuring it correctly.

In this article, I’ll analyze the pros and cons of a few popular control linkages that you might use on an average fixed-wing sport airplane.

z bend pliers are a super tool for creating tight
Z-bend pliers are a super tool for creating tight, compact Z-bends.

Universal Tips

No matter which type of linkage you choose for an application, there are some general things you can—and should—do to make sure that they work well.

One of the best things you can do is to ensure that the holes in your control horns and servo horns are appropriately sized for your linkages. An oversize hole creates lash that results in unwanted play in the control surface. An undersized hole might prevent the linkage from moving smoothly. It likely will frustrate your efforts to mate the linkage with the horn.

Unfortunately, there does not seem to be any standard sizing for these common interfaces. It is up to us to make sure that the horns and linkages are compatible. I suggest owning a pin vise and a good selection of mini drill bits. A caliper with precision to .001 inch/.001 mm is handy as well. With these tools, you can measure your linkages and enlarge (or create) holes to exactly the size you need.

Most linkages are designed for the pushrod movement to be perfectly perpendicular to the control surface hinge. There is more tolerance when using flexible pushrods than with rigid pushrods. In either case, using a skewed setup can cause binding and material fatigue.

Another concern with an off-axis pushrod is that the linkage will eventually ream out the hole in the control horn and create a sloppy connection. It pays to invest time in making your control setups as square as possible. In instances where right angles are not possible, choose a flexible pushrod or a linkage that can tolerate nonoptimal geometry.

Some control linkages, such as setscrew linkages and ball links, space the pushrod away from the control horn. In high-stress situations, this offset might cause the horn to twist. You want all of your servo power to go toward moving the control surface. A flexing control horn robs some of this power and gives sloppy control response. When using these types of linkages, make sure that your horns are strong enough to prevent any twisting.

01: z-bend

01: Z-Bend

The trusty Z-bend has been a staple of aeromodeling for decades. This type of linkage is strong and reliable. It is the very definition of rugged simplicity. The Z-bend is my preferred linkage type, and I use it whenever possible.

A Z-bend does not allow any adjustability in terms of pushrod length. For that reason, many modelers like to use a Z-bend on one end of a pushrod and some type of adjustable linkage on the opposite end.

I prefer to use a Z-bend on both ends of short pushrods, such as those commonly used for ailerons. The resulting stout and uncomplicated linkage is worth the extra effort required to precisely measure and bend the pushrod.

For most modelers, the biggest challenge with using a Z-bend is actually bending the shape into your pushrod. You want crisp, closely spaced 90° bends. I’ve never had much luck creating good Z-bends with normal pliers.

Luckily, there are tools designed specifically for this task. Great Planes and Hangar 9 offer Z-bend pliers that work perfectly on .062-inch diameter (2-56) pushrods and smaller. A Du-Bro E/Z Bender has swappable dies that allow you to put clean Z-bends in wire up to .093 inch (4-40). I recommend all of these tools.

02 90 bend

02: 90° Bend

A close cousin of the Z-bend is the 90° bend. As the name suggests, it is little more than a right angle bent into the end of the pushrod. An E/Z Link or a FasLink nylon keeper can be snapped into place to ensure that your pushrod stays engaged with the control horn.

The 90° bend offers most of the same advantages and limitations as the Z-bend. One notable convenience of the 90° bend over the Z-bend is that the pushrod can be removed without detaching the servo horn or control horn. You simply pop off the nylon keeper and the pushrod can then be detached from the horn.

As with the Z-bend, the hardest part about using a 90° bend is getting a clean, tight bend on your pushrod. Pliers will work if you’re careful. I always seem to have a curve in the long side when I use pliers. A better option is using a Du-Bro E/Z Bender. In addition to Z-bends, it can create compact 90° bends in pushrods.

Inspect the post, as well as the threads (or solder joint) where the clevis connects with the pushrod. Steel clevises might also deteriorate over time in certain climates. Replace any clevis that shows evidence of rust or corrosion.

Another area of potential loosening is the metal or nylon retainer that secures the linkage post to the servo horn. These retainers can slip over time and allow the linkage to become loose or even detach. Inspect them regularly.

03 nylon clevis

03: Nylon Clevis

If you want a linkage that allows some adjustability, a nylon clevis attached to a threaded pushrod is a lightweight and inexpensive option. Nylon clevises are available in a variety of sizes to fit many types of models. It is always a good idea to slide a small rubber band or heat-shrink tubing over the clevis to prevent it from opening unintentionally. Nylon clevises are generally robust, but they are prone to damage with rough handling. Be sure to inspect nylon clevises after a crash or any other kind of bump to a control surface. The weakest point is the small post that engages with the hole in the control horn.

Installing a nylon clevis onto a pushrod can be difficult. I’ve damaged my share of clevises by gripping them too tightly while trying to thread them onto a pushrod. My new method works much better. I hold the pushrod with pliers and grip the clevis with a Du-Bro E/Z Threader. This tool allows you to install a nylon clevis (or ball link) without overstressing the part.

a pin vise with a selection of small drill bits
A pin vise with a selection of small drill bits and a caliper are useful tools when optimizing the holes in control horns.

the author uses a small
The author uses a small, flat-blade screwdriver to carefully remove metal clevises from control horns.

most control linkages work best when the pushrod
Most control linkages work best when the pushrod moves at a right angle to the hinge line. A skewed pushrod, such as the one shown here, could cause binding or other problems.

04 metal clevis

04: Metal Clevis

Step up to steel clevises when you need something stronger than nylon. You can buy versions that screw onto threaded pushrods. Others are meant to be soldered to smooth pushrods. Both types engage with the servo horn the same way. As with nylon clevises, a rubber band or heat-shrink tubing over the installed clevis provides extra security.

Steel clevises have a strong grip. They can be difficult to disengage from servo horns and control horns. I typically use a small, flat-blade screwdriver to carefully pry the clevis arms apart and away from the horn.

Although much beefier than nylon clevises, steel clevises can be damaged in a crash or a hangar rash incident. Inspect the post as well as the threads (or solder joint) where the clevis connects with the pushrod. Steel clevises might also deteriorate over time in certain climates. Replace any clevis that shows evidence of rust or corrosion.

05 setscrew linkage

05: Setscrew Linkage

One of the most popular types of control linkages is also my least favorite: the setscrew. Some brand names include E/Z Connector and Screw-Lock Connector. These linkages are popular because they permit quick and easy adjustments to the pushrod length. You simply loosen the setscrew that anchors the pushrod to the linkage, make your adjustment, then retighten the setscrew.

The convenience of setscrew linkages comes with an increased risk of failure. That’s why I do not favor using them, but many modelers use these types of linkages without problems. When used correctly, the actual failure rate I have observed is low.

My biggest concern with setscrew connectors is that an improperly tightened setscrew can allow the pushrod to slip. I suggest using threadlocker on the setscrew and double-checking for adequate tightness after every adjustment.

the du bro ez threader makes it simple to install
The Du-Bro E/Z Threader makes it simple to install nylon clevises and ball links onto threaded pushrods

Another area of potential loosening is the metal or nylon retainer that secures the linkage post to the servo horn. These retainers can slip throughout time and allow the linkage to become loose or even detached. Inspect them regularly.

06 ball link

06: Ball Link

There are two types of ball links. Although each style is installed slightly differently, their operational attributes are essentially the same. A socket-type ball link consists of a nylon ball cup that is inserted onto a threaded pushrod or a threaded pushrod coupler. This cup snaps over a ball stud that is screwed onto the servo horn.

A swivel ball link has a brass ball embedded within a nylon socket, which is threaded onto the pushrod. A screw passes through the ball to connect the linkage to your servo horn. Both types of ball links provide a low-friction, slop-free connection with a wide range of motion. Ball links are ideal for those situations where it is impossible to have the pushrod movement perpendicular to the hinge line.

Several companies produce tools that are designed to install and remove ball cups from ball studs. These tools help reduce damage to the ball cup from repeated installation/removal cycles. Ball cups should be replaced as soon as they show signs of wear.

Final Thoughts

There are a lot of linkage choices when you set up the controls on a model airplane. Even if a kit or ARF includes control linkages, they might not be the best option. It’s a good idea to take a step back from time to time and evaluate all of the possibilities. You might find something that works better for you, or an improved way to configure your tried-and-true favorite linkage.

Sources:

Du-Bro

(800) 848-9411

www.dubro.com

Great Planes

(800) 338-4639

www.greatplanes.com

Hangar 9

(800) 338-4639

www.horizonhobby.com

4 comments

I use metal clevis's everywhere except the connection to the metal throttle link - don't want any RFI from the link rattling!

On using ball links... an excellent idea is to put a washer under the head of the bolt before putting it through the brass ball. This prevents the nylon link from coming off the brass ball insuring that the linkage system will not become unjointed.

Metal clevises threaded onto pushrods without a locknut are definitely subject to vibration fatigue.

The brass ball ball links can be prevented from disengaging by a washer under the bolt head.

Plastic clevises with metal pins are better than nylon clevises.

Excellent article and accompanying photos.

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