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Join Date: Oct 2006
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# Gear Ratio Myth?

10/20/2006 8:38 AM

I am doing gear design and wonder is there any limitation or special consideration regarding the gear ratio? What is the largest gear ratio does anyone ever use? I am doing my design based on 1:9 or 17teeth to 153teeth would there be any problem? People tell me that the ratio should keep below 7 is that true?

There is also a saying that even gear should mate with odd gear say 19:108 or 20:141, is it true or just a myth.

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#1

### Re: Gear Ratio Myth?

10/20/2006 11:05 PM

if you take the size, pitch etc of tooth and the diameter of the smaller toothed wheel you will find there is a minimum number of complete teeth that can be mounted and have them all properly shaped. The larger wheel can be almost any size larger you care to machine all the way up to a flat toothed rail as used in a cog railway.

Obviously ratios of axle speed enter into it. So if you have 10000 teeth to 25 teeth you will have a 400:1 speed ratio, as you would find in a winch.

As to odd numbers. Two odd numbers are best. the reason for this is to prevent any recurring impact pattern from happening. If you had 25 teel and 100 teeth and ne of the 25 teeth had a burr, if would them do harm to 4 teeth in the big wheel again and again. If you recuced one to 99, then the wear would not repeat on one set of 4 teetl, but would gradually wear all the teeth uniformly.

Of course, if you want specific integral ratios, like 4:1, 8:1, you must use those ratios.

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#2
In reply to #1

### Re: Gear Ratio Myth?

10/20/2006 11:42 PM

As one who worked for so many years with A.G.M.A. grade 10 & 12 gears, the best method for gearing whose end result does not require extremely accurate indexing is the employment of the "hunting tooth" ratio. A bit special because the tooth count selection will allow the pinion to progressively index the gear and change the tooth:tooth mesh so that it is many turns before the same teeth re-engage. This also promotes more even wear across all of the gear and pinion teeth, and increases the useful life on the gearset. That is, of course barring a severely damaged tooth on either the pinion or the gear.

As far as design, the pinion should have the largest number of teeth possible in order to increase tooth strength by avoiding undercut below the pitchline. And, it should be made of a harder material since its teeth are doing the most "work per tooth".

Ing. Robert Forbus

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#10
In reply to #1

### Re: Gear Ratio Myth?

05/21/2008 12:40 PM

hello aurizon

i am also stuck at same sitiution Actually I want to open & close the lid of large cylinder with an electric motor

So the motor which I want to use are 16 & 17 RPM and the carrying load (mean weight of lid) are approximately 200kg,and the distence i want to covered is 2500mm.closing and opening time of lid should be 5--10sec.

Moreover the gear of the motor will be drive another gear. So I want to find out gear ratio that which type (Dia & no of teeth) of gear use for this application.

plz help me in this matter

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#3

### Re: Gear Ratio Myth?

10/21/2006 12:39 AM

Short answer is "it depends".

On a lot of factors you have not supplied us with. 1:9 is certainly not out of line for some applications, it is for others. Input and output torque, rpm, type of drive (petrol, electric, hydrostatic, hand crank), unusual axial radial or lateral loads, gear type, material, pitch, running environment, lube type, braking or reverse torques and probably a bunch of things I haven't mentioned.

That said, at a minimum telling us the application should get you into the ballpark. Now just to be clear, are you talking a speed up reduced torque application? 153t drive, 17t driven? If so you may be better off using a prime mover designed for higher RPM if available and choose a lower ratio. You need to limit drive torque to avoid stressing teeth in a traction application, but it would be fine for say, turning a light wheel.

If however I misunderstood your question and your application is a gear down of 9:1 17t drive, 153t driven then the situation is better. Look at it this way, the higher the ratio the closer it is to a rack and pinion. Good when very slow output speeds and/or high accuracy is needed. Also needed when input RPM must be high to maintaing prime mover efficiency as in gas jet turbine.

Re: mixing odd and even gears. Not a rule. This could possibly be a reference to hunting vs. non hunting gears by someone that does not fully comprehend the principle. Yours is an example of a hunting ratio. Meaning that each tooth always meshes with the SAME teeth on the other gear every revolution. It 'hunts' them succesfully each time. All integer gear ratios are hunters. These are useful in test mockups and situations where exact position repeatability is a concern, timing gears etc. Non hunting gears (I think there are other names for them which I can't recall) are the goal of mixing odd and and even tooth numbers. Each revolution a given tooth meshes with a DIFFERENT set of teeth on the other gear. This generally improves break in properties and tolerance of imperfections and damage. Sometimes resulting in a less damaging failure mode due to decreased harmonic vibration. Contrary to some belief mixing odd & even tooth count does not always result in a non hunting gear set. Take for example any even integer gear ratio such as 2:1, 4:1, 6:1 ie. 17x136 = 8:1 which is a hunting gear set.

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#9
In reply to #3

### Re: Gear Ratio Myth?

10/24/2006 9:36 PM

Please forgive my confusion. This is not my field but I always understood that a hunting gear combination was where the teeth did not fall in the same place each revolution but rather ran up and down the combinations in the way that an engine hunting on a governor revs up and down. The teeth hunt but never find, just as the engine hunts for a stable rpm.

Is my understanding of this concept upside down or what?

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#4

### Re: Gear Ratio Myth?

10/21/2006 1:52 AM

Anecdote. Answering this got me thinking of a farm truck I once had. It had cascaded gearboxes that allowed a super low ratio. I just estimated the lowest gear ratio at 10000:1 based on a weird thing we used to do with it. But if you wanted to pull a stump or something you had to pick a gear somewhere in the middle of the range and not use the cascade box or it would break. We would put it at one end of a field in low gear with the hand throttle about 3000rpm and go to lunch. When we get back an hour and a half later the truck is at the other end of the field.

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#5
In reply to #4

### Re: Gear Ratio Myth?

10/21/2006 6:55 AM

It is obvious that the use of prime numbers on the gearing will result in non hunting gear sets. This is easy to do and I expect most gear makers adhere to this principal unless precise rational ratios are required.

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#6
In reply to #5

### Re: Gear Ratio Myth?

10/21/2006 9:50 PM

thanks you all, from all of your inputs, i can draw the conclusion now:

1. There is no ratio limit on gear mating

2. one should not choose a ratio that will result in the same pinion teeth mating the same gear teeth in cyclic. -> hunting

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#8
In reply to #6

### Re: Gear Ratio Myth?

10/24/2006 2:23 PM

well, one must consider the load transferred. With very light loads you can have very large ratios. With high power you must be practical in terms of tooth and gear strength.

If you want 1000:1 I do not think is wise to make a gear with 20 teeth mesh with one of 20,000 teeth. Better to have three 10:1 ratios = lots smaller.

In addition, gear backlash gets to be a problem with high ratios, so you must use backlash free gears or take some other means to reduce backlash.

The other thing is dynamic friction. If you step up in three stages to 1000:1 and hook up a large motor that runs at 1700 rpm to drive the low speed input the output will turn at 1,700,000 rpm...NOT BLOODY LIKELY.

You will in fact find that the machine will fail in some way because the drag of the gear that wants to go 1,700,000 will be torque magnified 1000 times and stall the motor at some lower speed. The motor may well not turn at all. If it ran at 1700 RPM the gear that goes 1,700,000 may feel high stresses and may well fly apart unless well made.

Sad, it would be so nice to break the light speed barrier with gear trains like this

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#7

### Re: Gear Ratio Myth?

10/24/2006 10:31 AM

dear sir

according to me what you have taken (17&153)is correct i used this in one of my gear box but it was inline gear train , yours must be a parrallel shaft gear train .

maximum ratio we can go up to 10:1

this is according to my experience.

and i am not clear about the cobination of gears weather that is internal or external gearing.

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