KEVOS® MAKEOS · Power Transmission
Step Thrust Bearing Design (Pad Layout, Film Thickness, Step Depth, Friction, Oil Flow, Temperature Rise)
For a step (Rayleigh step) thrust bearing carrying an axial load, what pad layout, film thickness and step depth result, how much friction power does it lose, how much oil does the film pass and how far does the oil temperature rise?
Step thrust bearing design by the handbook procedure, with its optimum proportions a = b, b2 = 1.2 b1 and step depth e = 0.7 h: outside diameter for a design unit load D2 = √(4W ÷ (π Kg p) + D1²), radial pad width a = (D2 − D1) ÷ 2, pitch-line circumference B = π(D1 + D2) ÷ 2, approximate number of pads B ÷ (a + s), pad length b = B ÷ i − s, pitch-line velocity U = B N ÷ 12, film thickness h = √(2.09 × 10⁻⁹ i a³ U Z ÷ W), friction power Pf = 7.35 × 10⁻¹³ i a² U² Z ÷ h, step position b2 = 1.2 b ÷ 2.2, hydrodynamic oil flow Q = 6.65 × 10⁻⁴ i a h U and temperature rise Δt = 42.4 Pf ÷ (c Q). It needs no chart readings.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Axial load | N | Axial (thrust) load on the bearing. |
| Inside diameter D1 | mm | Inside diameter of the thrust face, set by the shaft and its clearance. |
| Outside diameter chosen D2 | mm | Outside diameter you choose; for a lightly loaded positioning bearing it is chosen for convenient proportions. |
| Design unit load (for the diameter required) | kPa | Unit load used to find the outside diameter required; the handbook gives 200 psi (1379 kPa) normal and 500 psi maximum for step bearings. |
| Fraction of circumference occupied by pads Kg | Fraction of the circumference occupied by pads; usually 0.8. | |
| Oil groove width s | mm | Width of each oil groove; the handbook takes 0.062 in (1.6 mm) as the minimum. |
| Number of pads chosen | pads | Number of pads, the even number nearest the approximate number shown. |
| Speed N | rpm | Shaft speed. |
| Oil | ||
| Oil viscosity at operating temperature | cP | Oil viscosity in centipoises at the operating temperature. |
| Oil density | kg/m³ | Density of the oil. |
| Oil specific heat | kJ/(kg·K) | Specific heat of the oil. |
| Largest oil temperature rise you accept | °C | Largest temperature rise of the oil you accept; the handbook gives 50 °F (27.8 °C) as an acceptable maximum for thrust bearings. |
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: Axial load
- What it is
- Axial (thrust) load carried by the thrust bearing.
- Units and format
- Enter a number in N: newtons. The form accepts 0.000001 N to 10000000 N. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 111.21 N as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Raises the unit load and the diameter required.
- Get it right
- Include hydraulic, gear and impeller thrust at the worst running condition.
What this means: Inside diameter D1
- What it is
- Inside diameter of the thrust face.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 3000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 25.4 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger bore needs a larger outside diameter for the same area.
- Get it right
- Set by the shaft diameter plus running clearance.
What this means: Outside diameter chosen D2
- What it is
- Outside diameter of the thrust face or pads.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 3000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 76.2 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Sets the pad width and pitch circumference.
- Get it right
- Round the diameter required to a convenient size, or use the largest that fits.
What this means: Design unit load (for the diameter required)
- What it is
- Unit load used to find the outside diameter required for the thrust face.
- Units and format
- Enter a number in kPa: kilopascals. The form accepts 0.000001 kPa to 20000 kPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1379 kPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Only changes the diameter required shown.
- Get it right
- About 1379 kPa (200 psi) normal and 3447 kPa (500 psi) maximum for step bearings.
What this means: Fraction of circumference occupied by pads Kg
- What it is
- Fraction of the pitch circumference occupied by pads rather than grooves.
- Units and format
- Enter a plain number with no unit. The form accepts 0.000001 to 1. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.8 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A smaller fraction needs a larger outside diameter.
- Get it right
- Usually 0.8; 0.8 or 0.9 for tapered land bearings.
What this means: Oil groove width s
- What it is
- Width of each oil groove between pads.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.575 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Wider grooves shorten the pads.
- Get it right
- At least 1.6 mm (0.062 in); wider than any chamfer added later.
What this means: Number of pads chosen
- What it is
- Number of pads around the thrust face.
- Units and format
- Enter a number in pads: pads. The form accepts 2 pads to 100 pads. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 6 pads as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Sets the pad length and the unit load.
- Get it right
- Take the even number nearest the estimate shown.
What this means: Speed N
- What it is
- Rotational speed of the shaft.
- Units and format
- Enter a number in rpm: revolutions per minute. The form accepts 0.000001 rpm to 100000 rpm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5000 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Sets the pitch-line velocity, friction and film.
- Get it right
- Use the running speed at full thrust load.
Oil
What this means: Oil viscosity at operating temperature
- What it is
- Viscosity of the oil in centipoises at the operating temperature.
- Units and format
- Enter a number in cP: cP. The form accepts 0.000001 cP to 10000 cP. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 25 cP as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Higher viscosity thickens the film and raises friction.
- Get it right
- Read it from the oil’s viscosity–temperature chart.
What this means: Oil density
- What it is
- Density of the lubricating oil.
- Units and format
- Enter a number in kg/m³: kilograms per cubic metre. The form accepts 0.000001 kg/m³ to 2000 kg/m³. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 880 kg/m³ as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- With the specific heat, sets the heat the oil carries per litre.
- Get it right
- About 850 to 900 kg/m³ for mineral oils.
What this means: Oil specific heat
- What it is
- Specific heat of the lubricating oil.
- Units and format
- Enter a number in kJ/(kg·K): kilojoules per kilogram per kelvin. The form accepts 0.000001 kJ/(kg·K) to 10 kJ/(kg·K). Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.9383 kJ/(kg·K) as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher specific heat lowers the temperature rise or the flow needed.
- Get it right
- About 1.9 to 2.0 kJ/(kg·K) for mineral oils.
What this means: Largest oil temperature rise you accept
- What it is
- Largest temperature rise of the oil through the bearing you accept.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts 0.000001 °C to 100 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 27.778 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Fails the design when the rise is higher.
- Get it right
- The handbook gives 27.8 °C (50 °F) as an acceptable maximum.
Method and formulas
- D2 = √(4W ÷ (π Kg p) + D1²); a = (D2 − D1) ÷ 2; B = π(D1 + D2) ÷ 2; approximate pads B ÷ (a + s); b = B ÷ i − s; U = B N ÷ 12 ft/min.
- h = √(2.09 × 10⁻⁹ i a³ U Z ÷ W) in; step depth e = 0.7 h; step position b2 = 1.2 b ÷ 2.2 from the leading edge.
- Pf = 7.35 × 10⁻¹³ i a² U² Z ÷ h hp; Q = 6.65 × 10⁻⁴ i a h U gpm; Δt = 42.4 Pf ÷ (c Q) °F with c in Btu/gal °F.
- Inputs are converted to inches, pounds and °F for the handbook constants; results are shown in SI with the inch values.
- Example: 6 pads on 25.4 × 76.2 mm run with a 0.14553 mm film and a 0.10187 mm step, losing 0.098346 kW and raising the oil 15.267 °C.
Limitations
- The procedure assumes the optimum proportions a = b, b2 = 1.2 b1 and e = 0.7 h.
- If the oil flow is too small (temperature rise too high), add feed chamfers as in the flat plate thrust bearing design.
- Step bearings become sensitive to alignment as their size increases.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Design a small positioning thrust face for a shaft
- Check the film thickness and temperature rise of a step thrust bearing
- Size the step depth and position on each pad
Who it is for
- Machine designers
- Pump, compressor and turbine engineers
- Students
What you get
- Outside diameter required for the design unit load
- Pad width, pitch circumference, approximate number of pads and pad length
- Pitch-line velocity and actual unit load
- Film thickness and step depth
- Step position from the leading edge
- Friction power, oil flow and temperature rise
Interpreting the result
- The film thickens with speed and viscosity and thins with load.
- The step depth is fixed by the film: machine it to about 0.7 h.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2339, 2345–2347. Topic reference; no source tables or recommended settings are reproduced.
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