Bushing PV Calculator
Estimate plain bearing pressure, shaft surface speed, PV load, derated material limit, utilization, and allowable radial load from bushing geometry, speed, lubrication, and temperature.
Choose a starting case, then adjust the actual radial load, shaft speed, diameter, length, material PV limit, lubrication factor, and temperature derate.
Within derated PV limit
Oil bronze motor shaft | projected area D x L | normal variation service factor.
| Material type | Typical PV limit | Pressure guide | Speed guide | Best fit |
|---|---|---|---|---|
| Oil impregnated bronze | 50,000 psi-ft/min | Up to 2,000 psi | Moderate-high | General rotating shafts |
| Cast bronze, lubricated | 75,000 psi-ft/min | Up to 4,000 psi | Moderate | Pumps, gearboxes, pins |
| Graphite plugged bronze | 20,000 psi-ft/min | Up to 3,000 psi | Low-moderate | Dry pivots and dirty areas |
| PTFE lined steel backed | 25,000 psi-ft/min | Up to 8,000 psi | Low-moderate | Light duty, no grease |
| Acetal or nylon polymer | 12,000 psi-ft/min | Up to 1,000 psi | Low-moderate | Light guides and rollers |
| Fiber composite | 60,000 psi-ft/min | Up to 10,000 psi | Low-moderate | Oscillating pins |
| Lubrication condition | Factor | What it means | Common caution |
|---|---|---|---|
| Dry or uncertain | 0.45 | Little film, high friction heat | Watch startup and contamination |
| Light boundary film | 0.70 | Some oil or residue present | May not survive long high PV runs |
| Periodic grease | 0.85 | Greased interval with mixed film | Relube interval matters |
| Catalog oil condition | 1.00 | Comparable to many data sheet ratings | Confirm viscosity and supply |
| Forced oil film | 1.15 | Excellent cooling and film renewal | Do not exceed material pressure rating |
| Temperature band | Derate | Typical issue | Design response |
|---|---|---|---|
| Below 120 F / 50 C | 1.00 | Normal running | Standard PV check |
| 120-160 F / 50-70 C | 0.90 | Oil thinning begins | Add margin and inspect wear |
| 160-200 F / 70-95 C | 0.75 | Polymer softening or oxidation | Improve cooling or reduce load |
| 200-250 F / 95-120 C | 0.60 | Rapid lube degradation | Use high temp material and lube |
| Above 250 F / 120 C | 0.45 | Severe wear risk | Supplier review strongly recommended |
| Check item | Formula | Imperial units | Metric units |
|---|---|---|---|
| Projected area | D x L | in^2 | mm^2 |
| Bearing pressure | Load / area | psi | MPa |
| Surface speed | pi x D x RPM | ft/min | m/s |
| PV value | Pressure x speed | psi-ft/min | MPa-m/s |
| Derated PV limit | Limit x lube x temp | psi-ft/min | MPa-m/s |
The calculations of the PV value are often a necesary part of the decisions of which plain bearing to use for a particular application. The PV calculations can help to determine if a given combination of load, speed, and materials will remain within the limits that are published for those materials in relation to heat and wear. While many may only consider the PV calculations after a plain bearing begins to smoke or seize, PV calculations are used as a means of preventing such failures of plain bearings in applications as diverse as pumps, linkages, conveyors, and numerous types of hinges.
You can calculate the PV value by multiplying the values of the pressure and velocity of the system. Pressure is calculated by dividing the radial loads by the projected area of the bushing; the projected area of the bushing is the diameter of the bushing times the length of the bushing. Velocity is the rate at which the area of the bearing move at the diameter of the shaft.
How to Calculate PV for Plain Bearings
Multiplying these two values will yield the PV value for the plain bearing. This PV value will be a single number that indicates the amount of heat and wear that the plain bearing will experience. The limit for plain bearing materials is not a fixed number; the limit for plain bearing materials change according to a variety of different factors.
For instance, a plain bearing material may have a limit of fifty thousand psi-ft/min under ideal conditions. However, if the plain bearing is subjected to dry running conditions at one hundred and eighty degrees, it will not reach that limit. In these cases, you must apply derating factors for both lubrication and temperature to the limit for that plain bearing material.
One derating factor is for lubrication; the lubrication factor considers how well the plain bearing is lubricated. If the plain bearing is dry or undergoing boundary lubrication, fewer loading of oil will exist between the two moving parts. Forced lubrication will allow the material to reach its limit.
Similarly, if the temperature of the plain bearing increases, the polymers in the plain bearing will soften, the oil will thin, and the rate at which the plain bearing undergoes oxidation will increase. Thus, the allowable PV of the plain bearing will decrease with increasing temperature prior to comparing such an allowable PV to the PV that is calculated for that plain bearing. A third derating factor is for the service factor; plain bearings are rarely subjected to steady loads, and the loads that are applied to plain bearings are rarely applied in perfect accuracy with any given design.
Motors that are used to run plain bearings may, for instance, drive fans with relatively smooth and even torque, but other motors may drive crushers or other devices that experience shock loads when the motors are started or stopped. Thus, applying a service factor to plain bearings prior to calculating the load and PV will ensure that the plain bearing is sized to handle the expected loads. Different plain bearing materials has limits at different levels.
Oil-impregnated bronze is the most common plain bearing material; however, one may use graphited-plugged bronze in situations in which the plain bearing must run in dry and dirty environments. PTFE-lined sleeves are used for plain bearings that experience high levels of pressure and low velocities. Fiber composite plain bearings are used for situations in which the plain bearing experiences oscillation or edge loading.
Each of these materials has limits with relation to the pressure, velocity, and temperature of the plain bearing. However, the supplier data for individual plain bearing parts will feature limits for those same parameters that are more restrictive than the general descriptions of those materials. A person should not rely upon the PV value to be the only check performed on plain bearings.
Plain bearings can pass the PV value calculation yet fail in their use. Plain bearings may fail if the housing cannot remove the heat from the plain bearing, if the shaft has a rough finish, if the shaft is not aligned with the housing, and a variety of other factors. Oscillations of plain bearings may lead to fretting and brinelling at the areas in which the plain bearing shafts experience the change in movement from oscillations.
Such failures can occur even if the plain bearing passes the PV value calculation. A consideration of plain bearings is the consideration of the conditions under which the plain bearing is to operate; the PV value for plain bearings may be misleading in some situations. For instance, if the plain bearing is stationary and does not have any motion, the velocity of the plain bearing is zero.
Thus, the PV value for the plain bearing will be zero. However, if there is a high load during the moment at which the plain bearing begins to move, the plain bearing may fail. Thus, in situations with high loads during startup, a plain bearing made of a material that can endure the load without lubrication may be required, or lubrication may need to be provided prior to startup.
The length and diameter of plain bearings make up different considerations for plain bearing design. For instance, increasing the length of the plain bearing will reduce the pressure that is placed upon the plain bearing. Thus, increasing the length of the plain bearing will improve the PV value calculation for that bearing.
However, increasing the length of the plain bearing may increase the risk that the plain bearing will not be able to evenly distribute the load that is placed upon the bearing. Thus, short plain bearings with a wide diameter will reduce the risk of misalignment between the plain bearing and the shaft, but these plain bearings will experience increased pressure; the limit of the plain bearing material will become the determining factor in plain bearing design. Temperature effects upon plain bearings must also be considered.
The high temperature of plain bearings will reduce the lubrication factor that is allowed for the plain bearing, and the PV value of the plain bearing will be derated according to the temperature of the plain bearing. Additionally, plain bearings that increase in temperature may experience additional changes in the dimensions of the plain bearing and the housing; thus, the plain bearing may seize or wear at high temperatures. For instance, the plain bearing may run well at one hundred twenty degrees; however, if the housing reaches one hundred eighty degrees, the plain bearing may experience failure.
Plain bearings may be designed to have a PV value that is within a certain percentage of their derated limit. Many plain bearing designers may aim for a plain bearing utilization that is under eighty percent of its limit. This percentage can be calculated with the plain bearing screening process; the calculation will not only provide the designer with a figure that indicates the utilization of plain bearing materials, but will also indicate the load that can be placed upon plain bearing with certainty that the plain bearing will remain within the targeted percentage of its limit.
Plain bearings that are successful in their design are those that satisfy each of these conditions; the PV value calculation is the first of such checks to be performed in the design of plain bearings. However, the successful life of plain bearings may also depend upon factors beyond the PV calculation.
