×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS

GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS

GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS

(OP)
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

RE: GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS

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.

RE: GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS

(OP)
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

RE: GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS

A good start is this. You can download individual sections of their catalog. It shouldn't be too difficult to switch from that to the inch system.

RE: GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS

(OP)
That's great.

Thanks again.

Time to get my thinking cap on.

RE: GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS

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.

batHonesty may be the best policy, but insanity is a better defense.bat
http://www.EsoxRepublic.com-SolidWorks API VB programming help

RE: GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS

(OP)
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.

RE: GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS

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

RE: GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS

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.
http://en.wikipedia.org/wiki/Involute
http://en.wikipedia.org/wiki/Involute_gear

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.

batHonesty may be the best policy, but insanity is a better defense.bat
http://www.EsoxRepublic.com-SolidWorks API VB programming help

RE: GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS

(OP)
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

RE: GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS

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.

RE: GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS

(OP)
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

RE: GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS

Can you post a picture?

RE: GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS

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

RE: GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS

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.

RE: GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS

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.

RE: GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS

(OP)
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.

RE: GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS

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.

RE: GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS

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.


RE: GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS

Quote (MRSSPOCK)

....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.

RE: GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS

(OP)
Thanks everyone for your replies.

It all starts to make so much more sense.

I don't suppose anyone knows a way to simulate the rack cutting the gear using Catia V5 ?

I've been messing about with Power Copy but it isn't really a nice way to do it.

I'm only trying to do the simulation to give me a better grasp of how adjusting the various parameters effect the finished profile in reality.

Seeing the physical result, (albeit in CAD land), is really quite good.

It would be just nice to have a macro or something as opposed to using the Power Copy method which is a bit naff.

Thanks

RE: GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS

I found an older computation that I was working on in the day. Typically I would of hacked out the numbers as attached and then roughed it out in AutoCAD in order to get the geometry correct. Once I was certain with the layout, I would of updated it to a SolidWorks model in order to get the various quantities and do a little FEA on it in Simulation. The model coding would then get transferred to the CNC machines and the gear cut accordingly. I can't find any of those source files at the moment, probably archived, I have the machine G Codes, but they would be worthless to you at this point.

Anyway, hope this helps. Good luck with it.

Regards,
Cockroach

RE: GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS

(OP)
Hello Cockroach.

I'm presently working on a Catia file to try to include all your operations in one hit.

I have got something put together but it's still a bit messy.

Watch this space, but don't hold your breath smile

Thankyou

RE: GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS

I don't have Catia, but if you would drop here all the details of what you are trying to do I could leave a *.dxf file that you probably can import.

RE: GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS

(OP)
Thanks Occupant.

At this point in time I am just trying to create a macro or similar to create a 3D solid model of any spur gear. At the top of the specification tree in Catia there will be a list of parameters that can be adjusted to achieve the desired result. I don't have a real application for it presently, but it will be nice to know it's there and ready as and when I need it. I realise I have long way to go and lots of parameters still to add, but I've attached an image to illustrate the sort of thing I mean. This really is the bare bones, but it's just to help me figure out the best way to use Catia to achieve the best result. Once I get to that point I will add all the other relevant parameters to the list.

Thanks again for the offer.

RE: GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS

I often redesigned only the pinion to mate with the original gear design by using a 50 percent long addendum. It made for a stronger pinion and was very effective acting as a recess action gearing design. This was dependent on the fact that the pinion was the driving member. You may have to design a slightly larger long addendum to overcome the possible proper backlash in the system. Have fun.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources