Piston seals: Part 1
by Tom Gaylord
Writing as B.B. Pelletier
This report covers:
- Early leather seals
- What’s next?
- Now that you understand…
- No magic
- Don’t be depressed!
- Leather piston seals
- The better way
- It’s all the same
- Leather’s shortcoming
Pyramyd Air has shipped me the replacement Fortitude, so I will be restarting that report soon. Leapers is sending me a micro dot sight that I showed you recently. I wanted that to test on the Beeman P1 pistol that I stopped testing months ago, but now I also want to put it on the Chaser pistol and perhaps on a rifle or two. And yes, GunFun1, I am going to test the Gauntlet at 50 yards with the tightened shroud/barrel.
But today I want to talk about something different. As you are aware, this blog gets many new readers all the time. Often when they come in they have a question about a topic I have addressed in the past. If their question is easy to answer I will often just give them the links to the past report — if I can find it. But sometimes their question isn’t so easy to answer, and when that happens and I know that I have many other new readers who might perhaps benefit from it, I will write a special blog. Today is such a day.
Reader Arvizu joined us two days ago. He had some questions about various things, including the following statement.
“ I noticed, too, that the seal plays an important role to define performance (sometimes small variations in diameter makes the difference). I would like to clarify that this is only my appreciation and limited experience with airguns.
I would really appreciate an article from you, with your vast experience in this regards talking about this topic.”
I gave him several links to past articles, but I could not get his comment out of my mind. So today I would like to address the importance of piston seals.
Let’s start with a look at a popular toy that many of us have played with — the toy popper gun!
Pull the handle back and the cork is drawn into the muzzle by the string that’s attached. If the string is too long you can just stuff the cork in the muzzle yourself. Push the handle forward and the dowel it’s attached to pushes air in front of it to pop the cork out of the “gun.”
You can see the length of the dowel at the bottom.
The popper toy works exactly like a spring-piston airgun, except it has no piston seal. Just the close fit of the wood dowel inside the larger wood tube compresses enough air to pop the cork.
Early leather seals
If that is clear, the rest will be easy. You don’t need a wood dowel that’s as large as the compression chamber. A steel rod can be much smaller and it only has to “push” a small “head” that’s sized to the chamber.
This simple compression chamber and “piston” works exactly like the popper toy.
My graphic is interesting until you realize that the air, once compressed, has nowhere to go. Let’s add an outlet that we’ll call the air transfer port.
Now that we know how the air is compressed, it’s easy to see that it exits through the air transfer port.
Next we put something in front of the escaping air, like the cork in the popper. The popper pops when the cork is overcome by air pressure and can’t remain inside the wood tube any longer. That’s similar to what happens when a pellet blocks the air from an airgun air transfer port.
Now we have put a barrel in front of the air transfer port and blocked the escaping air with a pellet. The pressure will built in the compression chamber until the pellet has to move.
Now that you understand…
So the wooden dowel is sufficient to compress air to a point. And the “head” on a steel rod will do the same thing. If we put a seal on a steel piston instead of just a head the extra weight of the piston will give us more force for compressing the air. But, here is the important thing — the amount of air doesn’t change, regardless of what compresses it!
A wooden dowel will compress air but will also allow some air to escape around the sides of the dowel. A “head” with no piston body will compress air better, as long as its seal remains tight against the walls of the chamber. But the head alone may be too light to compress air beyond a certain point. If the pellet refuses to move we could end up with the sealed head resting against a bubble of compressed air inside the compression chamber.
By adding a heavy piston body we are adding force to the moving piston seal, ensuring that the tiny lead pellet cannot withstand the force of the compressed air behind it. But the piston body compresses no more air than the piston head, if the other things (diameter of the compression chamber and length of the stroke) are the same. And that fact, readers, is important to dwell on.
There is no magic in a piston seal. It can only compress the amount of air that’s in front of it. If you replace a mediocre seal with the best seal possible you will get some improvement, but how much? Let’s say the mediocre seal compresses 90 percent of the air in front of it and the best seal compresses 98 percent. What, then, is the difference? Eight percent!
And we know from experiments done in the 1970s by the father/son team of Cardews, that the pellet produces roughly a third of the potential energy that’s driving it. Many things like friction and the energy loss due to translation from momentum to compression diminish the force that’s generated. What I’m saying is a 100-pound mainspring can produce no more than about 33 pounds of energy at the muzzle. The actual amount, however, is usually far below even that, with the 100 pound spring producing about 20 pounds of energy maximum at the muzzle. So, an 8 percent gain in potential power becomes no more than a 3 percent gain at the muzzle and probably less.
Don’t be depressed!
Now that you understand what we are dealing with you can cheer up. There have been major advances in piston seals in the past 40 years. We don’t have time to look at all of them today, but we can certainly get started.
Leather piston seals
Leather piston seals have been around for centuries. The air pumps that filled airguns in the 1700s had either no seal or a leather seal. If they had no seal, they were worked-in by hand to the point that the fit between the steel pump rod (which served as the piston, as well) and the pump chamber was perfect. Just oil the pump rod and it worked without the hint of noise. I have operated several of these centuries-old hand pumps and could not feel anything beyond air being compressed. But hand fitting like this took countless hours to achieve, so a better way was created.
The better way
The better way was to attach a pad of leather to the end of the piston. Make it slightly larger than the bore it was going to work in and then oil it thoroughly and work it back and forth many times. In far less time than a steel piston seal takes to work in, the leather seal conforms to the inside of the pump walls and makes an airtight seal. As long as the leather is kept pliant with oil, this kind of seal works very well.
Dennis Quackenbush made this pump and reservoir with gauge so he and I could test the efficiency of vintage hand pumps.
The pump Dennis made doesn’t look that different than this real antique airgun pump.
It’s all the same
Don’t think that the seals in these hand pumps are any different than leather airgun piston seals. They’re not. They do the same thing and work the same way. Pistons are pistons and leather is leather. It needs to be supple to seal and when it is, it seals very well.
A leather piston seal can last for a century and even longer, all the time doing what it was designed to do. But leather does have a weakness. It doesn’t like to get dry or hot. And spring piston airguns get very hot. They can get up over 1,000 degrees, F for a millisecond or two when the gun fires. Only because the heat doesn’t last long do leather seals work at all. However, as the guns become more powerful, the peak heat rises to the point that it starts burning the edges of the leather seal. First it dries out all the oil through combustion (detonations) and then the leather starts getting hard and crumbly. Eventually such a seal will be reduced to powder if you persist.
The leather seal in a Diana model 27 that shoots a .22 caliber pellet at 475 f.p.s. may last for half a million shots, while a Diana 45 that shoots the same pellet at 850 f.p.s. may only last 10,000 shots — and only that long if it is kept oiled regularly. Leather piston seals have their limits.
We are not finished by a long shot. We still have to look at PTFE seals, older synthetic seals, newer synthetic seals, parachute seals, the fit of the seal to the chamber and more. I don’t know if I can finish this discussion in one more report or if there has to be a Part 3. We shall see.