Introduction: How to Modify a Hitec HS-65HB Servo W/Kryptonite Gears for Continuous Rotation

Picture of How to Modify a Hitec HS-65HB Servo W/Kryptonite Gears for Continuous Rotation

Presenting the Hitec HS-65HB, one of the the best micro servo's available with Karbonite Gears. So what's so special about this servo? Well how about 31 ounces/inch of torque and 0.11 sec speeds at 6 volts in a compact 23.60 x 11.60 x 24.00mm footprint, Super Strong Karbonite Gears capable of cycles in excess of 300,000 with Zero Wear and almost five times the strength of nylon gears, Top Ball Bearing for smooth and quiet operation, versatile enough for small and large applications, and best of all it's extremely easy to modify for continuous rotation. This is one servo Futaba, GWS and JR can't touch. A micro monster with whopping torque/speed and if you can afford the $21.00 price then you can't go wrong for the quality/durability you get with this fine product. So why bother with extra torque in a compact unit and not just get a cheap servo instead? Well let me give you an example. Torque complements speed and let's say you have a mobile robotic platform using cheap micro servos for drive. As you begin to add weight (eg. batteries, sensors, controllers) your platform begins to suffer and the speed is heavily decreased, not to mention the excess strain on those weak nylon gears. Having that extra torque output and the Karbonite gears gives you an advantage, the boost your platform needs to counter the effects of weight and keep it moving. This servo gets it done and does a fine job in its modified continuous rotation form. So how easy is it to modify? So easy that you only need to touch one gear. That's Right! No extracting of PCB boards, replacing of potentiometers with resistor networks or even cutting a single wire. Don't even mess with the electronics and save your servos in case you want to return to standard servo performance (see step 7). Why the steps are so easy you could just follow the photos. But please do read for "reading is knowledge" and well worth it. With that said, let's begin....................

Step 1: Tools Required

Picture of Tools Required

To perform this modification, you will need these tools:

1 x Phillips Screwdriver
1 x Precision Screwdriver 1.0m/m
1 x Needle Nose Plier
1 x Wire Cutter
1 x Drill
1 x Drill Bit 1/16"
(Optional) Small Hand File OR Sandpaper

Step 2: Removing the Almighty "X" Horn

Picture of Removing the Almighty "X" Horn

Use a phillips screwdriver and remove the horn on your servo.

Step 3: Unscrew the Case and Lift Off the Top

Picture of Unscrew the Case and Lift Off the Top

Using a small precision screwdriver remove the 4 screws beneath the case. Do this slowly as the screws have a tendency to strip easily. Now carefully lift off the top section of the case while paying attention to gear orientation. All gears should come out of the top section. If not, then just pull out any gears that have not remained intact. Also, if you suffer from "very short term memory" this would be a good time to take a snapshot of the gear orientation using a digital camera so you can reassemble everything back without going crazy. Trust me, It actually gets worse with smaller servos.

Step 4: Pull Off Main Gear and Cut the Stop Tab

Picture of Pull Off Main Gear and Cut the Stop Tab

Now with the case off, pull off the main gear. It is actually push fitted onto the D shaft of the potentiometer. Just pull it off and cut the stop tab. You can use a file or sandpaper to shave down the tab until flush but not necessary if you cut low enough. You will also notice the second gear on the left side of this image will already be loose and that's because it's shaft is inside the top section of the servo you removed in the previous step.

Step 5: Hold Top of Main Gear and Drill Completely Through Using a 1/16" Bit

Picture of Hold Top of Main Gear and Drill Completely Through Using a 1/16" Bit

Now you will need to drill through the main gear with a 1/16" bit. If using a high RPM drill, say about 1000 RPM's, you can simply hold the gear in you hand, but start the drill before you enter the gear. For slower RPM drills, I recommend holding the main gear firmly on the top half section only, where the servo horn aligns using a needle nose plier. 'Do Not Apply Too Much Force As That Can Damage the Teeth!' Just enough to hold it stable and prevent any turning. Also, 'Never Hold The Bottom Of The Main Gear While Drilling' You do not want to risk damage to the drive of the geartrain.

Step 6: You Are Done! Now Reassemble Your Servo.........

Picture of You Are Done! Now Reassemble Your Servo.........

You Are Done! Follow the steps on the image and reassemble the servo in the order shown. So how easy was that?

Please Note: If you want the servo to make a complete stop you will need to glue the potentiometer. You can do this now before reassembling by removing the gear that sits right under the Main Gear on the potentiometers shaft. Next using the needle-nose pliers turn the shaft left-to-right until you find the center point. Now just apply a little hot glue for a (non-permanent) modification. You can then simply pry the glue off if you wish to revert to standard servo performance someday (See Step 8).

Step 7: You Now Have a Continuous Rotation Servo. So Give It a Test Spin

Picture of You Now Have a Continuous Rotation Servo. So Give It a Test Spin

You have now converted your servo into Continuous Rotation without damaging or tampering with any of the electronics or the cuitting of a single wire. It does not get better than this! Here is some sample Basic code to give it a test spin at various speeds in both directions. Commands may differ slighly for each servo.

Please Note: If you want the servo to make a complete stop you will need to glue the potentiometer. You can do this by removing the gear that sits right under the Main Gear on the potentiometers shaft (See Step 6). Just apply a little hot glue for a (non-permanent) modification. You can then simply pry the glue off if you wish to revert to standard servo performance someday (See Step 8).
The "Stop" command can be found through trial-and-error if you glued your potentiometer as mentioned in Step 6.

'Servo Pin, Speed/Direction'

Servo 0, 99 (Very Slow Left)
Servo 0, 103 (Very Slow Right)
Servo 0, 95 (Slow Left)
Servo 0,105 (Slow Right)

Servo 0,80 (Very Fast Left)
Servo 0,130 (Very Fast Right)
Servo 0, 90 (Fast Left)
Servo 0,115 (Fast Right)

Step 8: Purchase a Replacement Main Gear and Return Your Servo to Standard Performance

Picture of Purchase a Replacement Main Gear and Return Your Servo to Standard Performance

Now comes the most awesome benefit. The best of both worlds. Just purchase yourself a set of replacement gears and simply install a new main gear and you're right back to standard servo performance. Remember, the main gear is push fitted onto the "D" shaft of the potentiometer so there is no chance for misalignment. It can only be inserted the correct way. Just too good to be true right? Hope you enjoyed this conversion..............


ktrantham (author)2014-06-22

One question what program is the code for!!!! Seems pretty basic missing key functions! Other than that Great job

bertus52x11 (author)2011-02-10

Very nice Ible, but now I'm a bit confused (just started with electronics and servos). When read all the I'Bles here on Instructables, there are 3 ways to hack a servo. Please correct me when I'm wrong.

1. "Rip out" the electronics so that your servo works like an ordinary DC motor (no control through a Digital Pin).

2. Desoldering the potentiometer and replacing it by 2 resistors. In this way you retain control over the servo trough a Pin.

3. Your method described in the above I'ble seems the most elegant one. However, one small remark confused me in step 6 (about fully stopping the servo). Why does the potentio meter has to be glued? And will the servo fully stop?

Zeitron (author)bertus52x112011-02-19

Hello bertus52x11,

Sorry for my extremely late response. I need to update my email address associated with this instructable.

Yes, My method for modifying a servo is the best and does not require any permanant alteration of the electronics. You only need to touch one Main Gear!

You asked why glue the potentiometer to make the servo fully stop? First off if you don't glue the potentiometer the servo will come very close to stopped except there may be a slight vibration that results in creeping but it's hardly noticeable.

Now, by adding a little hot glue to the potentiometer, not super glue, remember nothing permanent., you will be able to dampen the vibrations on the potentiometer thereby causing the servo to remain stopped completely when you command it to.

I realized this when I removed the case and the Main Gear and tested the servo were vibrations were much less and the servo would remain stopped on command. When the main gear is on and running you introduce vibrations that cause the servo to creep extremely slowly but like I said it's hardly noticaeble. So just add a little hot glue around the potentiometer and you can further eliminate the vibrations.

Remember, you have nothing to lose by trying my instructable. If you don't like the performance you could just buy a replacement gear set on ebay for $3.00-$5.00 and just put a new Main Gear back on and you will have your old servo performance back instantly. The best of both worlds! So have a little faith and you will be rewarded. Take it easy and make it easy on yourself!

bertus52x11 (author)Zeitron2011-02-19

No problem and thanks for your reply. 
I have read your I'ble several times and then felt confident enough.
When I opened my servo, I got a different idea al together and decided to deviate from you instructable. I even turned it into an Instructable (and gave some credits to you by presenting it as a variation of your idea). My variation is irreversible though.

Check it out here

Zeitron (author)bertus52x112011-02-19

Hello bertus52x11,

After looking you up I see you have checked out almost every servo modification on here.

I have read your version which does do permanant damage which is opposite to what I recommend but you have the freedom post whatever you want.

The point is all servos will never modify exactly the same. Some will be easy while others will be more difficult. Some will have gears on top of the potentiometer so you can't cut out or destroy it even if you wanted to. I chose my particular servo because it was simply one of my favorite models. My same instructable will also work for many of the other Hitec models including the HS-300 and HS-400 series standard size servos.

I'm sure I could figure out how to modify any servo but since everybody has their own favorite servo to use, you would have to make instructables for every single servo to please everybody. We do a few just to teach the different techniques and people have to decide which technique they like best. I personally don't want people to destroy or permanently alter their servos if they don't have to. It's great to be able to return to standard perfomance someday down the road. My modification has worked perfectly for my applications. Another modification might work best for some other application. There are many roads to take and I see that this was very important to you since you discussed it everywhere.

Did you know that you can buy servo's already modified for continuous rotation if you need it fast and don't want to do it yourself? Stores like Parallax, Lynxmotion, SuperDroidRobots, etc. sell these ready to go for people who don't want to mess with servos or simply want it done right. The only drawback is the servos they sell are limited models so they most likey wont have the brand or model you want to use. Then you're left with no choice but to do it yourself. Also, most stores don't sell micro servos in continuous rotation form which is why I did a Micro Servo Instructable. They say modifying smaller servos are usually harder than large servos. Not for my particular model but some are very small and delicate so it's easy to lose parts or even forget how to put it back together.

Anyway bertus52x11, I wish you luck on your project or whatever you plan to do with your servos and advise that you choose the best speed/torque output to match your application. Like they say, it's better to be overpowered than underpowered. So choose wisely before you decide to modify. The world is full of great servos..............

bertus52x11 (author)Zeitron2011-02-19

Thanks for your advice!

Eonir (author)2009-12-07

Great instructable. I have only one question. Why is there a need to drill through the main gear in step 5?

Zeitron (author)Eonir2009-12-07

Hello Eonir,

The Main Gear has a "D" shaped bore or hole which locks onto the "D" shaped potentiometer shaft. By drilling through the Main Gear you turn the "D" shape into a full circle so now when gear is placed onto the "D" shaped potentiometer shaft it will not lock but spin freely around thereby allowing continuous rotation. Pretty Easy Right!

With my modification, anytime you wish to go back to standard servo performance all you have to do is buy a replacement gear set and install the Main Gear back onto the "D" shaped potentiometer shaft. So you get the best of both worlds with this modification.............



Eonir (author)Zeitron2009-12-07

Thanks! Now all's clear.

amclaussen (author)2009-10-22

Even when I won't use this servo modification, because I use servos for model airplanes and ships, it is very nice to have a detailed description of the virtues/shortcommings of a servo. I have found that Hitec produces very good servos in general, offering best price/performance ratio.  The only thing I regret is that they don't offer their Karbonite gears as a retrofit for their nylon gear servos!

pitty (author)2009-07-21

Great job on the Instructable. Nice Photos it's good to see someone take the time to put good photos in. Thanks