Theory behind HW90 Inertia piston

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JayJay

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May 29, 2020
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Washington, United States
OK, I've been looking all over to find a decent explanation of how the HW90 inertia piston works. In one of the threads, Fischer posted a link to the Theoben patent. It was interesting but seemed like a complex mishmash of smoke and mirrors. I did some more searching on the internet but found nothing. 

The HW90 is famous for heavy cocking effort, but the net power output is below other magnum airguns that have similar effort. That indicates reduced efficiency. Is the inertia piston a factor?


 
The inertia piston is a sliding weight debounce device and doesn't affect cocking effort.

The '90 uses a licensed variation of the original, Theoben low pressure system that was later replaced by the HE system. Imho the LP system is superior. The HE system was touted as the latest and greatest with lots of improvements. Actually, it was a response to their expiring patent on the old system with just enough changes to get a new, longer, international patent. The version in the HW rifle is completely designed by Theoben.

One of the assets of the LP system is also a liability. it can be pumped up to higher velocities for power the hungry American market - as fully realized by the good Doctor with his RX series. But this puts more stress on the system and cocking them over multiple shots can be be taxing. 

If you keep it under 20 fpe, the cocking effort is comparable to any other untuned springer, although there is some stacking. If asked to recommend a single sporter springer, my response would be a '90 in .177 or .22. At a price of 600 USD you can have an out of the box platform of blued steel and wood of your choice that can be marvelously accurate.. It requires minimal maintenance and is so robust that your heirs will be shooting it generations later..
 
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That's interesting but it still doesn't address the basic question: What does the Theoben inertia piston do?

It obviously doesn't increase cocking effort. How could it? But its function could burn off energy as friction, thereby reducing HW90 efficiency. I suspect that's what's happening.

Let's take a closer look. It slides back and forth inside the main piston. It is suspended in the middle of the main piston, on a pair of o-rings. At each end of the inertia piston, the gap with the outer piston forms an air chamber. It has a port in the center which allows air to flow between these two chambers. When the main piston accelerates, it applies two forces to the inner piston. First is friction via the o-rings. Seccond, it compresses the chamber at the rear of the inertia piston, compressing the air and driving the inertia piston forward. The port allows some air to transfer to the forward chamber, reducing the force applied to the piston. At the end of the stroke this process works in reverse.

Therefore, the piston serves as a modulated damper. It's an internal, pneumatic shock absorber. It modifies the acceleration profile of the main piston, hopefully making a more comfortable shot cycle. 

The problem is that it bleeds off energy. Compressing and moving air always increases the temperature. The o-ring friction may also be significant. This may explain why the 90 is a few pound-feet shy of the ASP20, or other magnums that have equal cocking effort. 

I have read that some people remove the piston without adverse effect. 
 
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The HW90 is an amazing beast with a lot of things going for it. 

Check out the Weihrauch site specs page. https://www.weihrauch-sport.de/luftgewehre

First, the HW90 and HW80 both weigh 4.0kg. That's with the 500mm barrel.

Second, the HW90 is just slightly heavier than the HW35 (3.8 kg). It's lighter than almost all the HW77s and 97s.

In other words, the HW90 is relatively light despite having the highest power of all Weihrauchs and lugging around a heavy inertia piston. Why do most reviews characterize it as super heavy? 

The Weihrauch site has a link to a 4 way gasram comparison review. https://www.weihrauch-sport.de/test-report-of-our-model-hw-90-k?lang=en

The author says the following: "Recoil is barely noticeable. The action would put the most tuned springers to shame."
 
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Here are some interesting insights into the Theoben system by Gerald. M. Cardew in The Airgun From Trigger to Target. By the way, both of Cardew's books are available from scribd.com. 

Cardew spends quite a bit of time on the problem of piston bounce.

"If, however, it were possible to stop the piston from travelling backwards, this wasteful expansion of the air would be avoided and more energy would be imparted to the pellet... Theoben partially overcome the adverse effects of piston bounce by fitting an inertia piston inside the main piston of their gas ram rifles... When the rifle is cocked the inertia piston is pushed to the front end of the piston and is held there by the '0' rings. Immediately the piston is released and starts moving forwards the inertia piston, being fairly heavy, tries to stay in its original position relative to the outside of the gun, allowing the main plston to move over it. But, by the time the main piston has reached the front of its stroke the inertia piston has changed its mind and is moving forward too, Just In time to meet the main piston coming back as it bounces off a cushion of compressed air at its front. This sudden extra thump administered to the main piston by the inertia piston has many benefits, it increases the overall efficiency by reducing the piston bounce; also the main piston. can be lighter than normal which reduces recoil or more correctly the jerk of the rifle. There is another important factor in this system, the small hole drilled through the centre of the inertia piston allows air to pass in a controlled manner from one side to another. The size of this hole IS crucial to the correct working of a system which, although appears simple, is in fact mind blowing in its complexity,.. " Page 42

He also discusses piston seal design.

"Theoben Engineering have taken the concept of the plastic piston head a stage further with their Zephyr head. Its outline follows the normal design of a plastic head, but it has a number of shallow grooves cut into its front face radiating out from a shallow depression which matches the entry to the transfer port. As the piston completes its stroke, the air remaining in the cylinder is guided to the port increasing the efficiency of the system " Page 54

The HW90, like the HW35, embodies a lot of airgun history. We can be thankful that Weihrauch still produces these classic designs.
 
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I'm beginning to think that the inertia piston is intended to increase the gas-ram spring rate.

The inertia piston has significant volume. It reduces the gas-ram internal volume. This must increase the spring rate. So for a given power level, it would be easier to start cocking, but harder at the end of the stroke. It would reduce spring pressure at full gas-ram extension, possibly reducing reduce piston bounce.

I'm skeptical that the inertia piston dampens recoil. It's not mechanically fixed to the gun, but is free to move in the direction of recoil. For it to dampen recoil, the o-rings must cause significant drag on the inside of the gas-ram, or the difference in pressure between the fore and aft chambers must have some effect on the inertia piston. 

I read somewhere that the early inertia pistons were made of aluminum. If true, it indicates that Theoben was not concerned with weight or inertia but volume.

HW90's are known for cocking effort. This is despite having a long barrel for leverage, and moderate power levels compared to other gas-ram guns like the Gamo Magnum or Sig. Physics says that work is force x distance. The same power level (work) should require roughly the same average cocking force. Perhaps the problem is the spring rate. 

Unfortunately I don't have an HW90 to experiment with.


 
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