screw technology - ballscrew vs. leadscrew

Ballscrews and lead screws are the workhorses of today's motion-centric automation environment. In that light, making the correct choice for an application is extremely important.

Ballscrew technology allows balls to roll between the screw shaft and the nut to achieve high efficiency, usually above 90%, depending on lead angle. Its required driving torque is only one third of a conventional lead screw. As a result, ballscrews are capable of converting rotational motion to straight motion and vice versa. This screw type is suitable when smooth motion, efficiency, accuracy, and precision are a priority. The rolling elements eliminate sliding friction, so smaller motors can be used to drive ballscrews. And, because rolling motion is easier to control, accuracy and precision are also easier.

Lead screws are different in that there are no recirculating elements, and they are often used for simple transfer applications when speed, accuracy, and precision, are not as critical as rigidity and the load. On a positive note, more surface contact of the threads can make for a higher load rating of the nut over ballscrews. However, their metal-to-metal contact and high friction makes lead screws more suited to applications that do not require prolonged continuous movement or high speed.


Typically, application requirements dictate the type of screw that can be used. Neither component offers the best of all worlds, and so most designs using screws involve compromise. Think of a table with a ball bearing and a square block of material sitting on top; slightly lifting one side of the table will cause the ball bearing to roll, but the block will not slide until the table is lifted enough to overcome the friction between the block and the table. This demonstrates the efficiency differences between lead screws and ballscrews.

Ballscrew

> offers high efficiency, even with small leads, and will back drive

> used for high speed, since it has a low friction

> high precision

> longer lifetime, since the friction is low, the wear is also minimum

> difficult to manufacture, therefore standardized, and have narrower range of lead selection

Leadscrew

> it's efficiency depends on material and lead, so designers can control whether it will back drive

> basically self locking, which can eliminate the need for additional brakes in the system to support (for example, a vertical load during power loss), but also gives need of extra force to drive

> low cost and relatively easy to be made

> limited in rotational speed and duty cycle as well

> offers a wide range of lead

Planetary Roller screw
One advancement that's been around since the 1940s, but has only begun to take hold in the last 10 to 15 years, is the planetary roller screw. This screw type said to be combining some of the best features of the ballscrew and roller screw.
In a roller screw, a planetary arrangement of threaded rollers surrounds the main threaded shaft, increasing the surface area that takes the load and giving the screw its name. Planetary roller screws offer the highest possible thrust and lifetime of any screw-type actuator, operating with minimal maintenance and at high efficiency.

Of course, in the real world, all good things come at a price, and planetary screws are no exception. Their cost is the highest of the three lead-screw technologies. For demanding applications, however, planetary screws deliver the performance. “If a customer says they need 20 years of life out of the product -- it could be it in a remote location where there is no maintenance -- then maybe the roller screw would be the best choice,” says Frederick.

Of course being stronger and and more expensive is not enough, this type of screw also grow in diameter as it need more force. It's a trade-off, sometimes people will say it's Ok, but others might say no more space. It's up to designers to decide what's needed