Seal friction needed for pneumatic/hydraulic piston

[Snyder]

Hello, I have a project that requires me to calculate break force friction of a piston seal on the cylinder wall. I have a double acting cylinder at very low pressure, 3.5" diameter bore, one side of the piston is air, the other side is water. What seal type, seal compression, lubrication and bore finish do I need to achieve the least amount of friction possible?

I found this equation

"Seal friction = K • μ • (Pe)2 • V • A
where

• K = empirical factor specific to the design of seal installed and working under design conditions

• μ = coefficient of friction

• Pe = equivalent pressure equal to the interference pressure plus the fluid pressure

• V = speed

• A = surface seal face contact ( ≈ π • Diamater • RadialSection)

• Specific values of K factor are difficult to obtain unless evaluated on empirical lines or on the basis of comparative data. This formula can only be used directly to investigate possible differences in performance and friction on compression seals of the same type and material, but different size."

But, as the last bullet states, I think I need a better solution. My goal is to learn how to calculate seal friction and then apply it to different diameters. Any help in the right direction would be greatly appreciated.

 

[JRMichler]

Start by reading some seal catalogs. Here's a link to the Parker seal catalog: http://www.parker.com/literature/Praedifa/Catalogs/Catalog_PneuSeals_PTD3351-EN.pdf. It will give you an idea of some of the variables.

The information you are looking for is not published. You will need to contact a seal sales engineer from one of the major seal manufacturers. Before calling, you should know the maximum and minimum pressure of each fluid, maximum and minimum water temperature, maximum and minimum ambient temperature, how much movement (inches per year), how many times per year does it start from a dead stop, how long is it stopped, how much friction do you want, how much friction can you tolerate, how long do you want it to last, how much leakage can you tolerate, and anything else that could affect seal wear and life. Then ask the sales engineer about tolerances, bore finish, and anything else that affects friction and life.

Airpot is the ultimate in low friction air cylinders: airpot.com.

 

[betadave]

You might consider a rolling seal diaphragm. Since you are working with water, you are not at extreme temperatures or pressures. The rolling sleeve type diaphragm has no force between the wall and the piston so sliding friction will be very very low. The force required to roll the sleeve or to unroll the sleeve is not something I know but it is small. I had worked with a design of a water pump driven by low water pressure and the solution to overcoming static friction to begin piston movement was this type of seal. Piston movement in that design was very small and the rolling seal made tolerances between the piston and wall a trivial issue whereas a seal requiring contact needs a smooth wall and a precision fit. The pump I worked with was built with commercial pvc pipe and had no problems with sticking or leakage. Since I don't know more about your system, I can only suggest you consider a non traditional seal.

 

[JRMichler]

Bellofram cylinders use rolling seals: https://www.marshbellofram.com/bellofram-pcd/products/standard-diaphragm-air-cylinders/. They are good cylinders.

 

[Snyder]

Thank you both for the reply, the rolling seal sounds perfect! I feel like I have a good heading now. Cheers!

 

[NyxSoSo]

You might consider a rolling seal diaphragm. Since you are working with water, you are not at extreme temperatures or pressures. The rolling sleeve type diaphragm has no force between the wall and the piston so sliding friction will be very very low. The force required to roll the sleeve or to unroll the sleeve is not something I know but it is small. I had worked with a design of a water pump driven by low water pressure and the solution to overcoming static friction to begin piston movement was this type of seal. Piston movement in that design was very small and the rolling seal made tolerances between the piston and wall a trivial issue whereas a seal requiring contact needs a smooth wall and a precision fit. The pump I worked with was built with commercial pvc pipe and had no problems with sticking or leakage. Since I don't know more about your system, I can only suggest you consider a non traditional seal.

This seems like a very good idea! I have researched these for a project, after seeing this post, and was wondering if you could elaborate on how you used them in PVC. Did they have special fittings? I ask because my application will be PVC and I am not sure how to clamp the seal in PVC. Thank you!