Oil is the lifeblood of bearings. Grooves deliver more of it than holes.
Grooves move more lube
M. M. Khonsari
Mechanical Engineering Dept.
Louisiana State University
Baton Rouge, La.
E. R. Booser
Oil-fed sleeve bearings serve in a wide array of applications from automobile engines to household appliances to industrial pumps. Just a thin oil film separates bearing and shaft surfaces under load, keeps operating temperatures at reasonable levels, and expels air and contaminants. But delivering oil to bearings where it's needed and keeping it there without compromising oil-film pressure can be tricky.
Key is proper channeling of oil through various shaped grooves or supply holes. Bearings under steady load, such as a horizontal bearing supporting a rotor as dead weight, can be fed by a supply hole or an axial groove in the unloaded portion. Holes or grooves typically locate in the top half of a bearing which is characterized by large annular clearances and low internal oil pressures.
Supply holes work fine for small bearings and bushings, though bearing diameters or lengths exceeding about 1 to 2 in. should use axial grooves. Axial grooves deliver about 2.53 more oil than holes, which helps ensure the formation of a full oil film and better cooling. Axial grooves typically locate opposite load lines and extend axially from a bearing midplane to about 80% of bearing length, L. Some cylindrical bearings (to ease manufacture) may use two parallel axial grooves 90° to a load line at the split between halves.
A circumferential groove about a bearing midlength splits it into two identical segments. Then oil flow and power loss equal that of two half-length journal bearings. Circumferential grooves are appropriate when applied loads change direction over a fairly wide circumferential angle (>90°). Crankshaft main bearings in automotive and diesel engines use this arrangement, for example. Locating a groove off-center, however, lowers load capacity and raises eccentricity of the shorter half, possibly causing shaft axial misalignment.
Helical (spiral) grooves in vertical bearings help retain oil that otherwise tends to drain axially under gravity. Diagonal grooves extending from an oil hole or a central circumferential groove with a wick or oil ring feed also work on horizontal shafts and bearings. These diagonal grooves help distribute the limited oil feed along the entire length of a bearing bore.
As a rule, groove width and depth should be at least one-tenth and one-thirtieth that of shaft diameter, respectively. Angular width can be extended up to 30° for smooth lubricant flow exceeding 100 gpm in high-speed turbine or compressor bearings. In all cases groove depth should be set to maintain axial oil velocity below about 50 to 100 ips for good oil distribution.
Go with the flow
Total oil-flow rate from a feed hole or an axial groove is the sum of rotational and pressure-induced flow. Rotational flow, QR (in.3/sec), is caused by shaft rotation dragging oil along. Pressure-induced flow, QP, as the name implies, is the result of feed (supply) pressure in the inlet hole or groove. Note that circumferential grooves minimize rotational flow because there is little axial component of shaft surface velocity relative to grooves to drag oil into the bearing film.
Maximum rotational flow issuing from a hole or an axial groove can be estimated from:
QR,Max = (πDN/2)AC (1)
where AC = feed cross-section area (in.2), and πDN/2 = average film velocity between a stationary bearing bore, D (in.) and a shaft rotating at N rev/sec.
For an axial groove of length l, AC = lh, where h = film thickness (in.) at the feed location. Similarly, for a feedhole of diameter, d, AC = dh. Oil recirculating past the minimum film thickness to a feed zone lowers flow rates predicted by equation (1). A feed coefficient, kR, accounts for this effect:
QR = kRQR,Max (2)
Large kR values for feedholes or axial grooves apply to bearings with high loads or those with limited oil recirculation past areas of minimum film thickness.
A modified version of equation (2) models flow rate in a 45° spiral angle groove:
QR = kR(µDN/2)(sin 45°) wdG (3)
where w = groove width and dG = groove depth. Linear flow velocity along a spiral groove is about 40% of shaft surface velocity for common groove cross sections. Bearings used in vertical applications may employ three spiral grooves so total groove flow = 3QR. Oil feed in a spiral groove is typically not pressurized.
Pressure-induced flow rate, QP, (in.3/sec) can be estimated by:
QP = kPPSh3/µ (4)
where µ = viscosity in reyns (lb-sec/in.2), h = clearance between a shaft and bearing bore at the inlet, and kP = pressure-feed coefficient. Typically oil is supplied at about 20 to 50 psi then dropped by an orifice to a PS of 5 to 15 psi in a feedhole or groove. Supply pressure flow adds to rotational feed flow which boosts cooling. The addition of end-bleed ports to grooves works for applications needing more oil than available from combined rotational and pressure feeds. Conventional orifice equations help size bleed ports so total bearing oil flow exits bearings at about 140 to 160°F.
For more on the subject see F. A. Martin, Institution of Mechanical Engineers, Journal of Engineering Tribology, Vol. 212(J4), pp. 413-425 and M. M. Khonsari and E. R. Booser, Applied Tribology-Bearing Design and Lubrication, John Wiley & Sons, 2001.
Representative feed coefficients for single oil holes and grooves
|Spiral groove (45°)||0.3-0.5|
Comparing oil feed geometry
|QR||QP||QR + QP|
|Axial groove, 2.5-in. length||1.7||1.3||3.0|
|Oil hole, 0.375-in. diameter||0.3||0.5||0.8|
|Circumferential groove,QR = 0||-||1.2||1.2|
|Spiral groove, 45°, width = D/10, depth = D/30, PS = 0||4.0||-||4.0|
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