Rant!!

[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.

[Zomboid]

Yep heating expanding stuff...

That's what happens EVERY time I miss.

[BTDT]

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.

I'll need to have a think about that.

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.

I did state that I was specifically addressing the OP's problem, where the muzzle energy dropped, and stayed dropped, Rob.

Springer muzzle energy can indeed temporarily rise or fall with temperature, I believe entirely due to kinetic friction. That's very common.

[RobF]

I'll need to have a think about that.



I did state that I was specifically addressing the OP's problem, where the muzzle energy dropped, and stayed dropped, Rob.

Apologies Jim, I mssed that.

Springer muzzle energy can indeed temporarily rise or fall with temperature, I believe entirely due to kinetic friction. That's very common.

Yeh, aint it just