Leaking piston?

[Bill Becker]

Our shift piston reaches a mechanical limit before it reaches the end of the piston stroke. We loose air pressure rapidly in this condition and I am wondering if the piston doesn't hold air pressure very well mid-stroke or if there is some other culprit in the lineup. Anyone have any advice?

[dlavery]

This should not be happening. The piston should be able to maintain pressure at any extension position. It sounds like you may have a bad seal on the piston cup or end seal. On the style of pistons provided in the kit, these cannot be easily repaired. You will need to replace the piston.

-dave

[Bill Becker]

Thanks Dave! That is what I thought. It is actually a 3/4 bore, 1" throw piston. I bet it is in the one of the hose connections on the side that gets pressurized when the piston moves in that direction. I also have a spare piston I can put in if I need to.

[dlavery]

Yes - definitely check the hose connections first (sorry - I should have mentioned that to begin with, but mistakenly assumed that those were already known to be good). If the hose connectors are leaking, it will usually be from one of four problems:

1- the brass fitting/connector is not seated all the way down in the fitting hole in the piston. Make sure it is tightly wrenched into the fitting hole, and bottoming out on the connection.
2- forgetting to use teflon tape on the fitting threads. The brass-to-aluminum seal between the fitting and piston is not completely airtight by itself. You must use teflon tape (or a suitable alternative thread sealer) to completely seal the joint. One wrap completely around the threads is all that is needed. Too many wraps of the tape are just as bad as not enough.
3- the pneumatic hose is not seated all the way into the connector fitting. Be sure and push the hose into the fitting, past the little "bump" you will feel, and all the way to the bottom of the fitting (note: the pneumatic hose must end in a nice, clean, square cut to seat properly - do not have an angled or ragged cut at the end of the hose)
4- debris or foreign materials may have fallen in to the connector, preventing a good seal. Make sure the connector is clean and empty. Shake out any foreign material (and don't do things like drilling metal right next to an open connector, where the chips may fall in to the pneumatics).

To check and be sure where the leak is actually located (connector or piston seal), pressurize the piston and swab some very soapy water over all the joints. The leak will be easy to spot, as it shows up as a series of bubbles.

Your original message said that the leaks seem to be happening when the piston hits a mechanical stop before reaching the end of the full stroke. Does the leak also happen when the mechanical stop is not present, and the piston extends normally? If so, then I would look for a connector leak first. But if the leak only happens during an interupted stroke, and not at full stroke, then I would start to suspect a bad piston seal.

-dave

[KenWittlief]

by any chance is your design putting a side load on the piston? They really arnt designed for that, and this can be the result

[Mr. Ivey]

Just to throw out some help with locating a leak in one of your fittings, or quick connectors. You want to hit them with AirTech, it comes in an aresol can, and the stuff is dry. You shine a flash light on it, and it's reflects a jet of color up in the air from where the leak is. With AirTech there are no worries with getting a liquid on the electronics. I was told about this stuff from a mentor on my team, I still haven't gotten it yet, but he told me it was the stuff to use for finding leaks. If you can't get AirTech, windex is the other good stuff, it makes lot and lots of little bubbles where ever a leak is. Hope you find your problem.
ivey

[kevin.li.rit]

Originally Posted by KenWittlief

by any chance is your design putting a side load on the piston? They really arnt designed for that, and this can be the result

No side load. It begins to bleed very badly when were shifted into hi speed and the pistion retracts.