Top hats..

[clarky]

It seems that almost everyone has now accepted the Top hat as an aid to more consistent control over the mainspring at the nose end of a springer but its the material of this component that is more questionable...
Most seem to be going the Delrin route, either self made, or part of a purchased kit. I have produced a good number myself..
The advantage of one machined from Delrin is we get the spring centring effect but no additional weight with which to impact on recoil..but despite Delrins toughness, i have recently had to replace a few broken ones..my own and those from proprietary kits..

As a materials engineer, i know that Delrin is tough, especially in its POM H composition but Top hats and guides are quite thin walled and we ask a lot of it in a powerful air rifle..
With this in mind i set about testing this material...a little obsessively maybe but think i have come up with a formula to work to.
If you disagree, or have differing experience thats fine...its really for my own work and im just sharing...

Controlled tests were set up in a Rotation test machine which applies torsional loads....and impact torsional loads...with readouts and graphs at failure..
Cutting to the chase, i found that typical 1mm wall stems snapped for fun at the stem/flange interface if using a tiny 0.5mm base flange..Not suggesting anyone ever designed one that thin but wanting to get a picture at the extreme margin...Moving to 1mm thickness base flange increased the yield point almost 2 fold over 0.5mm before breakage occurred. 2.5 fold at the typical 3mm thickness (that many use) and approaching a realistic proposition at the now  250lbs force i was using...
The loading was increased until i reached 4mm thickness when the stem snapped 3 mm above the seating flange..The stem failing ...not the interface..
I repeated the experiment and achieved a similar result..where 4mm flange/stem interface held out.

Conclusion...
A 2mm flange is not quite thick enough if using Delrin....3mm is probably ok but 4mm a relatively safe bet and should reduce breakage potential for people going the Delrin route ..
Why...Well the answer is I dont know!
You would imagine stem/flange interface should be the same strength, regardless of thickness of the bearing flange but its clearly not.
The reason will be researched and form the basis of my part 2 to this post for those who are interested..

[avator]

The amount of back pressure applied to the piston will be a factor as well. If you shoot lighter/looser fitting pellets you'll be letting that piston slam harder into the front end of the air chamber. And if the guns is dry fired.. something will give.
IMO, too many people are power hungry. It's been my experience high 800 to low 900 fps produces the best accuracy in terms of max power. And we all know... a well placed shot is king.
Now before we get all crazy.. I'm speaking of the average over the counter pellets. Not these perfectly casted John Elway spiraling bullets.

[Motorhead]

How the SPRING ends are treated also is something most don't address.  While ground flat they all are, most ignore the SHARP edge of coil as it terminates . This edge DIGS IN and will create a burr which in turn becomes a stress raiser and where the fracture will most likely migrate from.  Grind this edge into slight down turned radius and the spring can freely rotate against the top hat and not dig in and create such issues.
Having made at this point likely 100's of guides & top hats from Acetal, can't say I've had issue as you describe tho my spring ends get dressed up a stated above FWIW and hats stay in the  2 - 4mm thickness's unless more is needed for preload.

Just adding to the conversation ...

[avator]

That is part of my normal home tune routine, Scott. I make sure both ends of the spring spin freely. Then I moly those ends and heavy tar the rest of the spring.

[nced]

It seems that almost everyone has now accepted the Top hat as an aid to more consistent control over the mainspring at the nose end of a springer but its the material of this component that is more questionable...
Most seem to be going the Delrin route, either self made, or part of a purchased kit. I have produced a good number myself..
The advantage of one machined from Delrin is we get the spring centring effect but no additional weight with which to impact on recoil..but despite Delrins toughness, i have recently had to replace a few broken ones..my own and those from proprietary kits..

As a materials engineer, i know that Delrin is tough, especially in its POM H composition but Top hats and guides are quite thin walled and we ask a lot of it in a powerful air rifle..
With this in mind i set about testing this material...a little obsessively maybe but think i have come up with a formula to work to.
If you disagree, or have differing experience thats fine...its really for my own work and im just sharing...

Controlled tests were set up in a Rotation test machine which applies torsional loads....and impact torsional loads...with readouts and graphs at failure..
Cutting to the chase, i found that typical 1mm wall stems snapped for fun at the stem/flange interface if using a tiny 0.5mm base flange..Not suggesting anyone ever designed one that thin but wanting to get a picture at the extreme margin...Moving to 1mm thickness base flange increased the yield point almost 2 fold over 0.5mm before breakage occurred. 2.5 fold at the typical 3mm thickness (that many use) and approaching a realistic proposition at the now  250lbs force i was using...
The loading was increased until i reached 4mm thickness when the stem snapped 3 mm above the seating flange..The stem failing ...not the interface..
I repeated the experiment and achieved a similar result..where 4mm flange/stem interface held out.

Conclusion...
A 2mm flange is not quite thick enough if using Delrin....3mm is probably ok but 4mm a relatively safe bet and should reduce breakage potential for people going the Delrin route ..
Why...Well the answer is I dont know!
You would imagine stem/flange interface should be the same strength, regardless of thickness of the bearing flange but its clearly not.
The reason will be researched and form the basis of my part 2 to this post for those who are interested..

I've personally never had a Delrin guide or top hat failure with my home turned kits, but then I've never used a top hat flange thickness less than .12" (3mm).
When cutting both the spring guide seat and top hat I always machine with a small radius where the flange and guide meet (no sharp corners)........

If I need a flange thinner than .08 (2ish mm) I make the top hat out of metal. As it's been mentioned, I also take the time to break and remove the sharp edge of the spring that's left after the initial "vendor grind". If the spring end grind isn't smoothed out the sharp corner of the last coil will cut a groove at the base flange exactly where it isn't wanted. While the tensile strength of Delrin is pretty good, the impact strength of Delrin is poor it won't tolerate a lot of "slamming".........
 

As far as minimizing top hat weight is concerned, the lightest top hat isn't always the best depending on the spring and pellet weight being used. It r00000000000000000eally is a game of balance and I've found that a bit of extra top hat weight is beneficial for higher stress tunes and heavier pellet use.

                                                             

[clarky]

Nice responses. Yes I here you with the radius at the interface etc.
My post was aimed purely at the material and tested somewhat above that of the pounding it actually gets in this situation.
Good to hear your experiences..

[nced]

Nice responses. Yes I here you with the radius at the interface etc.
My post was aimed purely at the material and tested somewhat above that of the pounding it actually gets in this situation.
Good to hear your experiences..

IMHO.....IF the springer tune is "balanced" there should be little "pounding" because the piston motion "should" come almost to a halt on the high pressure air cushion created just before the pellet "pops the leade". Matter of fact, if the pellet is too heavy relative to the spring power the piston will actually rebound on the "high pressure air cushion" and then smack the end of the receiver after the "pellet pops". If the pellet is too light relative to the spring power the pellet will "pop the leade" before a full cushion of air had built up causing the piston to slam onto the end of the receiver.