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andy_b
08-23-2010, 06:31 PM
On one of my many projects (sticking a small engine into an old bicycle frame), I need to build a gear reduction, as well as reversing the direction of rotation. On some of the bike motors the guys just use a belt and two pulleys for the reduction, but due to the orientation of the motor I want to use, the rotation of the motor output needs to be reversed. I was thinking of using two gears to do this (I am open to other ideas though).

Of course the nice thing about a belt is it doesn't need any lubrication (other than the bearings on the jackshaft, which is simple enough). For the gears, I would guess they won't last very long without some sort of gear lube on them. While I could probably just do a "total loss" system where gear lube drips from a tank onto the gears and then just flies off onto the ground, I'd prefer to be slightly more environmentally conscious (well, not really, but I'm cheap and don't want to have to be refilling a gear lube tank with oil all the time). So, how much oil is needed to keep the gears happy? If the big gear (the driven one) sets lower than the small gear on the motor crankshaft, how high does the oil level need to be? Do the gear teeth just need to barely touch it, or should the teeth be submerged some mount in it? Also, assuming this gear is turning at 2200 RPM and is 5" in diameter, a certain amount of oil will be flying around inside the case instead of resting in the bottom, so I would guess that needs to be taken into account somehow.

Trial and error may be the best method to go about this, and I would certainly start with what I think would be too much oil, instead of too little, but I have no idea what would even be a starting point. I have never had a gear reduction unit apart to see how much oil is in one of them.

andy b.

J. Randall
08-23-2010, 11:49 PM
Andy, turn your engine half around where the shaft exits the other side. Mount a short jackshaft behind it and input your power on the same side to what ever size sprocket or pulley you need. Now go the the other end of your jackshaft and pull your power for the rear wheel from it, if I am thinking right you should have reversed directions, and you can get your gear reduction by sizing your pulleys or sprockets on each end of the shaft.
James

andy_b
08-26-2010, 10:12 PM
That won't work. The engine head has to face forward (for cooling, exhaust, and space reasons), and the belt I need to drive will be on the opposite side of the engine. My plan is to use a jackshaft to get the power from the right side to the left side, but as I mentioned, the direction needs to be reversed. There are some engines that rotate in the correct direction, but they aren't the type I need for my project.

It appears the old "trial and error" method will be my initial option. I'm not really worried about too little oil (I'll start with more than I think I would ever need), but I'm guessing too much will cause some unnecessary friction losses. If I had 10HP to play with it wouldn't be bad, but dealing with only about 2HP means I have to try and be as efficient as possible.

andy b.

ADGO_Racing
08-26-2010, 11:23 PM
Most gear boxes use a level of about 50% full, maybe a little less 33%. Rear axles, transmissions, etc. You want to get oil to the bearings. More oil than that won't hurt anything, except it will likely push what it doesn't want, out the breather. This happens in race cars almost weekly, someone will overfill something. Don't forget, if building your own gearbox you will need a breather, or else the case will pressurize as it heats up in use. It will then find the weak spot to ooze from, and inevitably this will be a spot that you would rather it not ooze from.

More oil will kill the efficiency some. I would say bring it up the bottom of the highest shaft. Anything more isn't going to be beneficial.

OKChipmaker
08-27-2010, 01:35 AM
Use a belt and twist the belt into a figer 8 to reverse the rotation.A flat belt might work better for this.Remember bach in the line shaft days this was used a lot.

RobbieKnobbie
08-27-2010, 08:10 AM
That's pretty fast to be spinning those gears. I'd definitely use an oil with an anti foaming additive, and include a breather at the top of the gearbox or the pressure generated inside will blow out your seals.

As for oil level, too much and you'll just be creating more heat by churning oil around. Not very efficient. I'd set the running oil lever a little higher than the root of the bottom most tooth. When you fill it, spin the gearbox a few times and recheck the level so you can see how much oil is pulled from the sump and sticks to the walls.gears/etc. and re-fill to the correct level.

andy_b
08-27-2010, 08:11 AM
ADGO,

Thanks for the info. And even more thanks for the breather tip! I didn't even think of that.



Use a belt and twist the belt into a figer 8 to reverse the rotation.A flat belt might work better for this.Remember bach in the line shaft days this was used a lot.

I wish I had room for the figure-8 twist, as I was thinking of a way to do it. I can't do it on the drive belt (think old 1900s Harley and the belt that went to the large pulley on the rear wheel), and I don't think I have enough room down by the engine. I probably can get 12" of space, but I don't know if a modern Vee belt can be twisted in that short of a distance without the belt just rubbing itself to death in very short order. If I went with a 2" pulley on the motor crankshaft, and a 4" driven pulley, and put the shafts 8" apart, it would be a little under 12" long. Is that enough distance to do a twist and not have belt failure in 1000 miles? Yes, 1000 miles isn't that far, but this is for a simple motorized bike and it isn't like I will be riding it 50 miles per day. It is just for fun. If a belt only last me through the summer, what do I care.

Now if I build a gearbox using metal gears, I would hope the gearbox would last as long as the engine lasts.

andy b.

winchman
08-27-2010, 08:25 AM
When I was an early teenager, I put a vertical shaft lawnmower engine on the back of my bicycle. It was mounted on a swing arm that let me pull it tightly up against the side of the wheel. I had added an extra sidewall from an old tire as a friction surface. The reduction ratio was set by the diameter of the wooden drive wheel I put of the shaft. It worked pretty well.

I only got to use it in the cold months, when I wasn't earning money with the mower.

kendall
08-27-2010, 10:37 AM
use a serpentine belt, multi-V type. It's plenty forgiving of bends and angles.

Personally, I'd look into the planetary gears from an old automatic transmission, you can get reduction and reversal in a nice compact unit.

914Wilhelm
08-27-2010, 10:50 AM
When I was an early teenager, I put a vertical shaft lawnmower engine on the back of my bicycle. It was mounted on a swing arm that let me pull it tightly up against the side of the wheel. I had added an extra sidewall from an old tire as a friction surface. The reduction ratio was set by the diameter of the wooden drive wheel I put of the shaft. It worked pretty well.

By using a shaft as a friction drive against the wheel you are essentially creating a gear (with a bonus high reduction) without having to worry about oil cases and mess. If you are worried about the friction drive being too slippery then it's time to practice your knurling.

philbur
08-27-2010, 08:27 PM
The two reduction gearboxes in my lathe have the smaller gears running in the oil. I quess this reduces the amount of unecessary thrashing around. The lower gear sits only about 1/4 of the gear diameter in the oil. This small gear then feeds oil to the larger gear in a controlled manner and then the larger gear flings this controlled quantity of oil around the casing in order to oil the bearings etc. No antifoam or other additives just good quality machinery oil is the recommended lubricant, no breather either.

RPMs of the large gear in each box is approx. 2100 rpm and 1400 rpm.

Just a thought
Phil

J. Randall
08-27-2010, 10:12 PM
Depending on the complexity of the engine as to whether it is worth the trouble , a 2 stroke will run in either direction, it is just a matter of changing the ignition and cranking it the other way. I modified a little Tecumseh 2 stroke to run the other way for a mini bike yrs. ago.
James