GM5 13.5:1 Gear Motor Hack

The 1SORC Technologies GM5 (same as Solarbotics GM9) is a great motor for robot projects. It comes standard as a 143:1 gear reduction and the output shaft turns at approximately 78rpm @ 6v with a stall torque of 76oz*in. This is perfect for many applications, but there are times when speed counts more than torque. This hack will convert your standard GM5 (SB GM9) from a 143:1 gear train to a 13.5:1 gear train. This hack increases the speed to 826rpm, but reduces the torque to only 7.17oz*in.

The reduced torque is acceptable when it comes to some types of robots where speed is more critical. A good example of this is Mini-Sumo wrestling. Torque is less of a concern when all robots must be under 500gms and 10cm by 10cm. The best way to gain ground is by momentum and momentum is achieved from speed, not torque.
I have seen time and again, the robots that win are the fast ones, not the powerful ones.

This hack will take you through the process of disengaging one entire stage of gear reduction without destroying the gear box. In fact, you will still be able to put it back to the standard arrangement if so desired after this hack.

Tools needed:
Drill bit size #59
Small drill
Paper clip
Side dikes (heavy gauge wire snips)
Needle nose pliers
X-Acto knife
Phillips Screw driver
Optional:
Super glue

Supplies needed:
1/8th" copper tube (you can get this at most hobby supply stores)

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Step One - Remove motor strap & motor
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You can easily remove the motor strap with a pair of needle nose pliers by grabbing it just above the notch on the motor side and pulling the strap taught then slipping it over the notch.
Be careful not to break the notch that holds the strap!

Once you get it over the notch, the strap can be removed easily.

After the strap is removed, the motor will be free to pull out of its socket area.

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Step Two - Separate the two gearbox halves
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Remove the screws that hold the two halves together and separate the two halves as shown.

You can start the separation by sliding your thumbnail between the halves down in the motor socket area and then slide toward the other end. Be careful if you choose to use the X-Acto knife for this, because there are plastic guides that can easily get cut or break. These guides are needed to help hold the two halves together.
The shaft-less side should hold all the gears and the shaft side should be removed, be careful not to loose and parts!

You are now looking at the internal guts of the GM5 (SB GM9). Please take not of the gear arrangement at this time.

Gear stage one is called a crown gear (why? Because it looks like a crown) and it is driven by the pinion gear attached to the motor. Gear stage two is driven by the crown gear, this is the stage we will be removing. The stage two gear is hidden under the output gear in the photo above. Gear stage three is the blue colored gear which also contains a ratcheting clutch mechanism. This gear stage has two parts and can be separated. And finally, gear stage four is the output gear. There is a floating pin that holds the first and second stage gears in place and a fixed pin that holds the second and fourth stage gears in place. The fixed pin is press fit to the output gear and a retainer bearing, but allows the second stage gear to float freely.

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Step Three - Remove gear stages two and four
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We need to remove the press fit bearing in order to take stages two and four out of the gearbox. To do this, simply pull up on the output gear gently until the bearing falls out and the second and fourth gears come out.

It is not necessary to remove the first and third gear stages from the gearbox.

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Step Four - Replace or modify stage two gear
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Here's where you need to make a decision, do you want to be able to UN-modify this gear motor, or are you certain that this is what you want forever?
If you don't have a desire to go back, you can skip the next instructions on creating a spacer and move to the destruction of the stage two gear instead.

For those who want to have the option to go back, you will need that copper tube or something similar. First you will need to mark the tube at the length it needs to be cut as shown below.

The stage two gear is what's being replaced, so the spacer needs to be that length.
The best way to cut copper or plastic tube is with an X-Acto knife believe it or not. By simply rolling it back and forth on a table (not your mom's kitchen table!) with the blade of the X-Acto knife as shown below.

This will make the replacement spacer for the second stage gear that we are removing.

It wouldn't hurt to sand down any rough edges with some fine grit sandpaper. Slip it on the fixed pin with the output gear and set them aside for now.

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Alternate Step - Cutting the Stage two Gear
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Alternately, if you don't mind destroying the stage two gear (NOTE: YOU CANNOT UN-MODIFY IF YOU DO THIS!)
Take the stage two gear and a pair of side cutters and trim off the large diameter gears as shown below. If you do this you will not need the copper or plastic tubing, you can simply put the gear back on and now it won't engage with the first stage gear.

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Step Five - Pin the first and third stage gears together
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Now that you are done with the removal and replacement of the second stage gear, we can move on to pinning the first and third stages together. I have tried several methods including super glue and cutting notches, but this is by far the least destructive and most effective way to do this. If you do decide to go back, all you have to do is remove the pin and put the second stage gear back in place.

Notice here that the third stage gear has two parts, this is for the clutching machanism (kinda ratches if the motor stalls at high torque) We don't need to do anything to the clutch, because it will no longer have the torque required to cause the clutch to slip. So lets get started here, first remove the small blue gear from the larger blue gear as shown here.

Now you will need to get your drill bit and drill. I used a paper clip that had a diameter of 0.041", so I chose a drill bit size of #59 which allows for a snug fit. You don't want to use too large a diameter clip because you could take out too much material and cause the gear to be damaged.

Take a look at the small gear underneith the blue gear and try to line the drill bit up so that it will hit the small gear about half way between the outr edge and the center. Also, if you want, you can make two holes to give a little more strength, but it's not necessary.
You want to be careful not to go too deep, about half way through the first stage gear is plenty.
Now insert the end of your paper clip into the hole you drilled and mark it flush against the blue gear it will be cut, then remove everything from the gear box.

Cut the paper clip using a strong pair of side dikes (weaker dikes will be marred by the paper clip). Insert the cut piece into the hole and test to see if the clip sticks up any through the blue gear. If it does, you will need to recut. We want the clip to go through the blue gear, but not sticking up above the serface. Note here that my clip is exatly in the meatiest part of the white first stage gear, that's right were you want it!

This picture shows the clip sticking up too far, it has to be trimmed.

Now you can put the two gears back together and put it back in the casing as shown.

Notice that the clip is flush with the surface of the blue gear. Now you can put the second part of the third gear stage back in.

Make sure that this piece covers the pin so it can't work it's way out will you are using it.

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Step Six - Reassembly
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It's time to put this back together. Grab the assembly you made earlier and put it in as shown below.

Make sure you have good clearance, you don't want the new pieces to be too long as this will cause the output gear to rub and reduce the power of your motor or bind it completely.

Insert the retainer bearing to hold the output gear assembly in place as shown.

Now put the output side of the case back on to the rest of the assembly and check to make sure it all spins freely. You should feel no friction when you manually turn the output shaft.

Put the screws back in and check again.

For this last part, I have chosen to use one of the Solarbotics replacement motors to make this gearmotor a little better suited for my purpose. In the pictures below, I have used a 12v drop in replacement that you can special order from Solarbotics. You do not need this motor, but it IS a nicer motor.
Install the motor back into the cup socket, it may require that you gently turn the output shaft to get the pinion gear to line up with the crown gear.

Slip the retainer strap back on and pay attention to the 'D' shape. The motor wire tabs generally go towards the non-shafted side of the gearbox, but you can insert the motor either way if you want.

After you get one side on and have the 'D' lined up properly, you can stretch the plastic strap over the other notch. I have done it two ways, one way is to grab it with the needle nose pliers and pull it taught until it goes over the notch, another way is to use a small screwdriver as shown.

Be careful not to break the notch!

And there you have it, your very own modified speedy gear motor! This is fast even at 6v with the 12v motor. At 12v it really moves!