Gears has different shape of teeth. 1

[PeterKing]

HI,

Please check the attachment. Those gears has different shape of teeth. They are supr gear. Want to know what is the different in performance on those different teeth?

 

[dimjim]

To answer your question, we would have to know what they mate with and which is the driver. If the first is the driver, it is probably a recess action driver. If it is a follower, it probably has too much approach action. It would be a very strong tooth. The cutter would be withheld to produce this tooth form and have a long addendum.

I have to assume the second is a follower. It would have weaker teeth than the standard gear but may run quieter in mesh. It looks like a very short addendum gear where the cutter is sunk deeper to form these teeth and the gear o.d. blank is reduced accordingly.

 

[eromlignod]

They are different pressure angles.

Don
Kansas City

 

[PeterKing]

Sorry for lack of detail. They are both driver. Let's say the left one is A and right one is B.

A or B is smoother and quieter?

A or B is stronger?

A or B has larger pressure angles?

Thanks

 

[dimjim]

A is stronger and would have a larger pressure angle than B if the gearing were produced with standard operating center distances. They do look like 25 degree and 14.5 degree pressure angle gears.

However, these same type of gears could be produced using standard 20 degree pressure angle cutters but create that type of profiled looking gear by withholding the cutter to produce a gear looking like A. By sinking the same cutter deeper, you can produce a gear looking like B.

Normally smaller pressure angle produce quieter gears assuming no modifications are made to their mating gears.

But by using a gear shaped like A as the driver and like B as the follower or driven gear, they may be a very quiet operating gear set and this type of gearing would be called recess action gearing and have very little approach action in their mesh. This type of gearing used to be called long and short addendum gearing. The long addendum gear would be the driving gear.

 

[PeterKing]

dimjim: Thank you very much for your answer. That really helps me to learn a lot more. Since the smaller pressure angle the quieter, Want to know what is the smallest pressure angle I can make on gear? thanks!

 

[dimjim]

To be able to answer that, you have to know the number of teeth in each part and whether the driver is always the lesser number of gear teeth. 14.5 degrees was the practical lower limit for many years. It also helps to know the general envelope dimensions to know if you are talking about general spur gears or into the micro spur gear ranges.

 

[Matt51]

Typically you will get the quietest gear mesh with spur gears when the gears are designed with profile contact ratio 2.0. Usually a pressure angle in the range of 18-22 degrees can be designed to give this integer profile contact ratio, but it depends on the minium number of teeth on the pinion. Typically you need 20 teeth minimum to get a higher profile contact ratio (otherwise the tooth tips become pointed). A 14.5 degree pressure angle gear needs, as a guess, 25 teeth minimum, to avoid undercut. A higher pressure angle gives a stronger tooth, and can be designed with fewer numbers of teeth. So the 13 tooth pinion on the left looks conventional, perhaps with a 25 degree pressure angle, for a design with so few teeth.

The second pinion looks like it is undercut, which means it is a poorly designed involute gear, or it could be a correctly designed non-involute tooth form. The tooth thickness at the bottom portion of the tooth appears to be smaller than the tooth thickness at the pitch diameter. Look at the tooth at the 12:30 position, the first tooth to the right of the tooth at top dead center.

It is a sixteen tooth pinion, which is not enough for a 14.5 degree pressure angle system, which ususally needs at least 25-30 teeth. So my guess is you have a cycloidal tooth, which is sometimes used in gear pumps.

Take the parts to a gear shop, they can do an involute trace, and determine what type of gear teeth you have.

The pressure angle you select is directly tied to the minimum number of teeth selected for the pinion. The higher the pressure angle, the fewer the number of teeth you can use. You can't use any of the 14.5, 20,or 25 degree involute cutters, and have negative cutter shift to the extent shown, on a 16 tooth pinion, without having excessive undercut.

 

[Matt51]

I just tried to re-estimate the numbers of teeth from your picture, I probably underestimated before. What is the actual tooth count?

 

[dimjim]

Matt,
The gears look like 20 tooth and 24 tooth respectively.
I have a feeling these are only illustrations from a book illustrating different gear designs. I agree the second
does look like a cycloidal tooth form. I have never designed a spur gear with a 2.0 contact ratio. Interesting.

 

[MikeyP]

You can design integer contact ratio spur gears to minimise transmission error. However, they have to be very precisely manufactured to obtain the greatest benefit. Also the "working" contact ratio will only be integer for a single load.

M

--
Dr Michael F Platten

 

[Matt51]

Hi dimjim and MikeyP, thanks for the responses. It is somewhat standard practice to design gears with integral contact ratios for noise reduction in the automotive industry. Integer face and integer profile contact ratio. I have seen test results, and the noise is reduced as you approach the integer ratio from either side. So a profile contact ratio of 1.8 is better than 1.6, and 2.0 is better than 2.2. Nominal is 2.0, but you don't fall off a cliff if you are at 1.9 or 2.1.
A spur gear always has the problem it can only be optimized for one load, usually the maximum load. A helical gear is somewhat more flexible, if a corner modification is used, part of the profile can be left unmodified.
Designing for a contact ratio of just under 2.0 has the advantage you can still use AGMA rating standards.

 

[diskullman]

The gear on the left resembles a sprocket for silent chain.

Russell Giuliano