# Planetary Gear Box



## Captain Jerry (Aug 19, 2011)

Today I assembled a planetary gear box salvaged from a discarded portable drill. I didn't get any pictures of the process because there was no real plan, and lots of frustration and grief. I was only interested in the gears so the chuck and motor were just sawed off. The gearing in the drill used a double reduction to get the torque needed for a 3/8 drill from a high speed, low torque universal motor. The total reduction was about 25:1. The engine that I am going to use this with seems to get its maximum power output at around 600 rpm so I decided to use only one planetary gear set with a reduction of 5.25:1 so I should get an output shaft speed of around 150 rpm at full load.

I don't have any plans for a high load application yet but the challenge was fun. I'm not going to post any plans or dimensions because I doubt that all drills use the same gear specs. The project is really simple. Find some way to hold the internal ring gear. Find some way to support the input and output shafts and keep everything in line. To hold the ring gear (1.489" OD) I used an boring head in the mill to bore a close fitting blind hole in a block of aluminum. The ID of the ring gear at the tooth tip measured just a hair over 1.125" which was a real break because I have a supply of ball bearings with that dimension on the OD and a .500" bore. With the ring gear pressed into the bored cavity, the bearing is a tight fit in the gear and a little Locktite will keep it in place.

The planetary gear carrier shaft was less than .500" diameter but to close to make a bushing so I had to reduce it to .375" and the make a bushing with a shoulder to locate the carrier plate clear of the bearing face. The sun gear pinion had been mounted on the end of the motor armature but I was able to free it with some heat and persuasion and then turn a matching shoulder on a .250" steel shaft for the input. I had a pair of bearings with .250" bore and .750" OD so I turned a chunk of aluminum with a .750" bore to a 1" depth to hold the two bearings separated by a spacer. This piece was also bored out on one face to about 1.5" diameter to fit over the ring gear that protrudes about .250" from the base housing. A collar locates the shaft against the outer bearing and that is all there is to it.  Some cosmetic work on the lathe and mill and the results are shown below.

















I don't have screws long enough to clamp the two housing together so tomorrow will be a trip to town for some fasteners.

I believe that I read somewhere that small generators need to be spun up to get good output so with a 5.25:1 increase I should be able to turn up about 30,000 rpm. I have a scrap laser printer that should yield some motors that might work as a generator. 

More to come.

Jerry


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## agmachado (Aug 19, 2011)

Hi Captain,

Do you can tell me more about how build internal gears? 

Very cool your project... thanks for share with us...

Cheers,

Alexandre


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## Noitoen (Aug 20, 2011)

Alexandre Machado  said:
			
		

> Do you can tell me more about how build internal gears?
> 
> Alexandre



(Olá o carreto interno foi recuperado de um berbequim a pilhas :big: :big The gear was recovered from a drill.


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## steamer (Aug 20, 2011)

Hey Gerry,

You may have trouble with "Step Up" gear boxes ...generally they are not that efficient....ie they have enough friction to eat up a LOT of power....

What was the power output of your engine?....I'll see if I can dig the post up.

Dave


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## Captain Jerry (Aug 20, 2011)

Alexandre

All for the gears came from a battery powered drill. Most battery powered tools use planetary gearing to get the needed torque. Most plug in tools use simple spur gears. I could not hope to cut such a gear.

Dave

The engine develops about 2 inch pounds @ 950 rpm which works out to about 22 watts. I think you are right about the friction losses in a step up box. I think the biggest problem is on the output shaft where the torque is lowest. That's why I used 2 ball bearings on the output shaft. One is right at the pinion and the other one is about 1/2" away at the outer end of the support housing. The ring gear is used to keep the pinion centered. In the drill, the gear box is full of grease but I haven't decided on lubrication yet but I will probably use a light oil in the gearbox filled to about 1/8 volume.

Jerry


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## steamer (Aug 20, 2011)

OK

Last numbers I read was 2.36 in lbs @ 900 rpm

Power is torque x speed expressed in radians/sec

= 2 x pi x N x T

= 2 x pi x 900 x ( 2.36 /12in/ft) = 1112 ft lbs/min

= 1112/33000 = .033 hp = .033 x 747 watts/hp = 24 watts...

AT the shaft.....you might get 50% efficiency from a DC motor as a generator...if your careful

or about 12 watts........NOW subtract out the power to run that gearbox....you can see where I'm going with this...

Dave


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## Captain Jerry (Aug 20, 2011)

Dave

If I could light a 6 watt bulb, I would be happy.

How much power do you think the box will eat? Is a generator more efficient at 5,000 rpm instead of 980 rpm? 

I got it bolted up and running today. I hooked it up to the engine to check for alignment and to decide on how to mount it. It lined up pretty good so of course I had to see how it ran. It seemed to run fine with just my thumb to hold it down on the base. It did not seem to drag the engine down much if at all. with a small flywheel on the output shaft, I revved the engine up to about 1000 rpm. The flywheel was spinning at over 5000 rpm. I could have gone higher but the flywheel had never been run that fast and it seemed to be just slightly out of balance. I had never noticed that before. 

Before I try to run any load, It needs a firm mounting. Once I get it set, I can put the Prony brake on it and see what the output is. That will tell me what the actual loss is. There is some of course and it showed up as heat. After running for about 10 minutes intermittently over a half hour the gear box felt warm.

More to follow.

Jerry


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## steamer (Aug 20, 2011)

How much power do you think the box will eat? Is a generator more efficient at 5,000 rpm instead of 980 rpm? 


Power dissapation for a plain bearing goes up as the square of the rotational velocity....if you double the speed, you get 4 times the heat....triple the speed and you get 9 times the heat...ect.


Keep your lubrication as thin ( low viscosity) as possible ....hope for the best I guess.

Dave


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