 Hello friends, I am Mr. Sanjeev B. Knight, working as assistant professor in mechanical engineering department, Walton Institute of Technology, Salaapur. In this video, I am explaining about the power transmitting capacity of the worm gear drives based upon its beam strength and wear strength. So, at the end of the session, students will be able to design worm gears based on its beam strength rating and wear strength rating. As we know that the worm gear drive essentially consists of the worm and worm wheel. So, worm wheel essentially in the form of a screw, whereas the worm wheel is in the form of helical gear. So, basically it is transmitting power from a screw to gear. So, it is not gear to gear pair and that is why a linear motion of the worm is converted into rotary motion of the worm wheel. So, this is the basic difference between all other gear drives and worm gear drives. And that is why the design of the worm gear drive is based upon the beam strength of the worm wheel and wear strength of the worm wheel because always the corresponding beam strength and wear strength of the worm is always greater than the worm wheel because it is a continuous thread which is transmitting the power. So, design for strength for worm gear drives based on Lewis equation is applied to worm wheel teeth not for the worm. So, always worm wheel teeth is designed as it is weaker in its beam strength and wear strength. So, this is the basic difference and because of that and these are also available as standard mechanical equipments as a gear box worm gear boxes. So, gears are usually designed according to some national and international course and there are two basic equation as we know that beam strength concentration to avoid bending failure and wear strength concentration to avoid wear failure. So, two equations are used. So, pause the video for a while and just recall the beam strength and wear strength of power and helical gears. Now as I said that worm gear drives are normally designed according to some national and international course. So, here is according to IS 7443 1974 code the beam strength rating of the worm gears is defined as it is a maximum permissible torque that the worm wheel can withstand without bending failure and it is given by these two values that is a torque 1 and torque 2 where the equation is given like this one 17.65 XB1, SB1, MLR, D2, cos of gamma. So, here XB1 and SB1 these are the factors which are called as a speed factors and bending stress factors. So, these are obtained from standard chart based upon the speed and materials for the worm and worm wheel whereas all other terms are related to the terminology of worm and worm wheel that already we have seen in previous videos and that is why by knowing these values from a given gear gear pair of worm and worm wheel we can just refer the two factors XB1 and SB1 from the standard chart and we can calculate torque 1 and torque 2 which is the lower that much torque safely this worm gear pair having specifications can transmit a particular torque safely without bending failure. So, this is the way we can calculate the beam strength rating a particular maximum torque it can transmit without bending failure where LR is length of root of contact is calculated by this equation already we have seen by geometry. Now, as I said the factors SB1 and SB1 can be taken from standard chart for example if I want to know SB1 and SB2 which are the bending stress factors which depends upon the materials for the worm and worm wheel. For example, if I take SB1 that is the bending stress factor for the worm depends upon the worm material. So, the worm materials are normally steel or alloy steel so considering any of the materials out of these four materials corresponding value of SB can be calculated. Similarly, the material for the worm wheel is normally phosphor bronze and based upon its manufacturing method further we can consider its bending stress factors. So, SB1 and SB2 can be found out from this chart. Similarly, SB1 and SB2 which are the speed factors for the worm and worm wheel to be used in that equations are found out from this chart. So, for example, if I take 1000 rpm as the worm speed then corresponding value on this line or graph we can calculate as a speed factor. So, similarly the speed of the worm wheel can be considered and corresponding factors can be found out. So, that is why the SB1 and SB2 can be calculated and we can calculate the torque transmitting capacity based upon the beam strength rating as we have just seen it. Now similarly, the same code has suggested what can be wear strength rating of worm gas. So, the maximum permissible torque that the worm gear can transmit or worm wheel can transmit without pitting failure. So, pitting wear is a basic failure phenomenon in case of all the gear. So, similar is over here. So, pitting wear is considered and to avoid that failure what maximum torque it can safely transmit is given by these two equations according to this code. So, we can use it here. For example, torque is 18.64 Xc1, Sc1, Yz, d2 raised to 1.8 into m. So, once again this Sc1 and Sc1 these are the factors Xc1, Xc2 or the speed factors for the wear of the worm and worm wheel and Sc1 and Sc2 are the surface test factors of the worm and worm wheel under wear consultation whereas, the Yz is called as a zone factor. So, all these three factors are available as a standard values from the table under certain conditions whereas, d2 is a p-circle diameter of the worm wheel and m is the module of the worm wheel. So, by substituting all these we can get the two values of torque which it can transmit. Out of these two whatever the lower value we get that is a safe torque transmitting capacity of the worm wheel drive, worm gear drive avoiding the pitting wear. So, to avoid pitting wear failure we can make use of the lower value of this as a safe torque transmitting capacity from which we can calculate the power transmitting capacity. Now, here we can refer some standard charts which have been made available as a values of surface stress factor as I said Sc1 and Sc2. For example, the material for the worm wheel as I said is normally phosphor bronze but manufacturing method is different for which the corresponding worm material is considered. For example, if I take phosphor bronze centrifugal cast and corresponding material for the worm is B, what is B? It is a 40 C 8 material that is a plain carbon steel material under normalized condition. If I take combination of phosphor bronze as the worm wheel material and the worm material is a 40 C 8 as a carbon steel then the factor becomes 0.85 and that is what Sc2. If I take material for the worm and corresponding material for the worm wheel which is A only that is a phosphor bronze, so if I take material for the worm as steel normalized steel and material for the worm wheel as phosphor bronze then it is 1.1. So that is the way we can consider the factors Sc1 and Sc2 and then there is a similar speed factor just we have seen in case of beam strength rating similar chart is available to establish XC1 and XC2 and then the YZ zone factor which depends upon the number of start Z1 on the worm as a start of threads and Q is a diameter quotient. So normally the quotient is 10 or it may be in the range of A to 20 we can refer and number of starts of corresponding value is the value of YZ. So substituting all these things we can calculate the wear strength rating of the worm wheel as a torque 3 and torque 4. So finally we find that by using that quote we can calculate beam strength rating of the worm which is the lower value of these two and wear strength rating of the worm gas which is the lower value of these two. So if I want to design a worm gear pair then I have to design safe against bending failure as well as wear failure. So that is why what I have to do is that the safe torque transmitting capacity or safe power transmitting capacity of the worm gear pair depends upon the lower most torque out of these four values. So that it will be safe against bending failure as well as wear failure. So we calculate first beam strength rating based upon these two torque, wear strength rating upon these two torque and finally we can decide how much safe power it can transmit or torque it can transmit without bending failure as well as wear failure by considering the lower most value out of these four values and that becomes a safe torque the given gear pair or the worm gear pair can transmit without failure from which we can calculate further the power transmitting capacity of the worm gear drive. So this is the way we can use IS code, international code or Indian code 17, 1974 code as I have mentioned and we can just calculate the safe power transmitting capacity of the worm gear drive. This is my reference design function and memory and I have taken some figures from this site. Thank you.