Posts Tagged ‘gas spring’

nitropistoncomparison 11-09 (Medium)

A while back, Freddy King responded to one of my blogs and posted a question: “Could you give some info on Nitro Piston mechanism? How does this system work?”

Okay, Freddy, here goes. If you have ever seen the gas strut that lifts the back window of an SUV or the rear door of a hatchback automobile, you have seen the basic working mechanism of a Nitro Piston – also known as a gas ram or gas spring – airgun powerplant. At the heart of the gas spring is a cylinder with compressed gas. At the far end of the cylinder is a piston. The mechanism works the same way a conventional spring-piston airgun powerplant: when you cock a breakbarrel rifle equipped with a gas ram or Nitro Piston, you drive the piston down the cylinder (further compressing the gas inside the cylinder, which is comparable to compressing the spring in a conventional spring powerplant) until it latches. The powerplant sits there, under tension until the shooter pulls the trigger. When that happens, the gas expands, driving the piston down the compression tube, compressing the air in front of it until the compressed air squirts through the transfer port into the breech, propelling the pellet down the barrel.

To find out more about this technology, I spoke interviewed Ben Taylor, who invented gas ram technology, by telephone in England. In 1976, he and his business partner Dave Theobald were unhappy with the state of the art in spring-piston powerplants. They had made and sold eight spring rifles, and they all suffered from not keeping the energy that Taylor wanted. So he had a thought: what if you used compressed gas in a cylinder with a piston instead of a spring?

So he built one, using a brake seal for cars from Lockheed, and it worked! At first, he pressurized the gas ram with 150 psi air from the shop and got only about eight or nine foot-pounds of energy. “Then we tried 300 psi nitrogen from a bottle and got 1,000 fps in .177. I shot that same gun for five years with the same charge in it,” Taylor says.

In 1981, Taylor and his partner applied for a patent and tried to interest various airgun manufacturers in licensing the technology. A couple of times they came close, but ultimately no deals were consummated. “So we decided to manufacture it ourselves. It took 10,000 pounds to get set up. We sold 490 the first year, 1,000 by the third year,” he says.

Gas rams offer a number of advantages, Taylor says. “They are totally tireless. You can leave them cocked for as long as you like. Nothing wears out. The seals don’t wear. Recently I serviced gun number 25 from 1982, and it was the first time it had been serviced since it had been manufactured. You have to remember to shoot, or cock-and-decock, a gas ram every few months, otherwise the seal can get bonded to the bore, and that will cause failure.”

He adds, “We found that if a gas ram is going to fail, it will do so within the first week. Otherwise, it will last for years. Right now, there are more of our guns out there that have never been serviced than those that have been serviced.”

There are a few disadvantages to gas rams. Unlike a spring powerplant, which often will keep operating at reduced velocity even if the spring gets broken or bent, if a gas ram fails, it won’t work at all.

The biggest problem, Taylor says, is that, because Theoben gas ram powerplants had a valve where people could increase or reduce the pressure of the gas inside the powerplant, people, in a quest for more power, tend to overfill them.

“There is a sweet spot on the pressure vs. velocity curve,” Taylor says. “If you go beyond that, you increase the pressure, but you don’t get any benefit. The gun becomes hard to control and won’t shoot straight. In addition, there is the danger of burning the piston seal. We actually had to design our high powered guns so that over-pressurizing them wouldn’t create reliability problems.”

Taylor told of an interesting experience at the test range one day. “We had two 30 foot-pound guns of the same caliber shooting the same pellets. One was a gas ram and the other was a precharged pneumatic. We had chronographs set up at the muzzle, halfway down the range, and at the target. We found the gas ram was retaining energy much better at the target. When we recovered the pellets, they looked like they had come from two different manufactures. The pressure from the gas ram had flared the skirt of the pellet flat to the bore, so that it looked like a cylindrical pellet, and the gun was shooting flat like a laser!”

The Weihrauch HW90 incorporates the Theoben gas ram system.

The Weihrauch HW90 incorporates the Theoben gas ram system.

Although Theoben Airguns has gone out of business, you can still buy a breakbarrel rifle with a gas ram powerplant based on Ben Taylor’s design: the HW90.

I also interviewed Ed Schultz, Director of Engineering for Crosman Corporation, to get his views of Crosman’s Nitro Piston Technology.

“Nitro Piston offers an advantage in longevity in modern spring guns that operate at the velocities that people want,” Schultz said.

“When you are using a mechanical spring in an airgun, you are just doing bad things to the spring,” Schultz adds. “A rule of thumb in engineering is that you don’t want to stress a spring past 50% compression to maintain reliability, but that doesn’t work in a spring gun. Instead, you compress the spring almost 100%. You take up almost all the gap between the spring coils to get ultimate performance, and that tends to weaken the spring.  And if you leave it cocked, you’re taking some life out of the spring. So you use special materials and do special heat treatments to deal with that, but you’re basically fighting a losing battle.”

“But a gas ram, Nitro Piston, powerplant eliminates the weak link in the system. The gas doesn’t care if it is compressed, it’s not going to degrade the life of the powerplant,” he says. “A life of 5,000 shots is probably a good rule for estimating spring life in an average spring-piston powerplant. The life of a Nitro Piston powerplant is easily twice that, and at the end of that time, it will shoot close to the original numbers. It’s either working completely fine, or it’s not working at all.”

Schultz adds that a Nitro Piston powerplant has few moving parts, there is no spring torque, no vibration, no need for spring guides. “To make a spring powerplant really quiet and vibration free, you have to custom fit inner and outer spring guides because every spring is slightly different,” he says. “You don’t have to do that with a Nitro piston powerplant. There are billions of gas springs in use throughout the world. Automobile manufactures have adopted them because of their reliability, and we know how to make them with high precision. With a Nitro Piston gas spring powerplant in your airgun, you get a lot of the advantages of an expensive, custom-tuned powerplant at a more affordable price.”

Til next time, aim true and shoot straight.

–          Jock Elliott