Name: lubricant film thickness for shaft bearings
Uploaded: 2025-02-13T15:43:09.000Z
Duration: 1 min 23 s
Channel: JoVE

A rotating shaft operates inside a bearing housing. The shaft rotates at a specific speed and supports a load, requiring a lubricant with the correct viscosity to ensure smooth operation.
The objective is to determine the required thickness of the lubricating fluid to maintain the necessary shear stress within the fluid layer.
First, the shaft&#39;s tangential velocity is calculated by converting the rotational speed into angular velocity. This angular velocity is then multiplied by the shaft&#39;s radius, which provides the tangential velocity at the surface.
Next, the velocity gradient across the fluid is considered. The velocity gradient is determined by the difference in velocity between the rotating shaft and the stationary housing divided by the thickness of the lubricant layer.
The shear stress within the fluid is found by multiplying the velocity gradient by the fluid&#39;s viscosity.
Finally, the shear stress equation is rearranged to determine the fluid thickness. By dividing the tangential velocity by the shear stress, the appropriate thickness of the lubricant is calculated. The thickness value ensures proper lubrication between the shaft and the housing.

design example: deciding thickness of lubricating fluid in a shaft

Design Example: Deciding Thickness of Lubricating Fluid in a Shaft

Effective lubrication between a rotating shaft and its bearing housing is essential in rotating machinery to minimize friction, wear, and energy loss. With carefully controlled thickness and viscosity, the lubricant layer prevents metal-to-metal contact, ensuring smooth operation.
To calculate the required thickness of the lubricant layer, the tangential velocity at the shaft's surface must first be determined. This velocity is calculated by converting the rotational speed to angular velocity and multiplying it by the shaft's radius.
<img alt="Angular velocity formula \(V_{\text{shaft}} = \omega \times r_{\text{shaft}}\), equation image." src="/files/ftp_upload/18007/18007_Equation_1.svg" style="text-align: center; height: 32px;" data-altupdatedat="1762173717000" data-alt="Equation 1" />
The resulting tangential velocity v represents the shaft's linear speed relative to the stationary housing. The velocity gradient across the lubricant layer, which indicates how quickly velocity changes from the moving shaft to the stationary housing, is derived by dividing the tangential velocity by the lubricant thickness.
<img alt="Velocity gradient equation; fluid dynamics formula; demonstrates shear rate calculation." src="/files/ftp_upload/18007/18007_Equation_2.svg" style="text-align: center; height: 40px;" data-altupdatedat="1762173717000" data-alt="Equation 2" />
Shear stress within this fluid layer depends on the lubricant's viscosity and this velocity gradient, with higher viscosity providing greater resistance to flow and reducing metal-to-metal contact. To achieve the correct shear stress, the lubricant thickness must be calculated by rearranging the shear stress equation, expressing thickness in viscosity, tangential velocity, and the desired shear stress level. The necessary thickness t is obtained by dividing the tangential velocity by the product of viscosity and target shear stress.
<img alt="t=Vshaft&#215;&#956;/&#964;; formula; torque calculation; physics; equation; mechanical analysis" src="/files/ftp_upload/18007/18007_Equation_3.svg" style="text-align: center; height: 42px;" data-altupdatedat="1762173717000" data-alt="Equation 3" />
This optimal lubricant thickness ensures sufficient separation between the shaft and housing, allowing smooth operation and protecting the machinery components from excessive wear.

A rotating shaft operates inside a bearing housing. The shaft rotates at a specific speed and supports a load, requiring a lubricant with the correct ...

design-example-deciding-thickness-of-lubricating-fluid-in-a-shaft

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Fluid Properties

KEY TERMS AND CONCEPTS

lubricant film thickness for shaft bearings

Lubricant film thickness for shaft bearings is calculated to keep a rotating shaft from touching its bearing housing. A controlled layer of lubricant reduces friction, wear, and energy loss. It also helps the machine run smoothly by preventing metal-to-metal contact.

The calculation starts with the shaft’s tangential velocity. First, the rotational speed is converted to angular velocity. Then that value is multiplied by the shaft radius. The result is the shaft’s linear speed at its surface relative to the stationary housing.

Next, the velocity gradient across the lubricant layer is found. This gradient shows how quickly the fluid speed changes from the moving shaft to the fixed housing. It is calculated by dividing the tangential velocity by the lubricant thickness.

Shear stress in the fluid depends on the lubricant’s viscosity and the velocity gradient. Viscosity is the fluid’s resistance to flow. A higher viscosity can help reduce metal-to-metal contact by increasing resistance within the lubricant layer.

To find the needed lubricant thickness, the shear stress equation is rearranged. The thickness t is calculated by dividing the tangential velocity by the product of viscosity and the target shear stress. This gives an optimal lubricant layer that keeps the shaft separated from the housing and protects the parts from excess wear.

Lubricant Film Thickness for Shaft Bearings

lubricant-film-thickness-for-shaft-bearings

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Lubricant film thickness for shaft bearings is calculated to keep a rotating shaft from touching its bearing housing. A controlled layer of lubricant reduces friction, wear, and energy loss. It also helps the machine run smoothly by preventing metal-to-metal contact. The calculation starts with the shaft’s tangential velocity. First, the rotational speed is converted to angular velocity. Then that value is multiplied by the shaft radius. The result is the shaft’s linear speed at its surface...

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Learn how shaft speed, viscosity, and shear stress are used to calculate lubricant film thickness for bearings and reduce wear in rotating machinery.

Lubricant Film Thickness for Shaft Bearings | JoVE K12

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Lubricant Film Thickness for Shaft Bearings

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