17 Replies Latest reply on Nov 16, 2017 5:23 PM by Dennis Dohogne

    Involute Gear

    Maha Nadarasa

      I replicated an Involute Gear. It says m=2, I couldn’t find how m value is decided? It also says number of teeth N = P/m =76/2 = 38. P is a pitch diameter. I do not know why they call it pitch diameter.

       

      Tutorial: How to model involute gears in SolidWorks and show design intent. - GrabCAD

      2.png

        • Re: Involute Gear
          Zac Evans

          m is the "module" it's the radial distance from one tooth to the next in mm. similar to thread pitch in metric, distance from peak to peak, or trough to trough

          • Re: Involute Gear
            Newell Voss

            m is the module which represents a ratio or "size". you can either choose a starting ratio, or calculate it backwards based of what info you're given. pitch is given as a diameter because it is the nominal path (circular rotation not linear) on which both gears fully contact as they rotate.

            I would suggest researching gear design if you really want to know the different construction methods and reasoning for why the variables are used as there is much to be learned.

            • Re: Involute Gear
              Dennis Dohogne

              Maha,

              Since you didn't find it I dug around and found it for you.  Here is a good parametric file for metric gears.  This is in SWX2016 and uses the ability to set an arc length equal to a straight line segment.  This allowed me to make a geometric construction of an involute profile.  The necessary math is contained in the Design Table.  Here is a screen shot of the DT:

              The green cells are the inputs of module, number of teeth, pressure angle, and whether the teeth are standard or stub.

              • Re: Involute Gear
                Maha Nadarasa

                I replicated an Involute Gear. It says m=2, I couldn’t find out how m value is decided? It also says number of teeth N = P/m =76/2 = 38. P is a pitch diameter. I do not know why they call it pitch diameter.

                  • Re: Involute Gear
                    Maha Nadarasa

                    According to this thread m can be represented in diagram that is why I want know.

                     

                    1.png

                     

                      • Re: Involute Gear
                        Dan Pihlaja

                        I believe that it is referring to this (in red):

                        But not straight line distance.....arc length.   I think......

                         

                        Nevermind....I was wrong.  m is referring to the module, which is defined here:

                        It can be solved using this equation from Dennis's link:

                         

                        Where:

                         

                        So m = D/N

                          • Re: Involute Gear
                            Maha Nadarasa

                            I understand that m is a Module value therefore it can't be shown in the diagram.

                              • Re: Involute Gear
                                Matt Peneguy

                                The module is a "construct".  It is a ratio and is therefore unitless.  So, you can't show it in Zac Evan's diagram.  Think of PI, the ratio of a circles diameter to its circumference.  Well, the module is like that.  Does that answer your question?

                                  • Re: Involute Gear
                                    Dan Pihlaja

                                    Matt Peneguy wrote:

                                     

                                    The module is a "construct". It is a ratio and is therefore unitless. So, you can't show it in Zac Evan's diagram. Think of PI, the ratio of a circles diameter to its circumference. Well, the module is like that. Does that answer your question?

                                    I could be wrong, but I don't think that it is unitless.   I think that it will be in the same units as the Pitch Diameter, since m=D/N

                                    The N is unitless, since it is just a number of teeth, but Pitch Diameter will be in length units, therefore m needs to be in length units.

                                     

                                    Similar to a thread, in which Pitch = 1/Threads per inch.  Pitch will be in Inches.

                                      • Re: Involute Gear
                                        Zac Evans

                                        Maha, gears are one of those things that you could get a whole degree just one gear design. that's why it's so rare to have to design your own gear, when designing a gear system if it comes out that you need a custom gear 99.9% of the time it means you've done something wrong, unless you're in a field that designs air craft carriers.

                                        You just need to learn the basics of gears and how to pick the right gear, not necessarily how to design one from scratch.

                                          • Re: Involute Gear
                                            Matt Peneguy

                                            Zac,

                                            I think that goes for a lot of things... As an engineer, you should be using as many "off the shelf" parts as possible.  You can probably use a factor of 10x the cost for having a part custom made vs. buying it off the shelf. 

                                          • Re: Involute Gear
                                            Matt Peneguy

                                            Yeah, I think you are correct.  Thanks for correcting that. 

                                            If I remember right you need 3 specific criteria to define a gear, and then all of the other factors can be backed out. (Obviously, this is an over-simplification, you need to consider everything such as backlash, undercutting, profile shifting, etc.  But, you get the point.)  We generally use diametral pitch, number of teeth and pressure angle.  I'm sure if you had the module, you could eliminate one of the three (probably diametral pitch), but is that a common practice?  Have I been doing it wrong and nobody corrected me?

                                            Wikipedia says that module is usually specified in the metric...So, that may explain why I never bothered with it.

                                            • Re: Involute Gear
                                              Dennis Dohogne

                                              Dan Pihlaja wrote:

                                               

                                              Matt Peneguy wrote:

                                               

                                              The module is a "construct". It is a ratio and is therefore unitless. So, you can't show it in Zac Evan's diagram. Think of PI, the ratio of a circles diameter to its circumference. Well, the module is like that. Does that answer your question?

                                              I could be wrong, but I don't think that it is unitless. I think that it will be in the same units as the Pitch Diameter, since m=D/N

                                              The N is unitless, since it is just a number of teeth, but Pitch Diameter will be in length units, therefore m needs to be in length units.

                                               

                                              Similar to a thread, in which Pitch = 1/Threads per inch. Pitch will be in Inches.

                                              Module is for metric gears while Diametral Pitch is for inch gears.  Both relate to the pitch of the teeth.  Both have units of length: module is in mm, while Diametral Pitch is in inches.  Here is a screen shot from Machinery's Handbook showing relationships for some of the inch gear terms: