Plastic Sliding Screws

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1 NTN TECHNICAL REVIEW No.(1) [ New Product ] Plastic Sliding Screws Naonari TANIGAWA* Norio ITOU** Tomomi TONOMURA* Plastic parts have high design freedom and high corrosion resistance, can carry out weight saving and compactification and are low in cost. Therefore, plastic parts substitute metal parts in many industrial fields. Plastic s (plastic nut and metallic screw shaft) are one of them, and these are widely used for food machinery and medical machinery parts. This article introduces features and performance of plastic s. 1. Introduction Feed screws, which convert the rotating motion of motors to linear motion, are used in the load carrying mechanism of industrial machines, food machines, medical devices, etc. Feed screws are categorized into ball screws and s. s with copper-based nuts are used with grease. On the other hand, screws with plastic-based nuts can be used without lubrication. Table 1 shows the comparison of the performance of NTN's plastic s with other manufacturers' s and ball screws. Ball screws are characterized by large permissible load and screw efficiency; however, they are expensive. Since they are lubricated with grease, these screws are not suitable for use in environments where grease performance deteriorates, such as under water or in high temperatures. In contrast, plastic s have a lower permissible load capacity but are well suited for broad ambient conditions, such as in vacuums and high temperatures. In addition, they are quieter and inexpensive. Furthermore, they can reduce weight and size. However, corrosion resistance and heat resistance vary depending on the materials for screw shafts and nuts; therefore, appropriate selection based on their application is required. In this article, we will discuss NTN's plastic s (hereinafter, s) and NTN's high-load plastic s (hereinafter, high-load s) with increased permissible load capacity 1, ).. s s consist of plastic nuts and stainless steel screw shafts, as shown in Fig. 1. Table shows the materials and the specifications. Table 1 Comparison of features various s Lubrication Corrosion resistance Screw accuracy Screw efficiency Noise Heat resistance NTN plastic Not required Operable with lubricant oil-impregnated POM byanother manufacturer Not required Operable with lubricant Ball screw Required Grease Plastic nut Fig. 1 Plastic ** Engineering Dept. ** Planning Dept. -9-

2 NTN TECHNICAL REVIEW No.(1) The plastic nut (BEAREE AS5) is based on polyphenylenesulfide (PPS) and blended with special fluorocarbon resin and solid lubricants to improve friction/wear resistance properties. As the impact resistance is also improved, they are more durable than regular plastic s. Fig. shows the comparison of properties of BEAREE AS5, which is the material for our plastic nuts, and commercially available PPS-based sliding material with fluorocarbon resin. BEAREE AS5 is particularly superior in wear resistance and friction coefficient, in addition, improvement is made to impact strength, which is a weak point of PPS. Fig. 3 shows the cross section SEM image. The improved property of BEAREE AS5 against commercially available PPS with fluorocarbon resin was made possible by the three dimensional presence of the special fluorocarbon resin in the molding. A comparison test of our s against other manufacturers' s was conducted under the conditions listed in Table 3 up to a travel distance of km. In general, an increase of axial clearance of up to.3 mm due to operation is considered to be permissible. As shown in Fig., the increase of the axial clearance of the of another manufacturer exceeded.3 mm with a travel distance of around 7 km. In contrast, the increase of the axial clearance of our was. mm or less, even after a travel distance of km, which indicated it was still usable for continuing operation. Table Specification of our plastic s Part Material Plastic nut BEAREE AS5 SUS3 (thread rolling) Accuracy class Ct1JIS B 119 Cumulative lead error ±.13 Table 3 Test condition of wearing test Screw nominal diameter Nominal lead Number of threads Axial load Rotational speed Lubrication 1 5min -1 None (dry friction) Impact strength Elongation Large Superior Wear resistance Strength BEAREE AS5 Commercially available PPS sliding material with fluorocarbon resin" Low Friction coefficient Elastic coefficient Increase of axial clearance mm [Typical technical data] from another manufacturer NTN's plastic Travel distance km Usable Fig. Features of BEAREE AS5 Fig. Relationship between axial clearance increase and nut motion distance Special PTFE (fibrous form) Commercially available fluorocarbon resin (granular) 1µm µm BEAREE AS5 Commercially available PPS with fluorocarbon resin Fig. 3 Cross section SEM images -7-

3 Plastic Sliding Screws 3. Examples of applications The s are adopted by medical devices and food machines since they can be used with no oil or grease lubricant. Table shows a record of actual use. Among others, Fig. 5 shows an example of application in food fryers and noodle boilers. Field Medical Food processing machine Other Table Applications Name of device Vision tester Analysis equipment Dialyzer Anesthesia apparatus Vending machine Processing machine Wrapping machine Semiconductor manufacturing equipment Electronic microscope Adopted part Focusing XY axis Specimen carrying drive shaft Pump actuator shaft Elevator drive shaft Cup carrying drive shaft Fryer/noodle boiler elevator Packing elevator Substrate positioning driver Specimen positioning XY axis Table 5 shows comparison of load carrying capacity and Fig. 7 shows variation of shaft surface temperature during operation. The plastic nut material used in both nuts is BEAREE AS5 with improved friction/wear resistance. However the high-load sliding screw uses thin-walled composite molding which provides better heat dissipation; therefore, the temperature rise at the screw surface is approx. 1/ of the. In addition, the permissible axial load capacity of the high-load is double and the static fracture load is nine times higher than the, due to the composite molding effect. Composite nut Aluminum base material BEAREE AS5 Food fryer elevator Composite nut Fig. Plastic for heavy condition Fig. 5 Application for food fryer Converts rotating motion from motor to linear motion Permissible axial load Static fracture load of nut Table 5 Comparison of load limit N 7kN -load 1,5N kn. -load s Since the s use plastic nuts, there was a risk of flange failure or thread wear when the load significantly exceeded the permissible load capacity. However, lubrication-free and low-noise operation is also required for high-load applications, which have been addressed by s for low-load applications. In order to address this requirement, NTN has developed high-load s with metal composite material as shown in Fig.. BEAREE AS5, which was discussed in Section, is used for injection molded threads on top of the aluminum-based nut, as a composite nut, to improve permissible load capacity and heat dissipation. The aluminum is processed with special etching to improve adhesiveness with BEAREE AS5. Surface temperature of screw shaft C Nut traveling distance m -load [Screw test condition] Screw specification: shaft diameter: 1mm, lead: mm, thread: 1 Load: N, Rotational speed: min -1, room temperature, no lubrication" Fig. 7 Surface temperature of screw shaft -71-

4 NTN TECHNICAL REVIEW No.(1) Next, Fig. shows the increase of axial clearance after operation. The high-load has less wear, approx. 1/3 of the. Since the rise in surface temperature is small, the elastic coefficient of the BEAREE material is maintained and reduction of frictional shear is small. Therefore, there is little change in screw efficiency with good friction properties. Adhesiveness was evaluated during the environmental resistance test under the exposure conditions in Table. Adhesiveness was tested by measuring the maximum load when only BEAREE AS5 was shorn (Fig. 9). In addition to measurement of adhesiveness, the appearance of nuts was also observed. The result is shown in Fig In all of these exposure tests, reduction of adhesiveness was 5% or less. In addition, no abnormality was found, such as rust, in the appearance of the nuts after exposure tests. 1 Exposure to high temperature (15 C) Increase of axial clearance mm load Shear direction Plastic [Screw test condition] Screw specification: shaft diameter: 1mm, lead: mm, thread: 1 Load: N, Rotational speed: 1 min -1, room temperature, no lubrication, nut travel distance:.7km Fig. Axial clearance increase Table Test condition of environment resistance Exposure to high temperature Exposure to constant temperature/humidity Immersion in hot water Condition 15 C 5hrs C95RH 5hrs 9 C 5hrs Test jig Aluminum base material BEAREE AS5 Fig. 1 Adhesion strength variation in high temperature exposure test Exposure to constant temperature/humidity ( C, 95%RH) Fig. 11 Adhesion strength variation in temperature and humidity controlled test 1 Immersion in hot water Fig. 1 Adhesion strength variation in hot water test Fig. 9 Schematic of adhesion strength test -7-

5 Plastic Sliding Screws Fig. 13 shows the relation between the permissible axial load and rotation speed of s and high-load s. A test of sets was conducted, defining times of back and forth motion under a predetermined rotational speed and load as one set (the screws were cooled down to room temperature by stopping the motion between each set with a total of, times of back and forth motion). The permissible axial load was determined as the maximum load before the increase of axial clearance reached.1 mm without nut fracture. The high-load had a permissible axial load twice as high as the. 5. Conclusion In this article, we have discussed NTN's plastic s which convert the rotating motion of motors into linear motion in lubricant (oil or grease) free operation, and NTN's high-load plastic sliding screws with high permissible load capacity. Market needs for lightweight and small sliding screws will grow further in many fields, including medical devices and food processing machines. NTN will continue addressing those requirements by offering products to contribute to customer product development. Permissible axial load N 1, 1,5 1, 9 3 -load References 1) Takuya Ishii and Yoshihide Himeno, Functional Improvement of Resin Sliding Bearing Through e Combination with Metal, Journal of the Japan Society of Polymer Processing, 5 (13),, 73 ) Yoshio Oki, Product Introduction and Complex Technology of Resin, Sintered Metal and Magnet for Growth Markets, NTN TECHNICAL REVIEW No. (1), Rotational speed min -1 [Screw specification] screw shaft: 1mm, lead: mm, 1 thread [Test condition] Room temperature, no lubrication (dry), stroke ±1,1mm, A test of sets was conducted defining times of back and forth motion under the predetermined rotational speed and load as one set (screws were cooled down to room temperature by stopping the motion between each set with a total of, times of back and forth motion) The permissible axial load was determined as the maximum load before the increase of axial clearance reached.1mm without nut fracture. Fig. 13 Speed and axial load limit Photo of authors Naonari TANIGAWA Engineering Dept. Norio ITOU Planning Dept. Tomomi TONOMURA Engineering Dept. -73-