By Terry Dunn | terrydunn74@gmail.com | Photos by Terry Dunn
The servos that we use to control our RC models are amazingly resilient little gadgets. Many of us abuse our servos without mercy, yet failures are rare. Most of the breakages that happen can be traced to a crash or some other unplanned shock load that causes the servo’s internal gears to strip.
The good news is that stripped gears are not a death sentence for a servo. Most manufacturers sell replacement gear sets for a fraction of the cost of a new servo. Although some people are intimidated by the watch like collection of gears, the repair process is often quite simple. In this article, I will illustrate the basic steps for gear replacement with an E-flite DS76 (efliterc.com), a sub-micro digital servo.
Before I get started with the tutorial, I should point out that most servos have nylon gears. Some heavy-duty servos use stronger plastic, brass, or even titanium for the gears. If you find yourself stripping gears frequently in a particular application, you might want to upgrade to one of those heavy-duty types. You might even be able to find upgraded replacement gears for your existing servo.
Repairing a servo with stripped gears is a simple task that costs much less than a new servo.
Step 1: Assess the damage. It is usually quite obvious when a servo has one or more stripped gears. When you operate the servo, the rotational motion of the servo arm will be erratic. It will probably not sound good either. There are also times when the gears are okay, but the splined shaft that engages with the servo arm has become worn. If that is the problem, you can usually feel the splines slipping when you apply resistance against the servo arm. In either case, you need a new set of gears.
You will need a replacement gear set, as well as a high-quality screwdriver that properly fits the small screws.
Step 2: Gather parts and tools. Servos typically have small screws holding them together. Make sure that you have the proper screwdriver on hand for the job. Trying to get by with an oversize screwdriver will probably just lead to frustration and damaged screw heads. A #00 Phillips screwdriver is perfect for the DS76. I use a Wiha model 273 (wihatools.com). E-flite’s replacement gear set for the DS76 includes one of each of the servo’s five gears. I think it is a good idea to replace all of the gears, even if only one gear is broken. It’s better to be safe than sorry.
Step 3: Remove the servo arm. The first step in gaining access to the internal gears is to remove the servo arm. Even if the arm was not slipping on the splined shaft, this is a good opportunity to inspect the arm and the splines for wear. When in doubt, replace both parts. As you will soon see, the splined shaft is an integral part of the final gear.
Step 4: Disassemble the servo case. Loosen the four screws on the bottom of the servo case. This will allow you to remove the top and bottom caps of the case. For this job, you only need access to the top, so keep the bottom cap in place.
Removing four screws on the bottom of the servo allows access to the internal gears.
Step 5: Remove the gears. With the top cap removed, you have full access to the servo gears. Assess how the gears are arranged so that you can repeat it during reassembly. It helps to take a photo or two for reference. You might even want to mark each gear with a different color marker so that you can easily distinguish them. Carefully remove all of the gears. They should pull away easily. Note that the intermediate gears ride on small axles. These axles are loosely connected on some servos. Don’t lose them! The brass gear is attached directly to the servo motor. Leave it in place.
Step 6: Clean the servo. Now that the old gears and the new gears are accessible, you might want to mark the new gears with the same colors you used to mark the old gears. This will help you install the new gears in the correct order. Make sure that any debris from the damaged gears is removed from the top of the servo. Lingering gear fragments could foul the replacement gears. The white grease often used on servo gears can attract and hide severed gear teeth. I use a dental pick to poke around and make sure that the entire area is clean.
Step 7: Install new gear(s). Now it’s time to reinstall the gears on the servo. The color marks, along with the photos that you took during disassembly, should be the only references you need. If you somehow find yourself without adequate guides to put the gears in place, you can still get it back together. I start with the brass pinion gear and locate a gear with the proper diameter and gear pitch to mesh with it. There will probably only be one gear that fits that description. I then find the gear that meshes with the second gear, and so forth. Once you notice the subtle differences in each gear, it’s usually easy to determine where they should go.
Marking each gear with a different color will help you differentiate the gears while you work.
Step 8: Key the final gear. On the DS76 (as with most other servos), the intermediate gears spin freely on their axles, while the final gear is keyed to a rotating shaft. The shaft is connected to a potentiometer that tells the servo when to stop turning. It is vital that you engage this gear onto the shaft correctly. With the DS76, the upper end of the shaft has a flat spot. Likewise, the molded female pocket on the gear has a matching flat spot to mate with the shaft. Line everything up and push the gear into place.
It is a good idea to replace all of the gears, even if only one gear is broken.
Step 9: Apply servo gear grease. Make sure that the gears and axles are properly lubricated. In most instances, there is enough residual grease on the old gears and within the top cap to redistribute it onto the new gears. Then dental pick is again my tool of choice for this job. If you have to add grease, try Hitec Servo Gear Grease (part number 58450; hitecrcd.com). One tube should be all you need.
Step 10: Reassemble the servo case. Reposition the top cap over the gears and tighten the case screws. Make sure that you do not overtighten the screws and strip the case. If the top cap does not sit correctly, doublecheck to ensure that all of the gear axles are in place and the final gear is properly keyed to its shaft.
Step 11: Test the servo function. I like to verify that a repaired servo is working properly before putting it back in service. I connect it to a servo driver with a 4S NiCd/NiMH battery and run it through the full range of motion several times. I also give the servo arm a little resistance to make sure there is no skipping. If the servo passes that test, I put it back to use.
Test the servo after reassembly to ensure that it is working properly.
Go DIY. If you’ve previously balked at the idea of repairing servo gears because you thought it was a tough job, you are now officially out of excuses. In most instances, replacing gears requires only a few minutes of time. Your servo will then be as good as new. You’ll also see big savings compared with buying a whole new servo.
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