Guidelines for Bearing Design and Selection
Bearings with or without lubrication must be designed with consideration of the individual physical characteristics and operating conditions of each actual requirement. The design data below is provided as an introduction to bearing design.
PV value
PV value product (product of Bearing Pressure P and Velocity V) is an important parameter in selecting a OilLess Bearing. The friction Calorie Q generated in unit Area of Bearing for unit Time is obtained from the Energy Loss m, , P and V, using the following formula.
, , P and V, using the following formula.
Q = m · P · V
P = Bearing Pressure N/mm2 {kgf/cm2}
V = Velocity m/s {m/min}
m = Co-efficient of Friction.
Assuming that the Coefficient of Friction is a constant, the Friction Calorie Q, a criterion in selection of Bearings, is proportional to PV value. During continuous running of shaft, the friction heat generated at the Bearings and the heat lost by radiation maintains the Bearing temperature at a constant balance. After an extended period of operation, however, the Coefficient of Friction may increase due to changes in the sliding surface conditions. The existence of foreign particles, deteriorated lubricants, material fatigue and other factors will influence the Coefficient of Friction.
As a result, when the Bearing temperature rises, eventual damage to sliding surfaces or seizure is considered to occur. The lower the Bearing temperature stays, or in other words, the smaller the PV value is, the lighter the load conditions of the Bearing is considered to be.
Safety design of Bearings means that Bearings are to be designed to lower PV value, which leads to a longer service life.
Furthermore, under certain conditions, the PV value can exceed the maximum
Maximum Allowable PV Value
The maximum allowable PV value is defined as the maximum product of the Pressure projected on a unit Area of the Bearing and the Velocity allowable in the design of the Bearing. Also specified are the maximum allowed Load and Velocity.
In the case of a radial journal Bearing, the projected Area is obtained by multiplying the Inner Diameter of the Bearing by the Bearing Length.
Bearing Load
In general, the Bearing Pressure is obtained by dividing the maximum Load imposed on the Bearing by the Pressure Supporting Area of the Bearing. The Pressure Supporting Area is defined as the projected loading Area, which contacts with the shaft, projected in the direction of In general, the Bearing Pressure is obtained by dividing the maximum Load imposed on the Bearing by the Pressure Supporting Area of the Bearing. The Pressure Supporting Area is defined as the projected loading Area, which contacts with the shaft, projected in the direction of the load in cases of cylindrical and spherical Bearings.
Velocity
The main cause of generated heat is the work done at the friction surface of the Bearing. It is known from experience that the rise in temperature at the friction surface is affected more by the Velocity than by the Pressure. With the same PV value, the larger the V value is, the higher the Bearing temperature will be. When used in a high Velocity operation, it is recommended that the Bearings should be designed and used in such a manner, that the Coefficient of Friction be reduced by a positive supply of oil to enhance both cooling and lubricating effectiveness, in order to r to take advantage of their wear resistance and anti-seizure.
Direction of Motion and PV Value
In the case of continuous, one directional rotational motion of a radial journal, proper fluid film lubrication is easily accomplished for trouble free operation. In the case of a thrust Bearing, however, careful consideration must be given in designing the lubrication method and oil grooves, so as to ensure proper lubrication of the sliding surface. Moreover, when intermittent operations, oscillating motion or reciprocating motion is encountered. For a reciprocating motion or rotational and oscillating motion of a Thrust Bearing about half of the maximum allowable PV value should be employed. Even a lesser value is applied for an oscillating motion.
Operation Intervals
Operation may either be continuous or intermittent. Intermittent operations can be advantageous for general type Bearings, because of intervals, which allows generated friction heat to cool down. This enables a PV value to remain relatively high. The disadvantage of intermittent operations is that frequent operational intervals tend to cause inadequate lubrication, resulting in increasing wear amount. Moreover, seizure or scoring may occur when restarting. The heavy load imposed in an intermittent operation is liable to cause boundary lubrication condition. A Bearing should be selected which safely endures friction and wear in that condition. OilLess ZA#500 and ZA#1000 in particular, has a high load carrying capacity and displays excellent performance in intermittent operations with high load because of the tough film of the solid lubricants covers the sliding surface.
Oscillating Motion and Bearings
The oscillating motion is considered to be one of the most severe conditions to Bearings because it passes through points of zero velocity in each cycle of motion. Oil film is liable to be disrupted, fatigue and wear of material be accelerated and wear particles tend to remain longer. The ball Bearings, which are designed mainly for rotational motion, have a very small contact area causing, extreme high contact stress to develop at their pressure supporting areas. They are, thus, unsuitable for oscillating motions because of material fatigue. Sleeve Bearings, which have a large contact area, are generally considered better for this application.
Fretting Wear and OilLess Bearing
In some cases, like that of weaving machine application, slight oscillating and reciprocating motion are being manufactured yet without close study, it appears only as a slight vibration.
The type of wear developed in this situation is fretting corrosion or fretting wear. The cause of fretting corrosion is considered to be the fine wear particles generated by friction. Wear is further accelerated when these particles oxidize and create a highly abrasive surface. Effective measures to prevent fretting corrosion are to reduce the coefficient of friction and prevent wear particles from oxidizing. Plating shaft surfaces and applying grease may be effective. The NASA reported, that providing a solid lubricant film on the sliding surface is the most effective prevention for this type of wear.
Among OilLess Bearings, solid lubricant Bearings, OilLess JA#500 have proven to be effective for such applications. Since preventive effectiveness may differ, depending on actual surface conditions it is recommended that preliminary tests be performed to ensure that the most effective Bearing is selected.
Impact load and OilLess Bearings
OilLess Bearings` resistance to impact load is far superior to the general ball Bearings. Since impact load is a severe condition for Bearings and can vary according the environment, it is important to perform preliminary tests in order to find the suitable Bearing for practical use.
Rotating Bearings
When a Bearing is rotated under a high pressure such as a sheave metal or wheel Bearing, fatigue of Bearing material cannot be neglected. For operations under such conditions, it will be safer to limit the PV value to within one third of the maximum allowable PV value.
Oil Grooves and Oil Holes
When designing an oil hole and/or an oil groove, care should be taken as indicated in the following
1) Design the oil hole and oil groove in axial direction as one body, and locate them avoiding the area of maximal load.
2) Design the oil groove in axial direction to a length, 70 ~ 80% that of the Bearing.
3) Have no sharp edges in cross section of the oil groove and make properly rounded edges to protect oil film from breakage.
4) Prepare the circumferential groove on the OD of the Bearing to prevent inadequate lubrication due to misalignment of the oil hole in the case of oiling from the housing side.
Length and Thickness of Bearing and Chamfering
Since a shaft diameter is provided, the Bearing pressure (Kgf/cm) as a load condition for the Bearing is determined by its length. In general, the Bearing length is determined that L/d (Bearing length inner dia.) is 0.5 ~ 2.0. When high load or high-speed operations are the case, L/d is limited to 0.8 ~ 1.0 to avoid partial contact or excess heat generation due to high speed.
Wall Thickness of Bearings
A great advantage of sleeve Bearings is that they can be designed with a thinner thickness than the case of roller Bearings because of relatively small restriction imposed on the thickness. This is an important point indispensable to compact design of devices. OilLess Bearing makes available various structural modes of self-lubricating sleeve Bearings to facilitate compact design.
Bearings must be chamfered
To release stress concentration at both ends of Bearing, chamfering are applied to OilLess Bearings. Two types of chamfering are generally used, C-chamfer and R-chamfer. R-chamfer is applied to reciprocating motion and a composite motion mode of reciprocating and rotating.
