Applied Motion Products Step Motor Life & Reliability Applied Motion Products step motors are brushless motors with excellent bearings. As such, they can have a phenomenal life. Factors that determine motor life include: bearing life, insulation system temperature, shaft strength, and environmental conditions. The goal of this paper is to help you understand these, avoid motor life problems, and provide solutions if a standard motor does not meet your needs. Most life factors are rated in hours. This is the time the motor is actually running at its operating temperature. As a guide, life of 20,000 hours running is suitable for many applications. This typically represents 10+ years of field use (8 hours / day, 5 days / week, and 50 weeks / year). With good design motor life can be much longer once the actual working cycle, shaft loads, and motor temperatures are accounted for. Shaft Life: When a motor shaft is subjected to large radial loads it can break after many hours of use. This is due to the radial load causing the shaft material to fatigue as it rotates. Shaft life is a function of the magnitude and location of the radial load, as well as the shaft material and shaft size. As the radial load is moved further from the motor mounting face, the maximum radial force the shaft can withstand drops. Shaft Strength - Maximum Radial Load - Standard Shafts Shaft Radial Force Location Max. Radial Motor Diameter Force Series From Motor Mounting Size mm Inch mm Inch N Lbs. 8 HT08-220/221 4 0.157 End of shaft 20 0.79 18 4 HT08-020/021 Center of Flat 16.5 0.65 30 6.7 11, 14 5 0.197 5014-042/020 End of shaft 24 0.94 21 4.7 HT17-268/269/270 Center of Flat 16.5 0.65 30 6.7 17 HT17-271/272/273/274 5 0.197 HT17-275/276/277/278 End of shaft 24 0.94 21 4.7 HT23-560/593/594/595 Center of Flat 13.1 0.52 62 13.9 HT23-559/596/597/598 23, 24 6.35 0.25 HT23-599/600/601 HW23-598/601 End of shaft 20.6 0.81 44 9.9 HT34-504/505/506 Center of Key 20.8 0.82 300 67 34 HW34-506 12.7 0.50 HT34-495/496/497 End of shaft 31.8 1.25 200 45 A common cause for shaft (and bearing) failure, is high radial loads that are created when a pulley is attached to the motor shaft at a large distance from the motor mounting face, and the belt has high tension. To avoid this condition mount pulleys and gears as close to the face of the motor as possible, and avoid over tightening belts. This dramatically reduces the shaft stress, and increases the life of the bearings. Bearing Life: Bearing life depends on several factors including: axial and radial loads, motor speed, temperature, and the bearing ratings. Because the front bearing is positioned closest to the motor shaft, it usually carries a higher load and has the shortest life. There are two sets of bearing life curves below. The first set shows the maximum axial and radial shaft loads for 20,000 hours L10 bearing life at various speeds. These curves are for the radial load applied at the distance from the mounting face shown on the curve (usually the center of the flat / keyway). These curves are for the shortest motor in the series. Longer motors have a somewhat higher rating. Additional life curves are shown for larger motors, (size 17, 23, 34, 42) They show bearing life, at various speeds and radial loads. These curves are for the radial load applied at the distance from the mounting face shown on the curve (usually the center of the flat / keyway). Each page has six graphs for one motor size and length. There are multiple graphs for different axial loads, and separate graphs for English and metric units. Another limiting factor for bearing life can be the bearing grease life. Applied Motion Products uses special high-grade bearing grease. Normally Applied Motion Products motors operate for years without the grease being an issue. Typical grease life is 40,000 hours of operation. Insulation Life: Applied Motion Products standard insulation is class B, rated for 130 C. By industry definition, insulation life is rated for 20,000 hours, at rated temperature. For every 10C temperature reduction, insulation life approximately doubles. If the motor internal temperature is actually running at 120 C insulation life is 40,000 hours at 110C the life is 80,000 hours, etc. Because its rare for Applied Motion Products motors to be operated so that internal motor temperatures are continuously above 90 C, insulation life is usually not a limiting factor. Insulation and bearing temperature can be confirmed in the application by measuring the surface temperature of the motor, after running the machine under worse case conditions for several hours. The maximum internal temperature is about 15C hotter than the surface temperature of the motor. Thus, maximum temperature equals the measured motor body temperature, plus 15C, plus the difference between the maximum rated machine ambient and the test ambient.