Hertzian Pressure in a Bearing
Hertzian Pressure in a Bearing
(OP)
Hy,
I try to calculate the stress inside a journal bearing, if the shaft gets into contact with the bearing housing during shocks. I calculated with the formulas of Hertz (cylinder inside cylinder) and I found very low values for the pressure. This results from 1/r1+1/r2 (r1=radius of the shaft=100, r2=radius of journal housing=-100.3) in the numerator of the formula, which becomes very low due to the different signs in the radius. Some questions..
Is the calculation correct because the values are so low ?
Are there other formulas for this problem ?
Is the use of Hertzian pressure allowed for this problem ?
What other calculation/check would you make for this shock-contact-situation ?
I'm concerned, because the values for the pressure are so low for typical journal bearing geometries (radius nearly identical but different sign), that theoretically there would nearly never be problem..
Thanks for your comments
Frank
I try to calculate the stress inside a journal bearing, if the shaft gets into contact with the bearing housing during shocks. I calculated with the formulas of Hertz (cylinder inside cylinder) and I found very low values for the pressure. This results from 1/r1+1/r2 (r1=radius of the shaft=100, r2=radius of journal housing=-100.3) in the numerator of the formula, which becomes very low due to the different signs in the radius. Some questions..
Is the calculation correct because the values are so low ?
Are there other formulas for this problem ?
Is the use of Hertzian pressure allowed for this problem ?
What other calculation/check would you make for this shock-contact-situation ?
I'm concerned, because the values for the pressure are so low for typical journal bearing geometries (radius nearly identical but different sign), that theoretically there would nearly never be problem..
Thanks for your comments
Frank





RE: Hertzian Pressure in a Bearing
No such thing as a negative radius, unless you have bearings with imaginary cross sectional area. A radius is the (always positive) distance from the center of a circle to its circumference. Change the sign back to a positive value and re-compute.
RE: Hertzian Pressure in a Bearing
The reason I guess is that we are really talking about curvature.
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Hertzian Pressure in a Bearing
RE: Hertzian Pressure in a Bearing
frank
RE: Hertzian Pressure in a Bearing
EXAMPLE
A ball 1.50 in in diameter, in a race which has a diameter of 10 in and a groove
radius of 0.80 in, is subjected to a load of 2000 lb. It is required to find the
dimensions of the contact area, the combined deformation of ball and race at
the contact, and the maximum compressive stress.
Solution. The formulas and table of case 4 (Table 14.1) are used. The race is
taken as body 1 and the ball as body 2; hence R1 = -0.80 in, R01 = -5 in, and
R2 = R02 = 0.75 in. Taking E1 = E2 = 30,000,000 lb=in2 and n1 = n2 = 0:3, we get sigmac-252,000 lb=in2
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Hertzian Pressure in a Bearing
kd=D1*D2/(D1-D2)
sigmamax=0.591*sqrt(p*E/kd)
p is the load per unit length
so as D1 approaches D2 the stress approaches zero.
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Hertzian Pressure in a Bearing
black artscience of tribology was created to study the phenomena.RE: Hertzian Pressure in a Bearing
What do you use for P?
You really have to model the impact using momentrum and energy equations, make some assumptions on elssticity before you can apply that formula.
I haven't done this for some time but I know that any valid result will depend on these factors, and not simply writing an equation and making up some amplification of the load. I suppose the current literature has something on this.
RE: Hertzian Pressure in a Bearing