SKF compact inverted roller screws for powerful actuation
Roller screws for valve actuation
2017 January 31, 09:00 GMT
The control of large valves creates big challenges for conventional electromechanical actuators. According to Phil Nicholas, Business Development Manager – Linear and Actuation Technology at SKF, there’s one technology that addresses many of them.
Modern plants depend on a wide range of electromechanical actuators to provide the vital link between control logic and the process itself. For processes to perform safely and efficiently, those actuators must be quick, precise and reliable. To optimise plant layouts, and simplify installation and maintenance, they should also be small and compact. For the most part, today’s electromechanical actuators fulfil all these requirements, offering levels of performance that far exceed their hydraulic or purely mechanical predecessors.
In the case of very large valves, however, it is still difficult to achieve the right mix of performance characteristics. To generate sufficient force to control a large valve, actuators for these applications typically require a reduction gearbox and – if linear motion is required – a ball screw assembly.
Those extra parts create a variety of issues. Gearboxes require proper lubrication, pushing up maintenance requirements. And they are prone to wear, shortening the operating life of the actuator installation. The additional friction of complex mechanical drivetrains increases energy consumption and generates heat, especially in control applications where valves may operate continually for long periods. Ball screws, meanwhile, have a relatively low load carrying capacity, meaning units sized to match the load requirements of a big valve will often be large. Combined with the additional motor and gearbox components, the result is a bulky actuator, adding to design, installation and maintenance challenges.
One approach that resolves many of these issues is an actuator design based on the roller screw principle. Like a ball screw, a roller screw creates linear motion between a rotating externally threaded shaft and an internally threaded “nut”. The difference, as the name suggests, is in the design of the elements used to transmit loads while minimising friction and backlash. A number of threaded rollers sit around the shaft, held in position by guide rings at both ends of the nut. In the planetary roller screw design, gear teeth cut into the ends of the rollers mesh with internally toothed rings inside the nut, ensuring the correct rolling motion at all times.
The primary advantage of this design over the ball screw is its load carrying capacity. The load carrying capacity of a high efficiency ball or roller screw depends on the number and quality of surfaces at the points of contact. In ball screws, the load is transmitted from the nut to the shaft through small balls engaged in the groove. In a single start ball screw, the ball diameter is limited to approximately 70 per cent of the screw lead. As there is only a single helix of balls in a nut of a given length, the number of contacts is small. Ball screw variants with multi start thread forms have a greater number of smaller balls, providing more contact surfaces, but still do not achieve the required carry capacities. In roller screws, by contrast, the load is transmitted from the nut to the screw shaft through the surfaces of all the engaged rollers. The diameter of the contact surface is substantially increased, as is the number of points of contact. That means that for a given size, a roller screw can carry more load, for longer without wearing out. Roller screws can also be manufactured with an extremely short lead, offering high a high degree of linear precision and greater mechanical advantage – eliminating the need for intermediate gears between motor and actuator.
Roller screws come in various configurations, and are widely used in a broad range of applications, from manufacturing machinery requiring linear motion at high loads and speeds to high precision scientific equipment. A configuration commonly adopted in valve control applications is the inverted roller screw. In this design, typically either the nut or shaft are fixed in the axial direction and a set of planetary rollers translates along the inside of a nut during operation, moving a connected push tube that operates the valve. Either the nut or the shaft can be driven, but the former option allows the roller screw assembly to be built inside the driving motor, creating an extremely compact, powerful and accurate short-stroke actuator.
Modern planetary roller screw valve actuators can use brushless servo motors to actuate the screw. The combination of the two technologies delivers exceptional performance. Servo motors are rated for continuous operation, and these systems can offer in a working life of many millions of strokes, depending on the application parameters required. As a further benefit, servo motor control systems allow the straightforward implementation of condition monitoring technologies: continual monitoring of motor speed and current characteristics allows users to assess the health of the actuator and valve assembly, providing an early warning of developing problems.
The above technical article was originally published in the January 2017 edition of Process & Control magazine.