K'nex motors have a mechanism that makes attaching rods possible. This is a plastic part that is somewhat flexible and expands a little bit when you push the rod through the hole. I guess this mechanism also serves as a safety precaution that protects the motor by slipping the rod when it's too hard to turn the rod.
While this is nice it also causes the motor to under perform. Under heavy loads, even if the motor has enough power it doesn't turn the rod because of this mechanism. We can hear a clicking sound when this happens. (First video)
This was a great obstacle for my RC Tank - Altay.
There was another requirement for my RC Tank project, preparing the motors to be remote controlled. So I dismantled all the motors and made these two modifications.
Step 1: Getting the Motor Out
This is something that should be done out of the model. We will completely dismantle the motor so we take it out of the model first.
Step 2: Dismantle the Motor
There are 7 screws. Unscrew them all and open the motor. Be careful with the blue handle. It may get out of its place, put it back.
Step 3: Making the Motor RC Ready
First the easy part, making the motor RC ready. Cut some plastic as seen on pictures to make way for the power cable. When we have the power cable soldered it will be ready to be plugged to an ESC for remote control. AFAIK all k'nex motors are brushed motors, not the fancy brushless ones. So when you are picking an ESC it must be brushed motor ESC with the correct voltage parameters.
This is the HobbyKing X-Car 45A Brushed Car ESC which is brushed, 2 cell lipo (~7.4 V OK for 6v motor) with reverse capability. This is almost the only ESC I found on internet that can be used for this application.
There are two options about where to solder the cable:
1- Directly to the motor.
2- To the battery department.
Soldering to the battery department has an advantage of not bypassing the forward reverse switch of motor. So after you've done the RC modification you still have a switch to reverse the motor rotation. So I soldered to the battery department. One more side note, if you have multiple motors to modify solder all red/black cables the same way so once you set all the motors to forward position from the switch they all turn the same direction.
To get to the battery department soldering points you must remove another plastic panel that is used to divide the battery compartment in to two.
I used XT60 connectors at the other end by the way.
Step 4: Heavy Duty Modification
You must take out the gear that holds to the rod. Separate all the parts of that gear set. Take a good look, take notes or take photos before you dismantle the parts, you will have to assemble it all the way back.
At the end of dismantling there will be a plastic part that holds the rod. This circular part is not full circle, one side is cut to give it flexibility. There is also a spring around it to keep it tight. The condition of this spring determines how easy the motor will slip and make the clicking sound.
We will hunt down this flexibility with this modification. But since the flexibility will be gone so will the possibility attach or detach rods. So we will put the circular part on the rod we are going to use (black rigid rods in my case) and make the modification with the rod already attach and we will never try to take this rod out again.
The core idea is to make this circular part so tight that it holds the rod and doesn't let it slip. Wrap the circular part with adhesive tape ~10 rounds. Cut the extra pieces so that it can still attach to the other parts. Put on the spring back. Assemble all the parts back. Take care of the rc cable and the blue switch mechanism while closing the motor. Blue switch is the mechanical speed selector that changes the position of middle gear.
Note: In the video and some pictures you may see some white band pieces around the circular part, this is from the previous modification which has worn out in time.
Step 5: Damaged Interlocking Clip
There is also another point where you can *leak* power : the Interlocking clip. If you have a motor in good condition - that doesn't slip - and a heavy load then your Interlocking clip may slip the rod and if this starts to happen it wears out the Interlocking clip and if it's worn out it will happen more. You can see the difference between two Interlocking clips one is in good condition, other is dead. Solutions is once again to wrap it with adhesive tape.
Step 6: The Result
The result is a happy builder and a tank that moves the way it should...
Altay ready to move
Altay K'nex RC Tank instructable