The servos that we use to control our RC vehicles are amazingly resilient little gadgets. Many of us abuse our servos without mercy, yet failures are rare. Most of the breakages that do happen can be traced to a particularly hard 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. While some people are intimidated by the watch-like collection of gears, the repair process is often quite simple. In this article, I'll illustrate the basic steps for gear replacement with the Hitec HS-55, a very popular servo that is used in countless applications.
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 may want to upgrade to one of those heady-duty types. You may even be able to find upgraded replacement gears for your existing 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 probably will not sound very 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.
Step 2: Gather Parts & Tools
Servos typically have very small screws holding them together. Make sure that you have a proper screwdriver on hand for the job. Trying to get by with an oversized driver will probably just lead to frustration and damaged screw heads. A #00 Phillips screwdriver is perfect for the HS-55.
Hitec's replacement gear set for the HS-55 includes one of each of the servo's five nylon gears. You can replace them all or just the broken gears. I typically swap only the broken gears and save the others…which explains why the package shown here has a few extras.
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, we only need access to the top, so keep the bottom cap in place.
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 may 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 that are also easily removed. Don't lose them! The brass gear is attached directly to the servo motor. Leave it alone.
Step 6: Swap and Clean Gears
If you are replacing only the broken gear(s), lay all of the gears out and carefully inspect each one for damage. When you find a damaged gear, pull its replacement from the package. Make sure you pull the correct gear. The differences between some gears are very subtle. If you color-marked the gears during disassembly, transfer your marks to the respective new gears.
Make sure that any debris from the damaged gears is removed from the good gears. The white grease often used on servo gears can attract and then hide severed gear teeth. I use a dental pick to poke around and make sure that all of the undamaged gears are 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 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 be only one gear to fit that description. Then I 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.
Step 8: Key the Final Gear
On the HS-55 (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 HS-55, 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.
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. The dental pick is again my tool of choice for this job.
If you have to add grease, be very selective about the type of grease you use. Most people agree that any petroleum-based grease is bad news, so avoid those types. There is no clear consensus whether lithium, Teflon, or silicone grease is best for nylon-geared servos. You can avoid the debate and just go with Hitec's Servo Gear Grease. One tube should last forever.
Step 10: Reassemble 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 seat correctly, double check to ensure that all of the gear axles are in place and the final gear is properly keyed to its shaft.
Step 11: Test 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 4-cell Ni-Cad/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, it is put back to use.
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 will require only a few minutes of time. Your servo will then be as good as new. You'll also see big savings compared to buying a whole new servo.
Terry is a freelance writer living in Lubbock, Texas. Visit his website at TerryDunn.org and follow him on Twitter and Facebook. You can also hear Terry talk about RC hobbies as one of the hosts of the RC Roundtable podcast.