What Is a Sliding Screw?
A Sliding Screw is a type of feed screw, a mechanical element used to convert rotational motion obtained from a motor or other source into linear motion.
The rotational motion of a motor can be converted into linear motion on a linear motion mechanism such as a linear guide by the linear motion of the nut section that accompanies the rotation of the screw shaft. It consists of a combination of a screw shaft and a nut section. A material with good sliding characteristics is used for the nut section.
Sliding Screws have the same function as ball screws, which have a similar mechanism, but since they do not require balls, they can be realized with a simple mechanism and are relatively inexpensive. Because of these advantages, Sliding Screws are used in linear motion parts of precision equipment and various other electronic devices.
Applications of Sliding Screws
Sliding Screws are used in linear motion in precision equipment, typically in semiconductor manufacturing equipment. In the case of ball screws, there is a risk of balls falling out and getting mixed in with the manufactured product. However, with Sliding Screws, there is no need to worry about balls getting mixed in. For this reason, they are widely used in medical and food manufacturing equipment.
Principle of Sliding Screws
Sliding Screws are able to convert rotational motion into linear motion because the screw is formed in a spiral shape. Rotation of the screw shaft causes the nut part to move in the axial direction of the screw shaft.
Since force is converted by sliding between the screw shaft and nut part, the coefficient of friction and wear resistance of the contact surface are extremely important. In order to satisfy low friction and wear resistance, bronze casting BC6C or polyacetal resin POM is used for the nut part.
Carbon steel for machine structural use S45C and austenitic stainless steel SUS304 are used for screw shafts because of their excellent wear resistance. The combination of a stainless steel screw shaft and a polyacetal resin nut part has excellent corrosion resistance, making it useful as a linear motion mechanism for machines that are used in wet environments.
When fastening a component to a resin nut, the tightening torque must be adjusted to prevent damage to the nut section.
Types of Sliding Screws
There are three types of Sliding Screws depending on the shape of the screw.
1. Triangular Screws
This type of screw has an equilateral triangular thread profile and is also used for fastening. They are easy to process and provide high accuracy among Sliding Screws. On the other hand, its disadvantage is poor power transmission efficiency.
2. Trapezoidal Thread
Trapezoidal screws have trapezoidal threads. The angle of the slope, called the half angle of the screw thread, is 30 or 29 degrees. This type of screw is often used for Sliding Screws of processing machines and machine parts.
3. Square Thread
Square threads are screws with square threads. Although they are less precise in positioning, they have high power transmission efficiency, and are therefore often used in machines that transmit large axial forces, such as presses and jacks.
Other Information on Sliding Screws
1. Difference From Ball Screws
Sliding Screws are less expensive than ball screws because they do not require complex mechanisms. On the other hand, their positioning accuracy (explained in the next section) is lower than that of ball screws, equivalent to C10 class. Sliding Screws are also vulnerable to shear forces and moments other than in the axial direction, and their transmission efficiency is lower than that of ball screws.
Therefore, when Sliding Screws are used, they are sometimes used together with linear guides to limit the direction of the load. In addition, ball screws are suitable when high efficiency operation and high positioning accuracy are required.
2. Positioning Accuracy
JIS classifies ball screw lead accuracy using JIS B 1192 (ISO 3408). C0, C1, C2, C3, C4, C5, and C6.
C0, C1, C2, C3, and C5 are defined as precision ball screws, with smaller numbers indicating higher precision, and C7, C8, and C10 as rolling ball screws. As mentioned earlier, the positioning accuracy of Sliding Screws is equivalent to that of the C10 class. C10 allows an error of up to ±210μm for a travel of 300mm.