Hi, I was trying to simply use cylinders instead of actual gears in Inventor to help save some time.
What I’m asking is how do I figure out the diameter of the cylinder to draw so that the cylinders would touch where the gears would normally mesh? Would I use the pitch diameter? Or something?
I kept searching around and kept getting different results and it’s driving me crazy.
Thanks in advance for any help.
Yes, the pitch diameter is what you should use. If you don’t know it, it is the Diametral Pitch (the number describing the size of the teeth 20, 24, etc) divided by the number of teeth. 40 tooth 20 pitch gear has a 2" pitch diameter.
You may also want to draw the overall diameter of the gear to check for interferences.
yes, use the pitch diameter.
Ok, thanks a lot.
There are some great gear models at www.firstcadlibrary.com or www.cbliss.com, but I’ve found out that it’s just simpler to represent gears as cylinders using their pitch diameters - this allows inventor to run faster if you have a slower machine. Our first year of using inventor, we modeled everything down to the gear teeth and even the slots on the 8020. In the end, our model approached 1.5 GB and normally would take 10 minutes to load in - if the computer didn’t crash first.
Another tip, stack two cylinders on top of each other. one with the pitch diameter and one with the outside diameter of the gear. that way you can still check for interferences with other parts. The formulae:
OD = (N+2)/DP
PD = N/DP
OD = outside diameter
N = number of teeth
DP = Diametral Pitch
PD = Pitch Diameter
Pitch diameter plus voodoo.voodoo, n., A method of adjusting the actual centre distance from the theoretical amount of (D[sub]1[/sub] + D[sub]2[/sub]) ÷ 2 to something close, but not exactly the same.
So how’s that done? It depends on the gears themselves. For FIRST gearboxes, depending on the tolerances that I’m sure that I can maintain, I tend to say that I’d hope that the worst-case scenario would run freely without binding (or missing entirely!). That means, if you’re drilling gearbox plates on a drill press, without the benefit of a good solid vise, you may be looking at the possibility of missing by 0.020" or so, even if you centre-drill (how accurate is that, if it’s on the same drill press?), and exercise great caution. With the probable exception of the 32 pitch and 0.7 module gears, most everything *should *work under those conditions, but not necessarily well. If working with a good mill, perhaps 0.005" is more reasonable (actually, more accuracy is possible, but I wouldn’t count on it, without some decent expertise involved). Also, sometimes the gears have unusual shapes: motor pinions have undercut teeth in many cases, for instance, and some have extended-length teeth.
In any case, I tend to add an extra 0.005" to the centre distance, as a rule; but for the Bosch drills, I think that I added 0.007"; others (Joe Johnson, if I remember correctly, likes 0.003". He’s got better equipment than I did, I’m sure.) I’m not at all the most experienced one here, so you may want to consult a practicing engineer who may deal with this on a more regular basis. He’ll have his own voodoo reasons for the way he does it.
Now, if you’re just sketching, forget this. But if you’re going to build it, you might as well consider it now, rather than try to insert it in later. But: it may mean that the tangency constraints don’t quite work correctly in Inventor anymore (since the theoretical pitch circle circumferences aren’t necessarily tangent anymore). I like to set up an array of points, and dimension the separation between these, and then reference my gears based on these locations–I can then manipulate the points, rather than fighting with the tangency constraint (which is tough enough in Pro/E, and awful in Inventor). One last thing. If you fiddle with centre distances, make sure to check that the separation between parts is what you expect. A missed or incorrect constraint can cause things to miss or overlap unexpectedly, since they aren’t being constrained in tangency.
Right now I’m just sketching to see how I can layout the gears and motors, then I’ll CAD it, and so forth.
Yeah, now that you mentioned that I should have put that into the design from the beginning. Thanks for the tip, I’m gonna need to make those corrections to my drawings.
Another great place to get cad models of gears is www.sdp-si.com you can also get the specs for those gears too.
If I am understanding this correctly, you are taking the Diametral Pitch (20) and dividing it by the # of teeth (40)…
20/40 = 1/2
Am I wrong?
(Just trying to fix any confusion this could cause 
)
other way arround…(# of teeth)/DP= Pitch diameter
metric gears, on the other hand, use module: (# of teeth)*DP= Pitch diameter