# Gear ratio

Discussion in 'Off-Topic' started by Dritech, May 1, 2014.

1. ### Dritech Thread Starter Well-Known Member

Sep 21, 2011
756
5
Hi all,

Since this is an electronics forum, I decided to post this thread in this section. So, here is my problem. I have a motor connected to a drive gear marked M1 Z30 08-918. A driven gear (M1 Z40 08-512) is connected to the drive gear.
Now I need to calculate the number of teeth for a third gear which will be connected to the drive gear (M1 Z30 08-918). I want this new gear to turn 2.5 times with every single turn (360degrees) the driven gear (M1 Z40 08-512) makes.

Can someone please guide me on how to calculate this?

2. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
Count the number of teeth. It is what determines the turn ratio, assuming they mesh properly.

3. ### Dritech Thread Starter Well-Known Member

Sep 21, 2011
756
5
Hi,

So that mean I need to use a gear with 12 tooth ?
Also, what shell I look for to find a gear which mash with the drive gear?

Jul 18, 2013
10,836
2,510
The ratio is in directly proportionately to the gear teeth ratio.
In other words if you have a 12 tooth gear and you want the driven gear to go 2.5 times faster then it would be 12x2.5 = 30 teeth (gear up).
Keep in mind that the resultant torque is directly proportional to the ratio, so if you geared up by 2.5 the torque would be reduced by 2.5.
Gear down and it would be increased by 2.5.
Max.

Dritech likes this.
5. ### Alec_t AAC Fanatic!

Sep 17, 2013
5,971
1,135
Er, won't 30 teeth gear it down?

Apr 5, 2008
15,799
2,385

Jul 18, 2013
10,836
2,510
Momentary lapse.
The torque ratio is correct.
Max.

Last edited: May 1, 2014
8. ### shortbus AAC Fanatic!

Sep 30, 2009
4,092
1,674
But in a three gear train, the first and last gear are all that matters for the ratio. The second/middle gear is just along for the ride, an idler. And it does make the first and third turn the same direction.

GopherT and inwo like this.
9. ### inwo Well-Known Member

Nov 7, 2013
2,435
315
I never thought of that!

Works for more gears too.

Jul 18, 2013
10,836
2,510
The difference comes when the intermediate shaft carries two gears of different sizes for compact reduction/increase.
Max.

11. ### Brownout Well-Known Member

Jan 10, 2012
2,375
998
Since gears which mesh have the same pitch, and number of teeth are directly proportional to pitch and gear diameter, then you can use diameter ratios to calculate gear ratios. Hence, the gear you want will have a diameter of 1/2.5 = .4 of the driving one.

12. ### atferrari AAC Fanatic!

Jan 6, 2004
2,663
784
I learnt it the hard way! Assembled two equal gears at the ends with a different one in between. I was surprised to see that I was getting the same speed which was NOT what I wanted!

At least, now I know.

Jul 18, 2013
10,836
2,510
There are four diametric circles used in gears, the most common is the pitch diameter or Diametric Pitch.
Not all gears are rotary, as in a rack and pinion where the diametric pitch of the pinion is used to calculate the travel per revolution.
Max.

14. ### shortbus AAC Fanatic!

Sep 30, 2009
4,092
1,674
Yes, as long as they aren't 'compounded'. By that I mean two different gears physically linked together on a single shaft. On a simple train of gears only the first and last have to do with ratio. An even number of gears will reverse rotation. An odd number of gears will keep the same rotation.

15. ### Dritech Thread Starter Well-Known Member

Sep 21, 2011
756
5
Hi,
Thanks for the replies. Does 8 tooth 0.8 module gears exist please?

Last edited: May 3, 2014

Jul 18, 2013
10,836
2,510
There are two main differences in practice when matching gears is the pressure angle, those being either 20° or 14.5°, if 0.8 module are 20°?
Try SDP/SI Spindle drive products or Misumi.
Max.

17. ### Dritech Thread Starter Well-Known Member

Sep 21, 2011
756
5
To be honest I don't know the angle, but on the gear I have it say 0.8.

I found a 0.8 module 8 tooth gear on this site:

Is this a mistake or does such gears exist? I'm asking because this the the only 0.8module gear I managed to find with 8 tooth.

Jul 18, 2013
10,836
2,510
I believe it will be 20° for module 0.8.
Try these, I have used their miniature gears before www.wmberg.com.
Not sure if they have metric gears however.
Is there any chance of changing the gears you have for a more common size?
Max.

19. ### markdem Member

Jul 31, 2013
98
48
Here is a great program I use when working with gears.. Might help someone.
Download the demo and you can still use it for calculations. The demo will not allow printing or output to gcode or .stl.
http://www.gearotic.com/

Have Fun

20. ### MrChips Moderator

Oct 2, 2009
12,625
3,451
In electronics terms, think of the middle gear as a unity-gain inverting amplifier.