Rant!!

[flyingfish]

How is the breach seal looking on the 98?
Rich

It is brand new. Bought 2nd hand in March. Fitted the tbt maxi short stroke kit, new barrel shims and breech seal

Pete

[lensman57]

Bloody springers
Had a nice plink Saturday at LVAGC. Noticed that the 98 started shooting low after a 100 or so shots. TX worked well though. Got home cleaned guns, chrono the 98 650fps. Strange she shot well at the bash.
Chrono the TX. Even worse. Fitted 2 power washers still only just 10ft/lb. Stripped the TX today, can't see anything wrong. Re_assemble 11.25 - 11.17.
The 98 still low over chrono. Sort that tomorrow I hope

Rant over

Pete

It is the high temps. The metal expanding and the seal loosing grip is probably the culprit.

A.G

[RobF]

It is the high temps. The metal expanding and the seal loosing grip is probably the culprit.

A.G

Opposite. The metal hardly expands. Done all this with a dial gauge from 10-30 deg. But the seals expand real quick and add massive amounts of friction.

[rhyslightnin]

I will try pulling the barrel through. Its the short stroked one you shot at the bash Tony.
I think the heat must be involved but why did the TX cure itself?
Pete

Because it's a 'cooler' gun??! B'dum tssss!!

[CapitalBee]

Basically, everything has a co-efficient of thermal expansion (the amount it will grow per 1C temp increase). On a standard 25mm diameter bore on a hot day, without doing anything, a tube will increase in sizes 6 thousandths of a millimeter for a 10C temp increase. A seal will increase by 14 thousandths of a millimeter over the same temperature. The warmer the rifle gets, the tighter the fit of the seal in the tube. Now depending upon the design of the seal, it may distort and take up the growth or simply become harder to move in the tube, hence the power loss.

[RobF]

Basically, everything has a co-efficient of thermal expansion (the amount it will grow per 1C temp increase). On a standard 25mm diameter bore on a hot day, without doing anything, a tube will increase in sizes 6 thousandths of a millimeter for a 10C temp increase. A seal will increase by 14 thousandths of a millimeter over the same temperature. The warmer the rifle gets, the tighter the fit of the seal in the tube. Now depending upon the design of the seal, it may distort and take up the growth or simply become harder to move in the tube, hence the power loss.

Yep, plastic way more expansive than metal...

Think your maths could be out by 10... dunno, check mine

Linear coefficients of expansion for common metals are

aluminum : 0.000023 (m/moC)
steel: 0.000012 (m/moC)
PTFE: 0.000120 (m/moC)

so for every 10 degrees a steel bar 0.025m expands (0.025 x 0.000023 x 10) = 0.00000575m or 0.0000575cm or 0.000575mm... which is 6 ten thousandths of a mm rounded up.

Ally will be twice that, 12 ten thousandths of a mm, or 1/1000th of a mm = 1 micron

PTFE will be 6 times that.

common materials can be found here

http://www.engineeringtoolbox.com/li...ents-d_95.html

When I measured ours we so nothing on gauges that would register 1 ten thou of an inch, but the AA seal grew 0.2mm. Can't remember the temp range for that. It's on a post somewhere here. The friction went up massively. Which corresponded to the 80fps loss i saw going to Portugal and shooting there.

[CapitalBee]

There's a 100 cm to a metre, so your conversion was out by a factor of 10. The principles applied to the piston seal though explain the loss in power the hotter it gets and explains why some piston seal designs cope better with temperature increase.

[RobF]

There's a 100 cm to a metre, so your conversion was out by a factor of 10. The principles applied to the piston seal though explain the loss in power the hotter it gets and explains why some piston seal designs cope better with temperature increase.

ok...


where am i out?

0.025m = 2.5 cm = 25mm correct?

steel = steel: 0.000012 (m/moC) ( got that wrong and used ally's expansion before... but it's not out by 10)

10 degrees of change

so 0.025m x 0.000012 = 0.0000003meters per degree

x 10 degrees

= 0.000003 meters

= 0.000030 cm

= 0.000300 mm

= 3 ten thousandth of a mm (which would be 6 ten thousandths of a mm if it was ally)



Where am i out by 10? (just cos i can't see it that's all)

Certainly agree with the theory, and the observations i've seen, just can't find my mistake on my maths, and as it's not my strong point i'm just curious.

[CapitalBee]

0.000006 Metres

= 0.0006cm ; 100cm to the mtr so the decimal place goes over two

=0.006mm ; 10mm to the cm

[RobF]

0.000006 Metres

= 0.0006cm ; 100cm to the mtr so the decimal place goes over two

=0.006mm ; 10mm to the cm

ah yeh doh!

[rhyslightnin]

You lost me after the first decimal place!!

[CapitalBee]

Rhys the mild steel expand a gnat's cock when it gets warm and the polyurethane seal expands a smidgen, hence the problems

[BTDT]

The temperature of the seal has an effect on its kinetic (sliding) friction; the ‘hotter’ it is, the greater its diameter, the greater the friction. I measured the kinetic friction of a polyurethane closed face (more like HW than Air Arms) seal in a 25mm cylinder at -7C (1.4N), 20C (4.38N) and 44C (7.36N). That suggests a huge rise in the drag on the piston when the seal becomes hotter, but that’s only part of the picture, because the seal also expands in diameter (and to a far greater extent) as the air pressure in the cylinder rises during the compression stroke.

I repeated the kinetic friction test with the equivalent pressure on the seal face of 150 psi of air pressure, and the results were -7C (88N), 20C (111N) and 44C (115N). As the cylinder air pressure rises, the effect of elevated seal temperature on overall kinetic friction diminishes.

I doubt this explains the OP’s loss of muzzle energy; if it did, then every springer with a synthetic seal would have behaved the same on Saturday, and the forums would be awash with posts on the subject. I think a strong contender is evaporative losses from the cylinder lubricant due to the high temperature leaving a very thick, almost dry film, and other tests have suggested that piston seal kinetic friction is more than doubled with the loss of ‘wet’ lubrication. That would really bring the muzzle energy down.

[CapitalBee]

The temperature of the seal has an effect on its kinetic (sliding) friction; the ‘hotter’ it is, the greater its diameter, the greater the friction. I measured the kinetic friction of a polyurethane closed face (more like HW than Air Arms) seal in a 25mm cylinder at -7C (1.4N), 20C (4.38N) and 44C (7.36N). That suggests a huge rise in the drag on the piston when the seal becomes hotter, but that’s only part of the picture, because the seal also expands in diameter (and to a far greater extent) as the air pressure in the cylinder rises during the compression stroke.

I repeated the kinetic friction test with the equivalent pressure on the seal face of 150 psi of air pressure, and the results were -7C (88N), 20C (111N) and 44C (115N). As the cylinder air pressure rises, the effect of elevated seal temperature on overall kinetic friction diminishes.

I doubt this explains the OP’s loss of muzzle energy; if it did, then every springer with a synthetic seal would have behaved the same on Saturday, and the forums would be awash with posts on the subject. I think a strong contender is evaporative losses from the cylinder lubricant due to the high temperature leaving a very thick, almost dry film, and other tests have suggested that piston seal kinetic friction is more than doubled with the loss of ‘wet’ lubrication. That would really bring the muzzle energy down.

Would it have an effect if the internal volume of the cylinder had been reduced? Both of these had been short stroked I believe. Without excess volume the whole process becomes more sensitive I'd guess.

[RobF]

The temperature of the seal has an effect on its kinetic (sliding) friction; the ‘hotter’ it is, the greater its diameter, the greater the friction. I measured the kinetic friction of a polyurethane closed face (more like HW than Air Arms) seal in a 25mm cylinder at -7C (1.4N), 20C (4.38N) and 44C (7.36N). That suggests a huge rise in the drag on the piston when the seal becomes hotter, but that’s only part of the picture, because the seal also expands in diameter (and to a far greater extent) as the air pressure in the cylinder rises during the compression stroke.

I repeated the kinetic friction test with the equivalent pressure on the seal face of 150 psi of air pressure, and the results were -7C (88N), 20C (111N) and 44C (115N). As the cylinder air pressure rises, the effect of elevated seal temperature on overall kinetic friction diminishes.

I doubt this explains the OP’s loss of muzzle energy; if it did, then every springer with a synthetic seal would have behaved the same on Saturday, and the forums would be awash with posts on the subject. I think a strong contender is evaporative losses from the cylinder lubricant due to the high temperature leaving a very thick, almost dry film, and other tests have suggested that piston seal kinetic friction is more than doubled with the loss of ‘wet’ lubrication. That would really bring the muzzle energy down.

Surely though that would mean that when cooled the evaporation wouldn't be reabsorbed and the MV would stay low? In my experience it doesn't. It's really not uncommon in target shooters to see MV drop in heat, and increase in cold, and then return to back to where it normally is when the normal conditions are repeated, all without changing the lube.