Gear Ratios

Good question. I don't know if you're using a chain to run this gear set, but if you are, keep in mind that while the efficiency of a chain (or belt) is pretty high, it decreases with tension, friction and drastic turning radii. Just off the back of my head, I'd point to the first one being more efficient because it's going in less drastic "steps" but the only way to actually measure that is through a torque or power measuring system at both input and output to compare your losses in both cases. In short, that will be an interesting project for you.

In bicycling, research has it that big gear ratios, such as a 53T to an 11T are not as efficient as a less drastic combination that gives you the same ratio. However, the differences may be tiny.

My "off-the-top" assessment is the 4:1+1:1 for the following reasons:

• More commonality of gears, shafts, and bearings; thus, potentially lower cost for building and maintenance (spare parts)
• instantly reduces torque to 25% of input torque

I prefer 2:1 +2:1. It can be done with off the shelf engine cam drive gears. Plus 2:1 seems to be a common number in other things as well.

Also if the second gearset has a drag of 1 inch oz it will only give a 2 inch oz drag on the input drive. If the 4:1 is driving the 1:1 that also has the same 1 inch oz of drag then the input drive will see a 4 inch oz increase in drag.

Or at least thats how it was explained and demonstrated to me in my mechanical drive classes many years ago when I got out of high school and went to college the first time for an industrial maintenance tech degree.

As I recall its typically best to put the highest gear reductions last being that the output from it carries the highest torque multiplication which if put first would require all gear sets after it to be able to handle the maximum torque forces that it could produce and not the maximum torque forces the power source could produce. Which in a high reduction gearing system could mean substantial force multiplication in some instances.

You're talking of drag and such, perhaps I would have said the same thing if I had known what the OP's design application is. He didn't mention that did he? Or am I wrong?

What's also not been said is the possibility of higher wear on the second gearing proposal, because of the drastic decrease in gearing in one step.

I'd also go for the two ones with smaller ratio. In my oppinion, it will share better the load along the transmission. Also, you can buy exactly the same parts to install and replace, so, you may experience less maintenance issues. If its a large scale project, even better, reduced inventory, and, depending on the design, reduces even the number of tools required to assemble and disassemble. If you decide to replace contact gears by a chain or a belt (that would be my choice), even better, because you could use the higher efficiency and the possibility to have the output shaft in the same direction in only one reducing step. Assuming there are belts or chains for your power requirements, of course.

The input shaft is a drill press. The output is a chuck for a wood router bit. I'm trying to design an overhead router attachment for a drill press. All lateral forces would be on the output shaft, not the drill press spindle. This is just an idea I came up with for personal use. In principal, it will work. I don't intend to go commercial with such an idea.

I have a Cincinnati milling machine capable of 30k rpm. At speeds upwards of 15k I cannot use an unbalanced drill chuck.... the force will deflect my shaft. Using collets corrects this. Unless the chuck is somehow permanently attached my suggestion is NOT to do it on a drill press. They are not designed for lateral force applications.

The drill chuck is on the DP which will turn at about 4K. The input shaft is in the chuck and a router collet is on the output shaft. The DP chuck won't see high RPM's.

Your overall ratio is 4:1, the size of the intermediate gear has no bearing on the output efficiency at all, it simply facilitates the change of direction... except for any loss in the bearing of the gear. If the bearing is not too flash, then a larger intermediate gear might be less affected by its bearing innefficiencies, if any.

Just a thought: if you need the output shaft to run in a specific direction (say clockwise), then you can simply put the engine facing up or down - this will change the rotation direction and will enable you to use only one gear set.

another option is to use this kind of product: http://www.motovario.com/pages/SB_HSeries.html

and set it up to increase speed (it's common practice with these gears) and use a double stage design if you want to keep original direction or a single stage and connect the motor and gear with timing belt and pullies (this will save space and the use of an expensive coupling).

as for your original question: both designs are OK - it's such a small ratio that it doesn't realy matter...

Hope this helps.

1- If you do it for your own usage then the working intensity and total number of cycles is quite reduced. You can thus use gears of average quality

2- It is not recommended to use ratios which are an exact number since then same teeth came to contact and the wear is not uniform over the whole gear

3- For simplicity of the box design it is better to have 2 stages ratio ≈2 since you get two identical stages and the output shaft is coaxial with the input one, which means that you have only one shaft distance to do with the precision required by the gears. More you have only 2 gears types.

4- The intermediary shaft is stable on the 2 bearings but input and output shafts have to get bearings according to the fact that they are loaded by radial forces. The best solution is to introduce a wall between the gears and support in it bearings for the 2 shafts. If not possible then the bearings have to be able to support be,ding moments without excessive wear. 2 bearings at a convenient distance will do it.

1

Instinctively 4:2:1 seemed a better design, since the step down, even in the gear boxes we manufacture are in steps rather than in a single step whenever multi gear drive chain is used.

I just did an approximate analysis (no data available, so I ran the same values of pressure angle, μ, etc, put random, but proportional, values for pitch, OD and other parameters.

I got around 1% efficiency improvement in 4:2:1 gear chain with respect to 4:1:1 (98% Vs 97%).

At other , worse values, the improvement will be more.

The economics is however different aspect, I am commenting only on mechanical aspect.

I would use a toothed belt with two pulleys, one 4 x larger than the other. It could be chains and sprockets as well.

Gears may wear and will also intoduce some play in the drive....

As you neglected to mention the sort of power that needs to be transmitted, its difficult to be anymore accurate!!!

Why not use an internal gear and smaller spur gear, then you only require an input and output shaft, and do away with the intermediate shaft?