Converting rotary torque into linear motion
A
Power Jacks product story
Edited by the Engineeringtalk editorial team
Mar 22, 2006
Planetary roller screw, consisting of a roller screw (spindle) and a roller nut, converts rotary torque into linear motion, in a similar manner as acme screws or ball screws.
Spiracon is a planetary roller concept invented by Illinois Tool Works, USA, and developed by and licensed to Power Jacks.
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 can carry 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 suitable 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, and no contact between rolling elements.
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, 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.
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 left and right hand threaded roller screws can be mated with the same nut.
The nut can be manufactured in many forms to suit mounting requirements such as trunnion mounts, threaded end, flange and so on.
Each unit can be tailor made to suit the application, including the use of special materials, as each unit is manufactured to order.
However, to provide a capability guide there is a standard range catalogued.
There are 10 standard roller screw models, with diameters from 15 to 120mm, each with a choice of three leads.
Dynamic load capacities of over 1000kN and linear speeds of over 30m/min are possible.
Applications include as a screw and nut assembly or combined into a screw jack or linear actuator product before being installed on a machine.
Industries that use them include nuclear, steel, machine tool, automotive and aerospace.
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