Technical questions for technophobes and beginners

[andrewM]

I have read here and on other sites about top hats, power bands, washers and similar materials, often used for tuning. However, there is no ready explanation to be found about the effects these have and their use. It would be helpful if, possibly somewhere on the site - in the form of a 'sticky' perhaps - someone could provide a glossary of terms and a brief description of usage for each device. This would be most helpful for beginners, newcomers to this hobby and technophobes like me.

Meanwhile, I wonder if one of the experts here could kindly explain:

1 What is a top hat and what does it do? I gather it is added to the end of a mainspring. Is the purpose to safeguard the spring and increase its longevity?

2 What is a power band?

3 What effects do additional washers have? On the face of it, adding a washer would surely reduce the air space and sweep within the cylinder and also reduce the compression ability of the spring. That would, presumably, reduce the power of the rifle. However, I understand that washers can raise the power of a rifle. How is this feasible?

4 Does power vary between a heavy and a light piston?

These things are a mystery to me. This site has enthused me since I joined but there are still many things I have yet to discover!

Rgds to all,
A

[vbull]

Answers to your questions will certainly be of interest to me and many others I would say.

[Turnup]

OK I'll take a swing at:

3) Washers are fitted behind the main spring, which is well behind the piston so there is no effect on the swept volume. They pre-load the spring, so it generates more force on the piston and hence will accelerate it to a higher velocity during the firing cycle. Too many washers will prevent the spring from compressing and hence prevent the piston from latching during cocking. The spring is then said to be coil bound. Increasing pre-load can also result in the piston slamming at the end of its forward travel, and can cause prolonged bounce because the piston cannot transfer all of it's energy into the air/projectile.

4) The internal dynamics of a spring gun are extremely complex and the effect of a heavy or light piston are difficult to predict in general. All else being the same, a heavy piston will accelerate more slowly during the initial part of the firing cycle, but will decelerate more slowly during the later part of the firing cycle as the internal pressure increases. This will (probably) change the energy of the projectile, change the duration of the firing cycle, and change the felt recoil.

[Geezer]

1 What is a top hat and what does it do? I gather it is added to the end of a mainspring. Is the purpose to safeguard the spring and increase its longevity?

2 What is a power band?

A top hat supports the spring end, which is good for longevity and for smoothness. And it adds weight to the piston, which typically increases power. It is called a top hat because it looks a bit like one.

I believe people who talk about a “power band” mean a washer, whether metal or delrin, as in your Q3.

[Phil Russell]

To add a little to the above:
A 'power band' is a fancy name for a washer or slip washer. They can be steel or delrin or maybe another durable material. They can fit over the central rod in the piston (also termed the 'latch rod') so that they sit at the bottom of the piston and provide a bearing surface for the spring such that the spring can rotate during cocking / firing and so give a smoother action. This washer adds pre-load to the spring, the amount depending on the thickness of the washer and how many washers are used. If you have a 'piston liner / sleeve' then the tabs on the end of the sleeve that goes into the piston will be held in place by the washer: the sleeve tabs sit against the bottom of the inside of the piston with the washer on top of them. The term 'power band' comes from the fact that a washer will increase the compression of the spring once the rifle or pistol is assembled. Even a 1mm thick 'power band' can increase the power of the gun by quite a lot. Very often, particularly after a change of mainspring or addition of a tuning kit, the output power will need adjusting by varying the number or thickness of the 'power bands' or washers used. If a 'top hat' is used then a washer can be used between the face of the top hat and the spring.
Washers can also be used at the trigger block end of the spring, sitting between the spring guide and the spring.
Although not terribly common, beware that a gun can become 'oversprung'. This defines a state that increasing the compression of the spring beyond a certain level will actually decrease the power of the gun. It is a strange phenomenon and it seems counter intuitive to need to remove washers or indeed remove coils of a spring in order to raise output power. But it is a real effect. This is not the same as a spring being 'coil bound', which is explained above.
I mentioned 'piston sleeve / liner'. This is a thin sheet of steel (shim steel or from a beer can) or ptfe (typically maybe 0.5mm thick) fitted inside a piston between the piston wall and the mainspring. In so doing, it reduces the space between the spring and the piston wall and can eliminate annoying spring twang. It also helps prevent grease / oil from migrating through the slot in the piston to the front of the piston and the piston head / seal.

Cheers, Phil

[andrewM]

Thank you, all, for these most informative responses, which I will place into my 'airgun archive'. I wonder if those in charge of this site might consider a 'sticky', with a glossary of terms and an explanation of the various internal components of a spring gun. This would be enormously helpful to many.

Phil, you said: "The term 'power band' comes from the fact that a washer will increase the compression of the spring once the rifle or pistol is assembled. Even a 1mm thick 'power band' can increase the power of the gun by quite a lot."

I had no idea that a 1mm 'power band/washer' would have such a profound influence on power. Questions:

1 If the spring is compressed in such a way and is under pressure (I understand this to mean increasing the pre-load), will it not weaken the spring after a period? I understand some rifles have considerable 'pre-load', the FWB Sport being an example. Surely, after many years, the 'pre-load' will cease, as the spring weakens? Or could it be that using the rifle (and therefore the spring), keeps the spring 'tuned'?

2 Presumably, removing coils on the spring raises the swept volume within the cylinder. But from what you say, there is a point when removing coils so reduces the power of the spring that the increase in the swept volume is cancelled.

3 "It (a piston sleeve/liner) also helps prevent grease / oil from migrating through the slot in the piston to the front of the piston and the piston head / seal." I did not understand this. On my Mk3, I greased the inside of the piston, so the spring could move without friction at the top of the piston. However, I also greased the outside head of the piston, so it could slide more freely within the cylinder. I presumed, as the leather washer moved up and down the cylinder during cocking/firing, it would place oil behind the leather washer (via the wall of the cylinder) and place grease to the front of the leather washer. Is this a problem? Surely grease around the outside of the cylinder reduces wear (ie abrasion between piston and cylinder wall)? Or should I have not greased the outside of the cylinder?

Answers/views very gratefully received!

Rgds to all
A

[Benelli B76]

Removing coils from the spring has no effect on the swept volume. The piston rod will always latch on the sear in the same position. Therefore the piston head will always be at the same distance from the end of the compression chamber.

Baz

[Geezer]

1 If the spring is compressed in such a way and is under pressure (I understand this to mean increasing the pre-load), will it not weaken the spring after a period? I understand some rifles have considerable 'pre-load', the FWB Sport being an example. Surely, after many years, the 'pre-load' will cease, as the spring weakens? Or could it be that using the rifle (and therefore the spring), keeps the spring 'tuned'?

2 Presumably, removing coils on the spring raises the swept volume within the cylinder. But from what you say, there is a point when removing coils so reduces the power of the spring that the increase in the swept volume is cancelled?

1. Good springs don’t weaken with use. At least not in a linear way. A new spring with a little use takes a “set” (shortens a bit) and then works at the same performance level for many thousands of shots. Springs are at least as likely eventually to break from the stresses of repeated firing (especially if they are not well supported by the spring guide) as they are to weaken.

2. Nope. The piston will still be held in the same place by the sear when cocked, regardless of the length of the spring behind it. So the swept volume doesn’t change.

[rancidtom]

Removing coils from the spring has effect on the swept volume. The piston rod will always latch on the sear in the same position. Therefore the piston head will always be at the same distance from the end of the compression chamber.

Baz

I think you mean NO effect on swept volume Baz, (I know, hurried typing, forgetting to check before posting...)
Just to clarify for the OP, "Swept volume" is the volume of the compression chamber in front of the piston when cocked minus the volume of the compression chamber when the piston is at the end of its travel. It is usually easier to state "piston travel" in millimeters also known as "stroke".
Shortening or lengthening the spring, adding or subtracting washers has no effect on the piston travel.

[Phil Russell]

[QUOTE=andrewM;7600732]Thank you, all, for these most informative responses, which I will place into my 'airgun archive'. I wonder if those in charge of this site might consider a 'sticky', with a glossary of terms and an explanation of the various internal components of a spring gun. This would be enormously helpful to many.

Questions:

1 If the spring is compressed in such a way and is under pressure (I understand this to mean increasing the pre-load), will it not weaken the spring after a period? I understand some rifles have considerable 'pre-load', the FWB Sport being an example. Surely, after many years, the 'pre-load' will cease, as the spring weakens? Or could it be that using the rifle (and therefore the spring), keeps the spring 'tuned'?

It is true that with time and much use, a spring can get weaker, noticeable by a drop in power. This is why it is often necessary to replace the mainspring. The power can sometimes be raised to its original level by adding a washer or two but beware of becoming coil bound.

2 Presumably, removing coils on the spring raises the swept volume within the cylinder. But from what you say, there is a point when removing coils so reduces the power of the spring that the increase in the swept volume is cancelled.

Not so: The swept volume is the volume in the cylinder in front of the piston head when the rifle is in the cocked position. The spring sits inside the piston and has no influence on swept volume.

3 "It (a piston sleeve/liner) also helps prevent grease / oil from migrating through the slot in the piston to the front of the piston and the piston head / seal." I did not understand this. On my Mk3, I greased the inside of the piston, so the spring could move without friction at the top of the piston. However, I also greased the outside head of the piston, so it could slide more freely within the cylinder. I presumed, as the leather washer moved up and down the cylinder during cocking/firing, it would place oil behind the leather washer (via the wall of the cylinder) and place grease to the front of the leather washer. Is this a problem? Surely grease around the outside of the cylinder reduces wear (ie abrasion between piston and cylinder wall)? Or should I have not greased the outside of the cylinder?

It is all a case of balance. Excess grease / oil in front of the piston head can lead to dieseling when the grease / oil combusts during the high temperatures generated during air compression in the cylinder. This leads to excess pressure, an increased power for the shot, that 'not to be forgotten smell of combustion' and a loud 'crack' noise. all in all, not a desirable phenomenon as it can cause damage. It has been said, and generally accepted (I believe) that old style leather washers rely on a certain level of combustion of the lubricant oil used on the leather piston washers. Having a good fitting leather washer with proper lubrication leads to a very controlled release of combustible material. The Webley Service rifle used sealing rings akin to piston rings in a car engine. The recommended lubricant was motor oil. Older air rifles with leather piston washers invariably came with a recommendation in the Manual to add a drop or two of lubricant oil (usually a motor oil) through the transfer port into the cylinder after a certain number of shots to add more lubricant to the piston washer. However, you do not want excess material getting in front of the piston head during the compression stroke; a piston sleeve / liner heps reduce the chance of this happening as any lubricant on the spring is not flung through the cocking slot onto the cylinder wall where it can migrate in front of the piston. You do need some lubrication to allow easy movement of the piston; but not too much. I have heard some people run their modern piston heads without lubrication.

Hopefully this helps ... welcome to the 'dark arts'

Cheers, Phil

[thisisdonald]

Another reason people make to hats and guides, is to replace original, poorly fitting, badly finished metal guides with perfectly fitted, low friction high tech plastic ones. Not only does this keep the spring straight and reduce torque and twist upon firing, but it smoothes out the firing cycle and eliminates damage to the inside of the cylinder, due to badly fitted and finished original components.
You might also hear about piston buttons and piston rings. This is where the tuner (or manufacturer in a few cases) has reduced the diameter of the piston and fitted a low friction plastic (delrin/acetal) around the outer circumference of the piston, allowing it to ride inside the cylinder with no metal to metal contact.

[Benelli B76]

I think you mean NO effect on swept volume Baz, (I know, hurried typing, forgetting to check before posting...)
Just to clarify for the OP, "Swept volume" is the volume of the compression chamber in front of the piston when cocked minus the volume of the compression chamber when the piston is at the end of its travel. It is usually easier to state "piston travel" in millimeters also known as "stroke".
Shortening or lengthening the spring, adding or subtracting washers has no effect on the piston travel.

Yes, you are right thanks, I missed out "no" !

Baz

[andrewM]

Phew, this is all heavy going stuff for me! Thank you, all, for the further information. More questions, however:

1 Yes, I now see that swept volume is not raised by removing coils, because the cocking slot latches on to the sear at the same place - as you say Baz/Geezer/Phil. But how does removing coils raise the power of the rifle? How is this possible, given the swept volume remains unchanged? I am missing something in this.

2 Thisisdonald says: "This is where the tuner (or manufacturer in a few cases) has reduced the diameter of the piston and fitted a low friction plastic (delrin/acetal) around the outer circumference of the piston, allowing it to ride inside the cylinder with no metal to metal contact."

Everyone: Should the piston head and skirt actually be in contact with the cylinder wall? Given it is metal on metal, would this not cause abrasion? One would think a piston head and skirt would be considerably narrower than the cylinder but this seems not to be so, with almost a tight fit. One would think that only the leather washer needed to hug the cylinder walls (to prevent air leakage). It also seems that many here think that the piston head and skirt should not be greased/lubricated. Is there a definitive answer on this?

A

[thisisdonald]

If there is no contact between the piston skirt and the cylinder, when you cock the gun, it will cant to one side and start scoring. Metal on metal is only abrasive in the absence I'd lubricant and if there is some kind of foreign particles in there.
Look at semi auto and full auto powder burners - metal on metal contact everywhere. Look even at the old fwb rifles, I'm sure they actually have a metal ring instead of a piston seal.

[Geezer]

1 Yes, I now see that swept volume is not raised by removing coils, because the cocking slot latches on to the sear at the same place - as you say Baz/Geezer/Phil. But how does removing coils raise the power of the rifle? How is this possible, given the swept volume remains unchanged? I am missing something in this?

Removing coils almost always decreases power.

The key thing behind all your questions is that the physics inside a springer is really complicated. PCPs are comparatively simple. In a PCP the valve releases a big lump of pre-compressed air which expands to push the pellet down and out of the barrel.

In a springer, the mechanism in tiny fractions of a second compresses the air, heats it, sometimes burns some lubricant, persuades the pellet to start moving (so-called start pressure - different pellets start to move differently according to friction and mass), moves the pellet down the barrel, and finally the piston comes to a stop, sometimes by going forward against a cushion of compressed air and then “bouncing” back before finally stopping.

Different springers do all this slightly differently. Even different springers of the same make, model, and calibre are not 100% identical in their firing characteristics, because of minor differences in production tolerances.