Converting Rotary Torque Into Linear Motion

A planetary roller screw is a mechanism for converting rotary torque into linear motion, in a similar manner as acme screws or ball screws. It consists of a roller screw (spindle) and a roller nut. The roller nut contains rollers parallel to the axis of the spindle. As the spindle rotates, the rollers undergo planetary motion about the spindle. Unlike acme or ball screws, roller screws are capable of carrying heavy loads for thousands of hours in the most arduous conditions. The increase in performance is due to maintaining the high efficiency of rolling elements, similar to a ball screw, but increasing the number of contact points on the thread. This makes roller screws the ideal choice for demanding, continuous-duty environments. In comparison to ball screws roller screws have the following advantages: * Higher dynamic load capacity. * Larger diameters and higher leads. * Longer life. * Higher stiffness. * Higher rotational speeds * Higher rates of acceleration and deceleration. * Capability to operate in harsh environments. * Nut easily removed with rollers retained. * Less fatigue and more robust to shock loads. * No contact between rolling elements. Spiracon design summary - Spiracon is a unique planetary roller concept invented and patented by Illinois Tool Works, USA, and developed by and licensed solely to Power Jacks. The Spiracon system consists of a multi-start screw with an involute thread form and a number of planetary rollers with annular grooves, which engage with the screw providing full line contact. These rollers also engage with a grooved load bearing element, which transmits the load through roller thrust bearings, to the nut housing. The rolling action results in a high efficiency mechanism, while the line contact and hardened and ground construction achieves a high dynamic load carrying capacity, together with almost no axial backlash or wear. The axial load and torque take two different load paths. The axial load is from the screw to roller to load bearing element to thrust bearings to nut housing. While the load torque is transferred from the screw to roller through the needle roller bearings to the nut housing. An added advantage of using rollers with annular grooves means that both left and right hand threaded roller screws can be mated with the same nut. The nut itself can be manufactured in many forms to suit the customers mounting requirements such as, trunnion mounts, threaded end, flange, etc. Main features of Spiracon Roller Screws are: * High dynamic load capacity. * High efficiency. * High positional accuracy. * Long life and low maintenance. * Same nut fits both right and left-handed screws. * Hardened and ground rolling elements. * Clean operation. * Smooth running. * Low noise. * Each unit can be tailor made to suit the exact application requirements, including the use of special materials. The last point of product tailoring is worth noting, as each unit is manufactured to order Power Jacks can engineer it to meet the customer's exact requirements whether for performance or installation. However to provide a capability guide there is a standard range catalogued. There are 10 standard Spiracon roller screw models, with diameters from 15mm to 120mm, each with a choice of three leads. Dynamic load capacities of over 1000kN (100 tonf) and linear speeds of over 30m/min are possible. The Spiracon roller screws have proved to be successful in a wide variety of applications whether used as a screw and nut assembly or combined into a screw jack or linear actuator product before being installed on a machine. Industries that already use them include nuclear, steel, machine tool, automotive and aerospace. * About the author - Bruce Hamper is product and marketing director, Power Jacks Group.