Recirculating Ball Screws
Recirculating ball screw is the mechanical component increasingly used in movement transmission,used to transform a rotary motion into a translational motion. This mechanical component is precise and offer superior performances.
Between the threaded shaft and the nut screw are inserted steel balls which have the task to transform the sliding friction into rolling friction. The helically groove formed, has a hemispherical section having the same pitch of the screw.
The recirculation of spheres is obtained with the aid of recirculating elements (called inverter) that accommodated in the body of the lead nut, create an uninterrupted path for the spheres, ensuring the continuity of rolling and advancement within the raceways.
The denomination of recirculating ball screws is so called because gradually that the lead nut move forward ,the spheres are unloaded behind the lead nut, then get in position in front of the lead nut and automatically recharged.
In order to ensure the proper operation of the ball screw it is necessary to pay attention at the alignment of the screw with respect to the guides and external supports.
Some benefits guaranteed by the use of recirculating ball screws
- High efficiency and durability
- wear resistance
- axial rigidity
- reduced friction between parts in contact
The life of recirculating ball screw depends on multiple factors : design, quality, maintenance and from the dynamic axial load capacity. Factors that most affect dynamic axial load capacity are profile accuracy , the quality of the material and surface safety.
We can identify three types of ball screws :
- internal recirculation of the spheres : spheres are preloaded and their recirculation is returning them back after each round, at the previous. The number of the spheres is lower and thus are minor wear and friction.
- external recirculation of spheres : spheres, after several rotations, are bring again at the beginning of the circle through an outer pipe to the nut, which consequently is more bulky. The coupling parts must have a cavity for the passage of return tubes. This model is advantageous in the case of long pitch screws and it is adaptable, unlike the previous model, the addition of additives to increase the resistance at the corrosion and the
- recirculation at " end cap " : this type of ball screws uses the same mechanism as the external recirculating ball screws but with the difference that the recirculation tube is created inside the body of the lead nut as a through hole.
To ensure a proper operation of the ball screw, a correct and appropriate lubrication is required.
The choice of lubricant should be made taking into consideration:
- working temperatures
- speed of rotation of the screw / nut screw
- nominal diameter of the screw
- load size
- level of system security
It is recommended to use oil lubrication in all applications where there is a high speed of rotation of the nut or the threaded shaft and in which it is necessary to remove a relatively high amount of heat, given from the operating conditions of work or from the external sources.