A bearing is a machine element that supports another moving machine element known as a journal. It enables a relative motion between the contact surfaces of the members while carrying the load. While doing so a certain amount of power is wasted in overcoming frictional resistance due to the relative motion between the contact surfaces. We need to study the types of Bearings, design, and materials used for bearings briefly. In the previous article, we discussed the different types of Bearings. Hydrodynamic Lubricated Bearings are a type of bearing that comes under Sliding Contact Bearings. In this article, let us discuss the Bearing Modulus and Characteristic Variety for Journal Bearings.
A little thought will reveal that the comparative movement between the contact surfaces causes a certain amount of power to be lost in overcoming resistive resistance. There will be quick wear when the scratching surfaces come into direct contact. A part of fluid may be provided to lessen viscous resistance and wearing and, in some cases, to absorb the heat generated. The fluid used to distinguish the blog and holding is usually a nutrient oil refined from hydrocarbon, but veggie oils, silicone oils, greases, etc., may be used.
Bearing Characteristic Number for Journal Bearings
Because it provides a way to determine the loss of power brought on by bearing resistance, the coefficient of friction in bearing style is very important. Experiments have demonstrated that the coefficient of friction for a journal bearing that is fully lubricated is a function of three factors, i .e.
Thus, the tension factor can be expressed in this manner.
Where
μ = Coefficient of friction,
φ = A efficient marriage,
Z = Absolute viscosity of the lubricant, in kg / m-s,
N = Speed of the journal in r.p.m.,
p = Bearing pressure on the projected bearing area in N/mm2 = Loadonthejournal÷ l×d d = Diameter of the journal,
l = Length of the bearing
c = Diametral clearance.
The element ZN / p is termed as bearing quality quantity and is a variable number. The McKee brothers ( S. A. McKee and T. R. McKee )’s analysis of the coefficient of friction in a real test of friction shows the relationship between the operating values of the bearing characteristic number ( ZN/p ). The issue ZN/p helps to predict the effectiveness of a holding.
The part of the slope Rating represents the area of heavy film lubrication. Between Q and R, the viscosity ( Z ) or the speed ( N ) is so low, or the pressure ( p ) is so great that their combination ZN / p will reduce the film thickness so that partial metal-to-metal contact will result. Between R and S on the slope, there is a narrow video, limit friction, or inadequate stimulation. This is where the lubricant’s stiffness stops being a function of friction characteristics and its oiliness helps to prevent full metal-to-metal contact and parts-related seizure.
A decrease in viscosity ( Z ) will reduce ZN / p, which will be followed by a drop in bearing temperature that will raise the viscosity ( Z ), so it should be noted that the portion PQ of the curve represents stable operating conditions.
Bearing Modulus for Journal Bearings
The bearing should not be operated at this value of bearing modulus because a slight decrease in speed or slight increase in pressure will cause the journal to operate with metal-to-metal contact. The following chart ( Variation of coefficient of friction with ZN/p ) shows that the minimum amount of friction occurs at A. This will result in substantial resistance, wear, and heating.
The bearing should be designed to have a ZN/p value at least three times the bearing modulus ( K ) minimum value in order to prevent such a situation. If the holding is subjected to huge fluctuations of weight and heavy effect, the value of ZN / p = 15 K may be used.
According to what we know above, a bearing will operate under either hydraulic or thick film lubrication when ZN/p values higher than K. On the other hand, when the value of ZN / p is less than K, then the oil film may tear and there is a metal-to-metal call.
