Whether equipment in a foundry, a press in metal forming or guiding heavy logs in a sawmill, many applications require high-force linear actuators. Regardless of the application, the designer must specify a ball or roller screws for the electric high-force linear actuator.
In order to make an informed decision, there are several factors for a machine designer to consider:
- Budget (cost of ownership) for the actuator.
- Expected service life and duty cycle for the actuator.
- Space allowed for the actuator design.
Put simply, roller screws are more durable and quieter, but costlier than ball screws. Acme screws are the most affordable, yet have a lower expected service life. Here are some notes on each screw type:
Acme screws. These are not mechanically-actuated, but are simply “standard” (square-thread) screws with trapezoid-shaped teeth, which are rolled onto a steel shaft or leadscrew nut. As the shaft turns, its threads transmit linear force to the nut. Acme screws require more motor torque compared to other screw types and are less efficient and have a shorted service life.
Acme screws offer a low dynamic load rating (DLR) for expected service life, screw space needed is high and screw life is low. Acme threaded screws are some of the easiest to manufacture.
Ball screws. Named for the re-circulating ball bearings that fit between the circular screw threads, ball screws have ball bearings that transmit force and motion efficiently as they roll through the circuits in the nut. Ball screws are more affordable than roller screws, plus they create more noise.
Ball screws offer a medium dynamic load rating (DLR) for expected service life, screw space needed is medium and screw life is medium.
Roller screws (also called planetary or satellite screws). Available in standard or inverted options, roller screws use precision-matched rollers that have been machined to match the threads of the nut. These triangle-shaped threads provide multiple points of contact and offer less stress at each point of contact. The design of roller screws allow for both higher force and higher speed of operation in the machine, plus maintenance is also reduced.
For inverted roller screws, the function is the same as a standard roller screw except that the rollers move inside the nut instead.
Roller screws have a very high DLR, and extremely long expected service life and require less space than ball or acme screws.
Other important distinctions between actuator screw types include:
- Ball screws are more efficient than roller screws. This means that overall power requirements will be lower, process throughput is higher, and the need for external cooling is reduced when using ball screws. They typically operate at nearly 90% efficiency.
- Ball screws have lower back-driving force (the amount of force required to move the screw backwards in an application) compared to roller screws because of their lower internal friction.
- Roller screws should be chosen in two instances: where space is limited and an appropriate ball screw will not fit the application or where the required services lifetime of the screw exceeds what a ball screw is capable of.
- On average, a roller screw has 200 contact points in a 1 inch diameter, 1.5 inch long roller screw. The load is distributed over approximately 4 times the area than a ball screw, which can result in up to 15 times longer life and up to 15 times greater load carrying capacity. Roller screw design also delivers improved stiffness for greater precision and durability.
- Finally, ball screw noise is generated when the balls collide with the return tubes. The sound of a roller screw comes from the roller timing gears which are much higher frequency (typically 10 times). The noise of a roller screw does not grow exponentially with its speed, and ball screw noise does. Roller screws are generally much quieter than ball screws.
As you can see, there are many differences between acme, ball and roller screws. Depending on your budget, duty cycle for the actuator, and space allowed for the actuator design will all help determine the screws used on your electric high-force linear actuator. Still have questions? Contact GCC today!