---
title: "Types of lubrication and viscosity"
subject: "Machine Design I"
chapter: 7
key: "semester_6/machine_design_i:lubrication-types-viscosity"
source: https://www.bctengineer.com/q/semester_6/machine_design_i/lubrication-types-viscosity
site: BCT Engineering
---

# Types of lubrication and viscosity

Machine Design I · Chapter 7: Lubrication and journal bearings

- Asked in: year not given, 6 marks
- Video: [Machine Design: Lubrication - Lecture 21A](https://www.youtube.com/watch?v=SpfTuSPj2T4) — MEchanicsSimplified | By Dr. G, PE
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/machine_design_i/lubrication-types-viscosity
- Whole chapter: https://www.bctengineer.com/md/semester_6/machine_design_i/chapter-7

## Question

Explain the types of lubrication (hydrodynamic, boundary, mixed, hydrostatic and elastohydrodynamic). Sketch the variation of coefficient of friction with ZN/p for a journal bearing and explain stable lubrication. Define dynamic and kinematic viscosity with their SI units.

## Short answer

Hydrodynamic (thick film, wedge), hydrostatic (external pump), elastohydrodynamic (rolling contacts), boundary (molecular film, asperities), mixed (partial). Stable lubrication: operate right of minimum on f vs ZN/p; temperature rise lowers viscosity, reduces friction, prevents seizure. Dynamic viscosity μ (Pa·s) = stress/gradient; kinematic ν = μ/ρ (m²/s).

## Answer

Lubrication reduces friction and wear and carries away heat by putting a film of lubricant between sliding surfaces.

#### Types of lubrication

1. **Hydrodynamic (thick film):** the surfaces are completely separated by an oil film whose pressure is generated by the motion itself (wedge action). Friction is very low (coefficient 0.001 to 0.01) and there is no wear. Example: journal bearing running at normal speed.
2. **Hydrostatic:** oil is supplied under pressure from an external pump, so the film exists even at zero speed. Used for heavy slow machines such as telescopes and large turbines.
3. **Elastohydrodynamic (EHL):** in rolling contacts such as gears and rolling bearings, the high contact pressure deforms the surfaces elastically and raises the oil viscosity so a very thin film is formed.
4. **Boundary:** the film is only a molecular layer, and the surfaces touch at asperities. Friction coefficient 0.05 to 0.2. Occurs at low speed, high load or starting and stopping. Additives (anti-wear, extreme pressure) protect the surface.
5. **Mixed (partial):** a combination of boundary and thick film lubrication.

#### Friction against $ZN/p$

```text
   f
   |\
   | \                 /
   |  \      mixed    /  thick film
   |   \_           _/
   |boundary \_____/
   +------------------------> ZN/p
              ^
          minimum friction
          (critical value)
```

Here $Z$ is viscosity, $N$ speed and $p$ bearing pressure. To the **right** of the minimum, the friction rises slowly with $ZN/p$ and the bearing is in **stable** lubrication: if the temperature rises, viscosity falls, $ZN/p$ falls, friction decreases and the bearing recovers. To the **left**, a fall in viscosity increases friction and heat further, leading to seizure (unstable). Journal bearings are designed so that $ZN/p$ is about three times the critical value.

#### Viscosity

- **Dynamic (absolute) viscosity $\mu$ (or $Z$):** the ratio of shear stress to velocity gradient, $\tau = \mu\,\dfrac{du}{dy}$. SI unit Pa.s (N.s/m$^2$); 1 poise = 0.1 Pa.s; 1 cP = 1 mPa.s.
- **Kinematic viscosity $\nu = \mu/\rho$.** SI unit m$^2$/s; 1 stoke = $10^{-4}$ m$^2$/s.

Viscosity falls with temperature; the viscosity index measures how little it changes.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. In hydrodynamic lubrication, where does the pressure that separates the surfaces come from?
   A. An external pump supplying oil under pressure
   B. The motion of the surfaces itself, which drags oil into a wedge
   C. Elastic deformation of the surfaces under contact pressure
   D. Additives that form a molecular layer
   Answer: B. The film pressure is generated by the relative motion (wedge action). An external pump describes hydrostatic lubrication. (Key point: Hydrodynamic film)

2. Why can a hydrostatic bearing carry load even at zero speed?
   A. Its oil is fed under pressure from an external pump
   B. The oil viscosity rises sharply at rest
   C. Surface asperities carry the load
   D. The shaft climbs the bearing wall
   Answer: A. Because pressure is supplied externally, the film does not depend on motion, so it exists at zero speed. (Key point: Hydrostatic lubrication)

3. On the friction vs ZN/p curve, why is the region to the right of the minimum friction point stable?
   A. A temperature rise lowers viscosity, which lowers ZN/p and friction, so the bearing recovers
   B. A temperature rise increases viscosity and friction
   C. Friction is zero in this region
   D. The surfaces are in boundary contact there
   Answer: A. On the right side a fall in viscosity reduces friction and heat, a self-correcting effect. On the left, it raises friction and leads to seizure. (Key point: Stable lubrication)

4. Which pair gives the SI units of dynamic viscosity and kinematic viscosity?
   A. m^2/s and Pa.s
   B. N/m^2 and m^2/s
   C. Pa.s and m^2/s
   D. Poise and stoke
   Answer: C. Dynamic viscosity mu has unit Pa.s (N.s/m^2); kinematic viscosity nu = mu/rho has unit m^2/s. Poise and stoke are CGS units. (Key point: Viscosity definitions)

## Related in Machine Design I

- [Hydrodynamic lubrication theory and film formation](https://www.bctengineer.com/question.md?key=semester_6%2Fmachine_design_i%3Ahydrodynamic-lubrication-theory) — Machine Design I, chapter 7
- [Journal bearing design with film thickness check](https://www.bctengineer.com/question.md?key=semester_6%2Fmachine_design_i%3Ajournal-bearing-minimum-film-thickness) — Machine Design I, chapter 7
- [Petroff equation for bearing friction](https://www.bctengineer.com/question.md?key=semester_6%2Fmachine_design_i%3Apetroff-equation-bearing-friction) — Machine Design I, chapter 7

## Related in other subjects

- [Hydrodynamic lubrication and Petroff's equation](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Ahydrodynamic-lubrication-petroff-numerical) — Internal Combustion Engines, chapter 6
- [Hydrodynamic and thermal boundary layers](https://www.bctengineer.com/question.md?key=semester_5%2Fheat_transfer%3Ahydrodynamic-thermal-boundary-layer) — Heat Transfer, chapter 3
- [Torque to overcome bearing friction](https://www.bctengineer.com/question.md?key=semester_3%2Ffluid_mechanics%3Ajournal-bearing-torque) — Fluid Mechanics, chapter 1
- [Tribology in manufacturing: friction, wear, lubrication](https://www.bctengineer.com/question.md?key=semester_4%2Fmanufacturing_and_production_processes%3Atribology-friction-wear-lubrication-manufacturing) — Manufacturing and Production Processes, chapter 2