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Transcription
Olaf Zeiss discusses the technology used for electric cylinders, specifically comparing ball screws and planetary roller screws. He explains that while ball screws have a point contact between the balls, spindle, and nut, planetary roller screws have a line contact between the rollers, nut, and spindle. This difference allows planetary roller screws to transfer higher thrust forces, have a smaller pitch, and be more resistant to vibration. However, when comparing complete electric cylinders, the outside dimensions and dynamic load rating must be considered. The dynamic load rating represents the axial load the screw can handle for one million revolutions. Despite the advantages of planetary roller screws, the larger outside nut diameter results in a lower dynamic load rating compared to ball screws. This difference significantly affects the expected lifetime of the electric cylinder. For example, a cylinder with a planetary roller screw and integrated servo motor has a lower dynamic lo Welcome to Parker Huntington's Podcast. My name is Horst Bindner and I'm really looking forward to today's conversation with our guest Olaf Zeiss. Olaf works for the Electric Motion and Pneumatic Division at the Offenburg site and is responsible for electromechanical actuators. Olaf, what do you want to tell us about today? Hello Horst, I'm glad to be here. Today, I'd like to dig into the technology used for electric cylinders. Generally speaking, an electric rod-style cylinder converts the rotational movement of the motor into a linear movement of the piston rod. Two possible ways to realize this conversion process are using a ball screw or using a planetary roller screw. Both technologies are used for high thrust force electric cylinders. Can you give us a short description of the differences between ball screws and planetary roller screws? Looking inside a ball screw, there is a spindle and a screw nut. Between these two parts, small ball bearings are located, transferring the external load from the nut to the spindle. The balls do not slip between the nut and the spindle, which leads to the high efficiency of ball screws. The contact is between the balls, the spindle and the nut. This is called a point contact, where curved surfaces meet at one point. In comparison, a planetary roller screw also contains a spindle and a screw nut. But between these two parts, there are rollers, not balls. As a result, there is a direct contact between the rollers, the nut and the spindle. This is a line contact, not a point contact. And what does this difference between point contact and line contact make? In general, the permissible surface pressure is almost the same for the hardened steel used for both ball screws and planetary roller screws. Because the loaded surface of the line contact is bigger than the loaded surface of a point contact, a planetary roller screw can transfer higher thrust force than a ball screw of the same dimensions. Are there additional differences between the two screw technologies? Yes, because of the line contact, a planetary roller screw is also more resistant to vibration and shock loads during operation. In addition, especially at higher spindle diameters, planetary roller screws are available with a smaller pitch than a ball screw. This allows transfer of the motor power into higher thrust force at low linear speed without using an additional gearbox. So, Olaf, to summarize, a planetary roller screw can transfer higher thrust forces, can have a smaller pitch and is more resistant to vibration. Correct, these are the laws of physics. But, when looking in detail at a complete electric cylinder and not only at the screws, the picture changes. When comparing complete electric cylinders, we must look at the outside dimensions of the cylinders and compare cylinders with approximately the same outer square dimensions. We need to compare the dynamic load rating of the cylinders. The dynamic load rating is specified in units of Newton and describes the axial load the screw of the electric cylinder with a reliable handle for one million revolutions. As an example, I will compare our ball screw driven ETH 125 with a comparable planetary roller screw driven electric cylinder. The ETH 125 contains a ball screw with a dynamic load rating of 248,000 Newton at a pitch of 20 millimeters. A comparable size 125 electric cylinder equipped with a planetary roller screw has a dynamic load rating of 211,000 Newton at a pitch of 10 millimeters. But this means the load rating of the planetary roller screw equipped cylinder is roughly 15 percent lower than the same size ball screw driven cylinder. That doesn't fit with the general explanation of ball screw and planetary roller screws you gave us before. I totally agree, Rolfus. There is a contradiction. Nevertheless, both statements are right. Let me explain. The general comparison of planetary roller screw and ball screw starts with comparing the same screw size. Same screw size means the same spindle diameter for both ball screw and roller screw. But for integrating a roller or ball screw into a given size electric cylinder, the key is not the spindle diameter but the outside diameter of the spindle nut. The integration of the complex rollers into the roller screw nut needs more space than the integration of the balls into the ball screw nut. Therefore, the outside diameter of a roller screw nut is larger than the outside diameter of a ball screw nut for a constant spindle diameter. Coming back to our electric cylinders. To obtain the same outside nut diameter, the spindle diameter of the planetary roller screw must be smaller than the spindle diameter of the ball screw with the same outside nut diameter. The spindle diameter of the ETH125 ball screw is 63 mm. The spindle diameter of the same size planetary roller screw equipped cylinder is 48 mm. This difference in the spindle diameter results in a lower dynamic load rating of the roller screw cylinder compared to the ball screw cylinder. So, this means due to the larger spindle nut, the planetary roller screw can't play off its advantages against the ball screw solution in this case. That's it in a nutshell. Let's take this topic to extremes. You can find planetary roller screw equipped cylinders with integrated servo motors. Of course, this integration needs additional space, which requires a thinner spindle diameter. A cylinder with a roller screw and integrated servo motor with an outside square diameter of 177.8 mm has a dynamic load rating of 101,400 N at a pitch of 12.7 mm. So, that is less than half of the dynamic load rating of the ball screw equipped cylinder of the same size. How does this reduced dynamic load rating affect the customer in its application? That's a good question. The dynamic load rating dramatically affects the expected lifetime of the electric cylinder. The dynamic load rating represents the axial force, the screw drive will survive for one million revolutions. Using the pitch of the screw, the lifetime in kilometers can easily be calculated. Knowing that the axial force retroactively influences the lifetime in the third power, you can calculate the expected lifetime of the electric cylinder for every axial load. Let's have a look at the three cylinders we already discussed. If we take an axial load of 30,000 N as an example, the calculated lifetime of the cylinder with a planetary roller screw at 12.7 mm pitch and integrated servo motor is calculated to 491 km or 38.7 million revolutions of the roller screw. At the same axial load, the life expectation of the electric cylinder with a planetary roller screw at 10 mm lead is calculated to 3480 km or 348 million revolutions of the screw. The dynamic load rating of these two roller screws differs by a factor of 2.08, but the lifetime expectation differs by a factor of 9. This is the influence of the third power. 2.08 cubed is 9. Knowing the effect of the third power, you will not be surprised that the calculated life expectancy of the ball screw equipped ETH125 with 20 mm pitch is calculated as an impressive 11,300 km. I think you will agree. In terms of life expectancy, the difference becomes huge. So, Olaf, thank you very much for this fascinating insight into the details of electric rod-style cylinder technology. And one last point. Anyone interested can find further information about the ETH cylinder on the Parge website, parge.com. There is also a range of explanatory videos on the Parge YouTube support channel. Many thanks for your interest. We hope you join us soon for the next Parker podcast.
