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Fastening torque changes due to surface treatment

 

We are planning to go away from Chemical blackening on steel components to Zinc plating with yellow passivation.

The component is a screw of size M6*1.6g which is torqued to 220 lb-inches now. If we want to achieve the same bolt tension after zinc plating with yellow passivation, how much torque should we apply on this component.

If I can get any thumb rule, it'll be better.

V S Magesh
- Chennai, India

Dear Mr Magesh,

I think it would be advisable to contact any Speciality Lubricant Manufacturer to supply you a product which meets your torquing application. There may not be a thumb rule available.

Regards,

Khozem Vahaanwala
Saify Ind

Bengaluru, Karnataka, India

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The Rule of Thumb you are looking for is: T=KDL. Where T is the applied torque; K is the friction factor; D is the bolt diameter; and L is the load.

If you know the friction factor for the coatings you are using, one can calculate the required torque to generate a specific clamping load on the fastener.

If you do not know the friction factor for a given coating there are two ways to find this. One is to contact a coatings company who may provide this data. The second is to use a Torque - Tension devise (Skidmore). This machine allows one to apply a known torque and measure the clamping load. Given the bolt diameter you can then calculate the friction factor for a given bolt - nut combination.

Most any fastener testing house / lab can perform this for you if you do not want to buy a machine.

Matthew Horton
- St. Paul, Minnesota