All Springer/NP/PCP Air Gun Discussion General > "Bob and Lloyds Workshop"

Dual Tank Benchrest Rifle

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I made the rest of the valve parts today.... the cages/spiders that support the valve spring and balance chamber piston and allow air into the valve, through eight 5/32" holes (over twice the valve throat area).... and the adjustable pistons for the balance chambers....

I haven't cut the O-ring groove in the 1/4" pistons yet, but they are threaded and loctited to the 1/8" shafts, and are a loose fit in the 5/32" center hole of the spider, to avoid any alignment issues.... When the valve is closed, their normal position will be back towards the seat, with the distance between the end of the piston and the inside of the balance chamber in the poppet adjustable with the 5-40 Nyloc nut.... I filed flats in the piston so that I can use a 7/32" wrench on it to hold it when adjusting the locknut.... I can adjust the clearance from less than 0.070" to over 0.200", which varies the volume of the balance chamber, and so the amount of "blow-open" tendency of the valve....

The valve stem protrudes 0.200" from the end of the valve, and has at least 0.250" of travel before the spring goes coil bind.... The balance chamber has an O-ring for a bumper to absorb any impact and reduce the peak stress on the spider.... Under normal operation, the hammer will strike the valve stem, the lower pressure in the balance chamber will help to lift the poppet off the seat, the poppet will hit the end of the balance chamber piston and the two will travel together, against the spring force, and rising pressure inside the balance chamber, until the travel reverses and the valve closes.... It will be interesting to see how much affect adjusting the balance chamber volume has on the hammer impact required and the valve dwell....



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