### Author Topic: Difficult gear ratio  (Read 1312 times)

0 Members and 1 Guest are viewing this topic.

#### NGuggemos

• Jr. Member
• Posts: 36
##### Difficult gear ratio
« on: January 31, 2012, 08:51:03 AM »
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

• Supreme Robot
• Posts: 4,672
##### Re: Difficult gear ratio
« Reply #1 on: January 31, 2012, 12:56:19 PM »
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).
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Engineering is based on numbers - not adjectives

#### charlie_ehler

• Beginner
• Posts: 5
##### Re: Difficult gear ratio
« Reply #2 on: January 31, 2012, 01:36:08 PM »
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

• Jr. Member
• Posts: 36
##### Re: Difficult gear ratio
« Reply #3 on: January 31, 2012, 09:06:17 PM »
Thanks for the help! I think I'm going to use the two center gears on separate shafts.
« Last Edit: January 31, 2012, 09:11:18 PM by NGuggemos »

#### charlie_ehler

• Beginner
• Posts: 5
##### Re: Difficult gear ratio
« Reply #4 on: January 31, 2012, 09:31:36 PM »
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.

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

Charlie

#### Soeren

• Supreme Robot
• Posts: 4,672
##### Re: Difficult gear ratio
« Reply #5 on: February 01, 2012, 03:04:18 PM »
Hi,

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