packing material with resilience

[Tmoose]

We bought a machine with several tie rods that use a rod packing as part of an air seal. The packing is not adjustable. It is compressed by a plate bolted hard at assembly. The tie rods potentially move radially a few mm and jitter a few mm axially constantly when in operation. The packing is in a sub-housing with over 15 mm radial clearance to the main frame, so the packing should not have to absorb the radial motion, but it will transmit the radial force to move the sub-housing. Most commercial packings seem to rely on periodic adjustment to increase axial clamping to create a seal. We think packing material with some resilience or elasticity is required with this design. The machine is assembled and in operation, so disassembling the large tie rods and removing the seal to add springs or rubber elements to provide elastic axial force is not an option.

Service is less than 230 F operating temperature, and sealing air, but likely in contact with pulverized coal.
I'm in contact with some packing manufacturers, but the installation instructions and descriptions of the recommended products so far emphasize or brag about their lack of stiffnessnss, and need for continued adjustment.

Has anyone used packing materials that like an O'ring are installed with a defined amount of compression or preload?

thanks

Dan T

 

[1gibson]

Just to confirm, they don't rotate (no idea why tie rods would need to rotate but just checking.)

It looks like the packing is a compromise between support and sealing.

Any chance you could use a radial bearing (needle type, to allow axial motion) in place of the packing, with elastomeric bellows type seal on both ends of the housing?

 

[btrueblood]

"We think packing material with some resilience or elasticity is required with this design."

Agree there. Looks like it might be better as an O-ring or lip seal, with radial compression/squeeze designed in. You might need to do a bit of research and test to find the best elastomer material, and a lot will depend on how hot the joint really gets. (If you can keep it below about 180 F, then the nitriles may give good results).

 

[Tmoose]

no rotation.

Most or all other machines of this type from other manufacturers use a complicated housing with a pair of ball joints at each end of a tube to allow "toggling motion" and a bellows to track length variation.

I appreciate the relative simplicity of this design, but we already reportedly have a 50% failure rate (all the seals on one of a pair of machines) , and it will take some time to figure out if there is a basic design problem, an installation problem, or an incorrect packing material was used (by the OEM!).

The option for now is use the "best" packing material we can find, and be sure it is installed correctly.

thanks

Dan T