Broken Piston

[Corkbulb]

LOL! Wouldn't that same explanation be the reason for the FPS gain with the heavier piston weight?

[Sale]

This might be a case of calling the wolf too often. The argument got too heated and polarized before, and you refused to compare a weight and a lighter spacer of the similar size at that time.

 

I'd say give it some time and then come back with the results of the latest test. If you want to publish the results here, I would advise doing the test with a gun that shoots 1 joules or below.

 

In the mean time I'll see if I can give this test a go. I already measured some spacers/weights for the lack of better things to do:

 

Systema Bearing: 4.94 mm

G&P Aluminum spacer: 8.0 mm

Tokyo Marui spacer: 8.98 mm

 

One thing to consider is that aftermarket AEG springs are progressive. This means that if you get a 15 fps increase from a 3 mm spacer, you can't assume you'll get 30 fps from a 6 mm spacer. This was the fundamental problem with your previous tests, when the weights and spacers were of a different size.

 

-Sale

[Delfi]

As an example of this, think of a Newtons Cradle. When the balls are made of steel they go on for a long time. If they were made of plastic it wouldn't last so long because the plastic dissipates more energy than the metal.

 

I've been thinking about this and I'm not sure about anything I'm reading here.

 

Please bear with me.

 

Surely the purpose of the piston is to push air ... its mass, given the force imparted by the spring, is really neither here nor there. Any slight change in mass is even less significant. The spring is not just pushing a piston but is also pushing against a volume of air. The latter offers considerably more resistance than any small amount of additional piston mass which I suspect is a red herring that has little effect.

 

The Newton's Cradle analogy doesn't apply because that's to do with dissipation of impact energy through multiple deformations (aka lack of restitution ... amongst other things) and is nothing to do with any air resistance. In the same way, lightening an airsoft g/b piston in the manner suggested will likely have little or no effect. Its function is merely to apply a mechanical seal to push a resistive volume of air.

 

(For an example take the very large relative weights needed on a VSR 'real shock' piston to produce a minor kick for a small loss of muzzle energy)

 

If you want to improve performance by modding the piston then the BEST way must surely be to improve the fit of the piston to the cylinder thereby forcing more air in the right direction. In other words, the better the seal the better the performance. Other air seals will also help ... e.g. strategically placed PTFE tape etc.

 

Having said that there are material properties to take into account. A metallic piston is harder than a polycarb (or other non metallic) one and so if things do jam for some reason it will do more damage to the gear that it engages. And of course, the tighter the fit (or more generally the better the seal), the more the resistance so the greater the possible damage and likelihood of wear. The greater the efficiency of the seal the greater the stress on the parts doing the pushing.

 

Hope that makes sense.

 

D

[Corkbulb]

Thanks Sale, glad to see at least SOMEBODY is on my side. Unfortunately, I don't have a 1J gun, nor do I plan to get one

 

Anyway, If you my third piston weight thread, you'll see that the aluminum and brass G&P weights were the same size, just the mass was different. The FPS dropped by 15 with the aluminum one and when I re-installed the brass one, went back to 375 FPS with .25s.

 

Anyway, I'll make everybody happy. I'll shut up for a few weeks. I'll still read posts though. I've got some Half-Life maps I wanna work on anyway

[Stealthbomber]

I was only using Newtons Cradle to demonstate how different materials transmit impacts in different ways and show why a heavy, metal, piston will damage a gearbox more than a plastic one.

 

Nothing to do with airsoft gun power.

[Sale]

A heavier piston does deliver more stress to the gearbox, because it has more momentum. This is basic physics, and applies even if the amount of energy is the same (light piston moving fast, heavy going slower). As you may know, I'm not an academic guy and I can't be arsed to google the formula for y'all right now (it's 1:39 AM here), but I'm amazed why the formally educated people haven't mentioned this point earlier.

 

This is also why a heavy bullet knocks a steel plate over more easily than a light one, even if they are shot with the same energy.

 

-Sale

[Stealthbomber]

I think the sticking point is that (in theory) if you have 2 objects of different weight driven forward by the same spring then they should, in theory, be carrying the same kinetic energy even though it means the lighter one will be going faster with less mass and the heavier one will be the opposite.

 

I seem to recall that we've been here before though (I'm getting that a lot these days) and it WAS proved that a slower-moving heavy object carries more kinetic energy than a faster-moving light object.

 

I must admit, I was struggling to find the right word but, looking back, I'm glad to say I DID get the word "momentum" in there somewhere.

[tiff_lee]

A heavier piston does deliver more stress to the gearbox, because it has more momentum. This is basic physics, and applies even if the amount of energy is the same (light piston moving fast, heavy going slower). As you may know, I'm not an academic guy and I can't be arsed to google the formula for y'all right now (it's 1:39 AM here), but I'm amazed why the formally educated people haven't mentioned this point earlier.

 

This is also why a heavy bullet knocks a steel plate over more easily than a light one, even if they are shot with the same energy.

 

-Sale

<{POST_SNAPBACK}>

That is exactly the point I was trying to make.

[Delfi]

A heavier piston does deliver more stress to the gearbox, because it has more momentum.

 

Nope ... that breaks conservation laws. Same applied energy = same momentum.

 

This is possibly to do with inertia (Newtons first law).

 

D

[Sale]

Delfi: Sorry but you're wrong. Just calculate the momentum for a 0.25 gram BB at 293 fps, and a 0.2 gram BB at 328 fps. Both are exactly 1 joules, but the heavier BB has more momentum.

 

Or actually, let me calculate it for you:

 

0.25 x 89.3 = 22.325

0.20 x 100 = 20

 

http://id.mind.net/~zona/mstm/physics/mech...efinition1.html

 

See, same energy but the heavier one has more momentum.

 

I'm being nice to you because I didn't see you ridicule Corkbulb earlier. Pretty much anyone else would have gotten some flak at this point. I don't agree with Corkbulb and he's come off as stubborn (come on, he's 16 what do you expect?), but it's no reason for us to start arguing on that level. Just stick to facts and keep cool and you have a higher probability of actually promoting your opinion.

 

I find it ridiculous to bash someone when we (as a community, including myself) don't know everything either.

 

-Sale

[ViscountCharles]

Delfi's wrong.

 

Kinetic energy is 1/2 mass times the square of the velocity; momentum is mass times velocity.

 

Thinking about enery tells us why the OP was wrong, about weight making a difference. FPS are measured as a simple way to determine an increase or decrease in the kinetic energy of a pellet as it leaves the gun; for a known weight of pellet, a higher FPS means that it's received more energy.

 

The energy gets transferred to teh pellet by the sudden compresion of air in the cylinder, acting through the nozzle on the pellet. Assuming a perfect transfer of energy (which you don't get, thanks to noise, friction, imperfect seals etc), where does that energy actually come from? The spring.

 

Alter the spring, get a change in energy. Alter the air seals (barrel bore, hop-chamber, cyclinder head etc) and you get a change in energy transferrance. Alter the weight of the piston, you get - well, nothing that's affecting the energy in the spiring (unless it's acting as a spacer), and nothing that affects the quality of the air seal.

 

I suppose a lighter or heavier piston moves at a different speed through the cylinder which *might* have a miniscule effect on the amount of air lost at the O-ring, but if anything I'd have thought that the lighter, faster piston would be the more efficient.

 

What weights *might* achieve is greater shot-to-shot consistency. As well as increasing the energy LOST through the sound of the piston hitting the cylinder head a little harder, as it's a little heavier.

[Delfi]

I should perhaps have made it clearer ... for simplicity let's assume a lossless system. In other words the full potential energy of the compressed spring is transferred to the BB.

 

The air interface is also considered to be inelastic (and this is where at least one difficulty lies I think).

 

Under such ideal circumstances , momentum is conserved between the piston and the BB right (via an air interface)?

 

The potential energy of the compressed spring = the kinetic energy of the BB. Also conserved.

 

Also, the gearbox isn't affected by the mass of the piston in the manner that you suggest because of the way that it works. Think of it in terms of a catapult or bow. The spring is compressed much more slowly as it is being cocked in relative terms to what happens when it's released. Momentum and/or piston mass have nothing to do with gear or piston tooth damage. The only things that could possibly be damaged due to a piston mass change are the things in line with the spring. At the point of release in a properly working g/b the gears are not connected to the piston. Think of a gun with a powerful but ultra slow motor.

 

The issue is about what happens in a CLOSED system when a CONSTANT spring delivers its energy to a BB in a barrel. Where does this lost energy go because of a piston mass change? Why should piston weight affect things so dramatically?

 

It seems to me that IF there is an observable effect here then it's to do with the speed of momentum and/or energy transfer over the air interface between the piston head and the BB. The one constant is the volume of the (assumed incompressible) air interface. How that relates to the flow of air pushing the BB down the barrel with respect to time may be the key factor. A slower piston might explain less resistance (and possibly less loss in a non ideal system if loss is related to pressure) and that, in turn, might explain the results cited.

 

Think of this in terms of the effort required to blow down a large pipe as opposed to a hypodermic needle. It's easier to blow slower right?

 

My point is that the mass of the piston is not the key resistive force and that small changes may be negligible when compared to the complex air interface (involving a massive surface area change) between the piston and the BB. On the piston/cylinder side of things is there really enough change in velocity (due to piston mass) to cause such a change in muzzle energy?

 

D

 

PS ... And VC I'm not wrong :-)

PPS ... Is there a terminal velocity for the piston as it hits that wall of air?

[Delfi]

double post

 

deleted

[Delfi]

Better expressed as:

 

Conservation of Energy

 

PE (compressed spring ... piston at rest) = KE (spring and piston at full extension) = KE (BB at muzzle)

 

With the above in mind, it's easy to show that that Vbb (the velocity of the BB at the muzzle) is IDENTICAL regardless of Mp (the mass of the piston).

 

Any observed loss/gain in muzzle energy MUST therefore be due to the air interface between (and around) the piston and the BB.

 

D

[AirsoftHawaii90]

Systema duracon piston heads are good. Please note that if you do remove the weight, you will lose about 30 FPS with .25g BBs (like me). Everybody says it's a "spacer' and it compresses the spring more so the gun shoots faster, I disagree, but anyhow, removing it will result in a loss of 30 FPS with .25g BBs

 

BTW, don't buy the one with the bearings, I had two of them and even though they were THREADLOCKED, they came apart in the gun while firing, totaling my piston... twice 

<{POST_SNAPBACK}>

Could someone tell me why the bearings version of this piston head aren't good to use? I just got one of these and I read this. I am sorry if I am butting in.

 

Corkbulb, because you prefer not to post, PM me.

[Sale]

Face it Delfi, you were wrong where I said you were:

 

A heavier piston does deliver more stress to the gearbox, because it has more momentum.

Nope ... that breaks conservation laws. Same applied energy = same momentum.

 

A heavier piston is harder to bring into motion, thus it stresses the gears and piston teeth more than a lightweight piston. It also stresses the gearbox casing, which is why V2 owners most usually steer clear of any hard-recoil kits.

 

You are making a very big assumption when you assume the system to be lossless, and the air to be inelastic. These things are especially crucial, as the BB being shot is very lightweight, and thus sensitive to any slight changes in the system. The rule of conservation would otherwise apply, but the BB may exit the barrel before the piston is fully forward, so it is quite possible that under various circumstances the energy loss is air puffing out after the BB is out of the system.

 

-Sale

[Sale]

AirsoftHawaii90: All you need to do to get your answer is to read this thread over.

 

-Sale

[Delfi]

Face it Delfi, you were wrong ...

 

Yeah I was incorrect in assuming that momentum is the thing to consider. It isn't. This is all best explained in terms of force and energy. It's also worth noting that kinetic energy and momentum are intrinsically related (the latter being the first differential of the former). But that's just a simple point of interest and not particularly related to anything here

 

OK, taking your points roughly in order.

 

I think that we're all agreed that the material that the piston teeth are made of is key to wear and/or failure. Metal gear teeth against metal piston teeth is a recipe for bad things to happen.This is why many experienced engineers recommend sticking with stock TM polycarb pistons if you're upgrading.

 

Cocking Motion

 

Now, as for cocking the piston, it's your turn to be wrong There's absolutely no way that changing the mass of the piston seriously affects the teeth to teeth meshing as you suggest. In an empirical sense just imagine this without a spring in place. What resistance?

 

We have 2 possible dominant forces resisting the cocking of the spring and the mass of the piston is not one of them.

 

1. The first is the spring itself (Hookes Law) ... the stronger the spring and the greater the linear compression the greater the resistive force. This causes gear/piston tooth wear over time right?

 

2. The second is possible if the barrel (or similar) becomes blocked. This causes the piston to start creating a partial vacuum and is an *additional* force to that exerted by the spring on the system. This can cause all kinds of additional damage including tooth mangling, burnt out motors and blown fuses. The better the seals the worse this is potentially.

 

Forward Motion

 

Now, as for air in these circumstances being incompressible. The assumption is not as dangerous as it seems. It's most likely that other losses such as leaks will compensate for any increased pressure. The BB in the barrel (unless there is a blockage) is not an efficient plug.

 

In terms of efficiency of the barrel there are 2 factors here too. To be fully effective the barrel needs to have a volume >= the volume of air being pushed (less if you consider the air interface to be compressible which it isn't). If the barrel is too short the you will almost certainly lose energy at the muzzle BUT that has to be quite short! It's all about cylinder volume vs barrel volume.

 

A barrel that's too long for a given cylinder may also cause problems IF the piston cocking action starts to occur before the BB has left the barrel. I believe that this is one reason for using ported cylinders.

 

Again, the better the seals are the worse this effect will be and that includes the use of tightbore barrels

 

D

[Stealthbomber]

...but the BB may exit the barrel before the piston is fully forward...

<{POST_SNAPBACK}>

"May"?

 

I recently did the sums for this (for this thread?) and it works out that a BB travels 7.5 metres in the time it takes an average AEG to cycle once.

Obviously there's acceleration to account for but it seems to show that the BB should be long-gone from the barrel by the time piston cycles again.

[Sale]

I think that we're all agreed that the material that the piston teeth are made of is key to wear and/or failure. Metal gear teeth against metal piston teeth is a recipe for bad things to happen.This is why many experienced engineers recommend sticking with stock TM polycarb pistons if you're upgrading.

Here's the first disagreement. First off, a properly set steel-tooth piston like the Deep Fire is not a recipe for bad things to happen. As long as the parts are selected properly and you feed the gun quality BBs and keep the hop and barrel clean, it'll run longer than with a polymer piston. Systema uses a slightly milder steel for the piston rack in their PTW's, to retain its function as the replaceable part. The Deep Fire steel tooth rack isn't too hard either. (BTW. it is steel. Titanium doesn't stick to a magnet.)

 

I recommend stock TM pistons for most uses simply because the DF piston isn't drop-in, and the latter the only full metal tooth piston on the market at the moment. And also because a steel tooth piston is a bad combination with anything else but hard steel gears such as Systema.

 

Now, as for cocking the piston, it's your turn to be wrong There's absolutely no way that changing the mass of the piston seriously affects the teeth to teeth meshing as you suggest. In an empirical sense just imagine this without a spring in place. What resistance?

I never said that it would have any serious effect, and I never said it had anything to do with the meshing either. What resistance? Well, kick a football and then a bowling ball to see what I mean. It's harder to bring a heavy object in motion than a light one. This is why a heavy piston causes increased wear to the piston teeth and gear train. It's not a seriously big effect, but it's there.

 

We have 2 possible dominant forces resisting the cocking of the spring and the mass of the piston is not one of them.

Kick a football and bowling ball again. Neither need to be secured in place with friction or a spring or anything. They could be levitating in the air and you'd still end up with a broken toe.

 

I recently did the sums for this (for this thread?) and it works out that a BB travels 7.5 metres in the time it takes an average AEG to cycle once.

Obviously there's acceleration to account for but it seems to show that the BB should be long-gone from the barrel by the time piston cycles again.

Your calculations effectively disproved the assumed suckback from the piston starting to retreat while the BB was still in the barrel. That is not what I had in mind, but your statement is correct nevertheless.

 

I meant only the time between the piston starting to move forward after it's released, to the point where it meets the cylinder head. Not a full cycle. I used this argument to say that the entire energy of the spring is not necessarily conserved by the BB, because it could be out of the barrel before the piston is fully forward.

 

-Sale

[Stealthbomber]

Your calculations effectively proved the assumed suckback from the piston starting to retreat while the BB was still in the barrel. That is not what I had in mind, but your statement is correct nevertheless.

 

I meant only the time between the piston starting to move forward after it's released, to the point where it meets the cylinder head. Not a full cycle. I used this argument to say that the entire energy of the spring is not necessarily conserved by the BB, because it could be out of the barrel before the piston is fully forward.

 

-Sale

<{POST_SNAPBACK}>

No worries.

 

I'm not getting into this one. I just wanted to add that little dollop of information on a purely objective basis.

[Sale]

I'm not getting into this one.

I may have to follow you. Pints at the local pub after work?

 

-Sale

[Kman]

thread jack ftw?

[Sale]

Nope. We're still on the topic of pistons, and not only their weight either.

 

-Sale

[Kman]

Nope. We're still on the topic of pistons, and not only their weight either.

 

-Sale

<{POST_SNAPBACK}>

My original question was why did the piston break, freak accident or gearbox malfunction. I do not care about this piston weight garbage, and it really had nothing to do with my original question. Unless you are trying to tell me that due to the weight of my stock G&P piston it snapped the last tooth off, in which case continue. Otherwise, you would benefit from your own thread. Thank you.