 So in this video I want to talk about helical gears. Helical gears of course are very similar to spur gears. The main difference being that we now have a helix angle to consider. And this helix angle is right here. It's the angle that represents how skewed from a perpendicular axis the gear teeth are going to be. And of course you could think of a spur gear as effectively a helical gear with a helix angle of zero degrees. And basically everything that we're doing here would still apply. So because of this helix angle we have to modify some of our variables that we've talked about before with spur gears. So in spur gears we had you know for example the pitch P which was basically how many teeth we counted as we went around the gear. Well that number or that that distance excuse me is slightly different if I rotate my view and look at it from you can see this cross section that's cut here. N N cross section. And look at my gear teeth from that direction. That difference is going to be a little bit excuse me that distance is going to be a little bit different than it was from the other view from the RR cross section view. And the angle of the tooth is also going to be a little bit different there. So this is you know very similar to how we look at threaded rods or screw pitches when we talk about the angle being rotated slightly depending on how we cut that cross section. So the relation between these two things is similar to what we've done before it's purely geometry. So when we're trying to make this more clear that that's a lower case P the new pitch there is going to be related to the old one by that angle of the helix. And so we can write that that way. The new angle of the tooth phi sub n is going to be related to the old angle phi again by that helix angle. Probably of most interest to us is the diametral pitch capital P trying to make that as clear as possible which is related by the inverse of cosine psi. So now we have that and finally the module for SI unit systems related like that. We can still write you know gear diameter in terms of number of teeth and diametral pitch and just making the modification for the helix configuration by substituting in and similarly when we're talking about SI units something like that. Now when it comes to force analysis then for the gear everything looks again very similar to a spur gear but now if I have my crudely drawn helical gear mounted on a shaft I have my transmission force F sub t which isn't really changed. I still have a radial force or a gear separating force that's much the same but now due to this helix angle I also have a force acting in the axial direction which is going to be parallel to the direction of the shaft. So we need to take all those things into account and oftentimes we write things in terms of the transmission force again because the transmission force is related to the the power that's transmitted through the gear train so that's usually a useful quantity for us to know and we can write that axial force in terms of the transmission force as well. There is one more force that we might be interested in and that is the a force that would be normal to the direction of the tooth. So remember the teeth go kind of like this rather than straight across and this F sub t then is coming in at an angle but if we instead put a force on here we could call that F sub b and if I write that out basically like a like a bending force we get something like that F sub t over the cosine of psi. Now when it comes to stress analysis for the helical gear nothing much has really changed from our spur gear analysis it's the equation becomes only slightly different so if I write my stress equation it's going to look very similar to our spur gear equation however you will notice one difference here and that is that there's this factor of 0.93 out in front of the mounting factor and that's really just taking into account that helical gears are a little bit less sensitive to mounting accuracy than a spur gear is. The only other main difference then is in the geometry factor J which we now read from figure 16.8 in the textbook rather than the figure from the spur gear chapter and so it takes into account the helical angle or helix angle and kind of just adjust those those values accordingly. All right thanks.