Transfer port size !

[gedfinn 2]

This is mainly directed to Bigtoe & TonyL, & There excellent work on swept volume & Transfer ports !

Having a sort out today, I came accross a number of cylinder end caps with their various transfer ports,
I'lll start with the HW35 leaking problem ?
There is nothing wrong with the design of attaching them to the cylinder IE by Brazing, The problem is the operator,

I thought I would bore you a little with the problem & Answer,
Brazing is by the use of melted Brass as the medium, Brass is a combination of Copper & Zinc in various quantities,
Lets take a typical 60/40 ( Copper & Zinc )
The parts to be joined are heated , A Flux added & The lower melting brass ( In this mix Copper around 1060 C & Zinc around 420 ) Is added to the heated parts ,

These parts need a small gap for the liquid brass to enter ? I mention this as the adhesion betwix the adjoining parts is by the forming of a cohesive bond ( Thats stuck on to you Ha ha, )
Now soft Solder ( Thats Lead & tin ) Again lets say, Around 60/40 respectively, Works in a completely different way ? It needs small gaps between the parts to be joined & Capillary action ensures the fluxed liquid solder creeps along the adjoining parts ensuring a complete seal at a lower temperature,

A number of guns have the end of the cylinders sealed by Lead/Tin solder & work very well, ( Mainly Relum & Their counterparts )

To go back to the Brazed joint ?
The problem is Flux/Temperature/Flame , If overheated the Zinc in the brass will change from a Solid to a Liquid, To a Gas,
The joint is being overheated, The Zinc in the brass is volatizing, forming gas bubbles in the joint, Hence the leak,
The not overheated ones are fine !!!!!

A Brazed joint needs to be " Just " Enough heat, The use of a flux ensures adhesion &&&&& A Silica type layer over the molten metal to avoid the volatized Zinc from leaving ( In the form of a gas )
Furthermore , A slightly Oxidising flame is used when Welding or Brazing Brass for the same purpose " Not many people know that " Ha ha,

If I wanted to reduce transfer port size, I would remove the end cap,
Countersink it from the inside & Make a piston with a point ,
( Ala the early Airsporter ) This would virtually eliminate the transfer port size , PS Soft solder it on ?

Its time for my medication now ! A large whisky Ha ha, Ged.

[diipii]

Don't reduce the port size. I drilled mine out to 4mm. What does Cardew know ? I also lightened the piston by drilling holes in the shaft and raised the point of impact 2 notches on the rear sight and the gun (HW35) shoots very smoothly. p.s. you will get a zillion pet theories about this but that's all they are.

[TonyL]

Don't reduce the port size. I drilled mine out to 4mm. What does Cardew know ? I also lightened the piston by drilling holes in the shaft and raised the point of impact 2 notches on the rear sight and the gun (HW35) shoots very smoothly. p.s. you will get a zillion pet theories about this but that's all they are.

What Bigtoe and T20 will aim to do, on most examples, is not to reduce, but to increase the TP diameter. What they aim to reduce is the LENGTH of the TP, raising the static comp ratio, thus increasing efficiency.
If you look at the most efficient guns on the market, they have short transfer ports, like the HW77/97, HW80, Diana 52 variants and I'd guess the Air Arms TX and Prosport and the HW95.

The HW35, as much as I love them.....I had three until this morning, now four, is an old design and not as efficient. Lovely, lovely rifle with lots of character and charm, accurate and powerful and able to do the business today, but it isn't as efficient as some of these later models.

Until you look at Mick's HW35/LGV 25mm conversion with the short transfer port.

[bigtoe01]

Its not about solely increasing the port diameter or decreasing it, its all related to the static compression which dictates where the piston will bounce on the shot cycle.

Going to 4mm on a 35 being honest will border or slammy UNLESS you lighten the piston,when you lighten the piston you need less static compression to cushion the piston at the end of the shot cycle. Its still way to large imo, its better to reduce the swept volume to 39cc or so and cure the leaky breech and retain the 2.8mm port.

The 35 is not an efficient design though...it can be made to shoot nice however.

[QUIGLEY]

A nice bevel or radius on the cylinder side of the transfer port is a good aid to gas flow.
A smaller port with this works better than a slightly larger one without.
Edd.

[bigtoe01]

A nice bevel or radius on the cylinder side of the transfer port is a good aid to gas flow.
A smaller port with this works better than a slightly larger one without.
Edd.

you gain lost volume...so you are actually dialling in inefficiency, do you feel the gains in port flow outweigh the lost volume?. Air venturing work best when air is being dragged thru the port such as a carburettor on an engine, when air is being forced thru a port i think the gains are minimised somewhat. It actually looks better to have a tiny radius and a port large enough to ensure good flow but small enough to ensure the pressure build up is high enough to cushion the piston. The tiny radius minimises lost volume.

One thing I have considered and I mentioned this to Prof Mike at the Midland is a transfer port utilising the Bernoulli effect coupled with a venturi, there may be a way of increasing the amount of air behind the pellet so increasing the efficiency....not tested it though.

They use this system to blow up evacuation slides on aircraft, they have gas cylinders that if direct coupled would only fill around 1/3 of the slide, using a slick designed venturi they are able to tripple the amount of gas within the slide by dragging air into the system also.

how to fill a large bag with 1 breath... http://www.youtube.com/watch?v=UujAMPv3y-A


The trick would be closing off the ports that drag the air in as the air flow reverses and becomes negative.

[evert]

If one wanted to get good power, but still retain a soft and good shot cycle, what would the optimum transfer port diameter be on an old hw35?

And why reduce the swept volume to 39cc if the long port makes the gun inefficient in the first place?

My old hw35 gives me abot 9,2fpe, would like to see around 10,5-11, but still have a good cycle.

[TonyL]

Its time for my medication now ! A large whisky Ha ha, Ged.

I'm not surprised after all that, GED.

Cheers for that, not boring in the least.

Bigtoe is the man on this and who's willing to play and experiment. I'm just the Airgun-obsessed nutter with a gazillion theories and ideas racing around in my head constantly!

Think I might just do a little JD now!

[magicniner]

Brazing is by the use of melted Brass as the medium, Brass is a combination of Copper & Zinc in various quantities,

Brazing is a process for joining similar or dissimilar metals typically using a filler metal with a lower melting temperature that can typically, but not exclusively, include a base of copper, silver or nickel.

- Nick