1 Overview
A film applicator is a machine specifically designed for applying protective films and explosion-proof films in industries such as electronics, communications, and semiconductors. It ensures bubble-free and scratch-free application. The ball screw drive mechanism works as follows: the ball screw acts as the driving element, and the nut, following the rotation angle of the screw, converts this into linear motion according to the corresponding lead specification. The driven workpiece can be connected to the nut via the nut seat, thus achieving the corresponding linear motion. Figure 1 shows a model of the film applicator's screw drive shaft.
Figure 1 Model of the lead screw drive shaft of the film applicator
2. Ball screw strength calculation
Axial load: Fa = F + UW
Where F is the axial frictional force;
W—Workpiece weight plus worktable weight; W=10 x 10=100N
U—the coefficient of rolling friction on the rolling guide;
Therefore, F1 = 0.04 × 100 = 4 N, which is the minimum load of the lead screw.
Maximum load on the lead screw:
Axial working load:
Where Fmax and Fmin are the maximum and minimum axial loads of the lead screw, respectively;
When the load varies according to a monotonic law and the frequency of use at various speeds is the same, the equivalent dynamic load Cm is calculated. Where Kp is the load characteristic coefficient, taken as 1-1.2 for no impact, 1.2-1.5 for general conditions, and 1.5-2.5 for significant impact vibration; here Kp is taken as 1. Ka is the accuracy influence coefficient, taken as 1 for ball screws of accuracy grades 1, 2, and 3, and as 0.9 for ball screws of accuracy grades 4 and 5; here Ka is taken as 1. The equivalent dynamic load Cm must be less than the rated dynamic load Ca to determine the model and size of the ball screw pair. Based on the requirements and structural dimensions, an FFZD internal circulation washer preload nut type ball screw is selected, specifically model: FFZD2004-3. The ball screw in this device operates under low-speed conditions. A ball screw pair with a lead of 4 is selected.
3. Ball screw support and guide rail
The lead screw in this transmission system is axially fixed at both ends. The end closest to the motor uses a pair of 30203 tapered roller bearings face-to-face, while the other end uses a 6203 angular contact ball bearing. The guide rails are mainly divided into rolling guide rails and sliding guide rails; a linear sliding guide rail with a rectangular cross-section is selected.
3.1 Preload of ball screw assembly
There are four ways to support a ball screw pair on a worktable. This design uses a support method where one end is fixed and the other end is hinged. According to the preload type, it can be divided into single-nut no preload, single-nut variable lead preload, single-nut enlarged steel ball radial preload, double-nut washer preload, double-nut differential tooth preload, and double-nut threaded preload. Here, the double-nut washer type preload is selected, and its preload force is generally 1/3 of the axial load. Compared with sliding screw and nut pairs, ball screw pairs have many advantages: high transmission efficiency, high sensitivity, smooth transmission; low wear, long service life, elimination of axial backlash, and improved axial stiffness, etc.
3.2 Transmission efficiency of ball screw pairs
The transmission efficiency of the ball screw pair is:
In the formula, ψ is the helix angle of the ball screw.
ρ' — Equivalent friction angle
Based on the relationship between the equivalent coefficient of friction and the equivalent friction angle, the rolling speed is v = 50 mm/s, and gray cast iron with an HRC ≥ 45 is selected as the material. Simultaneously, ρ' = 7°58′.
tgρ'=0.14
Since Ph=4mm is the lead in the formula;
d=20mm is the nominal diameter of the lead screw.
The thread helix angle of the ball screw is then obtained as: ψ = 6.06°
The transmission efficiency of the ball screw pair is η = 0.425.
4. Conclusion
The ball screw drive mechanism of a laminating machine was designed. The strength of the ball screw was calculated, and tapered roller bearings and deep groove ball bearings were used. Linear guides were employed. The preload of the screw pair was analyzed, and the transmission efficiency of the screw pair was obtained and calculated.
