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
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
RE: GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS
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
RE: GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS
Thanks again.
Time to get my thinking cap on.
RE: GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS
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.
http://www.EsoxRepublic.com-SolidWorks API VB programming help
RE: GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS
RE: GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS
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
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.
http://www.EsoxRepublic.com-SolidWorks API VB programming help
RE: GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS
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
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
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
RE: GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS
RE: GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS
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
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
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
RE: GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS
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
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
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
Anyway, hope this helps. Good luck with it.
Regards,
Cockroach
RE: GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS
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
Thankyou
RE: GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS
RE: GEARS - SAME CENTRE DISTANCE - DIFFERENT RATIOS
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