BIZOMAT MAKEOS · Engineering
Thrust Bearing Pad Unit Load and PV
What unit load, sliding speed and PV does this pad or washer thrust bearing run at?
Pad area, unit load, mean sliding velocity and PV of a flat-land, tapered-land or tilting-pad thrust bearing (or a thrust washer), against allowable values.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Outside diameter | mm | Outer diameter of the pad ring. |
| Inside diameter | mm | Inner diameter of the pad ring. |
| Pad coverage of the ring | % | Share of the ring area covered by pads (100% for a plain washer; 75–85% typical for pads with oil grooves). |
| Thrust load | N | Axial load. |
| Speed | rpm | Speed. |
| Allowable unit load | MPa | Allowable unit load for the bearing type and material (hydrodynamic pads often 1.5–3.5 MPa; washers depend on material). |
| Allowable PV (0 = not checked) | MPa·m/s | PV limit for dry or boundary-lubricated washers; 0 to skip. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Outside diameter
- What it is
- The outer diameter of the ring of pads or of the thrust washer, the outside edge of the bearing surface.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 150 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the edge of the working surface, not of the housing.
What this means: Inside diameter
- What it is
- The inner diameter of the ring of pads or of the thrust washer, the inside edge of the bearing surface.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 80 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- It must be smaller than the outside diameter.
What this means: Pad coverage of the ring
- What it is
- The share of the full ring area actually covered by bearing surface, after oil grooves or gaps between pads are taken out.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 1% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 80% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Unit load and PV are inversely proportional to coverage.
- Get it right
- Use 100 for a plain washer; measure or calculate the pad area for a pad bearing.
What this means: Thrust load
- What it is
- The axial (thrust) force pressing the rotating collar against the pads or washer.
- Units and format
- Enter a number in N: newtons. The form accepts 0.000001 N or more. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 20000 N as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Unit load and PV are directly proportional to the thrust load.
- Get it right
- Include the largest thrust in service, such as from helical gears or pump impellers.
What this means: Speed
- What it is
- The rotational speed of the collar relative to the pads, used to find the sliding speed at the mean diameter.
- Units and format
- Enter a number in rpm: revolutions per minute. The form accepts 0.000001 rpm to 1000000 rpm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1500 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- PV is directly proportional to speed; unit load does not change.
- Get it right
- If both parts turn, use the relative speed.
What this means: Allowable unit load
- What it is
- The highest unit load allowed for this bearing type and material, from the bearing or material supplier or your design reference.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 1000 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Take it from the supplier for the actual bearing material and lubrication.
What this means: Allowable PV (0 = not checked)
- What it is
- The limit on unit load times sliding speed for dry or boundary-lubricated washers, from the bearing material supplier. Enter 0 to skip the PV check.
- Units and format
- Enter a number in MPa·m/s: MPa·m/s. The form accepts 0 MPa·m/s to 10000 MPa·m/s. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0 MPa·m/s as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- A PV limit applies to a stated material and lubrication; it does not replace the unit load check.
Method and formulas
- Area = π(Do² − Di²)/4 × coverage.
- Unit load = W ÷ area.
- Mean velocity = π (Do + Di)/2 × n ÷ 60 000; PV = unit load × mean velocity.
- Example: 20000 N on a 150/80 mm ring at 80% coverage is 1.9771 MPa.
Limitations
- Screening by unit load and PV; hydrodynamic film thickness and temperature rise need the full design method.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check a thrust washer or collar
- Screen a tilting-pad or tapered-land bearing size
- Compare pad geometries
Who it is for
- Design engineers
- Maintenance engineers
What you get
- Pad area and unit load
- Mean sliding velocity
- PV value
Interpreting the result
- If the unit load is too high, increase the outside diameter or the number of pads.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2338–2356. Topic reference; no source tables or recommended settings are reproduced.
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Method explained here · BIZOMAT original book
Thrust bearings
In Plain Bearings by the Numbers
What it covers: Check a thrust face's area, unit load, sliding speed and PV, estimate its friction heat and oil flow with the BIZOMAT thrust tools, and see why tapered lands and tilting pads carry load.
BIZOMAT tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.