# Seal friction needed for pneumatic/hydraulic piston

• Snyder
In summary, the conversation discusses the need to calculate break force friction of a piston seal on the cylinder wall for a project. The equation for seal friction is provided, but it is noted that specific values for the K factor are difficult to obtain. The recommendation is made to consult a seal sales engineer from a major manufacturer and provide specific information about the system to determine the best seal type, seal compression, lubrication, and bore finish for achieving the least amount of friction. The suggestion is also made to consider a rolling seal diaphragm, which has no force between the wall and the piston and can have very low sliding friction. Some examples of companies that use rolling seals are provided, and it is noted that they can be used in PVC pipes
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.

NyxSoSo
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.

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.

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

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!

## 1. What is seal friction in relation to pneumatic/hydraulic pistons?

Seal friction refers to the amount of force needed to overcome the friction between the piston and the seal in a pneumatic or hydraulic system. This friction is caused by the contact between the two surfaces as the piston moves, and it can impact the overall efficiency and performance of the system.

## 2. How does seal friction affect the movement of the piston?

Seal friction can increase the resistance against the movement of the piston, making it more difficult for the system to operate smoothly. This can result in slower or uneven movement, decreased accuracy, and increased wear and tear on the components.

## 3. What factors can impact the seal friction in pneumatic/hydraulic pistons?

The type and condition of the seal material, the surface finish of the piston and seal, the pressure and temperature of the system, and the lubrication of the components can all affect the level of seal friction in a pneumatic or hydraulic system.

## 4. How can seal friction be reduced in pneumatic/hydraulic pistons?

One way to reduce seal friction is by using high-quality seals and proper lubrication. Choosing a seal material with low friction properties, such as PTFE or UHMWPE, can also help. Additionally, maintaining the proper pressure and temperature in the system can minimize seal friction.

## 5. Can seal friction be completely eliminated in pneumatic/hydraulic pistons?

While seal friction can be reduced, it cannot be completely eliminated. Some level of friction is necessary for the seal to effectively prevent leakage and maintain the pressure in the system. However, with proper maintenance and selection of components, it can be kept at a minimum to ensure optimal performance of the system.

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