MolaKule
Staff member
A White Paper on Bearing Design
and Filtration Requirements
By MoleKule
Keywords: Journal bearings; lubrication; contamination; filtration.
1.0 General
Bearings reduce the friction that robs machines of horsepower. Bearings are classified by type as Roller bearings, Ball bearings, Pin/Needle bearings, Journal (sleeve) bearings, and sliding bearings. Roller and ball bearings are usually found in applications such as transmissions and differentials. Pin/Needle bearings are also found in automatic transmissions and manual transmissions that use Automatic Transmission Fluids, and in 2-cycle engines. Sliding bearings in engines can be found in cam/tappet assemblies and of course in the cylinder, where the ring pack interfaces with the cylinder liner. In this paper we will focus on the design and filtration requirements for the sleeve bearings that support journals, such as crankshafts, connecting rods, and camshafts.
1.1 Sleeve Bearing Design
A number of factors affect bearing design including loads (forces), diameter of journal/bearing, length of journal/bearing, rotational speed of journal bearing, eccentricity of journal-bearing system, clearance between journal and bearing, the minimum oil film, the viscosity of the oil film, the heat produced by all frictions, the coefficient of friction of the journal-bearing system, and the volume of oil flow.
The journal does not rotate inside the bearing in a concentric fashion, rather is has a slight wobble or “eccentricity” (denoted as “e”) that generates not only a wedge of lubricant film, but also “pumps” the oil around the wedge. Eccentricity is defined as the distance between the bearing center and the journal (shaft) center. Assume the shaft is rotating counterclockwise in the sleeve and the bearing sleeve has a hole in it, and the shaft and bearing are immersed in an oil bath. The oil will be drawn into the “diametral” clearance when the shaft is rotating and will force the wedge of oil around the inside of the sleeve. In most case, we provide both oil-in and oil-out passageways, with a higher pressure on the “in” side to keep a clean and cool oil supply flowing.
Bearing friction is determined by an empirical equation (derived by test data) called the “McKee” equation as: Eqn. 1, f = 1x10^-10[473(ZN/p)D/C] + k, where k is a constant determined from another equation or chart for various L/D ratios; L is length of bearing (in.), D is diameter of bearing (in.), N is speed of journal in rpm, N’ is speed of journal in rps, Z is absolute viscosity in cP, C is diametral clearance between bearing and journal in inches. For most bearings in automotive use, k = 0.002. p is pressure = W/LD, W is bearing load in lbs. An average value for D/C is 1000. In automobile and piston aircraft engines, the loads W, are: Main – 700-1700 lbs., Crankpin – 1,400-3,400 lbs., and Wrist pin - 2,000-5,000 lbs. The ratio of e to the radial clearance is called the “attitude” and is defined as: Eqn. 2, A = 2e/C = 1- 2Ho/C. Ho is the “Minimum Film Thickness.” So how does one find the viscosity required for a bearing? One uses the “McKee” equation, along with the “Lasche” equation, for bearing heat generation and dissipation, Hd and Hg, and equates these equations to find the viscosity required!
Typical minimum film thicknesses, in hydrodynamic lubrication for these types of bearings, are on the order of 0.1 to 1um, with most operating films (in hydrodynamic lubrication) on the order of 1um to 18 um or less. Bearing clearances range from 0.001 to 0.002 inches for every 2.54 cm (1”) of journal diameter. During normal operation the journal is supported on an extremely thin film of oil, and the two parts should have no contact. As the rotational speed increases, the lubricant is drawn into the bearing by the rotating action of the journal (shaft). Consequently, the oil film becomes thicker and the increase in friction is therefore less than directly proportional to the speed. Conversely, at lower speeds, the oil film is thinner. At extremely low speeds (such as starting or “lugging”) there is likelihood that the bearing may fail or be permanently damaged unless there is a “barrier” lubricant between the shaft and bearing that can take the load and not shear under loads.
1.2 Filtration.and Bearing Wear
In Duchowski’s paper, he shows that the range of most abrasive particles that cause bearing wear are in the range of 3 um to 32 um. He suggests that the filter have a beta of B >= 200 at 6 um, “Given the film thickness which exists under standard (steady-state) operating conditions, the filter element appropriate for the application should exhibit a high removal efficiency for particles down to 10 um in size. In addition, the selected filter element should also exhibit a significant removal efficiency for particle removal down to about 3 um in size to allow for thinner film which exists under startup conditions.”
References:
1. Hall, et. Al., “Theory and Problems of Machine Design,” Schaum’s Outline series, Chapter 23, “Lubrication and Bearing Design.”
2. John K. Duchowski, Examination of Journal Bearing Filtration Requirements,” Lubrication Engineering, September, 1998, pp. 18-28.
[ September 03, 2002, 09:37 AM: Message edited by: MolaKule ]
and Filtration Requirements
By MoleKule
Keywords: Journal bearings; lubrication; contamination; filtration.
1.0 General
Bearings reduce the friction that robs machines of horsepower. Bearings are classified by type as Roller bearings, Ball bearings, Pin/Needle bearings, Journal (sleeve) bearings, and sliding bearings. Roller and ball bearings are usually found in applications such as transmissions and differentials. Pin/Needle bearings are also found in automatic transmissions and manual transmissions that use Automatic Transmission Fluids, and in 2-cycle engines. Sliding bearings in engines can be found in cam/tappet assemblies and of course in the cylinder, where the ring pack interfaces with the cylinder liner. In this paper we will focus on the design and filtration requirements for the sleeve bearings that support journals, such as crankshafts, connecting rods, and camshafts.
1.1 Sleeve Bearing Design
A number of factors affect bearing design including loads (forces), diameter of journal/bearing, length of journal/bearing, rotational speed of journal bearing, eccentricity of journal-bearing system, clearance between journal and bearing, the minimum oil film, the viscosity of the oil film, the heat produced by all frictions, the coefficient of friction of the journal-bearing system, and the volume of oil flow.
The journal does not rotate inside the bearing in a concentric fashion, rather is has a slight wobble or “eccentricity” (denoted as “e”) that generates not only a wedge of lubricant film, but also “pumps” the oil around the wedge. Eccentricity is defined as the distance between the bearing center and the journal (shaft) center. Assume the shaft is rotating counterclockwise in the sleeve and the bearing sleeve has a hole in it, and the shaft and bearing are immersed in an oil bath. The oil will be drawn into the “diametral” clearance when the shaft is rotating and will force the wedge of oil around the inside of the sleeve. In most case, we provide both oil-in and oil-out passageways, with a higher pressure on the “in” side to keep a clean and cool oil supply flowing.
Bearing friction is determined by an empirical equation (derived by test data) called the “McKee” equation as: Eqn. 1, f = 1x10^-10[473(ZN/p)D/C] + k, where k is a constant determined from another equation or chart for various L/D ratios; L is length of bearing (in.), D is diameter of bearing (in.), N is speed of journal in rpm, N’ is speed of journal in rps, Z is absolute viscosity in cP, C is diametral clearance between bearing and journal in inches. For most bearings in automotive use, k = 0.002. p is pressure = W/LD, W is bearing load in lbs. An average value for D/C is 1000. In automobile and piston aircraft engines, the loads W, are: Main – 700-1700 lbs., Crankpin – 1,400-3,400 lbs., and Wrist pin - 2,000-5,000 lbs. The ratio of e to the radial clearance is called the “attitude” and is defined as: Eqn. 2, A = 2e/C = 1- 2Ho/C. Ho is the “Minimum Film Thickness.” So how does one find the viscosity required for a bearing? One uses the “McKee” equation, along with the “Lasche” equation, for bearing heat generation and dissipation, Hd and Hg, and equates these equations to find the viscosity required!
Typical minimum film thicknesses, in hydrodynamic lubrication for these types of bearings, are on the order of 0.1 to 1um, with most operating films (in hydrodynamic lubrication) on the order of 1um to 18 um or less. Bearing clearances range from 0.001 to 0.002 inches for every 2.54 cm (1”) of journal diameter. During normal operation the journal is supported on an extremely thin film of oil, and the two parts should have no contact. As the rotational speed increases, the lubricant is drawn into the bearing by the rotating action of the journal (shaft). Consequently, the oil film becomes thicker and the increase in friction is therefore less than directly proportional to the speed. Conversely, at lower speeds, the oil film is thinner. At extremely low speeds (such as starting or “lugging”) there is likelihood that the bearing may fail or be permanently damaged unless there is a “barrier” lubricant between the shaft and bearing that can take the load and not shear under loads.
1.2 Filtration.and Bearing Wear
In Duchowski’s paper, he shows that the range of most abrasive particles that cause bearing wear are in the range of 3 um to 32 um. He suggests that the filter have a beta of B >= 200 at 6 um, “Given the film thickness which exists under standard (steady-state) operating conditions, the filter element appropriate for the application should exhibit a high removal efficiency for particles down to 10 um in size. In addition, the selected filter element should also exhibit a significant removal efficiency for particle removal down to about 3 um in size to allow for thinner film which exists under startup conditions.”
References:
1. Hall, et. Al., “Theory and Problems of Machine Design,” Schaum’s Outline series, Chapter 23, “Lubrication and Bearing Design.”
2. John K. Duchowski, Examination of Journal Bearing Filtration Requirements,” Lubrication Engineering, September, 1998, pp. 18-28.
[ September 03, 2002, 09:37 AM: Message edited by: MolaKule ]
