Building for low friction.

[Hangtight]

Surprised myself today. I've revisited a build I did a few months ago after I did something stupid and bust a tooth off the piston. It was put together fairly carefully last time, and shot and cycled very nicely, but this time I spent some hours playing far more attention to the preparation of the components to get the smoothest possible running. And boy, has it paid off.

Is a fairly conservative spec. 16TPA motor, 13:1 gearing, 7.4v lipo and M100 spring. It used to have no overspin and needed 30ms of pre cocking. Now if you use even 10ms of pre cocking it double cycles. I've actually had to turn on the active braking to stop the overspin.

At some point I'll check to see if the ROF has improved, but certainly you can now pull the trigger on semi auto as fast and for as long as you like and the motor doesn't even start to get warm.

 

[Sitting Duck]

Always the way, the more attention to detail - the smoother it runs

(nearly always bar the odd bit of crap luck of stuff failing/$hit happens)

 

After doing the usual checks for piston binding, smooth/polish guides for piston/tappet plate

Often I may have two or three possible piston choices and choose the one that is just right

(or need least work before swissing (if I can be bothered - depends on build and my OCD))

Good wiring - very neatly arranged etc......

 

The spin test really speaks volumes, some boxes love SHS gears but not all

Some like cheaper ZCI/Core/Big Dragon gears that seem to run a bit smoother - though bevel not so robust imho

(heck I have even mixed a 16:1 SHS spur & ZCI sector up & found a nice 4 AR lug bevel than ran real smooth)

4 lugs x 5.75 bevel turns = 23 AR lugs per cycle which is ok

(wouldn't use a 4 lug bevel on a 13:1 set (about 18 lugs/cycle))

(keeping the SHS 6 lug bevel as spare bevel for 12 or 13:1 setups or DSG etc....)

 

Most of the time I start of spinning up the spur & sector first and roughly shim/space them out

If these two spin crap or are noisy then there is little point continuing with that set "as is"

(try a different set/make - heck even another ratio perhaps or experiment if all else fails)

Keep spinning those gears tilting box on its sides to see if sector's cam is rubbing on tappet rail etc....

lift sector and see if it pushes up the spur viewing the selector plate side = avoiding swirly marks on spur rubbing

(Most of all this is just done by holding the box together as you got a rough idea shimming play halves when tightened etc...)

 

If those two gears "seem" sweet, then add in bevel and yes there will be a little more noise of course

BUT at least you know where the little tiny extra noise is coming from....

How bad the bevel is - well hopefully not much extra noise but at times one gear can make all the difference I have found

 

Setting sector in other half of box - checking the operation of tappet plate, if sector lug catches on tappet guides

Ensuring tappet plate or fin doesn't catch/snag/hit - especially on re-enforced boxes etc.....

Trying to shim spur as low as possible but watching the outer edge of spur doesn't rub on bevel/sector bushings

(minor modding required with tiny dremel disc in some instances where thick bushings protrude into box)

 

BLOODY HELL - got well carried away as per usual just on installing 3 friggin' gears !!!!!

(no wonder I take too long at times)

 

Soz you know all this but just confirming - YES more attention = more smoothness = more efficiency/rps etc....

(and less friction/heat/amps etc.....)

I'd sooner take a couple of days to build a box than a couple of hours as the end result is worth it imho

[ImTriggerHappy]

Sometimes paying attention to the smallest details can work wonders.

 

Now try and work on variable drag for your piston. Fast at start slower at the end.

[Hangtight]

3 hours ago, ImTriggerHappy said:

Sometimes paying attention to the smallest details can work wonders.

 

Now try and work on variable drag for your piston. Fast at start slower at the end.

 

What will be the effect? The only way a can think of achieving that would be the right combination of O ring and angle on the rear face of the O ring groove?

At the moment I'm trying to size O rings so the piston falls down the cylinder under its own weight and the O ring only seals when it's accelerated sharply.

[Samurai]

7 minutes ago, Hangtight said:

 

The only way a can think of achieving that would be the right combination of O ring and angle on the rear face of the O ring groove?

Air break.

The guys at sniper ops are strongly believing that it is better to have the piston accelerate slower at first and gain momentum slowly. They say it has quite a bit of impact on accuracy (pun intended ).

Having an air break will result quieter piston impact and less stress on the cylinder head.

[ImTriggerHappy]

1 hour ago, Hangtight said:

 

What will be the effect? The only way a can think of achieving that would be the right combination of O ring and angle on the rear face of the O ring groove?

At the moment I'm trying to size O rings so the piston falls down the cylinder under its own weight and the O ring only seals when it's accelerated sharply.

Variable rate spring and tapering cylinder.  Haven't got it quite right yet but looks promising. 

Fast at start so it gets good backspin across the hop then slows so the rest of the air is travelling slower. Theory is that the slower air flow creates less turbulence around the bb and also ahead so everything is more stable. The air travels at a much higher rate than the bb which is more necessary at the start to give initial acceleration but less necessary once bb is in motion. Its more beneficial to have a slower slighty longer push. It also stops the piston slapping against the cylinder so hard. If I ever get round to fine tuning and finishing it will be interesting to see results. Would be too expensive and finicky to ever be more than an experiment though.

 

I have tried what Samurai is on about but found the slower speed across the hop to cause issues and had to give it more hop than normal to counter act it. Maybe the higher fps of sniper means that is less of an effect as the initial acceleration would still be quite high.

[Hangtight]

This corresponds to what I found in my Mancraft BASR. It was far more efficient when I reduced the initial gas volume and used a higher initial pressure, but I took this too far and got really inconsistent hop as I think the BB was being accelerated over the hop too violently. What eventually got the best result was the smaller gas volume to give the high pressure initial push with a fairly sharp decrease in pressure behind the BB, combined with a flow restrictor in the air nozzle to control the initial acceleration.

It's now very efficient (120+ shots from a 12g cartridge), very consistent and accurate, and a useful bonus is its almost silent.