325 -JGF 4QBO Ball screw's life is defined as: Total number of rotations, time, or distance where either the ball rolling surfaces or the balls begin to exhibit repetitive stress caused flaking. Ball screw's life can be calculated based on Basic Dynamic Load Rating with the following formula. -JGF )PVST -I 106 C 3 Lh= (hrs) 60Nm Pmfw ( ) Where: Lh: Life Span Hours (hrs) C: Basic Dynamic Load Rating (N) Pm: Mean Axial Load (N) Nm: Mean Rotational Speed (min−1) fw: Work Factor Impactless Run fw = 1.0~1.2 Normal Run fw = 1.2~1.5 Run with Impact fw = 1.5~2.0 *The basic dynamic load rating that satisfies the set life span hours is expressed by the following formula. *Setting life span hours longer than what is actually necessary not only requires a larger ball screw, but also increases the price. In general, the following standards are used for life span hours: Machine Tools: 20,000hrs Automatic Control Equipment:15,000hrs Industrial Machinery:10,000hrs Measuring Instruments:15,000hrs tBasic Dynamic Load Rating : C Basic Dynamic Load Rating (C) is defined as: An axial load which a group of same ball screws are subjected and 90% of the specimen will reach 1 million rotations (106) without experiencing any flaking of the rolling surfaces. See product catalog pages for the Basic Dynamic Load Ratings. 60LhNm C= Pmfw(N) 106 ( ) -JGF $BMDVMBUJPO &YBNQMF <Requirements> · Ball Screw Model BSS1520 (Ø15 Lead 5) · Mean Axial Load Pm 250N · Mean Rotational Speed Nm 2118 (min−1) · Work Factor fw 1.2 <Calculations> Since Basic Dynamic Load Rating C for BSS1520 is 4400N, 106 4400 3 Lh = = 24824 (hr) 60×2118 250×1.2 Therefore, Life will be 24824 hours. "YJBM -PBE Axial loads that apply on the screw shafts will vary depending on applicable motion profile such as acceleration, constant velocity, and deceleration phases. Following formula can be used. -Axial Load Formula- Constant Velocity · · · Axial Load (Pb)=μWg Acceleration · · · Axial Load (Pa)=W +μWg Deceleration · · · Axial Load (Pc)=W -μWg * Omit the "μ" for vertical applications. μ: Linear motion guide friction coefficient (0.02) W: Load Mass N g: Gravitational Acceleration 9.8m/s2 : Acceleration (*) m/s2 (*) Acceleration ( )=(Vmax/t)x10-3 Vmax: Rapid Feed Rate mm/s t: Acceleration/Deceleration Time s 'PSNVMBT GPS "WFSBHF "YJBM -PBE BOE "WFSBHF 3PUBUJPOBM 4QFFE Average Axial Load and Average Rotational Speed are calculated based on proportions of motion profiles. Average Axial Load and Average Rotational Speed for Motion profiles in Table 1. can be calculated with the formula 2. For machine tool applications, max. load (P1) is applicable for the “Heaviest cutting”. Regular Load (P2) is for the general cutting conditions, and Minimum Load (P3) is for the non-cutting rapid feeds during positioning moves. P13N1t1+P23N2t2+P33N3t3 Pm= (N) N1t1+N2t2+N3t3 ( ) N1t1+N2t2+N3t3 Nm= (min−1) t1+t2+t3 <5BCMF .PUJPO 1SPGJMF> <'PSNVMB "WFSBHF "YJBM -PBE $BMDVMBUJPO> "WFSBHF "YJBM -PBE BOE "WFSBHF 3PUBUJPOBM 4FFE $BMDVMBUJPO &YBNQMF <Requirements> <Calculations> (1) Average Axial Load 3433×1500×0.294+103×3000×0.412+3243×1500×0.294 Pm = = 250 (N) 1500×0.294+3000×0.412+1500×0.294 Therefore, the Average Axial Load Pm will be 250N. (2) Average Rotational Speed 1500×0.294+3000×0.412+1500×0.294 Nm = = 2118 (min−1) 0.294+0.412+0.294 Therefore, the Average Rotational Speed Nm will be 2118rpm. Motion Profile Axial Load Rotational Speed Hours Ratio A P1 (N) N1 (min−1) t1 (%) B P2 (N) N2 (min−1) t2 (%) C P3 (N) N3 (min−1) t3 (%) .PUJPO 1SPGJMF "YJBM -PBE 3PUBUJPOBM 4QFFE )PVST 3BUJP A 343N 1500rpm 29.4% B 10N 3000rpm 41.2% C 324N 1500rpm 29.4% (t1+t2+t3=100%) Ball Screws Selection of Ball Screws 3
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