1 SLEEVE BEARING DESIGN FOR slow SPEED APPLICATIONS . IN CEMENT PLANT. Sumit Singhal, Sr. Product Engineer, siemens Energy & Automation Norwood, OH, USA. ABSTRACT. Electric motor drives variety of APPLICATIONS in a cement manufacturing plant such as kilns, crushers, ID fans, separators, cooler fans etc. Motor manufacturers serving cement industry needs are often faced with challenging tasks of designing motors suitable for various operating conditions and requirements which are not only reliable but also cost competitive. Some of the drives such as a fan ID drive may be required to operate at SPEED as low as 50 rpm during turning gear APPLICATIONS which may last up to several hours. SLEEVE BEARING electric motors operating at low SPEED such as during turning gear operation require careful attention on BEARING DESIGN for reliable operations.
2 Safe operation of oil ring lubricated SLEEVE bearings rely on a generation of hydrodynamic oil film which separates motor rotor and BEARING and prevent metal to metal contact. Inadequate development of oil film thickness or boundary lubrication may lead to metal to metal contact between rotor and BEARING which can cause bearings to rapidly wear and fail. Although the cost of bearings is a small part of the motor, its failure may cause costly equipment damage and expensive downtime. This paper will discuss some of the key aspects of SLEEVE BEARING DESIGN for low SPEED APPLICATIONS and field practices to avoid BEARING failures. INTRODUCTION. Large electrical machines have been used in the cement industry since the turn of the century. As cement production has become more complex, so has the equipment and automation systems that support the manufacturing processes.
3 Electrical motors form the backbone of most of the processes involved in cement manufacturing plant such as kilns, crushers, ID fans, separators and cooler fans etc. Reliability of electric motors is important factor for the production and manufacturing of cement. Motor manufacturers serving cement industry are often faced with challenging tasks of designing motors suitable for various operating conditions and requirement which is not only reliable but also cost effective. Out of the several factors which may cause motor failure and reliability issues in cement plant, one of the most frustrating and involving is BEARING failure. Although the cost of bearings is a small part of the motor its failure may cause costly equipment damage and expensive downtime. It is not uncommon for a motor BEARING to operate fine during normal operation but fail during turning gear application on ID Fans.
4 A fan ID drive may be required to operate a de-energized motor rotor at speeds as low as 50 rpm during turning gear APPLICATIONS which may lasts up to several hours. SLEEVE BEARING electric motors operating at low SPEED , such as during turning gear operations, require careful attention on BEARING DESIGN for reliable operations. Sometimes bearings can be less forgiving at lower speeds than at higher operating speeds. A good understanding of various operating and ambient conditions, along with possible failure modes, helps the designer to DESIGN reliable bearings. Good DESIGN along with regular preventive maintenance practices can lead to longer BEARING and motor life. In the following sections a quick overview of the basic theory of oil lubricated SLEEVE BEARING DESIGN , various lubrication regimes, key DESIGN parameters along with possible failure mode and recommended field practice is presented.
5 THEORY. The nomenclature of oil lubricated SLEEVE BEARING shown in Figure 1. A SLEEVE BEARING consists of a stationary cylindrical body ( SLEEVE ) separated from a rotating shaft by a layer of lubricant. Operation of oil lubricated SLEEVE bearings relies on the generation of an oil film between rotating shaft and stationary BEARING babbitt due to hydrodynamic action. In oil lubricated SLEEVE bearings oil may be supplied to the bearings by gravity feed, external lubrication system or oil rings. Once the oil is supplied to the BEARING and it has a tendency to stick to the shaft due to its viscous properties. Oil which stick to the shaft is pumped into the clearance between shaft and babbitt by rotation of the shaft. Due to the hydrodynamic action of the fluid film , fluid pressure is generated within the clearance to counteract the weight of the shaft thus the fluid film developed lifts the shaft from the babbitt surface preventing metal to metal contact.
6 There are several key operating and geometric parameters which influences the generation of reliable oil film such as operating SPEED , load, clearance, oil viscosity, surface finish, radius and length of the BEARING . Some of the important parameters involved in the DESIGN of oil lubricated SLEEVE bearings are described below. A. Sommerfeld number (S). Is a non-dimensional DESIGN parameter which involves the geometrical and operating features of bearings such as rotational SPEED , load, oil viscosity, radial clearance, diameter and length of BEARING . 2. NLD R (1). S= . W C . where = viscosity of lubricant, N = rotor SPEED , L = length of BEARING , D = diameter of BEARING , W = weight of rotor, R= radius of BEARING , C= radial clearance between journal and BEARING . B. Eccentricity ratio ( ).
7 Is a ratio which gives overall radial clearance used by the shaft journal inside the BEARING during operation. Eccentricity ratio is strongly related to Sommerfeld number as shown in Figure 2. C. Film thickness parameter ( ). Is a ratio of composite surface roughness of mating surface to minimum film thickness . hmin =. ( ) 1 (2). R 2journal + R bushing 2 2. where hmin = minimum film thickness, Rshaft = rms surface finish of shaft, Rbushing = rms surface finish of bushing. Load C. BEARING Shaft . Clearance Circle hmin Lubricant Fig. 1. Schematic of journal BEARING 1. L/D=.25. L/D=.4. Eccentricity Ratio L/D=.5. 0. -2 -1 0 1 2. 10 10 10 10 10. Sommerfeld Number S. Fig. 2. Relation between Sommerfeld Number S vs. Eccentricity ratio . REGIMES OF LUBRICATION. In the theory of lubrication there are three different possible regimes of operation for oil lubricated SLEEVE bearings .
8 A. Boundary lubrication In this regime as shown in Figure 3a two mating surfaces are not separated by the lubricant film, fluid film is not developed and there is considerable asperity contact. Friction of coefficient is essentially independent of fluid viscosity. Boundary lubrication occurs when the film thickness is very small, typically less than composite surface roughness ( << 1 ). In this lubrication regime significant wear damage can be caused to shaft or babbitt due to metal - metal contact. Prolong operation of BEARING on this regime may eventually wipe out bearings. B. Mixed boundary-hydrodynamic regime Mixed lubrication regime is a transition zone from boundary lubrication to full film lubrication. In this regime a partial fluid film is developed between mating surfaces, there is asperity to asperity contact of peaks as shown in Figure 3b, coefficient of friction is highly dependent on operating conditions.
9 Prolonged operation of the BEARING in this regime with foreign particle or contamination in the lubricating oil may eventually lead to failure of bearings or journal. C. Hydrodynamic regime In full film lubrication regime or so called hydrodynamic lubrication regime the two mating surfaces are completely separated by the oil film with no asperity contacts as shown in Figure 3c. Coefficient of friction is viscosity of lubricating oil .Since there is no metal - metal contact there is no wear of either shaft or BEARING babbitt and a BEARING can run for an infinite time or not life limited . The oil film thickness is much greater than composite surface roughness ( > 5 ). Molecularily thin Asperity Film Contact Thick Oil Film (a) Boundary Lubrication (b) Mixed Regime (c) Hydrodynamic Lubrication Fig.
10 3. Regimes of Lubrication Figure 4  depicts the boundary, mixed and hydrodynamic lubrication regimes in terms of coefficient of friction and film parameter .While designing bearings, it is imperative to identify regimes of lubrication for the entire range of operating speeds and ensure that enough film thickness is developed to separate asperities of shaft and babbitt at all operating speeds. Fig. 4. Variation of friction coefficient with film parameter . DESIGN CONSIDERATIONS FOR slow SPEED BEARINGS. It is often the low SPEED problem that challenges the designer more than high SPEED BEARING DESIGN . The first step is to predict how slow BEARING can run and still have full film lubrication. It is imperative to understand the regimes of lubrication so that reliable bearings can be designed.