Gear ratio

Thread Starter

Dritech

Joined Sep 21, 2011
901
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?

Thanks in advance.
 

MaxHeadRoom

Joined Jul 18, 2013
28,619
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.
 

shortbus

Joined Sep 30, 2009
10,045
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.
 

inwo

Joined Nov 7, 2013
2,419
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.
:eek: I never thought of that!

Works for more gears too.:confused:
 

Brownout

Joined Jan 10, 2012
2,390
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.
 

atferrari

Joined Jan 6, 2004
4,764
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.
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! :mad:

Had I do the maths...

At least, now I know. :( :p
 

MaxHeadRoom

Joined Jul 18, 2013
28,619
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.
 

shortbus

Joined Sep 30, 2009
10,045
:eek: I never thought of that!

Works for more gears too.:confused:
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.
 

MaxHeadRoom

Joined Jul 18, 2013
28,619
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.
 

Thread Starter

Dritech

Joined Sep 21, 2011
901
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.
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:

http://www.smallparts.com.au/store/item/gs080080400b0300brn/gearsspurmodule08/

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.
 

MaxHeadRoom

Joined Jul 18, 2013
28,619
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.
 

MrChips

Joined Oct 2, 2009
30,712
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.
In electronics terms, think of the middle gear as a unity-gain inverting amplifier.
 
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