Difficult gear ratio

[NGuggemos]

Hi, I'm taking an Advanced Competitive Science class, and we are asked to build gear trains with certain gear ratios using Lego gears.
I am trying to make a 4.25:1 gear ratio with four gears only.

The different types of gears are the 8 tooth, the 16 tooth, 24 tooth, and the 40 tooth gear.

I've spent a long time trying to figure this one out, and I can't seem to figure this one out.
Maybe we have to use a different gear such as the worm gear or something?

Here is an example of a gear train made with two gears and a ratio of 5:1, a 40 tooth gear and an 8 tooth gear.

[Soeren]

Hi,

Hi, I'm taking an Advanced Competitive Science class, and we are asked to build gear trains with certain gear ratios using Lego gears.
I am trying to make a 4.25:1 gear ratio with four gears only.

The different types of gears are the 8 tooth, the 16 tooth, 24 tooth, and the 40 tooth gear.
Maybe we have to use a different gear such as the worm gear or something?

Other LEGO gears do exist, but
 40:16 = 2.5 and
 40:24 = 1.6666666..etc. which rounds to 1.7 with one decimal and
 2.5 * 1.7 = 4.25 (if the rounding is OK).

[charlie_ehler]

Hi!

If I am reading you correctly, you need to achieve a 4.25:1 reduction ratio with these four gears?
The two center gears become idler gears and do not change the gear ratio no matter which way they are installed.  With one idler gear the two gears will rotate in the same direction and with two they will rotate in the opposite direction. 
Knowing this, I don't see a possibility of achieving a 4.25:1 gear ratio with these gear sets as 8:16=2:1, 8:24=3:1, 8:40=5:1, 16:24=1.5:1, 16:40=2.5:1, and 24:40=1.6:1.
The only way you can change any of this is if the two center gears are fixed on one shaft and then you can increase or decrease the gear ratio based on the tooth differential for those gears occupying the same shaft.
So the question becomes; are you using the center two gears on separate shafts or fixed on the same shaft?

Charlie

[NGuggemos]

Thanks for the help! I think I'm going to use the two center gears on separate shafts.

[charlie_ehler]

NGuggemos,

OK.  I think you won't get your target gear ratio this way though.

On the other hand being a gear head, when I see 4.25:1 I think of RPM reduction.  Are you looking to reduce the final shaft rpm or increase it?  The reason I ask is Soeren's math is for an increase in final shaft RPM unless I'm reading it wrong.

This article on wikipedia may help if nothing else.

http://en.wikipedia.org/wiki/Gear_ratio

Charlie

[Soeren]

Hi,

The reason I ask is Soeren's math is for an increase in final shaft RPM unless I'm reading it wrong.

You're reading it wrong then.

The math on the difference is not attributed to any particular in- or decrease, it's strictly the difference/ratio.