# gearing question

#### bruce linde

Staff member
NAWCC Member
i have a larger gear with 61 teeth that makes a full revolution 1x per hour... counter-clockwise, looking from the front of the movement.

i have a pinion with 15 teeth mounted on the shaft of a 4 RPH synchronization motor that needs to be mounted on the back of the movement because of space considerations... which would drive the gear the wrong way.

the question is: if i use another pinion as an idler gear to reverse the effective direction of the motor.... does it matter whether the idler gear has 15 or 12 teeth? or do idler gears change direction without having to worry about teeth counts?

hope that makes sense... thx!

#### D.th.munroe

##### Registered User
Hi Bruce
Yep, it doesn't matter, idler gears just change direction without having to worry about the tooth count.
Dan.

Staff member
NAWCC Member
great, thx

#### Dick Feldman

##### Registered User
A 15 tooth pinion driving a 12 tooth idler will result in the Idler gear rotating 120% the speed of the pinion. That extra 20% will drive the third gear in the system as well as the rest of the entire train at 120%. If you run a 15 tooth pinion against a 15 tooth idler, the speed of the train will be maintained at 100% throughout, but in the opposite direction.
D

#### D.th.munroe

##### Registered User
Guys, you also have to change the 12-60 ratio which cancels out the 15-12 making it end up 100%

#### bruce linde

Staff member
NAWCC Member
so:

a 15 tooth pinion attached to a motor whose shaft is turning 4 RPH will cause a 12 tooth idler gear to go around 5 RPH.... which balances things out.

yes?

Yes

#### bruce linde

Staff member
NAWCC Member
awesome... i may have actually just learned something!

thx.

D.th.munroe

#### shutterbug

##### Moderator
Staff member
NAWCC Member
This is not making sense. If the motor turns at the proper speed, but just in the wrong direction the gear ratio has to be 1/1. Otherwise you are changing the speed of rotation as well as direction.

#### D.th.munroe

##### Registered User
Being just a gear it's ratio is 1:1 if it was a wheel and pinion then it would change speeds.
Very simple demonstration not mine
Dan

#### shutterbug

##### Moderator
Staff member
NAWCC Member
That one is a single pinion gear. Bruce is talking about two

#### bruce linde

Staff member
NAWCC Member
That one is a single pinion gear. Bruce is talking about two
no... there is a pinion attached to the motor shaft, the idler gear/pinion in the middle, and the larger gear. the idler is there only to change direction.

61 (CW) - 12 (CCW) - 15 (4 RPH CW MOTOR SHAFT)

i’m using the idler gear because i was unable to find a CCW motor

#### shutterbug

##### Moderator
Staff member
NAWCC Member
So in order for it to do what you want, I think you need a gear the same size and number of teeth as the pinion in the motor. Alternately, the same size and number of teeth as the other gear. Otherwise, in my mind at least, you are altering the speed of the larger gear. You can experiment and find out, but let us know. My curiosity is piqued now.

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#### Jerry Kieffer

##### Registered User
NAWCC Member
Bruce
As Shutterbug mentioned per attached sketch.

Or turn the motor over.

Jerry Kieffer

#### D.th.munroe

##### Registered User
You could put 100 gears between the first wheel and the last, with multiple different gear counts between as long as they they are gears 1:1 and not wheel pinion combinations, the only ratio that will matter is that of the first and last.
Dan

#### shutterbug

##### Moderator
Staff member
NAWCC Member
Dan - not to be argumentative here - but if that were true there would be no need of different combinations of gears in the time train to get the proper count at the escape wheel. If the number of teeth between any two wheels is not equal, there will be a change in speed. It can't be anything else. In your example above, the two end gears are turning in the same direction, but not at the same speed that they would if the center gear were larger or smaller than it is .... if the smaller gear is supplying the power.

#### D.th.munroe

##### Registered User
Well thats where the wheel pinion combinations come in, the wheels and pinions on the same shaft ave different counts making them no longer a 1:1 gear.

gmorse

#### ToddT

##### Registered User
NAWCC Member
Donor
Think of it this way. For every tooth that the motor increments the idle gear, the idle gear will increment the driven gear by one tooth. So it doesn't matter if the idle gear is 12 teeth or 100 teeth.

D.th.munroe

#### Jerry Kieffer

##### Registered User
NAWCC Member
Bruce
As Shutterbug mentioned per attached sketch.

Or turn the motor over.

Jerry Kieffer

View attachment 631785
Based on current discussion, my suggestion in post #14 should be explained.

My first thought was not the series gear function, but should have after many years of construction projects where it was utilized. Instead I was only thinking of a practical solution late at night.

It is of course true that when various Gears placed in series with various tooth counts, where one tooth space is moved at the beginning, will result in one tooth space moved at the end.

However in Bruces case, on technical level, we are most likely dealing with real life rather than just a mathematical calculation.

Prototypes for production are often perfected through trial and error where things are often done for a specific reason. These include strength, wear and friction. For example, in this case where the motor may run constantly, wear may be a major concern. As such, it is possible the idle gear may be fiber with the other two brass and or steel. If so, in order to maintain friction and wear characteristics its always best to maintain consistency in size and materials. In this case, if I were adding a reverse idler, the idler would be made from two materials duplicating the original condition.
Another words the top half would be driven against a fiber gear and the bottom half would drive the original fiber gear of the same material as the motor pinion. Thus the specific recommendation, but again without all of the details.

Jerry Kieffer

#### shutterbug

##### Moderator
Staff member
NAWCC Member
I believe the problem in understanding here is that the idler gear in Bruce's case is not a neutral idler gear. In Dan's video, the man was supplying power to a large wheel, and the gear between is a neutral idler. So even if the middle gear spins a hundred times for every turn of the large wheel, the other large wheel will remain at the same speed as the first. So the idler gear is neutral, and has no function other than reversing direction.
In Bruce's situation, the idler gear has two functions. I has to be a direction changer, AND it has to turn at a certain rate, equal to the motor speed. So unless it is exactly the same size and tooth count as the motor, it will not impart the proper ratio to the receiving wheel. It would be the same as changing the size of the gear on the motor.
In other words, if the center gear in the video was ALSO turning another wheel, the size of the wheel now is important, and any changes to it's size will be imparted to that fourth gear at a different speed.
For instance, lets look at Todds example. What he said is basically true, but is also wrong. Lets say the motor turns at one RPM. For every tooth, that the motor increments, there is a corresponding tooth turned on the mating wheel. So lets say the motor gear has only one tooth. The increment is one tooth per minute. But if it has two teeth, the increment is two teeth per minute - twice as many, and the second wheel turns twice as fast as it did with one tooth. If that second wheel is the idler wheel, and it has four teeth while the motor gear has only one .... well you can see the problem. Every thing is moving pretty slow.

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#### ToddT

##### Registered User
NAWCC Member
Donor
Let's recap. Bruce has a 15-tooth gear rotating at 4RPH, or 60 teeth per hour. He needs to drive a gear with 60 teeth that is supposed to rotate 1RPH. 15 @ 4 will engage with 60 to drive 60 @ 1. No problem there.

Bruce asks if he could use a 15 or 12 tooth pinion as an idler gear.

If he uses a 15, we have 15 @ 4 --> 15 @ 4 --> 60 @ 1. No problem there.

If he uses a 12, we have 15 @ 4 --> 12 @ 5 --> 60 @ 1. Yes, the idler gear is now moving at 5RPH, but with a lower tooth count. Either way, on the "out" side of the idler gear, the gear is progressing at 60 teeth per hour.

Going the other way (just for fun), if the idler gear had 120 teeth, the idler would rotate at 0.5RPH but it's output would still be 60 teeth per hour.

How fast the idler gear turns is immaterial as long as the correct number of teeth per hour are driving the final gear.

Shutterbug, I think what you were saying above, "if the center gear in the video was ALSO turning another wheel..." is if the idler gear was on the same arbor as another wheel that is driving other elements. In that situation, I completely agree with you. In that case, the rotation speed of the idler gear is important because it is setting the rotation speed of other wheels. But absent that, the rotation speed wouldn't matter.

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SuffolkM

#### shutterbug

##### Moderator
Staff member
NAWCC Member
Dang it Todd! You're making my head hurt, and you're starting to make sense. Let me think about it a bit.
Maybe you and Dan are correct....a tooth is a tooth. Hmmmm

Solga

#### shutterbug

##### Moderator
Staff member
NAWCC Member
That's certainly a lot of info, Teaclocks!

#### howtorepairpendulumclocks

##### Registered User
Maybe the case is closed now so sorry for chipping-in if it makes things even more confusing... )

I think any confusion here was between IDLER wheel and INTERMEDIATE wheel. The vast majority of clocks have intermediate wheels which normally comprise a wheel and a pinion sharing a common arbor. These DO change the overall gearing ratio. I think what the original enquiry was about was an idler gear simply to reverse the direction of rotation. In this case, it doesn't matter how many teeth are on it. The first person to respond had it right. If it is still tricky to understand, think of the idler wheel as a see-saw. As long as the fulcrum of the see-saw is exactly in the middle (as it is with wheels), when you push down on one end, the other end moves up by exactly the same amount. In example 2 on my sketch below, the value of idler wheel x is pretty much immeterial. In example 3, if the values of x and y on the intermediate wheel are the same, it is effectively a very long wheel and the same applies. If x and y are different, there is a change in the ratio. Hope this helps. Great thread BTW :+)

#### howtorepairpendulumclocks

##### Registered User
PS on the sketch, I put my '4' in the wrong column of course, it should b under the '0', not the '6'!!!

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