GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS 1

[MRSSPOCK]

Hi.

Does anyone know is it possible to cut involute gear teeth in such a way as to enable different ratios, but keeping the same centre distances?

I mean, suppose I have a pair of spur gears, 20 : 40, is there a way that I can cut a new smaller gear to have 19 teeth instead?

I realise that it would no longer be a rolling contact but slipping.

Is such a thing possible or am I just being silly?

Thanks

 

[Occupant]

Yes, of course you can. However, you have to shift the profile of the teeth on the pinion by a certain amount depending on the chosen diametral pitch. If you shift it too far, the teeth eventually will develop a knife edge - not a good thing.

 

[MRSSPOCK]

Thanks Occupant.

I have been looking for info' on this procedure but can't find anything, probably because I don't know the correct terminology to do a search. Can you recommend anywhere on the internet to read about this?

Thanks

 

[Occupant]

A good start is [link link

http://www.qtcgears.com/Q410/Q420cat.html

]this[/url]. You can download individual sections of their catalog. It shouldn't be too difficult to switch from that to the inch system.

 

[MRSSPOCK]

That's great.

Thanks again.

Time to get my thinking cap on.

 

[TheTick]

You can theoretically have just about any ratio you heart desires. Just divide the distance in proportion to the ratio and design accordingly.

Involutes (gear teeth profiles) are actually pretty forgiving of distance mismatch. If the distance varies a little from tangent-tangent contact of pitch circles, in most cases there is only a change in pressure angle, as long as the teeth still make contact and don't crash.

Honesty may be the best policy, but insanity is a better defense.

http://www.EsoxRepublic.com

-SolidWorks API VB programming help

 

[MRSSPOCK]

TheTick....Thanks, but is it possible to, let's say, have a common driven gear, that can be used with a choice of driving gears? For example, suppose I have a 48 tooth gear driven by a 24 tooth gear, can I create a 23 tooth gear that can be interchangeable with the 24 tooth gear, while still using the 48 tooth driven gear, and maintaining the same center distances? i.e. using the same gearbox, but being able to vary the ratio by swapping only one gear.

 

[Cockroach]

Essentially you are computing Dc = (Np+Ngf)/2P where Dc = distance between shaft centre, Np=pinion tooth number, Ng=gear tooth number, P=diametral pitch. So much of the commentary above is correct, you can vary gear ratio in the train.

However, the physical gear tooth geometry will be tougher to match, noting that standard involute of circle profiles and tooth depth may not be as easy to accomodate to your selection. Typically these are standard profiles, you may need to bastardize your teeth in order to get something that will work. Sort of like threads, the geometry of threading are standard for the various profiles, you can vary pitch but may get something crazy if you needed to find a particular solution to Box X Pin geometry.

Good luck with it. I often found gear trains challenging.

Regards,
Cockroach

 

[TheTick]

Maaayybee...

I once worked for a firm that made multi-output gearboxes. They could get pretty creative with custom gear tooth profiles. One custom gear in the right place could save a lot of shuffling around.

Mostly it depends on your constraints. Are you in a position to design custom gears, or are you dependent on off-the-shelf profiles?

A gear tooth profile (involute) is based on a base diameter, and the shape "unwraps" from that diameter. The base diameter is determined by pitch diameter and pressure angle.
[URL unfurl="true"]http://en.wikipedia.org/wiki/Involute[/url]
[URL unfurl="true"]http://en.wikipedia.org/wiki/Involute_gear[/url]

If you have a driven gear with 48 teeth, it may be possible to engage it with both a 23-tooth and 24-tooth driving gear in the same position. The gear mesh for each case could still work, but there would be differences in performance due to different pressure angles. At least one of the gears would be a non-standard profile.

Honesty may be the best policy, but insanity is a better defense.

http://www.EsoxRepublic.com

-SolidWorks API VB programming help

 

[MRSSPOCK]

Thanks for your comments.

My problem is that, although I know the basics of creating the involute forms from the two, let's say, regular base circles which are relevant to the 48 and 24 tooth gears, I can't understand how to then to create the pitch circle and base circle for the 23 tooth gear, since the pitch circle for the 48 tooth gear is already defined. Surely the only pitch circle which will be tangent to the 48 tooth pitch circle will be the 24 tooth pitch circle. It would be nice to see a worked example but I can't find anything anywhere. By the way, I don't have a real problem to overcome at the minute. I'm just trying to get a better grasp of the concept and maybe a better grasp of spur gears in general. Thanks

 

[TheTick]

Start with a sketch of the 24:48 gears with pitch circles, base diameters, and pressure angle.

Over the same sketch, create new pitch circles for the 23:48 gears. Use the same base diameter on the 48-tooth gear, and determine new pressure angle. This will give you base diameter for the 23-tooth gear.

 

[MRSSPOCK]

Thanks TheTick.

I've done that and have come up with a pressure angle of 22.08283° (initial pressure angle I used was 20°)

Does that sound like I've done the right thing?

Thankyou

 

[TheTick]

Can you post a picture?

 

[TheTick]

I was intrigued enough to try this in a SolidWorks sketch. I get the same answer.

 

[israelkk]

TheTick

How are you going to manufacture the 22.08283° pressure angle involute gear?
The common cheap procedure is to use profile shift (rack shift) using same 20% pressure angle hob or rack or any other 20% pressure angle standard tool to manufacture the gears. This will also allows the use of standard testing procedure such as testing against standard 20% pressure angle Master Gears.

 

[TheTick]

I'm not going to manufacture it. I'm only calculating it for MRSSPOCK. No one asked to manufacture, only if it was possible.

In my case, I typically work on small gadgets with molded plastic gears, so no biggie there. My past employer was able to make custom hob tools and had gobs of expertise in designing, measuring and inspecting.

 

[MRSSPOCK]

Sorry, I fell asleep on the job.

Here's the image attached.

Blue lines relate to 24:48 and green to 23:48

The two yellow lines are the common ones.

I sort of had in the back of my mind wire erosion as the manufacturing method, since I wasn't really thinking too much about production but rather special one off type items for motor sport.

 

[Occupant]

Yes, the working pressure angle will change with a profile shift, but what does that have to do with anything? I routinely design cams where the pressure angle exceeds 30deg without any problems and I don't think you'll ever face that situation with a mere profile shift. The tip of the teeth will turn into knifes before thet happens.

 

[benta]

You could simplify the whole thing by using modular gears.
This does limit your choices of ratio, but standard gears and cutters can be used.
Examples:
24/48, 23/49, 25/47 etc. as long as you keep the sum of the teeth constant, your centre distance will be the same.

Benta.

 

[tbuelna]

....but is it possible to, let's say, have a common driven gear, that can be used with a choice of driving gears?

Yes, it is possible. It is commonly done with the spur gear sets used in racing transmissions. The center distance between the input and lay shafts is fixed, and the gear geometries are designed so that the pinion can usually mesh with a gear of +/- 1 tooth. The performance of the gears are compromised a bit by doing this, but it also greatly reduces the cost of gear inventory a race team must maintain.

Here is an example of the gear ratios available for a racing transaxle. Note that there are several different gears available for each pinion.