Piston Seal/Wear Ring Combo 1

[FullMetalBracket]

Hello all,

We have situation where we using pressurized air to force a large steel piston down a cylinder in order to generate an impact. There is no rod attached to the piston, so the static load is completely supported by whatever seal/ring arrangement we have on it. The following are the operating conditions:

Max. Speed: 50m/s (164 ft/s)
Max. Pressure behind piston: 300psi
Max. temperature: 100C (200F)

Currently our piston has 2 wear ring grooves close to the front and back end, and an o-ring groove in the center. The idea is that the wear rings eliminate the possibility of any rocking and steel on steel contact while the o-ring provides the necessary seal.

We want to start life cycle testing of our materials, and so I'd like any recommendation you have on perhaps changing to another seal/ring configuration in order to increase seal life time. It has been suggested that we remove the wear rings altogether and that the piston simply ride on two seals, one at the front and one at the back.

Any advice or recommendations in terms of seal/wear ring configuration or materials would be greatly appreciated

 

[MikeHalloran]

O-rings are not bearings.

Double seals cause problems of their own, especially in cyclic applications like yours.

Test the configuration you intend to use.



Mike Halloran
Pembroke Pines, FL, USA

 

[iainuts]

I agree the wear rings are a good thing.

I don't understand what you're trying to test though. Are you testing the seal material (O-ring)? Or are you testing what the piston is impacting?

Do you want to increase O-ring life because it wears out too quickly? Seems like an O-ring is a very cheap, easy thing to replace. You could put a Teflon cap on it to reduce wear, add lubrication, change to a plastic material, but I'm not sure that's what you're after.

 

[jbeckhou]

are you coating the cylinder ID or leaving it "as is" after machining? I have used a polypac seal on pistons, which did have two wear rings. We applied QPQ to the ID of the cylinder and put it thru a API-6A PR-2 Test and the polypac never wore out. The actuator was opened and closed over 200 times.

Petrotrim Services

 

[dsheets05]

Fullmetalbracket,

What is the diameter of your floating piston?
How long is the piston?
Is the cylinder going to be vertical or horizontal?
I would not recommend an O-Ring at those speeds, your seal will fail quickly. Please provide me with this information and I will recommend a seal/wear ring design.

 

[Hidromar]

Hi!

This can be a problematic question, and application.

Depending on power consumption and permisable pressure loss can be acceptable.

Meaning:
O-Ring, and other rubber products has a very low speed allowance. If you intend using such a speed, O-Ring will tend to be damaged very soon. At the same time, rubber O-Ring will have a "stick-slip" effect, so, maybe, this will consum power on you application as soon as you change direction.

Alternatively, and always centered on speed, you have to think about plastics. You have plastics that will work great for such a high speed and temperature, but as less load you can apply, as much cycles you gan get going from a simple PTFE-energised seal up to a spring energised seal. The latter one having the faster effectiveness, lower wear, and higher pressure loss.

By the way, as tribology is a "matter of each one", test must be held before using on productions, even for your ideas.

http://www.hidromar.es

 

[kevindurette]

I'd go with either a PTFE cap seal energized by an o-ring or a single-acting rubber u-seal designed specifically for pneumatic piston sealing. Don't rely on seals to act as guide rings. There are guide rings available in PTFE, and depending on the size of your piston (and the resulting pressure loading on the wear strip), you might also want to consider a PTFE-filled nylon. The pressure loading scales down linearly with the width and OD of the wear strip, but it scales up quadratically with the OD of the piston and linearly with the length of the piston. The net result is that PTFE wear strips aren't recommended for larger bores--even in accumulator-like applications there is no loading from a rod.

I decided to stop checking the following email address, so if I give it out here, it's no big deal. Email me at "kevin the_nerd_@ya_hoo_.com" (removing the spaces and underscores) if you want to discuss the specifics a bit more. Post here when you email me so I can go check back at that account.

Durette