New Design Approach for a Scroll Compressor Anti-Rotation Mechanism

Transcription

1 Purdue University Purdue e-pubs International Compressor Engineering Conference School of Mechanical Engineering 1998 New Design Approach for a Scroll Compressor Anti-Rotation Mechanism S. Otake Sanden Corporation Y. Yoshii Sanden Corporation K. Tsurutani NTN Corporation Follow this and additional works at: Otake, S.; Yoshii, Y.; and Tsurutani, K., "New Design Approach for a Scroll Compressor Anti-Rotation Mechanism" (1998). International Compressor Engineering Conference. Paper This document has been made available through Purdue e-pubs, a service of the Purdue University Libraries. Please contact epubs@purdue.edu for additional information. Complete proceedings may be acquired in print and on CD-ROM directly from the Ray W. Herrick Laboratories at Herrick/Events/orderlit.html

2 NEW DESIGN APPROACH FOR A SCROLL COMPRESSOR ANTI-ROTATION MECHANISM Shinichi Otake, Yuji Yoshii R & D Dept. Yattajima Manufacturing Division SANDEN CORPORATION, Gunma, Japan Kanetsugu Tsurutani Bearing Engineering Department NTN CORPORATION, Mie, Japan ABSTRACT Various design concepts have been utilized for scroll anti-rotation mechanism. Each mechanism, however, has challenges which should be improved. The Ball Coupling mechanism is superior to other mechanisms when considering mechanical efficiency and balance. Based on the Ball Coupling mechanism, the "Eccentric Motion Bearing" (EM Brg) has been designed to meet various competing challenges. EM Brg unifies the Ring and Race of the Ball Coupling to single piece (unified race) and the balls are supported by the curved surface of the unified race. This unique design resulted in reduced stress between the race and the balls which made it feasible to reduce ball size diameter. EM Brg design made it possible to have a compact, light weight and simplified anti-rotation mechanism. Further NVH characteristics and productivity were enhanced. INTRODUCTION The Oldham Coupling, Pin & Ring Coupling and the Ball Coupling are some of the well known anti-rotation mechanisms used in compressors. The challenge is an anti-rotation mechanism that provides low noise, high mechanical efficiency, good balance, productivity, low cost and light weight. The above mentioned mechanisms fall short of meeting all the challenges (See Table 1 ). To achieve these challenges, a new scroll anti-rotation mechanism must be developed. The unique design of the EM Brg satisfied above objectives, thus making it feasible to design a compressor with compact, light weight and simplified anti-rotation mechanism (See Fig. 1 ). This paper will demonstrate the inherent design benefits gained by this design. Further, it will describe the testing and selection criteria utilized during the development process. CHALLENGES OF CURRENT SCROLL ANTI-ROTATION MECHANISMS At times, unfavorable NVH characteristics are observed with the Ball Coupling. This is due to "Backlash" which is generated by the clearance between the ring and the ball. The Oldham Coupling has low mechanical efficiency due to the frictional forces between the Orbiting scroll and the key. In addition, the Oldham Coupling has a fundamental limitation with regard to the balance of inertia forces that results from its reciprocal motion. The Pin & Ring Coupling has low mechanical efficiency due to it's inherent design in supporting the axial forces of the Orbiting scroll. It is evident that currently known anti-rotation mechanisms exhibit several weaknesses. DEVELOPMENT OF THE NEW SCROLL ANTI-ROTATION MECHANISM The Ball Coupling design offers inherent technical advantages in the areas of mechanical efficiency and balance of inertia forces. These features exceeds the performance of other known anti-rotation mechanisms. Therefore, the development process has been established to capitalize on these features while seeking solutions to the unfavorable characteristics: 219

3 1. Targets of Development: 1) NVH characteristics improvement 2) Light and compact assembly 3) Higher productivity 2. Areas of Evaluation During the Development of the EM Bearing 2.1 Orbiting Radius The race stress which is generated by the ball movement is the lowest when the offset between the orbiting radius of EM Brg ball and the orbiting radius of EM Brg is zero (See Fig. 2 and Fig. 3), the ball however, may not necessarily contact the bottom of the track considering parts tolerances. Therefore to minimize race stress, parts tolerances must be considered when setting the orbiting radius. When the ball orbiting position is set to "0 (zero) ± a 1 " maximum race stress "A" is realized. When the ball orbiting position is set to "13 ±a 1 " highest race stress "B" is lower than "A". Therefore, it is possible to decrease the maximum race stress by offsetting the ball orbiting position to the outside track of the EM Brg. Ball track contact can be selected by adjusting the offset of the EM Brg orbiting motion position. For example, when the EM Brg motion is offset to the inside of the track by "a 2 " the race stress "C" is higher than the race stress "D" when the EM Brg motion is offset to the outside of track by "a 2 ". According to above results, it is possible to decrease the maximum race stress by offsetting the EM Brg motion (EM Brg ball position) to the outside track in the EM Brg. Thus, the maximum race stress can be reduced and durability of the EM Brg can be improved. This can be accomplished by setting the orbiting radius of EM Brg to be smaller than that of Orbiting Scroll thus, shifting ball contact surface to the outside track. 2.2 Track Curvature of the EM Brg It is impossible that all the EM Brg balls movement to be at the same phase due to the dimensional tolerance of the EM Brg and the assembling accuracy of the parts. The balls will move easily between the races with larger track curvature, however, the ball behavior will become unsteady. On the other hand, if the rotational axis of the EM Brg is offset and a smaller track curvature is used, the race stress will increase excessively (See Fig. 4 ). To optimize design, the track curvature of one EM Brg race was selected to be relatively small for holding the balls and the second track curvature to be larger, thus reducing race stress. 2.3 Ball Diameter and Ball Quantity of the EM Brg In the Ball Coupling, the race stress is determined by ball diameter and ball quantity. In the EM Brg, however, it is found that the race stress is determined by combination of ball diameter, ball quantity and the offset of the ball orbiting position (See Fig. 5). Therefore the ball diameter and the ball quantity of the EM Brg should be determined considering the relationship between the offset of the ball orbiting position and race stress. If the offset of the ball orbiting position is "± a", the race stress of the EM Brg can be less than that of the Ball Coupling (See Fig. 5). 220

4 2.4 EM Brg Behavior at Compressor Start Up It is possible for the scroll compressor to maintain good performance and durability by utilizing "Compliant Drive Mechanism" which is able to adjust the orbiting radius of the scroll within a suitable range. The "Compliant Drive Mechanism" is the mechanism by which a variable orbiting radius is achieved. This is accomplished by swinging the Eccentric bushing around the Crank pin. The swing amount is determined by the clearance gap between the center hole of the Crankshaft and the projection of the Eccentric bushing (See Fig. 6). The EM Brg exhibits the following behavior with the operation of the "Compliant Drive Mechanism": When the Crankshaft turns from "A 00 ' (<D) to "COO'( ), the inertia of the Counter weight causes the eccentric bushing to swing around the crank pin and the orbiting radius is decreased from POR 1 to POR 3 As a result, the ball is moved towards the raised portion of the race center, and race stress is increased (Fig. 6). To prevent increase of the race stress, the reduction of the orbiting radius has been minimized by controlling the allowable swing amount of the Eccentric bushing. The above mentioned enhancement to the EM Brg design resulted in a durability level equal to or better than that of the Ball Coupling. The following formula of the EM Brg life has been developed. Utilizing this formula, the EM Brg design has been determined. The life of the EM Brg is greater than that of the Ball Coupling by 10 to 15% (approx.). L = ( 1 /(Z/L, 1o19))9'1o = L,/z9t1o L 1 = 10 6 /60n x (Ca/Pa) 3 L : Life of the EM Brg (Hrs.) L 1 : Life of a Ball (Hrs) Pa : Load on Ball (N) Z : Ball Qty. of the EM Brg Ca: Basic load rating (N) n : Speed (rpm) 3. Advantages of the EM Brg 3.1 Noise and Vibration Comparison (EM Brg vs. Ball Coupling) It is believed that the "Backlash" noise is seldom generated in the EM Brg because the balls are caught into the track of the race. This has been confirmed by testing; the noise level of the compressor with EM Brg is lower than that of the compressor with Ball Coupling (See Fig. 7). In general, vibration levels under 1kHz causes great NVH discomfort in the vehicle. It was found that the vibration level (under 1kHz) of the compressor with EM Brg is lower than that of the compressor with Ball Coupling (See Fig. 8). 3.2 Size and weight comparison (EM Brg vs. Ball Coupling) The basic design of the EM Brg is the integration of the Ball Coupling Ring and Race into a single piece. It was feasible to make the ball size of the EM Brg smaller than that of the Ball Coupling because race stress of the EM Brg was reduced by the unique design of the race curved surfaces. Changing design from Ball Coupling to EM Brg made it feasible to, reduce number of components by 2, and reduction in anti-rotation mechanism's weight and axial length by 46%, 31% respectively (Table 2). Further, the EM Brg design simplified compressor assembly. 221

5 CONCLUSION In the development of the EM Brg, the orbiting radius, track curvature and ball size has been designed to optimize compressor performance considering start-up behavior. The EM Brg has similar mechanical efficiency and durability as the Ball Coupling. Further the EM Brg offers productivity, size and weight advantages. The unique design and characteristics of the EM Brg makes it the best anti-rotation mechanism known today. REFERENCES 1. Jiro lizuka, et.al., "Improvement of Scroll Compressor for Vehicle Air Conditioning System", 1997 SAE Congress Paper Kiyoshi Terauchi, U.S. Patent No. 4,824,346. Table 1 Comparison of Anti-Rotation Mechanisms Mechanism/ New Ball Oldham Pin & Ring Item Mechanism Coupling Coupling Coupling Noise Mechanical Efficiency Inertia Balance Productivity NOTE : RANK 5 shows the best. Ball coupling EMBrg --~ : Twa ports, Race and Ring Unified Ia one pil!!t:e. Orbiting radius of EMBrg race Fig. 1 Structure Comparison Fig. 2 Orbiting Radius of EM Brg 222

6 e c... <D D () Ctl 0::: A 8 Inside of track -a:, 0[3 ' ' ~ -a:,+f3 i a:,+f3 Track bottom Outside of track Ball orbiting position Fig. 3 Race Stress of EM Brg Track curvature: a dw< b dw _. dw=ball diameter I Rotation axis is offset by 0.3mm e... <!) () Cll 0::: Orbiting race }. '., ' '""" I Same -... : Fixed race a dw curvature :-._.>.,.,... ~... ':_-:":_~c-~i~ed rac_e lb dw}different : Orbiting race a dw curvature.. - " Inside of track Track Bottom Outside of Track Angular ball position Fig. 4 Race Stress Comparison by Track Curvature Q)... l... Q) 0 co 0:: -a3 -a2 -a 1 0 a 1 Inside oft rack Track sottom a2 a3 Outside of Track Ball orbiting position Fig. 5 Race Stress Comparison Between EM Brg and Ball Coupling 223

7 G) Y.K K y K y II Center hole of crank shaft Counterwei ht X~ Y : Axis of rotation (Axis of crank shaftl J~K: Axis of EMBrg rotation (Axis of eccentric bushingl POR1 >POR2>POR; Fig. 6 Behavior of Compliant Drive Mechanism at Compressor Start-up - ~~;;;=========~------~...,. Test Condition :;(' Dis.. Press = 1.72 Mpa..._., Suo~ Preso = 220 ~Pa ot 1400 'I'm CO 90 S.H.=10K,S.C.=5K _,, ""0 Mic.rophone : 0.1 5m from Comp Body,... ~ -~/ ~ ~r """'-,t~ - Q) / ~ r e / ::l " ~ c.!! il g "-... /.... i EMSrg ::l I ----Ball ~;oufling I 0 70L ~--~--~ aooo sooo tiooo Compressor speed (rpm) Fig. 7 Noise Level Comparison Table 2 Comparison of Weight, Size and Parts Quantity Weight Axial assembled length Parts quantity for Same Compressor EMBrg Ball coupling 150g 276g 9. 8mm 14. 2mm 3 5 " Test condition Vibration pickup position: Vertical -- Dis_ Press.=1. 72MPa Sue. Press.=220KPa at 1400rpm -". S.H.=10K. S.0.=5K Oompresor speed (rpm) GO Oompresor speed (rpm) Ball Coupling EM Brg Fig. 8 Vibration Analysis Result of Compressors with Ball Coupling and EM Brg 224